Structure of Mass-Yield Distributions of 232Th Photofission Product by Bremsstrahlung at Energy 17.5 MeV


Relevance. One of the most promising areas for studying the fission process is to investigate its features under the action of photon radiation, since the interaction of gamma quanta with the nucleus is completely electromagnetic with well-known characteristics. Information on the yields of 232Th nuclear photofission products is of particular interest from the standpoint of experimental and theoretical studies. The nucleus of this element is located on the border between pre-actinides and light actinides.

Purpose. The purpose of this study is to experimentally investigate the structure of the mass distribution of yields of 232Th photofission products at a bremsstrahlung energy of 17.5 MeV (energy close to the threshold of the first-chance fission, where experimental data are not available).

Methods. 232Th photofission response was simulated on the electron accelerator of the Institute of Electron Physics NAS of Ukraine – M-30 microtron. The bremsstrahlung spectrum was modelled for the case of electron interaction (E=17.5 MeV) with a tantalum converter (1 mm) using the GEANT4 code 10.7. Yields of 232Th photofission products were measured by gamma-ray spectrometry. The yields of 232Th photofission products were modelled using the GEF 2020 / 1.1 and Talys 1.95 codes.

Results. The value of cumulative yields of 23 products (85mKr, 88Kr, 88Rb, 89Rb, 91Sr, 92Sr, 94Y, 95Zr, 97Nb, 99Mo, 101Tc, 131I, 132Te, 133I, 134Te, 135I, 138Cs, 139Ba, 140Ba, 141Ce, 142La, 143Ce, 146Ce) belonging to 22 isobaric mass chains (light: 85; 88; 89; 91; 92; 94; 95; 97; 99; 101, heavy: 131; 132; 133; 134; 135; 138; 139; 140; 141 ; 142; 143; 146 fragments) of the 232Th photofission was measured at a maximum bremsstrahlung energy of 17.5 MeV (average excitation energy ~ 11.3 MeV). The resulting mass distribution of heavy fragments indicates the presence of increased yields of products localized around mass 133-134, 138-139, and 143-144, which is associated with the influence of such a nuclear structure as the proximity of closed nuclear shells and the even-odd effect.

Conclusions. The measurement results indicate the presence of a fine structure in the resulting mass distribution of 232Th photofission product yields, which is manifested in increased yields of products localised in the mass regions 133-134, 138-139, and 143-144. The obtained theoretical output values calculated using the GEF 2020 / 1.1 and Talys 1.95 codes describe in general terms and predict the fine structure of the mass distribution of 232Th photofission products

Keywords: bremsstrahlung, microtron, 232Th, gamma-ray spectrometry, first-chance fission