Orateur
Description
Nuclear deformation has been a topic of interest for many decades. While most nuclei can be described using only quadrupole deformation, recent experimental results [1, 2], in agreement with theoretical predictions [3, 4], indicate that a more exotic type of deformation, namely octupole deformation, is needed to best describe nuclei in certain regions of the nuclear chart. In particular, the region of most enhanced octupole collectivity can be observed in the actinides around Z = 88 and N = 134.
In this framework and more generally for the study of heavy nuclei, a new decay station named SEASON (Spectroscopy Electron Alpha in Silicon bOx couNter) has been developed at CEA-Irfu. The online commissioning of the instrument was performed at the IGISOL facility of the Accelerator Laboratory of the University of Jyväskylä, Finland, in February 2026. SEASON is designed to meet the constraints of a high-energy-resolution decay station and an efficient counter for laser spectroscopy of heavy and superheavy nuclei. The detection system is made of 7 DSSD (Double-sided Silicon Stripped Detector) for the detection of alpha particles and conversion electrons, and is coupled with 2 HPGe (High Purity Germanium) detectors for the gamma-rays.
At IGISOL [5], a proton beam of energy 65 MeV induced fusion-evaporation reactions on a $^{232}$Th target, producing, among other neutron-deficient actinide isotopes, $^{225}$Pa, which decays to $^{221}$Ac by alpha emission. This reaction, previously studied at IGISOL with a different setup [6], allowed for the evaluation of SEASON performances and the better quantification of conversion electron factors, which are crucial to perform spin-parity assignments of the states. The resulting level scheme will provide insights into the nature of the deformation of $^{221}$Ac.
First, the detection characteristics of SEASON, including energy resolution and efficiency, will be presented. Then, preliminary online commissioning results will be shown. These results will provide precision on the limits of the static octupole deformation in the neutron-deficient actinide region, illustrating the potential of what could be achieved in the future with SEASON at IGISOL and later at the S3-LEB facility, in GANIL, of which SEASON will be an integral part.
References
(1) Gaffney, L. P. et al. Nature 2013, 497, 199–204.
(2) Verstraelen, E.; Teigelhöfer, A.; Ryssens, W.; Ames, F.; Barzakh, A.; Bender, M.; Ferrer, R.; Goriely, S.; Heenen, P. - H.; Huyse, M.; Kunz, P.; Lassen, J.; Manea, V.; Raeder, S.; Van Duppen,P. Physical Review C 2019, 100, 044321.
(3) Butler, P. A. Journal of Physics G: Nuclear and Particle Physics 2016, 43, 073002.
(4) Cao, Y.; Agbemava, S. E.; Afanasjev, A. V.; Nazarewicz, W.; Olsen, E. Physical Review C 2020, 102, 024311.
(5) Moore, I. D.; Dendooven, P.; Ärje, J. Hyperfine Interactions 2014, 223, 17–62.
(6) Rey-herme, E. et al. Physical Review C 2023, 108, 014304.