Recoil-Free Absorption Hyperfine Spectra of the 90-keV Mixed Transition inRu99

Abstract

The Mössbauer effect was employed to measure hyperfine splittings of the 90-keV transition in ${\mathrm{Ru}}^{99}$. A fully resolved magnetic hyperfine pattern was obtained with an absorber consisting of a solid solution of 2.3 at.% enriched ruthenium in metallic iron. Partially resolved quadrupole hyperfine spectra were obtained with absorbers of Ru${\left({\mathrm{C}}_5{\mathrm{H}}_5\right)}_2$ and Ru${\mathrm{O}}_2$. A single line source of ${\mathrm{Rh}}^{99}$ in metallic ruthenium was used for these measurements. From these spectra the following information was deduced. The ratio of the magnetic $g$ factors of the 90-keV level to that of the ground state, $\frac{g_1}{g_0}=0.759\mathrm{}\pm 0.016$. The 90-keV transition is a mixture of dipole and quadrupole ($M1\mathrm{}$ and $E2\mathrm{}$) radiation with a mixing ratio $\frac{E2\mathrm{}}{M1\mathrm{}}={\delta }^2=2.7\mathrm{}\pm 0.6$. By using the known spin and magnetic moment of the ground state, we infer that the spin of the 90-keV level is $\frac{3}{2}$, that $g_1=0.19\mathrm{}\pm 0.05$, and that the field at the nucleus of the ruthenium atom in the iron environment is 500 kG. The ratio of the quadrupole moment of the excited state to that of the ground state, $\left|\frac{Q_1}{Q_0}\right|\ge 3$, and $\left|Q_1\right|\ge 0.15$ b. With the use of a 30-kG superconducting solenoid the relative sign of the magnetic field at the ruthenium nucleus in the iron absorber was shown to be negative and an absolute measurement of the nuclear $g$ factors was made with the result that $g_1=0.20\mathrm{}\pm 0.02$. A measurement of the absorption spectrum of ferromagnetic Gd${\mathrm{Ru}}_2$ gave the result that the magnetic field at the ruthenium nucleus in this compound is <10 kG. Hyperfine measurements were made with the ruthenium-iron absorber magnetically polarized both parallel and perpendicular to the direction of the resonant radiation. Under these conditions it was possible to observe, for the first time, the interference between two multipole components for the individual hyperfine components of a radiative transition. From these measurements the sign of the conventional mixing parameter $\delta$ was determined to be negative, and an upper limit of 30% could be placed on the time-reversal-odd component of the gamma-decay matrix element. The properties of the 90-keV level, which are not compatible with a simple shell-model description, are compared with the predictions of a core-excitation description. If the latter description is correct, then these results give evidence that the magnitude of the time-average quadrupole moment associated with the 2+ core is ≥0.29 b.