Heavy Ions Physics Research Papers (original) (raw)

High energy gamma-ray production in heavy ion collisions has been the object of a number of recent studies between 15 and 86 MeV/Nucleon incident energies (Grosse,1985; Grosse,1986; Stevenson,l986; Stevenson.1987; Alamanos,l986; Hingmann,1987; Berthollet,l986,
Berthollet,l987; Kwato Njock,1986). Photons have attracted attention since they are not as seriously affected by absorption phenomena as pions, for example ; they can serve as unambiguous probes to study the reaction dynamics in the early stage of the collision. The main drawback of hard photon studies is the smallness of the -r-cross sections.
Several models have been proposed to predict the photon production yields. Some of them suggest that incoherent nucleon-nucleon bremsstrahlung is the main source of the -r-emission. These collisions can take place either in the initial stage of the reaction (Cassing,1986; Nakayama.1986; 8auer,1986; Bauer,1987; Che min Ko,1987; Remington.1986; Randrup,l988) or
within an equilibrated hot participant zone (Nifenecker,l985; Prakash,1987). Other models suppose that photons are produced by coherent bremsstrahlung where both nuclei or
substantial parts of them act as a whole (Vasak,1985; Vasak,1986; Stah1,1987) In the following, after a short overview of the experimental techniques involved we first
present the experimental results. Our main emphasis will be with the inclusive measurements of differential production cross-sections. These include the shape of the spectra, the angular distributions and the absolute cross-sections. We shall show, on qualitative
grounds, that these inclusive experiments tend to favor a picture where the photons are produced in first nucleon-nucleon collisions. Results from exclusive reactions which display
the impact parameter dependence of the multiplicity and spectra of the gamma-rays will be presented.
After summarizing the experimental status, we give a first, semi-classical, approach of a theoretical account of the photon production in many nucleon systems. The main aim of this
presentation is to show how, and in what limits. it mav be justified to schematized Nucleus- Nucleus reactions into an incoherent sum of independent nucleon-nucleon contributions. It will clearly appear that the understandino of the Nucleus-Nucleus reactions reauires both
knowledge and-understanding of the simple; nucleon-nucleon and nucleon-Nucleus cases. We shall, therefore, review the experimental and theoretical aspects of these simpler reactions
insofar as they relate to our main subject. Doina so we are conscious that we mav omit important deveiopments, both experimental or theoretical, but it would have been out of
scope of this review to give a full account of such an extended field. It will appear that the contribution of charged pion exchange currents to photon production is probably very
important. Finally, we present an overview of the available theoretical approaches, and try to balance
these different approaches with the experimental results, as well as with our knowledge of the more elementary processes.