Multiplicity of K X rays and collective structure in the transitional nuclei with A⋍200

Abstract

The multiplicity M_K of prompt K x rays emitted in (^6,7Li, xn) reactions has been measured for twelve targets in the mass range 180≤A≤209, at bombarding energies between 55 and 124 MeV. From the high multiplicities of x rays (1≤M_K≤3) characteristic of the observed evaporation residues, we infer that highly converting, low-energy M1 transitions constitute a substantial portion of the γ-decay cascades (especially at moderately high spin, (12≤I≤20) in a large number of nuclides in the transitional region between the shell closure at N=126 and the strongly deformed rare earths. The unexpectedly simple systematic variation of the M_K measurements with neutron number within this mass region suggests that the converting transitions occur predominantly among members of as yet unknown, mildly deformed, high-K, deformation-aligned rotational bands. From the measured values of M_K and the singles K x ray yields we extract absolute total cross sections for the (Li,xn) reactions. Expected statistical features of the x-ray multiplicity distributions and associated limits on the applicability of the present technique for evaporation-residue cross section determination are discussed.