Determination of the sign and magnitude of the nuclear quadrupole interaction byβ−γdirectional correlations

Abstract

Experiments are reported which demonstrate that the sign and magnitude of the quadrupole interaction of excited nuclear states can be reliably determined by means of time-differential $\beta -\gamma$ directional correlations with radioactive sources embedded in noncubic single crystals. The 828 keV leyel of ${}_{}{}^{115}{}_{}{}^{}\mathrm{In}$ and the 247 keV level of ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}$, fed by an allowed and a unique first-forbidden $\beta$ decay, respectively, have been investigated by this method. The coupling constants $\frac{e^2\mathrm{qQ}}{h}$ of these levels in Cd metal at 298°K, are -146(5) MHz for ${}_{}{}^{115}{}_{}{}^{}\mathrm{In}$ and +125(4) MHz for ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}$. The positive coupling constant for ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}$ in Cd coupled with a positive quadrupole moment $Q$ of this level implies a positive electric field gradient in Cd confirming recent theoretical predictions. The negative $e^2\mathrm{qQ}$ for the ${}_{}{}^{115}{}_{}{}^{}\mathrm{In}$ level in Cd metal correspondingly indicates a negative $Q$ for this level in conformity with its description as a member of a $K=\frac{1}{2}$ rotational band. The paper includes a discussion of the major concepts underlying the various methods available now for determination of the sign of $e^2\mathrm{qQ}$. Also, a detailed account of theoretical formulas necessary to evaluate allowed and forbidden $\beta -\gamma$ perturbed angular correlations and a brief discussion of the significance of the technique for further studies of nuclear quadrupole effects in materials are given.