Beta-gamma correlation measurements on Ir¹⁹²

Abstract

The β-γ directional correlation between β particles in the 536-keV group and the 605- and 612-keV γ rays in the decay of 72-day Ir¹⁹² has been measured at several β-ray energies. The measured correlation coefficient ε(W) rises from -0.074±0.015 at 185 keV to -0.096±0.010 at 255 keV and then starts falling; near the end-point energy the anisotropy is found to be very small. The large β-γ anisotrophy observed conclusively establishes that the parity of Ir¹⁹² is negative. Attempts have been made to extract information about the relevant matrix-element ratios governing the 536-keV β transition. The β-γ directional correlation between β particles in the outer group of 669-keV and the 469-keV γ ray (4^--β4⁺-γ2⁺) has also been measured; it shows nearly isotropic correlation in the β energy region 400-650 keV. The β-γ circular-polarization correlation in the above cascade has also been measured. It is concluded that while large cancellation among the vector-type matrix elements is responsible for the observed energy dependence of the β-γ anisotropy in the 536-keV β group, the outer β group of 669 keV apparently satisfies the quasi-allowed approximation. The nature of the 785-keV (4⁺) and 921-keV (3⁺) states in Pt¹⁹² is discussed in the light of these findings.