Study of the radiative decay of ²⁵²Cf spontaneous fission fragments

The nuclear fission is a reaction where a nucleus breaks into two lighter nuclei and emit a part of the released energy as neutrons and photons. This work is focused on the study of the photon emission following the spontaneous fission of ²⁵²Cf. The main difficulty of such study is to precisely identify the fragments created: a Frisch-Gridded Twin Ionization Chamber (FGTIC) was used with an ultra-thin backing to observe rare events where no neutrons are emitted (cold fission). In this regime the mass resolution is 0.68 a.m.u. allowing perfect mass identification. This FGTIC is coupled to 54 Sodium-Iodide (NaI) crystals to measure with great efficiency the emitted gamma-rays. The study of this photon emission in the cold fission allowed to perfectly identify two fragmentations: ¹²⁰Cd/¹³²Sn and ¹¹⁸Pd/¹³⁴Te. In the first one ¹³²Sn is measured in its ground state, meaning that the measured γ spectrum solely originates from ¹²⁰Cd. The reproduction of the spectrum showed great sensitivity to the initial spin distribution allowing its first experimental determination in cold fission. In the second fragmentation studied (¹¹⁸Pd/¹³⁴Te) the excitation energy repartition between the two fragments was the most sensible quantity for the spectrum reproduction. It was determined for the first time through a genetic algorithm. This repartition was well reproduced by a statistical distribution of the intrinsic energy at scission using microscopic level densities. (author)