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Description
Multi-humped fission barriers, as observed in the actinide region, give rise to isomeric fission [1]. Such barrier shapes can be described using various theoretical models [2]. Experimentally measured observables of nuclear fission isomers - such as the half-life, excitation energy, kinetic-energy spectra of fission fragments, and the isomer-to-ground-state population ratio - serve as important tests of theoretical models and help verify the influence of shell effects on nuclear structure.
Studies of fission isomer properties of 240,242Am were performed at the IGISOL facility of the JYFL Accelerator Laboratory, University of Jyvskyla, Finland [3]. Isomeric states were populated via deuteron-induced fusion–evaporation reactions on a 242Pu target. The decays of the isomers were detected using silicon detectors positioned downstream of a dipole magnet for mass separation.
The kinetic energy spectra of single fission fragments and the isomeric half-lives were measured using silicon detectors calibrated with a 252Cf fission source. A typical double-peaked kinetic-energy distribution characteristic of fission was observed. For the 242Am isomer, the two peak was found at EL= (78.10 ± 0.11) MeV and at EH = (105.93 ± 0.15) MeV. For the 240Am isomer, the corresponding values were EL = (78.9 ± 0.2) MeV and EH = (105.4 ± 0.7) MeV. The total kinetic energy (TKE) reached maximum values of TKE = (184.03 ± 0.04) MeV for 242Am and TKE = (184.3 ± 0.3) MeV for 240Am. A half-life of T1/2 = (14 ± 1) ms was determined for the 242Am isomer.
The mass distributions of 240,242Am were derived from the kinetic-energy spectra of single fission fragments using the 2E method. The results were compared with GEF (General Description of Fission Observables) model calculations and previous measurements, showing good agreement. The mass distribution for 242Am was found to be strongly asymmetric, with a heavy-fragment peak at A = 104.1 ± 0.2 and a light-fragment peak at A = 137.7 ± 0.2. For 240Am, the distribution exhibited a two-peak structure with a heavy-fragment peak at A = 101.9±0.9 and a light-fragment peak at A = 138.2±0.9.
The measurements of 240,242Am fission isomers at the IGISOL facility have provided detailed information on their kinetic energy spectra, total kinetic energies, and pre-neutron emission mass distributions. The results are consistent with GEF model predictions and confirm the asymmetric character of fission in these isotopes. These findings contribute to a better understanding of shell effects and barrier structures in the actinide region.
References.
1. P.G. Thirolf and D. Habs, Prog. Part, Nucl. Phys. 49, 325 (2002).
2. V. M. Strutinsky, Nuclear Physics A 95, 420 (1967).
3. S.A. Giuliani et al., Rev. Mod. Phys. 91, 011001 (2019).