HARD PHOTONS AND NEUTRAL PIONS RADIATED FROM COLLIDING NUCLEI

Abstract

A comparison of direct photon emission to pion production observed from nucleus-nucleus collisions in the Fermi energy domain shows some very interesting features. Valuable information is gained through the fit of a standard black body radiation formula to the data as well as by a comparison to classical bremsstrahlung theory. A production in the early phase of the collision is indicated. Radiative processes play an important role in atomic as well as in elementary particle physics whereas in nuclear physics the investigation of bremsstrahlung has been limited to a few cases, like beta-decay , proton-nucleus scattering , and only very recently also nucleus-nucleus collisions. In these especially at incident energies high above the Coulomb barrier the participating protons experience large velocity changes and are thus possible sources of electromagnetic bremsstrahlung'. Also for intermediate beam energies (B = 0.3) directly produced photons have been observed as well as electrons and positrons produced from them via external conversion. Theoretically the relevance of bremsstrahlung as a probe for the dynamics of a nuclear collision has recently been pointed out. For sufficiently hard photons, the energy for their production is available only in the reaction zone and only in the early stage of the collision. In that respect they have similar properties as pions which have been considered a particle probing the dynamics of nuclear collisions at intermediate energies. Respective experiments' l2 have supported that idea, which has been investigated theoretically on the basis of Fermi boosted NN-collisions with the inclusion of mean field effects* , Pauli blocking , or clustering , in a statistical model and by a semiclassical bremsstrahlung ansatz* . The interesting features resulting from this pion production work triggered our attempt to also study the photons directly produced with medium-energy heavy ions and their analysis as a radiative process accompanying the nuclear collision. Because of their negligable interaction with the surrounding nuclear matter photons seem to have some advantage as compared to pions. To study the features of the neutral pion and the direct photon emission from nuclear collisions we performed a series of experiments using C ions from the CERN synchrocyclotron of 84, 74, 60 and 48 MeV/u, impinging on targets from C to 2 3 U. To furthermore study the mass dependence we investigated the (nearly) symmetric systems 0+Mg at 48 MeV/u and compared them to data from *'Ar+*°Ca as well as *Kr+*Zr, taken at the 44 MeV/u beam of GANIL. The directly produced photons were registered with the same detector ls as the decay TS's from simultaneously n ' 1 2 produced ir's. The inclusive photon production cross section integrated within different energy bins increases charArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986423