Precision nuclear orientation measurements for determining mixed magnetic dipole/electric quadrupole hyperfine interactions

We report the temperature dependence of the gamma-ray anisotropies from oriented¹⁶⁰Tb nuclei produced by neutron activation of the central portion of a high-purity single crystal Tb slab. The magnetically saturated sample was studied over a wide temperature range from 18 mK to 150 mK. The temperatures were determined using precision resistance thermometry within situ calibration by a magnetically shielded six-element superconducting fixed point device. Temperature stability during data acquisition was better than 0.1%, and least-squares fitting of the resulting temperature dependences of 0° and 90° anisotropies allowed both the magnetic dipole and the electric quadrupole hyperfine interaction frequencies to be determined with good accuracy. The weighted averages for 18 gamma rays arev _M=1393.8 (8.1) MHz andv _P=178.0 (2.1) MHz, in excellent agreement with NMR results on ion-implanted samples.