Perturbed angular correlation spectroscopy as a radiotracer tool

When a radionuclide decays by emission of gamma-radiation in cascade and the two gamma-rays are detected by a coincidence spectrometer, the coincidence counting rate may depend strongly on the angle between the directions of propagation. This angular correlation may be perturbed by hyperfine interactions, i.e. coupling of nuclear moments with extra-nuclear electromagnetic fields provided the nucleus moments with extra-nuclear electromagnetic fields provided the nucleus remains in the intermediate excited state for a sufficiently long time. The extent of this perturbation depends on the electron distribution at the site of the probe nucleus. Thus, perturbed angular correlation (PAC) spectroscopy provides an opportunity to obtain non-invasive information on the direct chemical environment of the probe nucleus. This contribution provides an introduction to the principles of PAC spectroscopy, the advantages and limitations, and its opportunities for application of this radiotracer method in biology and chemistry. (author)