A study of photoproton reactions in heavy nuclei

Abstract

In 1934 Chadwick and Goldhaber (Ch35) disintegrated the deuteron using the 2.6 MeV gamma rays from ThC". This was 9 followed by the disintegration of Be by Szilard and Chalmers (3z34). The early work was limited to using gamma rays from naturally occurring radioactive sources and it was not until high energy electron accelerators were developed that it became possible to investigate the systematics of nuclear photodisintegrations. The problem of accurately determining photonuclear cross sections is complicated by the use of bremsstrahlung radiation which makes the data much more difficult to interpret than the data obtained using monochromatic gamma rays. However, recent improvements in the energy stability of electron accelerators, now reported as low as 5 keV (Go54), together with better knowledge of the bremsstrahlung spectrum, have made it possible to measure photonuclear cross sections with greater accuracy. Also the problem of producing a monochromatic variable energy Y-ray is being investigated (Mi59) and the results are encouraging. A recent measurement (Se60) produced cross sections which were very similar to other results obtained by using the conventional bremsstrahlung radiation. With the advent of suitable monochromatic gamma rays it will be possible to make a detailed investigation of the region above the giant resonance, where at present any resonance structure is masked by the yield from the giant resonance. The theoretical. approach to nuclear photodisintegration has been analogous to that of the atomic photoeffect and it has been possible to explain the main features (of the nuclear photoeffect) by using non-relativistic wave functions. Although it has been realised that meson fields must play a part in the explanations, Siegert (Si37) has shown that they can be ignored in the giant resonance. At high energies it is no longer possible to use nonrelativistic wave functions and here a different approach must be made to the problem. So far the theoretical predictions have been limited to the gross features, such as the position of the giant resonance, its width, magnitude and, in the case of deformed nuclei, the splitting of the giant resonance. The remainder of this chapter gives a brief outline of the present state of photodisintegration theory and how it fits the experimental facts. The discussion deals mainly with the medium and heavy nuclei and with photon energies below the meson threshold.