High energy gamma ray emission in heavy ion collisions

The study of high energy γ-rays has become a very active field of research, both in experimental, and theoretical nuclear physics. The discovery by Newton et al. (1981) of the Giant Dipole Resonances built on high-spin states has opened a new field of research recently. It was found that the γ-ray spectra following heavy ion induced fusion reactions carry an important signature about the shape of the hot, fast-rotating compound nucleus from which it was emitted. This information can form a crucial test about the present understanding of the shapes of nuclei and the reliability of the extrapolation of the models to nuclei at high temperatures and/or high angular momenta, and is not available from some other measurement. In this paper, first the procedures involved in the measurements of the γ-ray spectra in the medium energy (5 MeV < Eγ < 30 MeV) range will be presented. This will be followed by a discussion of results of some measurements at the VECC, in Calcutta during the last few years, aimed at a better understanding of the origin of the Ultra Dipole Radiation emitted in reactions of medium energy α-particles with various targets. At the TIFR Pelletron a systematic program of the study of the shapes of hot nuclei has been taken up and some results from this work complete this paper. These results are however tentative. (author). 8 refs., 8 figs