Lifetimes of levels in 21Ne

Abstract

Attenuated Doppler shifts have been measured for the decay of the levels in 21Ne at 1.74, 2.80 and 2.87 MeV using 24 MeV 16O ions from the Harwell tandem Van de Graaff accelerator and the reaction 9Be( 16O, αγ) 21Ne. From these results, the mean lifetimes of the 1.74 and 2.87 MeV levels were found to be, in units of 10 −13 sec, 1.5 −1.0 +1.5 and 1.1 −0.7 +1.5 respectively. The observed Doppler shift for the decay of the unresolve doublet at 2.80 MeV is consistent with the recent lifetime limits and branching ratios given by Smulders and Alexander for levels at 2.790 and 2.797 MeV. The data are combined with branching ratio and multipole mixing ratio data to obtain the partial mean lifetimes of eight transitions in 21Ne. The results are discussed in terms of the systematics of transitions in the 2s-1d shell and in terms of several nuclear models. The lifetimes and branching ratios of the 1.74 and 2.87 MeV levels are consistent with the predictions of the Nilsson model with K-band mixing for spin and parity assignments of 7 2 + and 9 2 + , respectively, to those two levels. The results for the 2.797 MeV level are consistent with the predictions of the Nilsson model, the Chi-Davidson asymmetric core model and the Dreizler excited core model if a 1 2 + spin and parity assignment is made to this level. However, the results for the 2.790 MeV level are inconsistent with its interpretation as the 5 2 + level predicted in this energy region by the Nilsson and Dreizler models, since these models overestimate the M1 transition probabilities for decay by several orders of magnitude and fail to give the correct branching ratio.