Abstract
The probability of emission of a particle or quantum of radiation from a single radioactive nucleus depends on the angle between the nuclear spin axis and the direction of emission. In a normal radioactive sample the nuclei are randomly oriented, and there is no overall preferred direction of emission. Hence, the radiation pattern is isotropic. However, in a sample in which the nuclei are not randomly oriented, there can be preferred directions for emission. | This work deals with one method for selecting ensembles of oriented nuclei which happen to lie in a certain direction at the time of measurement. If the nuclei decay through successive emission of radiations R1 and R2, the radiation R1 can be detected in a specified direction ->k1 with one detector, and this measurement picks out an ensemble of similarly oriented nuclei. Then the subsequent measurement of R2 in the direction ->k2 will show a definite correlation with ->k1. In beta - gamma angular correlation studies a radioisotope emits in rapid succession a beta particle followed by a gamma-ray. The relative probability that the gamma-ray is emitted into the solid angle of a movable detector at an angle theta with respect to a fixed beta detector is described by the directional correlation function, W(theta). Theoretical predictions for W(theta) have been worked out for most cases of interest. In an actual experimental situation (Figure 1), the beta - gamma coincidence counting rate, C(theta), is measured as a function of theta, the angle subtended by the axes of the two detectors. Since the detectors have a finite solid angle omega, these numbers C(theta) are actually averages over angles within omega, This means that the C(theta) must be suitably corrected for the finite extent of the detectors. The distribution C(theta) is compared to W(theta) to extract the desired nuclear structure information.