Deviations from the Porter-Thomas Distribution due to Nonstatistical $\gamma$ Decay below the $^{150}$Nd Neutron Separation Threshold

TL;DR

This study introduces a new method using nuclear resonance fluorescence to analyze gamma decay fluctuations in $^{150}$Nd, revealing deviations from the Porter-Thomas distribution due to nonstatistical effects.

Contribution

The paper presents a novel experimental approach to study partial transition width fluctuations and demonstrates deviations from Porter-Thomas distribution in $^{150}$Nd.

Findings

Average branching ratio for gamma decays is 0.490(16).

Degree of freedom for widths is 1.93(12), deviating from Porter-Thomas.

Nonstatistical gamma decay effects contribute 9.4% to 94%.

Abstract

We introduce a new method for the study of fluctuations of partial transition widths based on nuclear resonance fluorescence experiments with quasimonochromatic linearly polarized photon beams below particle separation thresholds. It is based on the average branching of decays of $J=1$ states of an even-even nucleus to the $2^+_1$ state in comparison to the ground state. Between 5 and 7 MeV, a constant average branching ratio for $\gamma$ decays from $1^-$ states of 0.490(16) is observed for the nuclide $^{150}$Nd. Assuming $\chi^2$-distributed partial transition widths, this average branching ratio is related to a degree of freedom of $\nu = 1.93(12)$, rejecting the validity of the Porter-Thomas distribution, requiring $\nu=1$. The observed deviation can be explained by nonstatistical effects in the $\gamma$-decay behavior with contributions in the range of 9.4(10)% up to 94(10)%.