The decays of 21.55 min 162gTm and 24.3 s 162Tm (a new isomer)

Abstract

The decay of the 21.55 min ground state and of the 24.3 s isomeric state of 162Tm was investigated with semiconductor detectors. The γ-ray spectrum was investigated with a Compton-suppression Ge(Li)-NaI(Tl) arrangement. A Si (Li) detector, mounted in an electron transport solenoid, was used to investigate the conversion electron spectrum. Three-dimensional coincidence measurements were performed with large-volume Ge(Li) detectors. The 162Tm ground state has spin-parity 1 − and Nilsson assignment p[411]↓− n[521]↑. An allowed β-transition (log ft ≈ 6.4) was observed to a 2 −, 2 octupole vibrational level at 1572.84 keV. The Q-value determined from positon-gamma coincidence measurements is 4705 ± 70 keV. The discrepancy of the experimental K / β + ratio with theoretical predictions might possibly be explained by a large number of unobserved weak γ-rays besides the total of 315 stronger ones observed in this study. The average β-strength function was calculated to be 1.2 × 10 −5. Among the 50 levels observed in the decay, the 2 +, 4 + and 6 + members of the ground-state band, the 2 +, 3 + and 4 + members of the γ-band, several 0 + and 2 + members of the K = 0 β-bands and 1 −, 2 − and 3 − octupole vibrational levels were identified. Parameter values Z γ (0) and Z γ (2) determining the mixing between the γ-band and the ground-state band, allow no conclusive evidence about unequalness of the intrinsic quadrupole moments of the ground states and the γ-band. The Z(0) parameters, determining the mixing between the β-bands and the ground-state band, and X parameters determining the ratio of E0 to E2 transition probabilities, were deduced. A previously unreported 24.3 sec isomer in 162Tm was observed to decay in 10% of the cases by an allowed unhindered (log ft = 4.7) β-ray transition to a level at 1712.20 keV in 162Er. The Nilsson configurations assigned to the isomeric and 1712.20 keV levels are p[523]↑ + n[521]↑ 5+ and n[523]↓ + n[521]↑ 4+ respectively. The isomeric level decays in 90% of the cases by an E3 transition ( E IT < 125 keV) to a p[404]↓ −n[521]↑ 2− level at 66.90 keV in 162Tm, which decays by an (M1+ < 40 % E2) to the 21.55 min 162Tm 1 − ground state.