The role of pionic currents in gamma emission during nuclear encounters

Abstract

A review of experimental data on Nucleus-Nucleus-Gamma reactions is given. Spectra display an exponential shape above 30 MeV, with an almost linear variation of the exponential parameter with incident beam energy. The laboratory angular distributions show strong forward peaking, as well as a remarkable insensitivity to the mass ratios of target and projectile. Transformed into the nucleon-nucleon center of mass, these angular distributions display forward backward symmetry. They are mostly isotropic, with a slight admixture of a dipole component. In first approximation, the intensity of the spectra appear to be proportional to the number of first nucleon-nucleon collisions, which leads to a simple scaling law. Exclusive measurements display an interesting correlation between the hardness of the spectra and the centrality of the collision. Experimental results put stringent constraints on theoretical models. The scaling laws appear to contradict the prediction of collective models where both nuclei behave like entities. The angular distributions do not seem to be easily reconciled with emission by thermal sources. Semi-classical dynamical calculations of the BUU or Landau-Vlassov type, where the collision term is the source of incoherent photon emission, have been used extensively to reproduce the experimental results. These calculations were able to reproduce most of the experimental features. Photon emission in Nucleus-Nucleus collisions is, thus, intimately related to that occuring in the elementary n-p reaction. Recent measurements and calculations show that most of the photon production is, in this case, related to the charged pion exchange between the two nucleons. Therefore, Nucleus-Nucleus collisions may be a unique probe of the exchange currents in hot and dense nuclear matter.