Identification of a four-particle state in the N = 152 nucleus 250Cf

Abstract

Examination of the first-forbidden beta decays in very heavy (A ⩾ 245) nuclei leads to the first identification of a four-quasiparticle (4qp) state in a nucleus of this ‘frontier’ region. The 1658.0 keV 2+ state in the N = 152 nucleus 250Cf, fed in β− decay from the 3.21 h 2− 250Bk parent and in ϵ-decay from the 2.22 h 1− 250Es parent, is assigned the 4qp configuration 2+(p1:7/2[633] ⊗ p2:3/2 [521] ⊗ n1:1/2[620] ⊗ n2:9/2 [734]) on the basis of the observed beta decay features. This assignment is supported by the observed gamma decays of this 250Cf state, excitation energies of the constituent one-, two- and three-quasiparticle bands in core nuclei and the experimental log ft values for the corresponding n:1/2 [620] ⇌ p:3/2[521] and the n:9/2 [734] ⇌ p:7/2[633] transformations in nuclei of this region. It is further shown that the same underlying single-particle transformation populates the 1qp, 2qp, 3qp and 4qp states with comparable log ft values in fast (log ft = 6.5 ± 0.6) first forbidden beta transitions in this ‘frontier’ region. An operative selection rule for such transitions is suggested.