Thermal neutron activation cross sections for the isomeric states in odd indium isotopes and the j-selection rule

Abstract

The thermal neutron activation cross sections for the two isomeric states in 114In were measured using a 2π flow counter for the long-lived and the pulsed activation method for the short-lived isomer. The values obtained are σ act = 7.5±0.7 b for 114 m 1 In (49 d) in agreement with a value measured by Keisch, and σ act = 3.1 −0.7 +0.4 b for 114m 2In (42 msec). The isomeric cross-section ratios were analysed using the statistical approach of Huizenga and Vandenbosch. It follows that both isomeric states should have the same spin value 5, in spite of the fact that they are connected by a pure E3 transition. The same situation prevails in 116In. Possible explanations are (i) E1 and M2 transitions are so strongly hindered that they are in fact completely suppressed; and (ii) the statistical approach in evaluating isomeric cross-section ratios breaks gamma-ray down because of the channellizing action of the ( g 9 2 −1) p ( h 11 2 ) n multiplet which directs the gamma-ray cascade to its member with the lowest excitation energy (i.e. the isomeric state). Both explanations are founded on the action of the j-selection rule, which forbids transitions of low multipole order between the configurations [( g 9 2 −1) p( h 11 2 ) n]1 − … 10 − and [( g 9 2 −1) p( S 1 2 + d 3 2 ) n] 4 +, 5 + .