# Alpha-gamma angular correlation in Bi212 (ThC)

## Abstract

The α-γ angular correlation function, W(θ), of the cascade Bi212(ThC)→α(6.04Mev)Tl208*→γ(40kev) Tl208(ThC′′) was determined by experiment to be consistent with a predicted law of the form W(θ)=1+Acos2θ. The ratio W(90°)W(180°) was measured to be 1.2993, ±0.0095 due to statistics, and ±0.010 due to experimental corrections. Comparison of this result with predictions of α-γ angular correlation theory shows that Bi212 cannot have zero spin. Of fifty-four possibilities considered, it is shown that the following spin and parity assignments are most consistent with presently available experimental and theoretical evidence from several sources: the ground state of Bi212 is 1(-), the 40-kev state of Tl208 is 4(+), and the ground state of Tl208 is 5(+). These assignments determine the orbital momenta of α particles emitted in the decay of Bi212 to be a mixture of 3 and 5 units for the transition to the 40-kev state of Tl208 and to be 5 units for the transition to the ground state of Tl208. Using these values of orbital momenta in the Gamow theory, and in the Weisskopf-Devaney theory of α fine-structure, the theoretical ratios of decay probabilities for the two groups, λ(40kev)λ(0,) are found to be in good agreement (10 to 50%) with the observed value. This agreement affords, in the case of Bi212, an explanation for the prohibited decay to the ground state relative to the first excited state found for many non-even-even nuclei by Perlman, Ghiorso, and Seaborg.1 Since the emission of the α particle with five units of orbital momentum hinders the decay Bi212 → Tl208 (ground state) by a factor of 16, this helps to explain the unusual size of the "departure factor" (1000) given to this decay by Perlman et al. © 1956 The American Physical Society.