Gamma Rates and Intensities;l-ForbiddenM1Transitions

Abstract

A search for $l$-forbidden $M1\mathrm{}$ transitions of the type ($1g_{\frac{7}{2}}\leftrightarrows 2d_{\frac{5}{2}}$) has been made in ${\mathrm{Pm}}^{149}$, ${\mathrm{Pm}}^{151}$, and ${\mathrm{Sb}}^{125}$. Lifetimes of the first excited states were obtained from beta-gamma and gamma-gamma time-delay distributions that were measured with a fast time-to-height converter. The measured half-life for the 114-keV level in ${\mathrm{Pm}}^{149}$ is (2.62±0.10) nsec and that for the 326-keV level in ${\mathrm{Sb}}^{125}$ is ≤0.2 nsec; hindrance factors relative to Moszkowski limits for these transitions are 350 and ≤300, respectively. In the deformed nucleus ${\mathrm{Pm}}^{151}$, the half-life of the 118-keV first excited state is ≤0.3 nsec and that of the 256-keV level was measured to be (0.91±0.03) nsec. Lifetime limits for additional levels were obtained. Because of uncertainties in the level schemes of ${\mathrm{Pm}}^{149}$ and ${\mathrm{Pm}}^{151}$, the gamma-ray intensities from the decay of ${\mathrm{Nd}}^{149}$ and ${\mathrm{Nd}}^{151}$ were measured Ge(Li) with a Ge(Li) solid-state detector. From the 28-h ${\mathrm{Pm}}^{151}$ daughter activity, a half-life of (0.48±0.03) nsec was measured which is associated with the 105-keV level in ${\mathrm{Sm}}^{151}$ and an upper limit of ≤0.2 nsec was placed on the half-life of the 345-keV level.