Orateur
Description
Despite the development of different theoretical models [1] and simulation codes based on experimental data, such as GEF [2], the fission process is not reproduced with enough accuracy along the nuclear chart. Given that the fission mechanism is sensitive to the interplay between both microscopical quantities, such as nuclear structure of the fission fragments, and macroscopic effects; a fully microscopical description of the interaction has not been obtained so far. In order to further constrain the models, a large set of experimental needs to be provided.
Following the advantages of inverse-kinematics, the VAMOS group and collaborators decided to perform fission studies more than 10 years ago [3,4]. 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 [5]. This configuration enables the isotopic identification of complete fission fragment distributions. The magnetic spectrometer is combined with a highly stripped silicon detector named PISTA [6], 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.
Taking advantage of the VAMOS++ setup, A new experiment was conducted using 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. The produced nuclei lay on a region closer to the known transition between asymmetric to symmetric fission in the actinides [7]. 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 [8]. Moreover, experimental results show that the isotopic distributions around Th isotopes deviate from the general actinide behaviour [9].
In this work, the isotopic and mass fission fragment yields of several nuclei such as 232Th or 234U will be presented. These distributions have been obtained as a function of the excitation energy, which allows to study the shell effect evolution. The comparison between Thorium yields and heavier actinides gives new experimental insight into understanding the so-called "Thorium anomaly" [10].
References
[1] Schunck, N and Robledo, LM , Reports on Progress in Physics 79 (2016) 116301.
[2] Karl-Heinz Schmidt and Beatriz Jurado , Reports on Progress in Physics 81 (2018) 106301.
[3] 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.
[4] Ramos, D., et al. Isotopic fission-fragment distributions of 238U , 239Np, 240Pu, 244Cm, and 250Cf produced through inelastic scattering, transfer, and fusion reactions in inverse kinematics. Physical review C, 97(5), 054612.
[5] M.Rejmund et al., Nuclear Instruments and Methods in Physics Research A 646 (2011) 184-191.
[6] Bégué-Guillou, L., Lemasson, A., Morfouace, P., Ramos, D., Taieb, J., Frankland, J. D., ... & Tonchev, A. P. (2026). Performance of the Particle-Identification Silicon-Telescope Array Coupled with the VAMOS++ Magnetic Spectrometer. arXiv preprint arXiv:2601.20907.
[7] Möller, P., & Randrup, J. (2015). Calculated fission-fragment yield systematics in the region 74 ≤ Z≤ 94 and 90≤N≤ 150. Physical Review C, 91(4), 044316
[8] Scamps, G., & Simenel, C. (2019). Effect of shell structure on the fission of sub-lead nuclei. PhysicalReview C, 100(4), 041602.
[9] 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.
[10] 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.