Level structures inNp240

Abstract

Whereas the 7.2 min 240 Np, first identified over 60 years ago, has been assigned conflicting configuration by various investigators, the 62 min 240 Np is assigned a "probable" configuration, deduced in each case by primarily focusing on just a single β-connected pair of levels in either 240 Np (β − ) 240 Pu or 240 U (β − ) 240 Np decay. We evaluate the level energies of physically admissible 2qp configurations in 240 93 Np 147 employing a three-step procedure, with experimental inputs at each step, and using a well tested two-particle rotor model with inclusion of residual n-p interaction and other contributions. This exercise clearly establishes that the 62 min 240 Np and the 7.2 min 240 Np isomers constitute a Gallagher-Moszkowski (GM) doublet corresponding to the two-quasiparticle (2qp) configuration 5 + {p5/2 + [642] ± n5/2 + [622]}0 + with J π K = 1 + 0 for the higher-lying 7.2 min isomer. This assignment is conclusively confirmed in a level-by-level analysis of data on 23 β transitions in these decays.Structures of a few other 240 Np levels populated in 240 U β decay are also discussed.