7th Workshop on Nuclear Fission and Spectroscopy of Neutron-Rich Nuclei

9 Mar 2026, 08:00 → 13 Mar 2026, 20:00 Europe/Paris

Salle Totem, Le Bachat, Chamrousse, France

Fission 2026:  7th Workshop on Nuclear Fission and Spectroscopy of Neutron-Rich Nuclei

We are pleased to invite you to participate in the 7th Workshop on Nuclear Fission and Spectroscopy of Neutron-Rich Nuclei (FISSION 2026) to be held from 9th-13th March 2026 in Chamrousse, France. The scope of the workshop is large and intended for the international communities of both nuclear fission and the nuclear structure of neutron rich nuclei, to promote synergies and exchange expertise. 
 

The topics covered are:

- Fission dynamics and theory
- Prompt emission in fission
- Nuclear structure of neutron-rich fragments
- Fission yields (mass and charge)
- Facilities and experimental techniques
- Quasi-fission and superheavy elements
- Fission isomers
- Applications

The workshop will be held for the first day (9th march) at the Institut Laue-Langevin (ILL) in Grenoble, with a guided tour of the reactor facility given in the morning and the first session of talks in the afternoon. Buses will then be organised between Grenoble and Chamrousse in the evening. 

There will be no workshop fee, but participants are expected to cover their own costs of travel and accommodation.

Some financial support for young researchers presenting research performed at EUROLABS facilities is available. Please contact the organisers for more details.

Important dates:

Registration and abstract submission for the workshop are now open.

The workshop will consist of both invited talks and shorter submitted contributions. The deadline for abstract submission is 9th January 2026. Accepted speakers will be notified before January 16th 2026. Registration closes on 31st January 2026. 

       

 

            
 

Monday, 9 March

08:30 → 09:30 Registration

09:30 → 09:40 Introduction to the workshop

09:40 → 10:30 session 1 (Chair: C. Michelagnoli)

09:40 From shape isomers to superdeformation at high spins

Extreme shape coexistence, of which shape isomers observed in actinide nuclei at spin zero are peculiar examples, is also at the origin of the phenomenon of superdeformation in rapidly rotating nuclei. Nevertheless, the studies of superdeformation have developed virtually in a completely independent manner from other shape-coexistence investigations, mainly because they were carried out almost always in the high-spin regime, while the others were related to low-spin phenomena [1,2].
In this talk, we will focus on Ni isotopes, where example of shape-isomer-like 0+ excitations have been found in 66Ni and 64Ni [3,4], and we will discuss new results on 62Ni, in which superdeformed rotational bands at high spin, decaying out at around spin 8, are known [5]. Following a neutron capture experiment with the FIPPS spectrometer at ILL, ten 0+ excited states below 6.5 MeV have been observed in 62Ni, with a fragmented gamma decay, in close agreement with predictions from state-of-the-art Monte Carlo Shell Model calculations. This allowed to identify, among them, the band-head of the superdeformed rotational band at high spins, which turns out to be fragmented over mainly two highly deformed and triaxial 0+ configurations.
The work allows to make a connection between shape coexistence at spin 0 and superdeformation at high spins, and to trace, microscopically, the origin of the fragmented decay pattern of the superdeformed band.

[1] S. Leoni, B. Fornal, A. Bracco, Y. Tsunoda, T. Otsuka, Prog. Part. Nuc. Phys. 139 (2024) 104119.
[2] S. Leoni, B. Fornal, N. Marginean, and J. N. Wilson, Eur. Phys. J. Spec. Top. 233 (2024)1061.
[3] S. Leoni, et al., Phys. Rev. Lett. 118 (2017) 162502.
[4] N. Marginean, et al., Phys. Rev. Lett. 125 (2020) 102502.
[5] M. Albers et al., Phys. Rev. C 94 (2016) 034301.

Speaker: Silvia Leoni (University of Milano and INFN Milano)

10:05 Fission studies with the nu-Ball2 array

A series of recent experiments to perform high-resolution gamma ray spectroscopy of nuclear fission have been carried out with the ν-Ball2 spectrometer at the ALTO facility of IJC Lab in the framework of the EUROLABS transnational access project [1][2]. ν-Ball2 is a state-of-the-art hybrid array developed and constructed at ALTO and used by a large international collaboration. Several open questions are currently being addressed, such as the evolution of fragment yield distributions in the sub-actinide region [3], the emission of high energy gamma rays in nuclear fission with potential population of collective excitations (PDR, GDR, etc.) in the emerging fragments [4]. The experiments have also explored other outstanding questions, such as the angular momentum carried away by neutron emission [5] and angular correlations between the spins of fission fragment partners and measurements of angular distributions of gamma rays with respect to the fission axis [6][7]. Finally, the potential energy landscape before fission occurs can also be studied by gamma spectroscopy of fission shape isomers [8]. An overview of these new studies from the ν-Ball2 experimental campaign will be given and selected results will be presented along with future perspectives.

References
[1] G. Pasqualato and J.N. Wilson, Nuclear Physics News, 34 16-20, (2024)
[2] https://web.infn.it/EURO-LABS/
[3] K. Miernik et al. Phys. Rev. C 108, 054608 (2023)
[4] H. Makii et al. Phys. Rev. C 100, (2019) 044610
[5] D. Gjestvang, J.N. Wilson et al. Phys. Rev. C 108, 064602 (2023)
[6] J. Randrup, Phys. Rev. C 106, (2022) L051601
[7] G. Scamps et al., Phys. Rev. C 108, L061602 (2023)
[8] C. Hiver, J.N. Wilson et al., Acta Physica Polonica Vol. 18, 2-A25 (2025)

Speaker: Jonathan Wilson (IJC Lab, Orsay)

10:30 → 11:00 Coffee break

11:00 → 11:15 Welcome from ILL Director

11:15 → 12:05 session 2 (Chair: A. Lemasson)

11:15 Fission Yields for reactor applications: Development of a new program at CEA-Cadarache

Fission yields play a crucial role in improving our understanding of the fission process and are essential for numerous reactor physics applications, such as decay heat calculations and the determination of delayed neutron multiplicities. In this context, the research group at CEA-Cadarache is actively involved in fission yield studies, with a focus on three complementary aspects.

Fission yield measurements using the Lohengrin recoil mass spectrometer
Taking advantage of the high performance of the Lohengrin recoil mass spectrometer and a close collaboration between CEA-Cadarache, the Laboratory of Subatomic Physics and Cosmology (LPSC, Grenoble, France), and the Institut Laue-Langevin (ILL, Grenoble, France), a new experimental procedure has been developed. This procedure aims (i) to improve the accuracy of experimental fission yield data and (ii) to provide the corresponding experimental fission yield correlation matrices.
In addition, a new time-of-flight device coupled to the Lohengrin spectrometer has been developed and characterized in order to specifically investigate the symmetric and highly asymmetric mass regions. The first results obtained with this new setup demonstrate very promising performance.

Evaluation of fission yields in the thermal energy region
A new methodology for fission yield evaluation has been developed at CEA-Cadarache and applied to thermal-neutron-induced fission reactions on the following target nuclei: ²³³U, ²³⁵U, ²³⁹Pu, and ²⁴¹Pu. These evaluations, which have been included into the recent JEFF-4.0 nuclear data library, provide consistent independent and cumulative fission yields. For each fission product, the evaluated yield and its associated uncertainty are provided, together with the correlation matrices between fission product yields.

From pre-neutron to post neutron fission yields to deduce various fission observables
The evaluated post-neutron mass yields Y(A) are related to the pre-neutron mass yields Y(A) through the probability P(|A) that a fission fragment of mass A emits 𝜈 prompt neutrons and becomes a fission product of mass A. Using the generalized -method and assuming that this probability follows a Gaussian distribution, we deduce the average number of prompt neutrons as a function of fragment mass < (A)>, commonly referred to as the saw-tooth curve. The Monte Carlo code FIFRELIN was then employed to calculate the main post-fission observables relevant for reactor applications.

Speaker: O. Serot (CEA-Cadarache)

11:40 Excitation energy, angular momentum and deformation of fission fragments.

We present experimental results on the study of the radiative decay of fragments produced in the neutronless channel of 252Cf(sf).

We use a twin ionization chamber loaded with an ultra-thin Cf sample and apply the 2E method to determine fragment masses, yielding excellent resolution (0.7 u) for neutronless events[1].

Specific fragmentations with the heavy fragment at N=82 were unambiguously separated from the rest of the data by applying simple energy gates.

An array of 54 large volume NaI detector allows measurement of the fragment de-excitation; photons being the only way to dissipate angular momentum and excitation energy.

The measured gamma ray spectra constrain the angular momentum and excitation energy distributions of the fragments. Combined with theoretical models, we infer the fragment deformation that best reproduce the observation[2,3].

[1] A. Francheteau et al., PRC 111, 034608 (2025).
[2] A. Francheteau et al., PRL 132, 142501 (2024).
[3] A. Francheteau et al., PRC - To be published.

Speaker: Laurent Gaudefroy (CEA)

12:05 → 13:45 Lunch break

13:45 → 16:30 ILL visit (15 min introduction+guided tour)

16:30 → 17:00 Coffee break

17:00 → 18:30 Bus transfer to Chamrousse

Tuesday, 10 March

08:30 → 10:05 session 3 (Chair: A. Andreyev)

08:30 Properties of the $^{236m}$U Fission Isomer

Peter Reiter, University of Cologne

We report on measurements of delayed $\gamma$ decay and delayed fission of the shape isomer $^{236m}$U, as well as a search for isomeric decay in $^{233}$Th, performed with the Heidelberg--Darmstadt Crystal Ball spectrometer. For $^{236m}$U, the data demonstrate that delayed $\gamma$ decay and delayed fission constitute two competing decay modes of the same isomeric state. This conclusion is supported by the identical half-lives observed for both decay channels, the same excitation energy of the isomer, equal missing energy in the prompt energy balance, and consistent prompt energy spectra populating the second minimum. The excitation energy of the isomeric ground state in the second minimum is determined to be $2814 \pm 33$ keV. The $\gamma$ back-decay proceeds via multi-step cascades involving known $1^{-}$ states of normally deformed $^{236m}$U while a direct single-step $\gamma$ transition is excluded within the experimental sensitivity. In contrast, no evidence for isomeric $\gamma$ decay was found in $^{233}$Th. An upper limit of $<10^{-6}$ is established for the branching ratio populating a potential shape isomer in the second minimum relative to the first minimum. These results place stringent constraints on the existence and population probability of a second minimum in $^{233}$Th and provide valuable benchmarks for theoretical models of shape isomerism in actinide nuclei.

Speaker: Peter Reiter (Germany)

08:55 Fission studies at FIPPS

Neutron-induced fission of actinides is a well-established experimental method to probe the structure of neutron-rich nuclei. While this technique provides a wealth of information about several species in a single experiment, the collected data sets are very complex and require multiple coincidences in order to disentangle radiation originating from different fission fragments. A further complication comes from an additional background from the neutron capture as well as from the beta decay of the created fission fragments. As a result, to extract the desired spectroscopic information, the use of highly granular experimental setups is necessary.

In this talk, I will introduce FIPPS, the Fission Product Prompt gamma-ray Spectrometer, a permanent highly granular gamma-ray detection setup at the Institut Laue-Langevin in Grenoble, France. Its capabilities, results from the previous fission campaigns as well as future experimental and development plans will be presented. A particular focus will be put on the results obtained in the shape-coexistence region around Zr-100 and the development of the future diamond-detector array for fission-fragment tagging.

Speaker: Marek Stryjczyk

09:20 Investigating the gamma back-decay from the shape isomer in 236U

Shape isomers (SI) in actinide nuclei are poorly understood metastable states lying in the second super-deformed prolate potential well. Their unique characteristics include a super-deformed shape, a low spin - similar to that of the normally-deformed ground state - and a main decay mode that is spontaneous fission. Although many such isomers have been discovered in the actinides since their first discovery in the early 1960's (Polikanov, 1962), information about the second potential well remains nowadays very limited (Bjørnholm and Lynn, 1980) (Metag, 1980).

The unexpected absence of such short-lived spontaneously fissioning isomers below Plutonium isotopes has tentatively been explained by a competing $\gamma$ back-decay branch, depopulating the SI back to normally deformed states. Amongst the many experiments aiming at directly observing this transition, only one produced strong evidences in favor of such a transition, performed on the shape isomer of 236U at GSI using the Crystal-Ball detector (Reiter, 2025).

The main aim of this work is to reproduce and refine this experiment using state-of-the-art technology. In 2022, 236IIU was produced by the 235U(d,p)236IIU at the ALTO facility of IJCLab and its decay detected by the nu-Ball2 hybrid spectrometer (made of 24 HPGe Clovers, 64 PARIS detectors and a DSSD for light particles).

The full characterization of these back-decay gamma-rays would enable a unique and precise way to determine the parameters of the fission barriers, which play an essential role in fission's theory. Moreover, spectroscopy in the second well can allow for a better understanding of nuclear structure of these mysterious highly-deformed states.

Here, the results of this fission shape isomer experiments will be presented, along with a proposition of a re-analysis of the isomeric fission half-lives systematics.

References :

SM Polikanov et al., JETP 42, 1464 (1962), Soviet Phys. JETP 15, 1016
(1962).

S Bjørnholm and JE Lynn, Reviews of Modern Physics, 52(4):725, 1980.

V Metag et al., Physics Reports, 65(1):1–41, 1980.

P Reiter et al., The European Physical Journal A, 61(7):158, 2025.

Speaker: Corentin Hiver (Heavy Ion Laboratory, University of Warsaw)

09:35 Recent β-delayed fission studies at ISOLDE

In the process of βDF, an excited state populated in β decay close to the top of the fission barrier in the daughter nucleus undergoes fission (in competition with γ-ray or particle emission). The achievable excitation energy is limited by Qβ value of the parent nucleus, which is typically less than ~10 MeV in the lead region, or even less than ~5 MeV in actinides. Therefore, βDF represents the so called low-energy fission, which is sensitive to nuclear structure. It enables investigation of fission properties (such as fission fragment mass distributions, fission barriers, etc.) of isotopes for which other approaches to low-energy fission studies would be extremely difficult or currently impossible [1,2].

There is also a particular interest in βDF in the neutron-rich side of the nuclear chart, since alongside other types of fission, βDF is responsible for termination of r-process nucleosynthesis and for fission recycling, see for example Ref. [3]. This contribution will present our search for βDF in neutron-deficient 178Au [4] and neutron-rich 230,232,234Ac [5] isotopes performed at ISOLDE-CERN. For 178Au, presumed to be located in the new region of asymmetric fission discovered in the vicinity of 180Hg [1,2], we employed selective power of Resonance Ionization Laser Ion Source (RILIS) to study its high-spin and low-spin β-decaying state separately. In the case of actinium isotopes, we measured the whole Fr → Ra → Ac decay chain, thus, obtaining new data also for francium nuclei. No βDF events were observed for isotopes of interest despite collection of high statistics. Therefore, upper limits of βDF probabilities were determined. For 230Ac, where identification of βDF was reported in the past [6], the limit was more than order of magnitude lower than the literature value, thus, questioning the observation of this decay mode. The results will be discussed in the context of experimental systematics of βDF probabilities and partial half-lives, and compared with calculations using TALYS code [7].

[1] A. N. Andreyev, M. Huyse and P. Van Duppen, Rev. Mod. Phys. 85, 1541 (2013).
[2] A. N. Andreyev, K. Nishio, K.-H. Schmidt, Rep. Prog. Phys. 81, 016301 (2018).
[3] S. Goriely et al., Phys. Rev. Lett. 111, 242502 (2013).
[4] B. Andel et al., accepted in Phys. Rev. C.
[5] S. Bara et al., Phys. Rev. C 111, 065803 (2025).
[6] Y. Shuanggui et al., Eur. Phys. J A 10, 1 (2001).
[7] A. Koning, S. Hilaire, and S. Goriely, EPJ A 59, 1 (2023).

Speaker: Boris Andel (Comenius University in Bratislava)

Andel_presentation.pdf

09:50 Improvements of the knowledge of actinides on the HPRL for nuclear energy

Neutronics codes accuracy is now driven by the knowledge of their nuclear data used as input, and the structure of nuclei plays a very important role in nuclear reactors, whether for experimental or theoretical physics. Several cross sections are listed on the High Priority Request List (HPRL) [1] for improvements. In this presentation, we will show how improving the knowledge of the nuclear structures of nuclei is important for this purpose through two examples of cross sections listed on the HPRL, the first one via an experimental analysis and the second one via an applied theoretical analysis.
In the first part, we will improve the knowledge of the 238U level scheme by analysing the g-g coincidence matrices of the two nu-Balls campaigns, which coupled the nu-Ball g-spectrometer [3] and the LICORNE neutron source [4, 5] of the ALTO facility, Orsay, France.
This level scheme is important to improve the 238U(n,n’) cross section which plays a key role in neutron population and their moderation. When measuring (n, xng) cross sections via prompt g-ray spectroscopy coupled to time-of-flight measurements, structure’s information is mandatory to infer the total (n, xn) cross section [2]. Moreover, when calculating the (n, n’) cross section, knowledge of the structure is needed to obtain the (n,xng) cross sections. In total, 110 g-transitions and 60 levels registered in ENSDF have been confirmed and 158 new -
transitions and 58 new levels have been found.
In a second part, we will study the states lying in the second fission well of the 242Pu. This nucleus is created in the current reactors cores and will be present in the future ones. Moreover, 242Pu fission chambers are candidate instruments for on-line measurements of high neutron energy flux such as in the future JHR reactor (CEA, Cadarache). Past neutron cross-section studies are mainly based on resonance analyses performed by Auchampaugh et al. [6,7] and Weigmann et al. [8]. However, no very specific analysis of the resonant structures observed in the 242Pu fission cross section was recently performed. We will revisit the topic by methodically analyzing and modeling Class-II states fluctuations observed in experimental fission cross sections in order to deduce the fission-isomeric energy EII, using complementary features of three nuclear reaction model codes: the CONRAD code [9] to treat Class-II states in the resolved resonance range by introducing Lorentzian shapes in the penetration factor; a home-made version of TALYS [10] to treat the unresolved resonance structures above 1.15 keV using a similar approach; and the combinatorial Quasi-Particle-Vibrational-Rotational
Level Density (QPVRLD) method [11] implemented in the AVXSF-LNG code to generate a sequence of Class-II states compatible with the experimental Class-II state sequence established in this work. This sequence has then been compared to a Constant Temperature Model, from which a fission-isomeric energy EII ranging from 1.90 to 1.95 MeV was deduced.

[1] OECD-NEA, “Nuclear data high priority request list”, 2018, online:
http://www.nea.fr/dbdata/hprl/
[2] Kerveno et al., Phys. Rew. C 104, 044605, 2021
[3] M. Lebois et al.ter, Nucl. Instrum. Methods in Phys. Res. A 960, 163580, 2020
[4] M. Lebois et al., Nucl. Instrum. Methods in phys. Res. A 735, 145, 2014
[5] J. N. Wilson et al., Phys. Proc. 64, 107−113, 2015
[6] G.F. Auchampaugh, J.A. Farrell, D.W. Bergen, NPA 171, 31-43 (1971).
[7] G.F. Auchampaugh, C.D. Bowman, PRC 7, 5, (1972).
[8] H. Weigmann, J.A. Wartena, C. Bürkholz, NPA 438 333-353 (1985).
[9] C. De Saint Jean et al., EPJ Nuclear Sci. Technol. 7 10 (2021).
[10] A.J. Koning and D. Rochman, Nuclear Data Sheets 113, Issue 12, 2841-2934 (2012).
[11] O. Bouland, J.E. Lynn and P. Talou, Phys. Rev. C 88 054612 (2013).

Speaker: Ms Carole Chatel (CEA, DES, IRESNE, DER, SPRC, Physics Studies Laboratory, Cadarache, F-13108 Saint-Paul-lez-Durance, France and Univ. Bordeaux, CNRS, CENBG, UMR 5797, F-33170 Gradignan, France)

Fission2026_chatel.pptx

10:05 → 10:35 Coffee break

10:35 → 11:30 session 4 (Chair: N. Pillet)

10:35 Fission isomer studies at FRS/GSI and IGISOL/JYFL-ACCLAB

The structure of fission barriers in actinide nuclei is characterized by pronounced multi-humped shapes, which give rise to long-lived isomeric states trapped in secondary minima of the potential-energy surface. These complex barrier profiles emerge from the interplay between macroscopic liquid-drop behavior and microscopic shell effects. In the region around the deformed magic neutron number 𝑁 = 146 (𝑍=92–97, 𝑁=141–
151), this mechanism leads to the formation of a distinct “island” of fission isomers. To date, 35 such isomers have been experimentally identified, with half-lives spanning from a few picoseconds to several milliseconds. As strongly deformed low-spin configurations stabilized by shell corrections, fission isomers provide a sensitive probe of nuclear structure at extreme deformation and offer an important benchmark for the shell effects expected to govern the stability of superheavy nuclei.

Until recently, studies of fission isomers relied almost exclusively on light-particle–induced reactions. In a first experiment at the Fragment Separator (FRS) at GSI, we demonstrated for the first time the use of projectile fragmentation of 1 GeV/u 238U beams to populate and investigate fission isomers. This approach provides access to isotopes that are difficult or impossible to reach with conventional reactions and, combined with in-flight separation, enables the study of short-lived isomers with high beam purity and event-by-event identification. Two complementary detection techniques were employed to cover half-lives from about 50 ns to 50 ms: (i) implantation in a fast plastic scintillator and (ii) thermalization in the cryogenic stopping cell of the FRS Ion Catcher with subsequent decay detection.

Since this initial measurement, the analysis of the first GSI experiment has been essentially completed, a second improved experiment at the FRS has been successfully performed, and a first experiment at IGISOL in Jyväskylä has been carried out, providing first results. At IGISOL, light-ion induced reactions are used, but contrary to past experiments, the production and detection of fission isomers is spatially separated, allowing a quasi-background-free detection of the fission isomers.

Speaker: Timo Dickel (GSI Helmholtzzentrum für Schwerionenforschung GmbH(GSI))

11:00 Angular distribution measurements of fission fragment gamma rays

H. S. Haug1, S. Marin2, N. P. Giha2,3, J. N. Wilson4, I. A. Tolstukhin3, A. Al-Adili1,5, D.Gjestvang1, S. Siem1

1 Department of Physics, University of Oslo, N-0316 Oslo, Norway
2 Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA
3 Physics Division, Argonne National Laboratory, Lemont, Illinois 60439, USA
4 Universit´e Paris-Saclay, CNRS/IN2P3, IJC Laboratory, Orsay, France
5 Department of Physics and Astronomy, Uppsala University, Uppsala 75120, Sweden

In nuclear fission, a heavy nucleus normally splits into two fission fragments. Fragments are typically created with about 6-7 ¯h of angular momentum on average [1]. The fragment spin directions relative to the fission axis are not well understood, with the only available data from 1976 by A. Wolf and E. Cheifetz [2]. The angular distributions of γ rays give with respect to the fission axis give us experimental information about spin directions. In this work, we present measurements of the angular distributions of discrete γ rays in fission fragments from spontaneous fission of 252Cf. The data were taken at Argonne National Laboratory with the Gammasphere spectrometer. The set up combined a twin Frisch-gridded ionization chamber, which gives fission fragment mass, kinetic energy and directions, together with the Compton-suppressed high-purity germanium (HPGe) detector array Gammasphere that provides high-granularity, high-resolution γ-ray energy measurements [3]. Around 2.5·109 fission events wert recorded over 180 h of experiment. This set-up allows accurate measurements of gamma ray angular distributions with respect to the fission axis.

References
[1] J. N. Wilson, D. Thisse, M. Lebois, N. Jovancevic, D. Gjestvang, and et al., “Angular momentum generation in nuclear fission,” Nature (London), vol. 590, no. 7847, pp. 566–570, 2021.
[2] A. Wolf and E. Cheifetz, “Angular distributions of specific gamma rays emitted in the deexcitation of prompt fission products of 252Cf,” Phys. Rev. C, vol. 13, pp. 1952–1960, May 1976.
[3] N. P. Giha, S. Marin, I. A. Tolstukhin, M. B. Oberling, R. A. Knaack, C. Mueller-Gatermann, A. Korichi, K. Bhatt, M. P. Carpenter, C. Foug`eres, V. Karayonchev, B. P. Kay, T. Lauritsen, D. Seweryniak, N. Watwood, D. L. Duke, S. Mosby, K. B. Montoya, D. S. Connolly, W. Loveland, I. E. Hernandez, S. D. Clarke, S. A. Pozzi, and F. Tovesson, “Measurement of spin versus total kinetic energy of 144Ba produced
in spontaneous fission of 252Cf,” Phys. Rev. C, vol. 111, p. 014605, Jan 2025.

Speaker: Mr Henrik Haug (University of Oslo)

11:15 New experimental insights into the "Thorium anomaly" from isotopic fission fragment yields of 232Th produced in inverse-kinematics

Since the discovery of fission in 1939, several theoretical models were developed to properly explain the observations [1]. It wasn’t until the introduction of shell effects by Strutinsky [2], that the microscopic - macroscopic models [3] could match the measured yields of heavy actinides like 252Cf or 254Es [4]. The interplay between both quantities prevents, so far, from a fully microscopical description of the interaction. Despite the development of different theoretical models [5] and simulation codes based on experimental data, such as GEF [6], the fission process is not reproduced with enough accuracy along the nuclear chart. A large set of experimental data is needed in order to constrain the models.

Following the advantages of inverse-kinematics, the VAMOS group and collaborators have performed fission studies for more than 10 years [7,8]. The VAMOS++ spectrometer, composed of a pair of magnetic quadrupoles and a dipole, is coupled to a set of Multi-Wire Proportional Counters (MWPCs) before and after the optical modules and an Ionization Chamber (IC) positioned at the end of the focal plane [9]. This configuration enables the isotopic identification of complete fission fragment distributions. The magnetic spectrometer is combined with a highly stripped silicon detector (PISTA), which allows the identification of the fissioning system and the reconstruction of its excitation energy with high resolution. The combination of both devices permit to systematically study the fission process.

This setup was used in a new experiment conducted with the newly accelerated 232Th beam at Coulomb energies. Transfer reactions performed with a 12C target permitted to populate fissioning systems from 230Th up to 244Cm. This allows the systematic study of the shell-closure effects occurring for different deformation parameters, like octupolar deformation, recently proposed to be responsible for the asymmetric fission in the actinides region [10]. Moreover, experimental results show that the isotopic distributions around Th isotopes deviate from the general actinide behaviour [11].

In this work, the atomic number and mass fission fragment yields of 232Th will be presented. These distributions have been obtained as a function of the excitation energy. The comparison between Thorium yields and heavier actinides give new experimental insight into understanding the so-called "Thorium anomaly" [11].

References
[1] N. Bohr and J. A. Wheeler, "The mechanism of nuclear fission", Physical Review, vol. 56, no. 5, p. 426, 1939. Review, vol. 89, no. 5, p. 1102, 1953.
[2] V. Strutinsky, "Shell effects in nuclear masses and deformation energies", Nuclear Physics A, vol. 95, no. 2, pp. 420-442, 1967.
[3] B. Wilkins, E. Steinberg, and R. Chasman, "Scission-point model of nuclear fission based on deformed-shell effects" , Physical Review C, vol. 14, no. 5, p. 1832, 1976.
[4] K. Flynn, J. Gindler, L. Glendenin, and R. Sjoblom, "Mass distributions for the spontaneous fission of 253Es and the thermal-neutron-induced fission of 254Es", Journal of Inorganic and Nuclear Chemistry, vol. 38, no. 4, pp. 661-664, 1976.
[5] Schunck, N and Robledo, LM , Reports on Progress in Physics 79 (2016) 116301.
[6] Karl-Heinz Schmidt and Beatriz Jurado , Reports on Progress in Physics 81 (2018) 106301.
[7] M. Caamaño, O. Delaune, F. Farget, X. Derkx, K.-H. Schmidt, L. Audouin, C.-O. Bacri, G.Barreau, J. Benlliure, E. Casarejos, et al., "Isotopic yield distributions of transfer-and fusion-induced fission from 238U+ 12C reactions in inverse kinematics", arXiv preprint arXiv:1304.2647, 2013.
[8] Ramos, D., et al. Isotopic fission-fragment distributions of 238U , 239N p, 240P u, 244Cm, and 250Cf produced through inelastic scattering, transfer, and fusion reactions in inverse kinematics. Physical review C, 97(5), 054612.
[9] M.Rejmund et al., Nuclear Instruments and Methods in Physics Research A 646 (2011) 184-191.
[10] Scamps, G., & Simenel, C. (2019). Effect of shell structure on the fission of sub-lead nuclei. PhysicalReview C, 100(4), 041602.
[11] Schmidt, K. H., et al. (2024). Identifying and overcoming deficiencies of nuclear data on the fission of light actinides by use of the GEF code. Annals of Nuclear Energy, 208, 110784

Speaker: Alex Cobo Zarzuelo (GANIL)

Fission_2026_10_03_AlexCoboZarzuelo.pdf

11:30 → 16:00 Mid-day break

16:00 → 17:20 session 5 (Chair: S. Leoni)

16:00 Effect of nucleon exchange on fission fragment angular momenta

This work was carried out in collaboration with Pavel Nadtochy, Christelle Schmitt, and Katarzyna Mazurek. It explores the role of nucleon exchange for the generation of the fission fragment angular momenta.
For a number of typical fission cases, samples of 10,000 shape evolutions were generated by Langevin simulation and, subsequently, for each of these evolutions, the nucleon exchange transport theory previously developed for damped nuclear reactions was applied, yielding the time development of the fragment spin-spin distribution within the Fokker-Planck transport framework.

The characteristic evolution of both parallel and perpendicular fragment spin components is discussed. A common feature is that the rotational modes fall out of equilibrium before scission because further nucleon exchange is being increasingly suppressed due to the shrinking neck, while the rapidly rising temperature causes the equilibrium fluctuations to keep growing.
A number of fission observables are extracted from these event ensembles: the distribution of the magnitude of the fragment spin and its orientation relative to the fission axis, as well as the correlation between the two spins and the distribution of their opening angle. The dependence of these observables on the mass asymmetry is also examined.

Speaker: Jorgen Randrup (LBNL)

16:25 Experimental Fission Campaigns at VAMOS++

The fission process has intrigued physicists for a long time from both experimental and theoretical perspectives. From a qualitative point of view, it is well established that nuclear structure strongly influences the production of fission fragments at low excitation energies. However, the large deformations reached by the system and the complex fission dynamics that drive it from a single object to two separated fragments have so far prevented a quantitative microscopic description of the process.
Considerable effort has been devoted over the past decades to modeling nuclear fission. In this context, the ongoing inverse-kinematics fission program at GANIL, using the VAMOS++ magnetic spectrometer, has contributed by providing new and unique experimental data. These data combine information from the entrance channel—such as measurements of the excitation energy of the fissioning system—with information from the exit channel, including isotopic fission-fragment yields.
The experimental setup has undergone continuous improvements, allowing increased accuracy in the measured observables and providing access to new quantities, such as pre-neutron evaporation data. In addition, new experimental techniques have been implemented to expand the range of accessible fissioning systems.
A global overview of the experimental techniques used at VAMOS to study both actinide and pre-actinide fission will be presented, together with the most significant results obtained so far.

Speaker: Diego RAMOS (GANIL - IN2P3/CEA)

16:50 Medium spin states in the 87Se isotope produced in neutron induced fission of 233U and 235U targets

The area of the nuclear chart lying northeast of the doubly magic nucleus ⁷⁸Ni represents a key benchmark for shell-model–based theoretical descriptions. Although closed-shell nuclei in this region have been extensively investigated for decades, the double-magic character of ⁷⁸Ni was only recently confirmed experimentally by Taniuchi et al. [1]. Their results also revealed a weakening of the N = 50 and Z = 28 shell closures beyond ⁷⁸Ni, driven by the appearance of deformed configurations, in agreement with modern large-scale shell-model calculations [2]. These observations underscore the importance of further experimental studies in the vicinity of ⁷⁸Ni.

In this context, the selenium isotopic chain, located six protons away from the closed shell, displays clear signatures of shape coexistence. In particular, the tentative identification of a 3⁺ state in ⁸⁶Se [3] points to the emergence of collectivity. The present work investigates neutron-rich selenium isotopes northeast of ⁷⁸Ni, with special emphasis on ⁸⁷Se, for which spectroscopic information has so far been very limited, with only three known γ-ray transitions [4].

The ^86,87,88Se isotopes were populated through thermal-neutron–induced fission of ²³³U and ²³⁵U targets at the Institut Laue–Langevin. In ⁸⁷Se a level scheme extending up to 3.5 MeV in excitation energy was constructed from high-fold γ-ray coincidence data recorded with the high-efficiency HPGe array FIPPS [5]. Using cross-coincidence analysis, seven previously unobserved γ-ray transitions were identified and their relative intensities determined. Spin assignments for low-lying states were inferred from γ-ray angular-correlation measurements. The observation of an E3 transition connecting the (11/2^-) and (5/2⁺) states suggests enhanced octupole correlations and motivates systematic searches for similar E3 excitations in neighboring even-mass selenium isotopes.

In addition, our analysis, validated the presence of an emerging collective band in ⁸⁶Se, until now observed only in β- decay [3] and confirmed 589-keV gamma ray as deexciting the first 2⁺ state in ⁸⁸Se. These findings and validations in ^86,88Se isotopes contribute to more vivid picture of evolution of a collectivity along even-even Se isotopic chain.

References:
[1] R. Taniuchi et al., Nature 569, 53-58 (2019).
[2] F. Nowacki et al., Phys. Rev. Lett. 117, 272501 (2016).
[3] T. Materna et al., Phys. Rev. C 92, 034305 (2015).
[4] T. Rząca-Urban et al., Phys. Rev. C 88, 034302 (2013).
[5] C. Michelagnoli et al., EPJ 193, 04009 (2018).
[6] K. Gajewska et al., Acta Phys.Polon.Supp. 18 (2025) 2, 2-A41.

Speaker: Katarzyna Gajewska (IFJ PAN)

17:05 Neutron rich nuclei and fission studies at the IGISOL facility

The Ion Guide Isotope Separator On-Line (IGISOL) facility at the Accelerator Laboratory of the University of Jyväskylä (JYFL-ACCLAB) has since 1984 utilized particle induced fission as means to produce neutron-rich nuclei for various spectroscopy studies and mass measurements [1]. Committed research work include β- and β -ɣ spectroscopy, total absorption ɣ-ray spectroscopy (TAGS), β-delayed neutron spectroscopy, and collinear laser spectroscopy. Examples of each kind studies can be found in references [2-7]. In addition, fission yields (FY) and isomeric yield ratios (IYR) have been measured using the IGISOL facility [8-11].

The presentation gives an overview on the current status of the fission related work at the IGISOL facility, focusing in particular on the recent technical developments such as the commissioning of the Multireflection Time-Of-Flight (MR-TOF) device and structure and performance of the new, sputtered self-sustaining uranium targets [12]. In addition, the developments for fission yield measurements with examples from preliminary analysis of proton induced fission yields of 232-Th will be discussed.

1. I.D. Moore, P. Dendooven & J. Ärje, Hyperfine Interact 223, 17–62 (2014).
2. M. Vilen et al., Phys. Rev. C 105, 034312 (2022).
3. J. Kurpeta et al., Phys. Rev. C 105, 034316 (2022).
4. V. Guadilla et al., Phys. Rev. Lett. 122, 042502 (2019).
5. R. Caballero-Folch et al., Phys. Rev. C 98, 034310 (2018).
6. L. J. Vormawah et al., Phys. Rev. A 97, 042504 (2018).
7. A. Pérez de Rada Fiol, et al., Physical Review C 111, 044312 (2025).
8. H. Penttilä et al., Eur. Phys. J. A, 52:104 (2016).
9. V. Rakopoulos et al., Phys. Rev. C 99, 014617 (2019).
10. A. Al-Adili et al., EPJ Web of Conferences 256, 00002 (2021).
11. S. Cannarozzo, et al., Physical Review C 111, L031601 (2025).
12. H. Penttilä et al., in Proc. European Research Reactor Conference RRFM 2021, https://www.euronuclear.org/scientific-resources/conference-proceedings/.

Speaker: Heikki Penttilä (The University of Jyvaskyla)

Fission2026-Penttila.pptx

Fission2026-Penttila.pptx

17:20 → 17:50 Coffee break

17:50 → 18:55 session 6 (Chair: J. Dudouet)

17:50 Lifetimes of medium-high spin states in neutron-rich fission fragments

The experimental investigation of the structure of atomic nuclei reveals the presence of different shapes as, for example, spherical or ellipsoidal. The latter can have sizable deviation (i.e., deformation) with respect to the spherical shape. Nuclear deformation is found especially far from the magic numbers of nuclear stability. The evolution of nuclear shapes in different regions of the nuclear chart is the subject of extensive studies, by means of different experimental and theoretical techniques.
Recent experimental results on the deformation of neutron-rich nuclei with mass A~100 at medium-high spin (8-10h) will be presented. Those follow the measurement of the lifetimes of excited states to determine transition strengths, from which the magnitude of the deformation can be inferred. Particular focus will be on a novel implementation of the Doppler Shift Attenuation Method (DSAM) for the measurement of lifetimes of excited states in fission fragments. This method has been applied to the first set of data taken with an active fission target coupled to an array of germanium detectors. The nuclei have been populated via neutron-induced fission on U-235, dissolved in a liquid scintillator (fission tag via active target). This reaction, combined with a high-resolution gamma detection system, has allowed for high-statistics studies, complementary to the those performed, for example, at radioactive ion beam facilities. Thermal neutrons have been delivered by the Institut Laue-Langevin (ILL) nuclear reactor and the FIPPS (FIssion Product Prompt gamma-ray Spectrometer) instrument has been used for gamma-ray detection. The active target has allowed to "tag" the fission events, suppressing the gamma rays produced via the beta decay of the fission fragments. The experimental data have been compared to simulations obtained using a Geant4 Monte Carlo code developed for FIPPS. Different event generators, in particular the one for fission fragments, based on the FIFRELIN database, have been included in the simulation code, as well as the full geometry of the detection system and the gamma decay through complete level schemes. New results have been obtained for the lifetimes of excited states in Zr-(97,101) and Nb-(100,102) nuclei, together with the re-evaluated values for Zr-(99,100,101,102). These have been compared with the previously reported measurements in the literature, via an accurate evaluation of all the systematic errors.
A development of a plunger device for lifetime measurements in neutron-induced fission experiments will be also presented in the final perspectives.

Speaker: Giacomo Colombi (University of Guelph)

18:15 Microscopic description of fission process including intrinsic excitations

The microscopic description of fission is very challenging due to the numerous degrees of freedom involved in the process: collectivity, pairing, individual excitations, dynamical effects. Furthermore, this phenomenon happens in heavy nuclei in which the number of nucleons prevents from an exact solution to the many-body problem. Several approaches based on mean-field approximations have been devised to capture its main characteristics. However, a unified description is still not available.
In this talk, I will present recent developments that open up new horizons for understanding fission within the framework of TDGCM approaches, namely the inclusion of the scission area and intrinsic excitations. To achieve these objectives, new protocols based on overlap constraints have been invented to generate continuous adiabatic and excited potential energy landscapes constructed from HFB vacua [1]. They enabled the first implementation [2] of the SCIM formalism [3] in the case of the 1D asymmetric path of Pu240. I will discuss in particular the properties of fragments close to scission as well as the energy balance, including intrinsic excitation energy.

[1] P. Carpentier, N. Pillet, D. Lacroix, N. Dubray, and D. Regnier, Phys. Rev. Lett. 133, 152501 (2024).
[2] P. Carpentier, « Microscopic and dynamical description of the fission process including intrinsic excitations », PhD thesis, Université Paris-Sacaly (2024), https://theses.hal.science/tel-04761482v1.
[3] R. Bernard, H. Goutte, D. Gogny, and W. Younes, Phys. Rev. C 84, 044308 (2011).

Speaker: Dr Nathalie Pillet (CEA DAM DIF)

18:40 Lifetime measurement of 235U fission products using Doppler-shift methods

The fission process leads to excited states in neutron rich nuclei at high spin and excitation energy. Obtaining new data for nuclear lifetimes and transition probabilities is a step towards a better understanding of the structure of neutron-rich nuclei and nuclear theory, in general. This presentation deals with the use of the Doppler-shift Attenuation Method (DSAM) coupled with the coincidence method, with the goal to present new values for lifetimes in the sub picosecond range of excited states in neutron-rich nuclei of mass \textit{A $\approx$ 100} and \textit{A $\approx$ 140}. These nuclei are produced with good yields in the neutron induced fission of $^{235}$U. In this work, new results will be shown for the high-spin states of $^{104}$Mo and $^{134}$Te, which have not been studied using DSAM before. Complementary results using the coincidence method have been obtained for $^{97,99,100,101,102}$Zr, as well as $^{100,102}$Nb, which were the result of a previous work using the same analytical setup. The related experiment has been done at ILL, using the FIPPS high-resolution $\gamma$-ray spectrometer array, by using a beam of thermal neutrons on a $^{235}$U target, dissolved in a liquid scintillator. In parallel, the \textit{GEANT4} simulation toolkit has been used to reproduce the experimental conditions at FIPPS and create Monte-Carlo generated spectra. Simulations have been performed for different lifetimes and the lifetime of a given nuclear state has been extracted via a $\chi^2$ analysis. $\gamma$ coincidences have been used both in experimental and simulated data to increase the selectivity in the lineshape analysis.

Speaker: Basile Massimino

19:30 → 22:00 Workshop Dinner

Wednesday, 11 March

08:30 → 10:05 session 7 (Chair: A. Lemasson)

08:30 The Physics of the CGMF Code

The CGMF code generates binary fission events and follows the decay of the two scission fragments through the evaporation of prompt neutrons and photons. It implements the Hauser-Feshbach theory of nuclear reactions to follow the competition between neutron and photon emission at every stage of the decay and for every excitation energy, spin and parity the fragments are in. It is written in C++, and a comprehensive toolkit has been written in Python to facilitate the analysis of its output files. It is a standalone code, as well as a component of the MCNP-6.3 transport code. In this talk, I will describe the main features, physics models, input parameters, and assumptions that are present in CGMF. I will illustrate its usefulness, accuracy and predictability, or lack thereof, through various fission studies: time evolution of prompt $\gamma$-ray emission [2], angular anisotropies in prompt neutron emission [3], inferring the neutron multiplicity distribution from $\gamma$-ray studies [4], correlation studies between fission observables [5], corrections on experimental data using simulated neutron emission [6], to name but a few. I will also discuss plans and options for future releases of the code.

References

[1] P. Talou, I. Stetcu, P. Jaffke, M.E. Rising, A.E. Lovell, and T. Kawano, Comp. Phys. Commun. 269, 108087 (2021)
[2] P. Talou, A.E. Lovell, I. Stetcu, G. Rusev, T. Kawano, and M. Jandel, Phys. Rev. C 112, 014601 (2025); G. Rusev et al., Phys. Rev. C 111, 064613 (2025)
[3] A.E. Lovell, P. Talou, I. Stetcu, and K.J. Kelly, Phys. Rev. C 102, 024621 (2020)
[4] A.E. Lovell, I. Stetcu, P. Talou, G. Rusev, and M. Jandel, Phys. Rev. C 100, 054610 (2019)
[5] J. Randrup, P. Talou, and R. Vogt, Phys. Rev. C 99, 054619 (2019)
[6] N. Fotiades, P. Casoli, P. Jaffke, M. Devlin, R.O. Nelson, T. Granier, P. Talou, and T. Ethvignot, Phys. Rev. C 99, 024606 (2019)

Speaker: Patrick Talou (Stardust Science Labs)

08:55 Shape evolution in the A≈100 region studied by fission-fragment γ-ray spectroscopy: recent progress and perspectives

Nuclear fission provides broad access to neutron-rich nuclei in the $\mathrm{A}\approx 100$ region and enables detailed $\gamma$-ray spectroscopy across the $\mathrm{N}\approx 60$ shape-transition landscape. This region is characterized by a sharp onset of deformation at $\mathrm{N}=60$ near $\mathrm{Z}\approx 38-40$, an abrupt loss of collectivity when moving to lower $\mathrm{Z}$, and an increasing role of triaxial degrees of freedom toward higher $\mathrm{Z}$. These features provide stringent constraints on microscopic and configuration-mixing descriptions of shape evolution.

Selected recent results are presented using two complementary experimental strategies. Event-by-event isotopic identification in inverse-kinematics fission at GANIL with the VAMOS++ spectrometer coupled to the AGATA $\gamma$-ray tracking array provides clean, nucleus-resolved spectroscopy. Within this framework, prompt $\gamma$-ray spectroscopy along the neutron-rich Kr chain has been used to map the evolution of collectivity across $\mathrm{N}\approx 60$ and to delineate the low-$\mathrm{Z}$ limit of this deformation region.

In parallel, high-fold $\gamma-\gamma$ coincidence spectroscopy with large $\gamma$ arrays (e.g., FIPPS and Gammasphere) enables complex level schemes to be disentangled and weak branches or isomer-linked structures to be accessed. Br and Nb case studies are used to illustrate how combining isotopic identification and high-fold coincidence approaches yields a more complete picture of shape evolution and coexistence, and constrains the interplay between axial and triaxial structures on the high-$\mathrm{Z}$ side of the transition.

Speaker: Jérémie Dudouet (IP2I)

09:20 Building Continuous Potential Energy Surfaces for Nuclear Fission Dynamics

Nuclear fission is a complex many-body process involving a very large number of degrees of freedom. A widely used microscopic framework to describe this phenomenon is the Time-Dependent Generator Coordinate Method, where the generating functions are constrained Hartree-Fock-Bogoliubov states. In this context, elongation and mass asymmetry appear as essential collective degrees of freedom.

The restriction to a limited set of collective coordinates may induce discontinuities along the fission path, since continuity in energy does not necessarily imply continuity in the underlying nuclear structure obtained from constrained energy minimization. Recently developed methods based on wave-function overlap constraints [1] ensure continuity of both energy and structure from for the adiabatic and excited paths, from the ground state deformation up to scission and beyond, thereby enabling a consistent description of fission dynamics. These methods have been successfully applied to the one-dimensional fission pathway of 240Pu, yielding fragment properties in good agreement with experimental data.

In this presentation, we investigate the behavior of the standard and continuity-preserving protocols in the construction of potential energy landscapes for various heavy nuclei. The perspective of this work is to extend this framework to two-dimensional collective spaces.

[1] P. Carpentier, Microscopic and dynamical description of the fission process including intrinsic excitations, PhD thesis, Université Paris-Saclay (2024).

Speaker: Pablo Nieto Gallego (CEA)

09:35 Fast-timing@nu-Ball2 fission campaign: new results for the neutron-rich isotopes 134,136Te

J. Fischer1, A. Blazhev1, J. Jolie1, N. Warr1,2, C. Hiver3, R. Lozeva4, J.N. Wilson4,
A. Messingschlager5, M. von Tresckow5, S. Pascu6, L.M. Fraile7 and A. Korgul3 for the nu-Ball2 N-SI-120 collaboration.
1 IKP, University of Cologne, Germany
2 Oliver Lodge Laboratory, University of Liverpool, UK
3 University of Warsaw, Poland
4 IJCLab, Orsay, France
5 TU Darmstadt, Germany
6 University of Surrey, UK
7 UCM, Madrid, Spain

Neutron-rich nuclei far away from the valley of stability contribute decisively to our understanding of nuclear characteristics. At the IJCLab in Orsay, a variety of nuclei were produced in a fast-neutron-induced fission reaction 238U(n,f) as part of the nu-Ball2 fission campaign in 2022. The measurement was performed with the nu-Ball2 spectrometer, a hybrid γ-spectrometer equipped with HPGe and LaBr3(Ce) detectors, which provide excellent energy and timing resolution, respectively. In comparison to the first fission campaign in 2018, nu-Ball1, several improvements on the spectrometer and the beamline were made. An important gain was the tripling of the LaBr3(Ce) efficiency (from 0.7% to 2.1%). Together with the factor of 10 increased beam intensity, this led into almost two orders of magnitude more of HPGe-LaBr3(Ce)-LaBr3(Ce) coincidences. The excellent time resolution of the LaBr3(Ce) detectors allows lifetime measurements in the ps-regime using the fast-timing technique. The nu-Ball2 LaBr3(Ce) data was properly time-walk
calibrated, which allowed the application of the more precise centroid-shift method instead of the convolution and slope methods used for nu-Ball1 data analysis [1]. The fast-timing analysis procedure was benchmarked by re-evaluating known lifetimes of low-lying excited states in 134,136Te. The new results for 134,136Te will be presented, compared with literature and theory, and discussed. While for most of the lifetimes only an improvement of the error bar was achieved, the newly determined lifetime of the 6+state in 136Te disagrees with the previous result. Currently, none of the presented theoretical calculations can consistently reproduce the new set of experimental B(E2) strengths for the low-lying 6+ → 4+, 4+ → 2+ and 2+ → 0+ yrast transitions in 136Te.

[1] G. Häfner et al., Phys. Rev. C 103 (2021) 034317

Speaker: Julia FISCHER

09:50 Improved modelling of nuclear fission with the TDGCM and projection techniques

The demand for an accurate, predictive model of nuclear fission continues to grow today, driven not only by the development of new experimental facilities but also by the need to describe exotic reactions relevant to stellar nucleosynthesis or superheavy element formation. However, given the complexity of the fundamental quark-gluon interactions, theoretical attempts to simulate nuclear fission must apply approximations to strike a balance between computational feasibility and physical relevance.

This presentation introduces a new description of fission based on the Time-Dependent Generator Coordinate Method (TDGCM), formulated without the commonly-used common Gaussian Overlap Approximation with the aim of achieving enhanced accuracy and compatibility with future extensions. While development of the new model is ongoing, intermediate results suggest that the choice of generator states for the method affects the resulting dynamics in ways that have not been previously considered.

A notable drawback of TDGCM and fission models more widely is the breaking of symmetries of the nuclear system, resulting in uncertainties in observable quantities such as particle number and angular momentum. Projection techniques are typically used to restore these symmetries and observables. However, projecting instead onto one of the constrained dimensions of the system reveals some innovative new uses. Applications to improve the depiction of fission dynamics and to consistently modify the Hamiltonian of the system will be demonstrated. Furthermore, by extending the previous work of Scamps and Hagino (2017), projection can be used to implement an imaginary absorption potential which leads to a simple but effective calculation of spontaneous fission half-lives.

Speaker: Ngee Wein Lau (Laboratoire des 2 Infinis Toulouse (IN2P3/CNRS))

Chamrousse_2026_presentation.odp

Chamrousse_2026_presentation.pdf

Chamrousse_2026_presentation_v2.pdf

10:05 → 10:35 Coffee break

10:35 → 11:30 session 8 (Chair: D. Hinde)

10:35 On the properties of superheavy nuclei

Superheavy nuclei (SHN) with extremely large atomic numbers (Z > 103), whose existence is due to their underlying nuclear stucture, remain one of the central and interdisciplinary research topics in science [1].

To date, SHN with proton numbers up to Z = 118 and neutron numbers up to N = 177 have been successfully synthesized and identified [2,3]. These nuclei are produced in heavy-ion induced fusion reactions at atom-at-a-time production rates and are identified through their α-decay chains and spontaneous fission.

Experimental data on the partial half-lives of α-decay and spontaneous fission confirm the enhanced fission stability of SHN thanks to their nuclear structure. However, many important properties (fission hindrance, fragment mass distributions etc.) of fission from SHN remain largely unexplored experimentally. These properties are essential for improving theoretical descriptions of the highly complex fission process within semi-empirical, macroscopic–microscopic, and fully microscopic approaches.

Intensive research programs dedicated to probing the shell structure and fission stability of SHN are ongoing worldwide, including efforts within the SHE Chemistry Department at GSI, Germany [4,5].

I will present/discuss the recent progress on the fission properties of SHN.

[1] Yu.Ts. Oganessian, A. Sobiczewski, G.M. Ter-Akopian, Phys. Scr. 92(2), 023003 (2017).
[2] F.G. Kondev et al., 2021 Chinese Phys. C 45 030001 (2021).
[3] Yu.Ts. Oganessian et al., Phys. Rev. C 106, 064306 (2022).
[4] J. Khuyagbaatar, et al., Phys. Rev. Lett. 125, 142504 (2020), Phys. Rev. C 104, L031303 (2021), Phys. Rev. C 106, 024309 (2022), Phys. Rev. C 109, 034311 (2024), Phys. Rev. Lett. 134, 022501 (2025).
[5] P. Mosat et al., Phys. Rev. Lett. 134, 232501 (2025).

Speaker: Dr Khuyagbaatar Jadambaa (GSI Helmholtzzentrum für Schwerionenforschung)

11:00 The spectroscopy of the shape isomer in 238U by Nu-ball spectrometer

We report on a new investigation of the fission shape isomer in ²³⁸U [1-3], performed with the Nu-Ball high-efficiency hybrid γ-ray spectrometer [4,5] at IJCLab, Orsay, France. The Nu-Ball array combines 24 clover HPGe detectors, 10 coaxial HPGe detectors with BGO shields, and up to 20 LaBr₃(Ce) or PARIS clusters, offering excellent efficiency, energy resolution, and timing performance. When coupled to the LICORNE directional neutron source at the ALTO facility, this setup provides a unique opportunity for precision spectroscopy of neutron-induced reactions with sensitivities far exceeding those of earlier experiments.
In this study, the ²³⁸U shape isomer was populated using inelastic neutron scattering, enabling a reexamination of previously reported γ-decay branches originating from the superdeformed second minimum. With significantly higher population cross sections and a substantially larger HPGe detection volume than in the original measurements, the new dataset allows a comprehensive assessment of the decay pathways associated with the isomer.
We will present the results of this investigation, together with their implications for the interpretation of earlier observations and in the broader context of recent studies on actinide shape isomers. These findings contribute to a more robust understanding of isomeric decay modes and fission dynamics in heavy nuclei.
[1] P. Russo, J. Pedersen and R. Vandenbosch, Nucl. Phys. A 240, 13, (1975).
[2] J.Kantele, W. Stroffl, E. Ussery, et al., Phys. Rev. C 29, 1693,(1984).
[3] Polikanov and Sletten, Nucl. Phys A 151 (1970) 656.
[4] N. Jovancevic et al., Act. Phys. Pol. A 50, p.297, (2019).
[5] M. Lebois et al., Act. Phys. Pol. A 50, p.425, (2019).

Speaker: Nikola JOVANCEVIC (University of Novi Sad)

11:15 Future fission experiments at the Oslo Cyclotron laboratory

S. Siem 1,2, A. Al-Adili 3, L.Csige 4, A. Görgen 1,2, H. Haug 1,2, J. Heines 1,2, M.Hunyadi 4,
V. W. Ingeberg,1,2, N. Kumar1,2,, M. Torsvoll 1,2, G. Torvund 1,2, J. Wilson 5

1 Department of Physics, University of Oslo, N-0316 Oslo, Norway
2 Norwegian Nuclear Research Centre, Norway
3 Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
4 Institute for Nuclear Research, Hungary
5 IJC-Laboratory, Orsay. France

Abstract. Fission is one of the five Research Themes in the newly established Norwegian Nuclear Research Centre [1]. The new improved experimental set-up for fission experiments at OCL will be presented and plans for new experiments. A new target chamber dedicated for fission experiments with actinide targets has been designed and is now being built. Also a new array of solid-state detectors for fission fragments, designed and built by collaborators in Hungary [2], will be replacing the previous gas-filled detectors (PPACs) and are planned to be tested as soon as the new target chamber is ready. One of the planned experiments is to compare different initial spin of the fissioning system by comparing proton and alpha induced fission data. Previously we have measured prompt fission gammas (PFG), as a function of excitation energy, at OCL using the CACTUS array (NaI detectors) and PPAC fission detectors [3,4]. With CACTUS it was not possible to separate out the contribution from neutrons from the prompt fission gamma rays experimentally, so assumptions had to be made regarding the contribution from neutrons in the PFG spectra. With the new detector array OSCAR, consisting of 30 large volume LaBr3 detectors, with excellent timing, we can now distinguish neutrons and gamma rays using time of flight. By repeating one of the experiments run with CACTUS, we can measure the neutron contribution using OSCAR and the new fission set-up and use this to make a “recipe” to correct the CACTUS data. In the past we have performed many experiments on actinide targets with the gamma detector array CACTUS, mainly to measure nuclear level densities and photon strength functions and using the PPAC fission detector only as a veto. These data can with this “recipe” be re-analysed to get the PFGs. We would also like to take the opportunity to discuss possible new experiments at OCL with the fission experts at this conference.

[1] Norwegian Nuclear Research Centre: http://www.nnrc.uio.no
[2] L. Csige and M. Hunyadi et al., Institute for Nuclear Research, Hungary
[3] S. J. Rose, F. Zeiser, J.N. Wilson, A. Oberstedt, S. Oberstedt, S. Siem, G. M. Tveten et al., Phys. Rev. C 96, 014601 (2017),
[4] D. Gjestvang, S. Siem, F. Zeiser, J. Randrup, R. Vogt, J.N. Wilson et al., Phys. Rev. C 103, 034609 (2021)

Speaker: Prof. Sunniva Siem (University of Oslo)

11:30 → 16:00 Mid-day break

16:00 → 17:20 session 9 (Chair: S. Oberstedt)

16:00 Pandemonium free data for reactor applications

Is this talk I will cover the systematic study of beta decays performed by our collaboration in relation to reactor applications. The main goal is to provide Pandemonium free decay data to improve decay heat and antineutrino spectrum calculations. Particular emphasis will be devoted to our recent beta spectrum shape experiments, that allow us to validate the corrections needed for the calculations of the antineutrino spectrum in reactors.

Speaker: Alejandro Algora (IFIC (CSIC-Univ. of Valencia))

16:25 Study opportunities with the LOHENGRIN fission fragment separator

The recoil mass-separator LOHENGRIN at Institut Laue-Langevin (ILL) selects fission fragments produced by actinide targets in a thermal neutron flux of about 5x1014 n/cm2/s in an in-pile position of the high-flux reactor. The experimental study of nuclear fission and fission products is of particular interest for both fundamental and applied physics. The performance of the LOHENGRIN spectrometer, allowing ion selection with excellent mass- and energy resolution, contributes to

• improving the understanding of the nuclear fission process with fission studies (fission yields and dynamics);
• giving access to many neutron-rich medium-mass nuclide enabling investigation of nuclear structure by nuclear spectroscopy: beta- or isomeric decays, gamma-ray and conversion electron spectroscopy, fast-timing, etc.;
• detector characterisations where ions of various masses, from protons to very heavy ions, and kinetic energies, from MeV to above 100 MeV, can be selected.

A large panel of experiments performed with the LOHENGRIN spectrometer will be presented and discussed.

Speaker: Jean Michel Daugas (ILL)

16:50 Multi-phonon gamma vibration study from AGATA+Gammasphere complimentary setup

Rapid shape change occurs in neutron-rich nuclei in the A∼100 region. Some of the nuclei in this region also exhibits ellipsoidal shape oscillations, known as gamma vibrations and linked to triaxiality and gamma softness. Furthermore, the two-phonon gamma-vibrations also provide tests of Pauli principle. To study of the evolution of the shapes and level structure, the 104Nb was investigated from two complementary experimental methods: i) high statistics triple- and four-fold gamma coincidences from 252Cf spontaneous fission using Gammasphere and ii) prompt gamma from the induced fission of the 238U+9Be reaction with isotopic fragment identification using the VAMOS++ and the AGATA spectrometers. Observation of multi-phonon gamma vibrations and shape coexistence of this odd-odd nucleus will be presented.

Speaker: Enhong Wang (Shandong University)

fission2026-ehwang.pdf

fission2026-ehwang.pptx

17:05 Cluster radioactivity fission valleys along isotopic and isotonic chains of heavy and super-heavy nuclei

Most heavy and super-heavy nuclei decay through fission or alpha emission, but other decay modes can also be noticed. In the 1980s, an exotic decay of cluster radioactivity was observed in actinides [1, 2, 3]. A light nucleus, but heavier than an alpha particle, is emitted in this process. The heavy mass residue is a doubly magic $^{208}$Pb in all observed decays of this type.

The theoretical description of this process as a very asymmetric fission has been successfully performed in the HFB model [4]. The analysis of cluster radioactivity fission valley on the potential energy surface has been extended to heavier isotopes. It has been found that the analogue process, with lead as one of the fragments, can be noticed even in super-heavy nuclei [5]. Moreover, the asymmetric fission of cluster radioactivity type plays a non-negligible role in this region. In some cases, it may be even the dominant fission channel.

Here, we will delimit the region in the chart of nuclides where super-asymmetric fission paths exist and set the limits for the cluster radioactivity decay mode [6]. Selected isotopic and isotonic chains in the region of heavy and super-heavy nuclei will be investigated to find the evolution of the super-asymmetric fission valley with increasing number of protons and neutrons.

[1] H. J. Rose and G. A. Jones, Nature (London) 307, 245 (1984).
[2] A. Sandulescu, D. N. Poenaru, and W. Greiner, Sov. J. Part. Nucl. 11, 528 (1980).
[3] R. Bonetti and A. Guglielmetti, in Heavy Elements and Related Phenomena, Vol. II, edited by W. Greiner and R. K. Gupta (World Scientific, Singapore, 1999), p. 643.
[4] M. Warda, and L. M. Robledo, Physical Review C 84, 044608 (2011).
[5] M. Warda, A. Zdeb, and L. M. Robledo, Physical Review C 98, 041602 (2018)
[6] M.Warda, A.Zdeb, and R Rodriguez-Guzman, Physical Review C, submitted

Speaker: Michał Warda (Maria Curie-Skłodowska University, Lublin)

17:20 → 17:50 Coffee break

17:50 → 20:15 session 10 (Chair: C. Simenel)

17:50 An Alp-Sized Neutron Absorber: Recent Discoveries and Measurements around Zirconium-88

It was discovered in 2019 that the thermal neutron absorption cross section of zirconium-88 was roughly 800,000 barns when 10 barns was expected. This has set off a flurry of activity and interest in the nuclear community. This talk will describe the ongoing efforts to resolve this mystery. These activities include measurements of the metastable state of zirconium-89 at the University of Texas NETL reactor in April 2024, a search for the underlying neutron absorption resonance at the CERN neutron Time-Of-Flight (n_TOF) experiment in August and September 2024, and spectroscopy of the neutron absorption at ILL FIPPS in August through October 2025.

Speaker: Will FLANAGAN (University of Texas / University of Dallas)

18:15 The SOFIA experiments, an overview

The SOFIA project relates to the measurement of fission yields. DUring the course 3 experiments run at GSI, we could investigate the fission of numerous fissioning systems, both close to the beta stability valley and , on the neutron deficient side, down to Platinium isotopes. I will sum up some key finding of the programme in my talk.

Speaker: Julien TAIEB (CEA DAM IdF)

18:40 Dual Shape and Intruder evolution between 73Zn and 75Zn isotopes

G. Duchêne,1,∗ J. Dudouet,2 F. Didierjean,1 D.D. Dao,1 F. Nowacki,1 E. Clément,3 A. Lemasson,3 C. Andreoiu,4 G. de Angelis,5 A. Astier,6 C. Delafosse,7 I. Deloncle,6 B.Z. Dombradi,8 C. Ducoin,2 G. de France,3 A. Gadea,9 A. Gottardo,5 D. Guinet,2 B. Jacquot,3 P. Jones,10 T. Konstantinopoulos,6 A. Korichi,6 I. Kuti,8 F. Le Blanc,6,1 S.M. Lenzi,11,12 G. Li,13,14 C. Lizarazo,15 R. Lozeva,6,1 G. Maquart,2 C. Michelagnoli,16,3 B. Million,17 D.R. Napoli,5 A. Navin,3 R.M. Pérez-Vidal,9 C.M. Petrache,6 N. Pietralla,15 D. Ralet,18,15 M. Ramdhane,19 N. Redon,2 M. Rejmund,3 K. Rezynkina,11,1 O. Stezowski,2 C. Schmitt,1,3 D. Sohler,8 and D. Verney6
1 Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France
2 Universite Claude Bernard Lyon 1, CNRS/IN2P3,IP2I Lyon, UMR 5822, Villeurbanne, F-69100, France
3 GANIL, CEA/DRF-CNRS/IN2P3, BP 55027, 14076 Caen Cedex 5, France
4 Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
5 INFN, Laboratori Nazionali di Legnaro, Viale dell’Universtità 2, I-35020 Legnaro (PD), Italy
6 Université Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France
7 ESRIG, University of Groningen, Zernikelaan 25, 9747 AA Groningen, The Netherlands
8 HUN-REN Institute for Nuclear Research (ATOMKI), Pf.51, H-4001, Debrecen, Hungary
9 Instituto de Física Corpuscular, CSIC-Universitat de València, E-46980 Valencia, Spain
10 iThemba LABS, National Research Foundation,P.O.Box 722, Somerset West,7129 South Africa
11 INFN Sezione di Padova, I-35131 Padova, Italy
12 Dipartimento di Fisica e Astronomia dell’Università di Padova, I-35131 Padova, Italy
13 GSI, Helmholtzzentrum für Schwerionenforschung GmbH, D-64291 Darmstadt, Germany
14 Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, 73000, China
15 Institut für Kernphysik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany
16 Institut Laue-Langevin, 71 avenue des Martyrs, 38042 Grenoble Cedex 9, France
17 INFN, Sezione di Milano, I-20133 Milano, Italy
18 MIRION Technologies Canberra, 1 Chemin de la Roseraie, 67380 Lingolsheim, France
19 LPSC, Université Grenoble-Alpes, CNRS/IN2P3, 38026 Grenoble Cedex, France

The area of the nuclear chart from Ni to Se and from N=40 to 50 is known to be transitional with shape evolution from spherical to axial or triaxial on the one hand, developing shape coexistence on the other hand [1-7]. Part of these nuclei are situated along the r-process path contributing to the first peak around A~85-90 [8]. The present study concentrates on neutron-rich Zn isotopes which are located at the border of this intriguing region.
The spectroscopy of the N=41-49 Zn isotopes was performed using AGATA coupled to VAMOS++ at GANIL. The isotopes were produced using the 238U (@6.2 MeV/u) + 9Be fusion-fission reaction. The gamma rays were detected in AGATA and the light fission fragments were identified in VAMOS++. As from these data Zn is the lowest Z chemical element for which gamma-ray spectra could be generated, the statistics is low. The isotopes from mass 73 to 79 could be studied.
In this talk, we focus on 73Zn and 75 Zn isotopes. A detailed analysis of the data enables us to propose 73Zn and 75Zn level schemes extended towards larger spins than known so far. Large-scale shell-model calculations were performed using the LNPS-U interaction. In a previous study, shape coexistence has been evidenced in the even-even 74Zn nucleus [7]. In the present work, the structures observed have been interpreted as due to the coupling of one neutron hole (73Zn) or one neutron particle (75Zn) to both the ground-state band and the excited 0+ and 2+ bands of 74Zn taken as a core. Our results evidence also a transition from triaxial intruder configurations in 73,74Zn to axial natural configuration in 75Zn whose features emerge naturally from underlying dynamical Nilsson-SU3 symmetries.

[1] S. Suchyta et al., Phys. Rev. C 89, 021301 (2014)
[2] R. Taniuchi et al., Nature 569, 53 (2019)
[3] M. Niikura et al., Phys. Rev. C 85, 054321 (2012)
[4] A. Illiana et al., Phys. Rev. C 108, 044305 (2023)
[5] A. D. Ayangeakaa et al., Phys. Rev. C 107, 044314 (2023)
[6] K. Rezynkina et al., Phys. Rev. C 106, 014320 (2022)
[7] M. Rocchini et al., Phys. Rev. Lett. 130, 122502 (2023)
[8] M.P. Reiter et al., Phys. Rev. C 101, 025803 (2020)

∗ gilbert.duchene@iphc.cnrs.fr
G. Duchêne,1,∗ J. Dudouet,2 F. Didierjean,1 D.D. Dao,1 F. Nowacki,1 E. Clément,3 A. Lemasson,3 C. Andreoiu,4 G. de Angelis,5 A. Astier,6 C. Delafosse,7 I. Deloncle,6 B.Z. Dombradi,8 C. Ducoin,2 G. de France,3 A. Gadea,9 A. Gottardo,5 D. Guinet,2 B. Jacquot,3 P. Jones,10 T. Konstantinopoulos,6 A. Korichi,6 I. Kuti,8 F. Le Blanc,6,1 S.M. Lenzi,11,12 G. Li,13,14 C. Lizarazo,15 R. Lozeva,6,1 G. Maquart,2 C. Michelagnoli,16,3 B. Million,17 D.R. Napoli,5 A. Navin,3 R.M. Pérez-Vidal,9 C.M. Petrache,6 N. Pietralla,15 D. Ralet,18,15 M. Ramdhane,19 N. Redon,2 M. Rejmund,3 K. Rezynkina,11,1 O. Stezowski,2 C. Schmitt,1,3 D. Sohler,8 and D. Verney6
1 Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France
2 Universite Claude Bernard Lyon 1, CNRS/IN2P3,IP2I Lyon, UMR 5822, Villeurbanne, F-69100, France
3 GANIL, CEA/DRF-CNRS/IN2P3, BP 55027, 14076 Caen Cedex 5, France
4 Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
5 INFN, Laboratori Nazionali di Legnaro, Viale dell’Universtità 2, I-35020 Legnaro (PD), Italy
6 Université Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France
7 ESRIG, University of Groningen, Zernikelaan 25, 9747 AA Groningen, The Netherlands
8 HUN-REN Institute for Nuclear Research (ATOMKI), Pf.51, H-4001, Debrecen, Hungary
9 Instituto de Física Corpuscular, CSIC-Universitat de València, E-46980 Valencia, Spain
10 iThemba LABS, National Research Foundation,P.O.Box 722, Somerset West,7129 South Africa
11 INFN Sezione di Padova, I-35131 Padova, Italy
12 Dipartimento di Fisica e Astronomia dell’Università di Padova, I-35131 Padova, Italy
13 GSI, Helmholtzzentrum für Schwerionenforschung GmbH, D-64291 Darmstadt, Germany
14 Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, 73000, China
15 Institut für Kernphysik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany
16 Institut Laue-Langevin, 71 avenue des Martyrs, 38042 Grenoble Cedex 9, France
17 INFN, Sezione di Milano, I-20133 Milano, Italy
18 MIRION Technologies Canberra, 1 Chemin de la Roseraie, 67380 Lingolsheim, France
19 LPSC, Université Grenoble-Alpes, CNRS/IN2P3, 38026 Grenoble Cedex, France

The area of the nuclear chart from Ni to Se and from N=40 to 50 is known to be transitional with shape evolution from spherical to axial or triaxial on the one hand, developing shape coexistence on the other hand [1-7]. Part of these nuclei are situated along the r-process path contributing to the first peak around A~85-90 [8]. The present study concentrates on neutron-rich Zn isotopes which are located at the border of this intriguing region.
The spectroscopy of the N=41-49 Zn isotopes was performed using AGATA coupled to VAMOS++ at GANIL. The isotopes were produced using the 238U (@6.2 MeV/u) + 9Be fusion-fission reaction. The gamma rays were detected in AGATA and the light fission fragments were identified in VAMOS++. As from these data Zn is the lowest Z chemical element for which gamma-ray spectra could be generated, the statistics is low. The isotopes from mass 73 to 79 could be studied.
In this talk, we focus on 73Zn and 75 Zn isotopes. A detailed analysis of the data enables us to propose 73Zn and 75Zn level schemes extended towards larger spins than known so far. Large-scale shell-model calculations were performed using the LNPS-U interaction. In a previous study, shape coexistence has been evidenced in the even-even 74Zn nucleus [7]. In the present work, the structures observed have been interpreted as due to the coupling of one neutron hole (73Zn) or one neutron particle (75Zn) to both the ground-state band and the excited 0+ and 2+ bands of 74Zn taken as a core. Our results evidence also a transition from triaxial intruder configurations in 73,74Zn to axial natural configuration in 75Zn whose features emerge naturally from underlying dynamical Nilsson-SU3 symmetries.

[1] S. Suchyta et al., Phys. Rev. C 89, 021301 (2014)
[2] R. Taniuchi et al., Nature 569, 53 (2019)
[3] M. Niikura et al., Phys. Rev. C 85, 054321 (2012)
[4] A. Illiana et al., Phys. Rev. C 108, 044305 (2023)
[5] A. D. Ayangeakaa et al., Phys. Rev. C 107, 044314 (2023)
[6] K. Rezynkina et al., Phys. Rev. C 106, 014320 (2022)
[7] M. Rocchini et al., Phys. Rev. Lett. 130, 122502 (2023)
[8] M.P. Reiter et al., Phys. Rev. C 101, 025803 (2020)

∗ gilbert.duchene@iphc.cnrs.fr

Speaker: Mr Gilbert Duchene (IPHC-CNRS-Université de Strasbourg)

Thursday, 12 March

08:30 → 10:05 session 11 (Chair: A. Goergen)

08:30 Shell effects in Quasi-fission: a theoretical perspective

Quasifission occurs in fully damped heavy-ion collisions following a significant mass transfer between the fragments, without formation of a compound nucleus. It is the primary reaction mechanism hindering the formation of a superheavy compound nucleus after the collision partners have reached contact. As in fission, quasi-fission is expected to be affected by quantum effects leading to asymmetric mass splits.

In addition to shell effects in the compound nucleus, quantum shells stabilising fission fragments with octupole shapes have been invoked as a factor determining the distribution of nucleons between the fragments at scission, explaining the fact that the centroid of the heavy fragment charge distribution is found around Z=54 protons in fission of actinides. A supersymmetric fission mode influenced by shell effects in the 208Pb region is also predicted in superheavy nuclei.

Similar shell effects are predicted in microscopic studies of quasi-fission. In particular, time-dependent Hartree-Fock (TDHF) calculations have been performed for reactions forming actinides and superheavy compound nuclei, favouring formation of fragments with Z~54 and 82, respectively. In the absence of clear experimental confirmation, however, one should ask whether the strong influence of shell effects in quasi-fission predicted by TDHF would still persist in beyond-mean-field approaches. Perspectives for such calculations will be discussed.

Speaker: Cedric Simenel (Australian National University)

08:55 Fission with PARIS @ VAMOS

As obvious from the intense experimental and theoretical work done over the past decades, nuclear fission is complex. A major reason of this is the interference of various aspects, from both reaction dynamics and nuclear structure, which determines the observables that can ultimately be measured in the laboratory. The last years showed that high-fold coincidences between as many as possible observables are crucial to unravel the intricacies of fission, a mandatory condition for un-ambiguous interpretation. In this context, an innovative experimental approach was set up at GANIL coupling for the first time a heavy-ion spectrometer such as VAMOS++ and a new-generation scintillator gama array like PARIS. While the former is capable of identifying accurately in mass and charge the fragments emitted in fission, the latter gives access to the properties of the coincident gamma rays over their full dynamical range with unprecedented quality,as well as information about the coincident neutrons. In this contribution, the first experiment with PARIS@VAMOS dedicated to fission induced by fusion and nucleon transfer in 238U+9Be collisions is presented. A selection of results is used to illustrate the performance of the set up in terms of efficiency, resolution, and sensitivity. The variety of aspects that can be addressed is highlighted. In particular, the so-called fission gamma bump and its highly probable connection to the Pygmy Dipole Resonance is demonstrated by means of calculations employing microscopic nuclear level densities and strength functions. The impact of this connection is double, as it makes the gamma bump a relevant signature of the dynamics after scission, as well as it proposes fission as a new probe of soft dipole modes, complementary to the conventional approaches.

Speaker: Michal Ciemala (IFJ PAn)

09:20 Fast-timing lifetime measurements in A = 83 nuclei near the N = 50 shell closure

Fast-timing measurements have been performed to investigate the nuclear structure of the A =
83 isobars 83Se and 83Br, with the aim of studying the evolution of collectivity and possible shape
coexistence in the vicinity of the N = 50 shell closure.
The experiment was carried out at the LOHENGRIN recoil mass separator at the Institut
Laue-Langevin. Neutron-induced fission products were separated and implanted, and γ-rays were
detected using a fast-timing setup based on LaBr3(Ce) scintillators. Lifetimes were extracted
using both Generalized Centroid Difference (GDC) and Advanced Time-Delayed (ATD) techniques,
providing sensitivity in the picosecond range.
In 83Se, the lifetime of the state at E = 963.4 keV, depopulated by the 734.9 keV (3/2+ → 1/2−)
transition, was measured for the first time and determined to be τ = 13 ± 3 ps using the GDC
method and τ = 17 ± 6 ps using ATD. In 83Br, new lifetimes were measured for the state at
E = 356.7 keV, depopulated by the 356.5 keV (5/2− → 3/2−) transition, yielding τ = 12 ± 2 ps,
and for the state at E = 866.9 keV, depopulated by the 509.9 keV (7/2− → 5/2−) transition, with
τ = 7 ± 3 ps, as determined via GDC analysis, with compatible values obtained using ATD.
These new lifetime data extend the experimental knowledge of nuclei close to the N = 50 shell
closure and provide important constraints for the interpretation of structure evolution and possible
shape coexistence in the A = 83 mass region.
This work was performed in collaboration with the University of Cologne and INFN.

Speaker: Ludovico Lapo Luperi (Institut Laue-Langevin)

09:35 Fission of Hg-194

We present results of experiment focused on a previously unknown fission characteristic of Hg-194 perfromed at the ALTO facility during the campaign with the nu-ball-2 spectrometer. The nuclide of interest was created in a fusion reaction of C-12 beam on a thick W-182 target. Independent fission yields of even-even nuclei were determined by detecting gamma-gamma cascades in the fission fragments. The number of emitted neutrons was determined from the fragment distribution, as well as from the study of fission partners observed by gamma-gamma coincidences. Finally, an average momentum carried by the gamma-rays was determined by the Manchester method.

The results were compared with calculations by M. Mumpower et al. and the GEF model. The latter was found to describe the fragment distribution more closely. However, in this case, we found that other observables - the number of emitted neutrons and the angular momentum of fragments - are overpredicted. It seems that the mechanism of fragment excitation based on the deformation of fragments might not be the correct one, or the parameters tuned for the actinides region are not universal.

These observations, helpful in addressing the long-standing question of the excitation mechanism of fission fragments, might not be accessible with other experimental methods, where access to the full mass and charge distribution of fragments and their excitation energy is more limited. This proves that accessing information about fission with gamma spectroscopy is a valuable and complementary method that should be employed simultaneously with other experimental techniques.

Speaker: Krzysztof Miernik (University of Warsaw, Faculty of Physics)

miernik_ill.pdf

old

09:50 Angular Momentum in Fission through 241Am(2nth, f ) Isomeric Ratio measurements with the Lohengrin spectrometer

Despite decades of study, accurate modeling of nuclear fission remains difficult, as a wide variety of theoretical approaches—microscopic, macroscopic, and phenomenological models—are based on different underlying assumptions. To date, despite this diversity of approaches, none of the currently existing models can predict the angular-momentum generation mechanism with satisfactory accuracy. To complement and compare with existing studies, we adopt an indirect approach based on the measurement of isomeric ratios (IRs), defined as the independent yield of the isomer divided by the total independent yields of both the ground and isomeric states. IRs are particularly valuable because they retain information about the fragment’s initial angular momentum immediately after prompt-particle emission. A recent experimental campaign was conducted at the LOHENGRIN recoil spectrometer of the Institut Laue–Langevin (ILL) in Grenoble to measure the kinetic-energy dependence of the isomeric ratio for several fission products produced from the 241Am(2nth, f ) reaction. The IRs were extracted using γ-ray spectrometry with two clover High-Purity Germanium (HPGe) detectors operated in coincidence with an ionisation chamber located at the focal plane of the spectrometer. These measurements provide a sensitive probe of the total angular momentum generated in the fission fragments. The choice of the 241Am(2nth,f) reaction represents a unique case study, as the first neutron capture populates 242Am in both a long-lived metastable state with t1/2,m = 141 y and Jπ = 5−, and a ground state with a half-life of t1/2,g = 16 h and Jπ = 1−. These two competing states, which differ significantly in both half-life and angular momentum, undergo fission independently after capturing a second neutron and forming the compound nucleus 243Am∗. Furthermore, due to variations in reactor power over the course of the measurements, it is possible to determine the relative contributions of fission originating from the metastable and ground states of 242Am as a function of time. The time evolution of the IR, as well as its dependence on kinetic energy, has been extracted for several isotopes. Promising preliminary results show, for the first time, a clear kinetic-energy dependence of the IR for the isotope 100Nb. To interpret the experimental data, the FIFRELIN Monte Carlo code will be used to simulate the de-excitation of the fission fragments. By combining the measured IRs with FIFRELIN calculations, we aim to determine the angular-momentum distributions of the fission fragments as a function of their kinetic energy. This analysis will ultimately enable a deeper understanding of angular-momentum generation mechanisms in nuclear fission.

Speaker: Anna SKOULOUDAKI (CEA Cadarache)

FISSION_2026_SKOULOUDAKI_Anna.pdf

FISSION_2026_SKOULOUDAKI_Anna.pptx

10:05 → 10:35 Coffee break

10:35 → 11:30 session 12 (Chair: A. Setaro)

10:35 Shape isomerism in uranium isotopes

The unexpected observation of a metastable spontaneously fissioning state
in 242Am put in question the common knowledge of the fission barrier. The inability of the available fission models back then to explain the observed decay mode, based on a single-humped barrier, led to the consideration of shell effects modulating the liquid-drop energy as a function of deformation. In 1967, Strutinsky achieved a breakthrough by incorporating shell effects into the calculation of the nuclear energy at large nucleus deformations. As a result, the modulation of the energy surface leads to an additional local, so-called super-deformed, minimum, at a deformation corresponding to an aspect ratio of about 2:1, when approximating nuclear shapes as spheroids.

In the forthcoming decade systematic investigation led to the discovery of about 30 shape isomers in nuclei with Z > 93, except for 236,238U and 237Np. In the latter case, the exceptionally low population probability of the shape isomer promoted the idea that an internal transition to the normal ground-state takes the main strengths of the decay. This decay branch had indeed been observed in 236U and 238U. In the meantime, the so-called γ back-decay has been confirmed for 236U, whereas this decay mode has been put into doubt for 238U after several further experiment campaigns.

At the JRC Geel attempts have been made to search for shape isomers in odd uranium isotopes, for which half-life predictions ranges from ns until hundreds of ms. While a shape isomer had been discovered for 235U with a reasonable population probability, a recent measurement campaign hints to the existence of a shape isomer in 237U with a probability comparable with that in 237Np.

The present situation around shape isomers will be discussed and possible future efforts presented.

Speaker: Stephan Oberstedt (European Commission)

11:00 Fission Yield Analysis of Neutron-Induced Fission on Th-232

Thorium-based molten salt reactors have recently attracted increasing interest as one of the promising Generation-IV reactor concepts and as a potentially safer alternative to Uranium-fuelled systems. However, the fission properties of Thorium are still insufficiently understood, particularly due to the limited availability of experimental fission yield data. In this work, we analyze γ-ray spectroscopy data from neutron-induced fission of Th-232 performed with the nu-Ball1
spectrometer at the ALTO facility to obtain its fission fragment yields. The yields were first extracted using a conventional spectroscopy method, and then further improved by introducing a Cf-252-based normalization approach. Using the well-characterized Cf-252 spontaneous fission dataset, we established the fraction of specific γ transitions relative to the total transitions intensity for major isotopes, and applied these ratios to the Th-232 data. This method enables more reliable yield extraction, particularly for odd-Z and odd-A nuclei with complex decay schemes where conventional spectroscopy often fails. The results demonstrate that the Cf-252-based normalization provides a valuable complementary strategy for yield reconstruction, enhancing accuracy for isotopes with complicated level structures.

Speaker: Shiyu Liu (IJCLab)

11:15 Results on 235U(nth,f) isotopic fission yields using prompt and delayed gamma rays at the FIPPS spectrometer of the ILL

Although nuclear fission has been known and studied for more than 80 years, it remains a very active field of research. A deeper understanding of the fission process can be achieved by investigating the prompt and delayed gamma-ray cascades emitted by fission fragments.

We report on the results of a measurement campaign performed with the FIPPS gamma-ray spectrometer at the ILL, using an active target consisting of 235U dissolved in a liquid scintillator. Prompt gamma rays were used to determine absolute, independent isotopic fission yields for a selected set of well-produced even-even nuclei from the 235U(nth,f) reaction, which were subsequently compared with the JEFF-3.3 evaluated data.
This set includes the doubly magic nucleus 132Sn, for which we observe a pronounced deficit with respect to JEFF-3.3, in agreement with results obtained using a similar technique in the fast fission of 238U. Using the FIFRELIN fission fragment de-excitation code, we interpret this apparent anomaly as evidence that 132Sn is predominantly produced in its ground state, either at scission or after neutron evaporation.

Delayed gamma rays recorded in the same experiment were also analysed for a set of fission products to assess the potential of such experimental data for identifying discrepancies in nuclear databases in the context of the reactor antineutrino anomaly. No deviation is observed in the cumulative fission yield of 132Sn with respect to nuclear databases. However, an excess of about 40% is found for the 1118.7-keV transition in the beta decay of 90Kr, which may originate from an overestimated ground-state feeding of 90Rb in the evaluated data. This observation confirms a result previously reported using MTAS measurements.

Speaker: Thomas Materna (IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France)

slides_Thomas_Materna.pdf

11:30 → 16:00 Mid-day break

16:00 → 17:35 session 13 (Chair: J. Wilson)

16:00 Shell-driven Fission across the Nuclear Chart

Results are presented from a broad, systematic study of heavy-ion induced fusion-fission mass distributions carried out at the Australian National University, covering a significant part of the chart of the nuclides. Fission characteristics of isotopes of every even-Z compound nucleus (ZCN ) from 14464Gd to 21290Th were measured. Systematic evidence of shell-driven structure is present in every fission mass distribution. The results for the heavier systems are consistent with proton fusion-fission measured at lower excitation energies, near the fission threshold. In heavy ion fusion the structure is, as expected, significantly less pronounced, meaning great care must be taken in the data analysis and fitting process.
The changing shape of the heavy-ion fission mass distributions with ZCN can be visualised through the residuals from single Gaussian fits. These results are consistent with quantitative fitting of the measured 2-D mass and total kinetic energy spectra using multiple components. Both approaches demonstrate that fragment proton shell gaps at ZFF ~ 34, 36 and at ZFF ~ 44, 46 are major drivers of fission mass distributions for nuclei below the actinide region.
For all systems, a second more mass-asymmetric fission mode is required to fit the fission mass distributions. If driven by a single shell gap, it appears to be in the light fragment around ZFF ~ 28, 30 or possibly N ~ 36.
The apparent universal influence of shell gaps on fusion-fission raises questions about the influence of shells gaps in quasi-fission, where recent results suggest that shell effects do not play such a strong role as previously believed.

Speaker: David Hinde

16:25 Simultaneous measurement of fission, gamma, and multi-neutron emission in surrogate reactions at a heavy-ion storage ring

Neutron-induced reaction cross sections of short-lived nuclei are crucial for our understanding of nuclear fission and nuclear structure, with important applications for nuclear astrophysics and a wide range of nuclear technology. However, direct measurements of these cross sections are extremely challenging, or even impossible, due to the difficulty of producing and handling the required radioactive targets. In addition, measuring cross sections for radiative capture (n,gamma) and multi-neutron-emission (n,xn) of fissionable nuclei is extremely complicated due to the intense background of gamma-rays and neutrons generated by the fission fragments.

We are developing a novel approach that, for the first time, employs surrogate reactions in inverse kinematics at a heavy-ion storage ring [1,2]. This method enables the measurement of all de-excitation channel probabilities as a function of the excitation energy of nuclei formed in surrogate reactions, allowing the indirect determination of the neutron-induced cross sections.

In this contribution, I will present our methodology and the results from our last surrogate-reaction experiment conducted at the ESR storage ring of the GSI/FAIR facility in Darmstadt, Germany. In this experiment, we investigated the (d,p) and (d,d') surrogate reactions on U-238 and achieved a major milestone: for the first time, fission,
gamma-ray emission, single-neutron emission, as well as two- and three-neutron emission probabilities were measured simultaneously.

The simultaneous measurement of all competing decay channels places stringent constraints on key nuclear properties such as fission barriers, gamma-ray strength functions, and nuclear level densities. These quantities, in turn, allow us to infer neutron-induced cross sections for the (n,f), (n,gamma), (n,n'), (n,2n), and (n,3n) reactions.

[1] M. Sguazzin et al., Phys. Lett. 134, 2025, 072501.
[2] M. Sguazzin et al., Phys. Rev. C 111, 2025, 024614.

Speaker: Boguslaw Wloch (GSI Darmstadt)

16:50 Directional distributions of prompt fission neutrons from 252Cf(sf)

The study of particle (n, p, α, and γ) emission in fission phenomena is crucial to understand the involved timescales, the energy dissipation, and the underlying mechanisms of the process. Among these, neutron emission is one of the most experimentally exploited process. Earlier studies of neutron-fragment (n-f), neutron-neutron (n–n) correlations were primarily focused on scission neutrons. More recent measurements have explored the average energy of neutrons detected in coincidence with others of specific energies, analyzed as a function of the angle between them [1,2]. These investigations offer indirect insights into the partitioning of excitation energy between the fission fragments during the splitting without measuring the fragments. In addition, three-neutron angular correlations have been examined under various energy and angular conditions [3]. Currently, efforts are been made to understand the effect of fragment spin on the neutron and γ-ray emission [4]. To gain a deeper understanding of the neutron generation in the fission process, a dedicated experiment was carried out to examine the energy and angular correlations of emitted neutrons with respect to the direction of the fission fragments in the spontaneous fission of the 252Cf nucleus.
The measurement was performed out using the ELIGANT-GN [5] array at ELI-NP. In contrast to previous measurements with this setup, where the trigger was provided by prompt fission γ-rays, the array was complemented with a vacuum chamber in order to detect fission fragments. A ²⁵²Cf spontaneous fission source was placed inside the vacuum chamber at the center of the array. The 16 × 16 double sided silicon strip detector (DSSSD) was mounted at a distance of 9 cm from the source for the detection of fission fragments. The neutrons emitted in the fission were measured in coincidence with the fission fragments using thirty-six EJ-301 liquid scintillators or the detection of fast neutrons as well as twenty-five 6Li-glass detectors for the detection of low-energy neutrons. The distance between the source and detectors was 150 cm and 100 cm, respectively. All neutron detectors were arranged in the upper hemisphere and provide high efficiency with excellent timing resolution for time-of-flight energy calculations. The lower hemisphere of the array houses thirty-four large-volume LaBr3:Ce and CeBr3 detectors mounted at a distance of 30 cm to measure the γ-rays emitted in a fission event. Data were collected over a period of six months in order to achieve sufficiently good statistics.
The angular distribution of neutrons relative to the detected fission fragments confirms the kinematical focusing of neutrons in the direction of the emitting source. The energy-angular distributions of the neutrons correlated with light fission fragments show good agreement with literature. Moreover, the data are consistent with FREYA model [5] calculations for the fission of the 252Cf nucleus. The analysis is further extended to investigate two-neutron correlations with respect to the light fragment. A detailed description of the experimental setup, along with an investigation of neutron generation in fission focusing on one- and two- neutron energy-angle correlations relative to the detected fission fragment, will be presented and discussed in the framework of the FREYA model.
Acknowledgment: This work is supported by the Romanian Ministry of Research, Innovation and Digitalization under research contract PN 23 21 01 06, and the ELI-RO program, contract number ELI-RO/RDI/2024-007 ELITE.
References:
[1] J. M. Verbeke et al., Phys. Rev. C 97, 044601 (2018)
[2] P. F. Schuster et al., Phys. Rev. C 100, 014605 (2019)
[3] D. Choudhury et al., Phys. Rev. C manuscript under review
[4] S. Marin et al., PRC 109, 054617 (2024)
[5] P.-A. Söderström et al., Nucl. Instrum. Meth. Phys. Res. A 1027, 166171 (2022)
[6] J.M. Verbeke, J. Randrup, R. Vogt, Comput. Phys. Comm. 222, 263 (2018)

Speaker: Dr S. Dhuri (Extreme Light Infrastructure - Nuclear Physics, IFIN-HH, 077125 Magurele, Romania)

17:05 Magnetic Moment Measurements of 98Y Isomers Using the TDPAC Method at Lohengrin

The neutron-rich A~100 region is known for a sudden change in nuclear shape deformation from nearly spherical to highly-deformed as A goes from 97 to 99. The most severe change (as measured by the electric quadrupole deformation parameter) seems to occur in the region centered on 98Y. To better understand this phenomenon, the magnetic moments (another probe of nuclear deformation) of the 2−1 and 4−1 isomeric states of 98Y have been measured using the Time-Differential Perturbed Angular Correlation (TDPAC) method on data collected from the Lohengrin fission fragment spectrometer at Institut Laue-Langevin. The results of the measurements have been compared to two theoretical calculations, the Interacting Boson Fermion Fermion Model (IBFFM) and the Complex Excited Vampir Model (EXVAM), which both compare somewhat favorably.

Speaker: David Friant (Institut Laue-Langevin / CEA Saclay)

17:20 Status of S3 and possible studies on spontaneous fission

The in-depth study of the regions of Superheavy elements and the proton dripline around 100Sn are two major challenges of todays’ Nuclear Physics. Performing detailed spectroscopic studies on these nuclei requires a significant improvement of our detection capabilities. The Super-Separator-Spectrometer S3 is part of the SPIRAL2 facility at GANIL. Its aim is to use the high stable beam currents provided by the new LINAC to reach rare isotopes by fusion- evaporation.

S3 is designed to provide the best rejection power along with a high transmission and a mass resolution of around 400. The use of high-acceptance superconducting multipoles provides a high transmission thanks to large gaps and higher-order optical corrections. These features, connected to a high-power target station, will provide access to nuclei with fusion-evaporation cross-section down to the picobarn region and below.

This presentation will describe the technical capabilities of S3 and give a status of the construction of all systems and its commissioning plan. An overview of the Physics Cases planned for the first experiments will also be given with a focus on the study of spontaneously fissioning systems.

Speaker: Julien Piot (GANIL)

17:35 → 18:05 Coffee break

18:05 → 19:15 session 14 (Chair: S. Siem)

18:05 Approaching the dominance of electron-capture delayed fission in 234Bk and 230Am

The first identification of the exotic decay process of Electron-Capture Delayed Fission (ECDF) of the very neutron-deficient isotope 234$Bk was performed, and improved ECDF properties for its alpha-decay product 230Am were obtained at the gas-filled recoil separator SHANS2. The isotope 234Bk was produced in the fusion-evaporation reaction 40Ar+197Au->237Bk*->234Bk+3n.
By using the method of temporal and position correlations, different decay channels of 234Bk and 230Am isotopes were investigated and a wealth of new experimental information was obtained.
The highest ECDF probabilities, P(ECDF)(234Bk)=0.55(15) and P(ECDF)(230Am)=0.35(11), among all beta- or EC-delayed fission cases known so far are reported, showing the tendency of approaching the expected saturation towards P(ECDF)=1. The comparison of the P(ECDF) systematics with two theoretical fission models shows significant discrepancies in respect of corresponding fission barrier values. Meanwhile, the analysis of the ECDF probabilities in the heavy actinides and lead region suggests the similar ECDF mechanism in both cases. The need for a theoretical framework that can provide realistic beta-delayed fission probabilities for astrophysical predictions is strongly underlined.

*on behalf of the York-IMP(Lanzhou)… collaboration

Speaker: Andrei Andreyev (University of York, UK)

18:30 Delayed gamma-ray spectroscopy as a tool for fusion-fission studies

Fusion-fission reactions provide valuable insights into nuclear structure and fission dynamics. They are studied with various techniques, including direct mass and charge measurements and gamma-ray spectroscopy. Prompt gamma-ray emission is commonly used, requiring sophisticated detector arrays and coincidence analysis. We present a complementary spectroscopic method based on medium- and long-lived fission products, offering independent verification of fragment yields.

The method exploits delayed gamma radiation from nuclides in fusion-fission and competing channels. Spectra are recorded after irradiation, with target kept in situ or moved to other setup for longer measurements. Absolute yields are determined through decay curve analysis. Success depends on matching measurement timescales to fragment half-lives for separating fission products from fusion-evaporation and activation backgrounds.

Three datasets covering different systems and timescales are presented. 215Fr fission studies [1] demonstrate capability to extract fragment yields comparable to prompt measurements. Analysis of 12C+182W reaction [2], including medium- and long-lived measurements, shows capabilities and limitations of the method, especially regarding experimental parameters such as measurement length.

Results demonstrate delayed gamma-ray spectroscopy serves as a valuable alternative to classical prompt analysis for fusion-fission studies.

[1] Miernik, K. et al. Fission of 215Fr studied with gamma spectroscopic methods, Physical Review C 108, 054608 (2023)
[2] Miernik, K. et al. Gamma spectroscopy of 12C + 182W fusion-fission reaction (submitted to PRC)

Speaker: Piotr Garczynski (Faculty of Physics, Uniwersity of Warsaw)

18:45 Total gamma-ray energy in 252Cf spontaneous fission

Prompt fission gamma-ray emission is a complex phenomenon at the interplay of nuclear reaction and nuclear structure, that depends on the highly excited fission fragment entry state, subsequent neutron emission, and the nuclear structure of the resulting fragment. In the last few years there has been a renewed interest about this observable, as it is a sensitive probe to study the nuclear fission process, and especially the spin distribution of the fission fragments [1-4].

In that respect, the VErsatile SPectrometer Array (VESPA) was built at EC-JRC Geel to study gamma-rays emitted in the spontaneous fission of 252Cf. It consisted of 8 LaBr3(Ce) gamma-ray detectors with excellent timing and energy resolution surrounding a position-sensitive twin Frisch-grid ionization chamber. The first experimental campaign was successfully carried on in 2019-2020. The goal of this campaign was to provide multi-parameters measurements to improve our understanding of the fission process by constraining nuclear fission models such as the FIFRELIN code [5]. In particular, we could obtain the mass and total kinetic energy (TKE) dependence of the prompt fission gamma-ray multiplicity of 252Cf [6].

In this work, we will present the next-order properties associated to these data, that are the mass- and TKE-dependent total gamma-ray energy. Such properties could help understanding the excitation energy sharing between the fragments, as well as the gamma-neutron competition during their de-excitation. The data analysis technique used to extract these properties will be described, and the experimental results will be presented and compared to existing data. Finally, preliminary comparison with FIFRELIN calculations will be discussed.

[1] : A. Chebboubi et al., Phys. Lett. B 775, 190 (2017)
[2] : J.N. Wilson et al., Nature 590, 566 (2021)
[3] : V. Piau et al., Phys. Lett. B 837, 137648 (2023)
[4] : N.P. Giha et al., Phys. Rev. C 111, 014605 (2025)
[5] : O. Litaize et al., Eur. Phys. J. A 51, 177 (2015)
[6] : M. Travar et al., Phys. Lett. 817, 136293 (2021)

Speaker: Valentin Piau (Subatech)

Fission2026_VPiau.pdf

Friday, 13 March

09:00 → 09:50 session 15 (Chair: A. Andeyev)

09:00 Study of the fission process in 263Bh and 269Bh on the path toward the Island of Stability

The aim of this work is to investigate the fission process in the transitional region of the nuclear chart, with the goal of progressively approaching the predicted island of stability through both the systematic analysis of newly available experimental data and an improved theoretical description. Two Bohrium isotopes, 263Bh and 269Bh, located between the heavy actinides and the superheavy nuclei near the island of stability, have been studied through their most relevant observables: the mass and total kinetic energy distributions of the primary binary fragments, reconstructed from measured flight times and hitting position of fragment using a two-arm time-of-flight spectrometer TOSCA (ToF-2V method).
To interpret these data and to achieve a more comprehensive understanding of the fission process, the original Pashkevich routine implemented in the mymash code was extended to build a consistent theoretical framework suitable for describing the fission decay of superheavy compound nuclei formed in heavy-ion induced reactions, systems produced at moderate excitation energies and angular momenta. The resulting model, referred to as SAF (Stationary Approach to Fission), incorporates both the damping of shell effects with increasing excitation energy and the rotational contribution associated with angular momentum.
The analysis focuses on the evolution of fission modes in 263Bh and 269Bh, with the aim of determining whether the transition between elongated asymmetric and compact symmetric modes persists in these isotopes and whether it is consistent with the experimental observations. A major difficulty in this mass region arises from the overlap between fusion–fission and quasi-fission processes. One of the objectives of this work has been therefore perform the experimental campaign at different excitation energies to study the evolution of fission modes and exploit SAF calculations also to constrain the fusion–fission component in the experimental distributions, enabling a more reliable interpretation of the data.

Speaker: Pia Antonella Setaro (Università degli Studi di Napoli Federico II- INFN sezione Napoli)

09:25 Role of Macroscopic Effects in Dynamics of Nuclear Fission

Mass-asymmetric fission in nuclear fission processes has traditionally been considered a consequence of microscopic properties associated with the internal structure of the nucleus. However, dynamical model calculations indicate that the competition between mass-asymmetric and mass-symmetric fission in induced fission is strongly governed by liquid-drop–like properties, i.e., macroscopic effects, that develop during the fission process. On the basis of these results, an interpretation of the dynamical description of induced nuclear fission is discussed.
As a representative example of induced nuclear fission, we consider the fission reaction of 235U induced by thermal neutrons. The kinetic energy brought into the compound nucleus by the incident thermal neutron is converted into internal excitation energy, causing the compound nucleus to undergo strong thermal fluctuations that enable it to overcome the fission barrier. The interpretation of the fission process driven by such thermal fluctuations is consistent with the physical picture of Brownian motion, and the nuclear fission process has therefore been described using the Langevin equation [1,2]. The transport coefficients appearing in this equation, namely, the inertia tensor and the friction tensor calculated within macroscopic models, strongly influence the dynamical behavior. Recent dynamical calculations have revealed that these tensor quantities exhibit characteristic properties [3].
The dynamics governing which saddle point is selected by a trajectory are of crucial importance and depend sensitively on the description of thermal fluctuations of the nuclear shape. The trajectory evolution is not determined solely by the structure of the potential-energy surface, but is also strongly influenced or constrained by the directional properties of these tensor quantities. In this work, we address the liquid-drop–like characteristics of the transport coefficients and discuss their role in the mechanism of induced nuclear fission.

[1] Y. Abe, C. Gregorie, and H. Delagrange, J. Phys. Colloq. 47, C-4-329 (1986)
[2] T. Wada, Y. Abe, N. Carjan, Phys. Rev. Lett. 70, 3538 (1993); N. Carjan, T. Wada, Y. Abe, AIP Conf. Proc. No. 250 （AIP, NY, 1992）; G.-R. Tillack, Phys. Lett. B 278, 403 (1992)
[3] Y. Aritomo, A. Iwamoto, K. Nishio, and M. Ohta, Phys. Rev. C 105, 034604 (2022)

Speaker: Dr Yoshihiro Aritomo (Faculty of Science and Engineering, Kindai University, Higashi-Osaka, Osaka 577-8502, Japan)

09:50 → 10:35 Coffee break

10:35 → 11:15 session 16 (Chair: C. Michelagnoli)

10:35 A Comparative Analysis of Langevin and Random Walk Methodologies within a 4D Framework for Fission Fragment Mass Distributions in Actinide Nuclei

We present a comparative study of Langevin dynamics and a density-of-states-based random-walk approach for modeling fragment mass distributions in neutron-induced fission of thorium-229, thorium-232, uranium-235, plutonium-239, curium-245, and californium-249. We ensure a consistent theoretical framework by using the same four-dimensional potential energy surface derived from a macroscopic-microscopic model with Fourier-over-Spheroid shape parametrization. We adapt the random-walk methodology to simulate fission dynamics and analyze its relationship to the Langevin approach. We show that Langevin dynamics converge to the random-walk model in the low-friction limit. Calculations for several actinides indicate that both methods accurately reproduce the positions of the asymmetric fission peaks. The random-walk model, guided solely by the potential energy surface, predicts zero symmetric fission yield at low energies, in line with experimental trends. The Langevin model, by incorporating fluctuations, allows a small symmetric yield even at low energies. Differences in the predicted widths of the mass distributions—Langevin typically producing peaks only slightly narrower than experiment, while the random-walk approach yields narrower peaks—highlight the importance of dynamical effects such as inertia and friction, which are explicitly included only in the Langevin formalism. the importance of dynamical effects like inertia and friction, which are explicitly included only in the Langevin formalism.

Speaker: Michal Kowal (NCBJ)

11:00 Evolution of triaxiality in the mass 110 region

Andreas Görgen and Johannes Sørby Heines for the GANIL E704 Collaboration

Strong triaxiality is rare in the nuclear chart, and such nuclei can serve as useful tests of theoretical predictions. The breaking of axial symmetry also enables phenomena which cannot occur in symmetrically deformed nuclei, making it an interesting phenomenon to study. The neutron rich region around mass 110 presents several cases of strong triaxiality, namely in the ruthenium, molybdenum and palladium chains. Ruthenium especially is considered one of the best examples of strong triaxial deformation near the ground state in the entire nuclear chart.

This contribution presents new results from an experiment performed at GANIL to measure lifetimes of excited states in this region with the recoil distance Doppler-shift method. The studied nuclei were populated in a fusion-fission reaction, and identified event by event in the Variable Mode Spectrometer (VAMOS++). The Advanced Gamma Tracking Array (AGATA) provided high resolution gamma spectra of the decaying fission fragments.

We present new B(E2) values for transitions in 108–112Ru, and compare them with phenomenological triaxial rotor predictions and fully microscopic symmetry conserving configuration mixing calculations. Triaxiality is shown to be increasing from 108Ru to 112Ru, with the latter exhibiting near maximum triaxiality. The results are consistent with a simultaneous transition from γ soft to γ rigid deformation. New results for molybdenum and palladium isotopes will also be presented.

Speaker: Prof. Andreas Görgen (University of Oslo)

11:15 → 12:00 End of the workshop

12:00 → 13:30 Transfer to Grenoble railway/bus station