Study of the stability of the ground states and K-isomeric states of 250Fm and 254102 against spontaneous fission

Abstract

By employing the 249Cf(4He, 3n) and 208Pb(48Ca, 2n) reactions, experiments to study the stability against spontaneous fission of the nuclides 250Fm and 254102 as well as of the two-quasi-particle (2q-p) K isomers 250mFm (T1/2 = 1.8 ± 0.1 s) and 254m102 (T1/2 = 0.28 ± 0.04s) have been performed. The ground-state spontaneous fission of the two nuclides has been discovered and the corresponding branching ratios bsf and partial half-lives Tsf, respectively, have been determined to be: (6.9 ± 1.0) × 10−5, 0.83 ± 0.15 yr for 250Fm; (1.7 ± 0.5) × 10−3, (3.2 ± 0.9) × 104s for 254102. As a by-product of these studies, new data about cross sections of the 206,208Pb(48Ca, xn) reactions have been obtained. Experiments designed to search for the spontaneous fission decay of the 2q-p K-isomeric states in 250Fm and 254102 have not revealed the effect in question. The lower limits of the ratios of the partial spontaneous fission half-lives for the 2q-p K-isomeric states to those for the respective ground states, Tsf*/Tsf, have been established to be ⩾ 10−1 for 250mFm/250Fm and ⩾ 5 × 10−3 for 254m102/254102. This means that the stability of the 2q-p K-isomeric states in 250Fm and 254102 against spontaneous fission is practically not inferior to that of the ground states of these nuclei. In accord with the experimental findings, the theoretical estimates of Tsf*/Tsf made in the present paper show that, due to the influence of the specialization and blocking effects on the potential energy and the effective mass associated with fission, spontaneous fission from 2q-p K-isomeric states cannot be facilitated but, on the contrary, should be essentially hindered compared with ground-state spontaneous fission.