The Alpha-Gamma Angular Correlation in Radiothorium

The alpha-decay of radiothorium is followed by an 83.3-kev gamma-ray whose angular correlation with the alpha-particle was investigated, using scintillation counters and fast coincidence circuits. The correlation curve does not agree with any of the theoretical ones derived under the assumption that the angular momenta of the three states involved are $0-J-0$. Possible reasons for this somewhat unexpected result are discussed.