Nuclear orientation and angular correlation studies of gamma ray cascades in155Gd populated by the decay of155Tb

Abstract

The low-lying positive-parity levels in 155Gd have been investigated using the two complementary techniques of directional correlation and directional distribution of radiation emitted by an oriented source. The orientation was produced using the hyperfine field in ferromagnetic gadolinium to polarize an ensemble of 155Tb nuclei at temperatures below 10 mK. The resultant directional distribution of gamma rays depopulating levels below 488 keV was measured using higher resolution Ge detectors. The correlation measurements used a NaI(Tl) or an intrinsic Ge detector in combination with a Ge(Li) detector. Most of the cascades investigated contained either the 86 or 105 keV E1 transition to the ground state. Thus the gamma ray of interest was the first member of the cascade in the coincidence measurement. In the case of the nuclear orientation measurements, the gamma ray was the second member of the cascade. This effective reversal of temporal order produces two unique values for the coefficient describing the transition, which in this case, is almost always an E2/M1 admixture. Comparison of the two coefficients for a given gamma ray often enables unambiguous spin assignments to be made. In addition, the mixing ratios involved can be deduced. The decay scheme is discussed and experimental results are compared with the band mixing calculations of Lovhoiden et al. (1970).