Covariance analysis of selection rules governing the γ−decay cascades of the rotational nucleus 164Yb Original Research Article

The γ-cascades feeding specific intrinsic configurations of 164Yb are studied making use of a statistical analysis of counts fluctuations in γ–γ coincident spectra. In particular, variance and covariance spectra are obtained starting from the measured (0,+), (0,−) and (1,−) gate-selected matrices. The analysis of the first ridge of the spectra yields about 25 excited bands carrying rotational energy correlations for the total spectrum, and about 10 in coincidence with each of the (α,π) configurations. The covariance of count fluctuations between the different (α,π) gates is expressed by an experimental correlation coefficient, r, which measures the degree of sharing of decay paths through the excited bands, and thereby the extent to which selection rules are obeyed by the decay. The correlation coefficient is around r≈0.3 for the ridge, and also in general rather small for valley fluctuations. Only at the highest valley energies, a tendency to larger correlation coefficients is seen, indicating some weakening of the selection rules over longer decay cascades. The results of the experimental analysis are compared to rotational damping model predictions based on cranked shell model plus residual interactions, and rather good agreement is found.