J.D. Bierman, M.P. Kelly, J.F. Liang, K.A. Snover and A.A. Sonzogni

The GDR and bremsstrahlung observed in heavy-ion collisions, especially in the region of 1-3 MeV/u excitation energy, has been the subject of considerable experimental and theoretical study in recent years.^1,2 In particular, the behavior of the GDR width in the region of high temperature and spin is not understood and the treatment of bremsstrahlung in competition with GDR emission in reactions which populate this region of excitation energy has been cursory at best.

Our current studies of -rays produced in ^16,18O+^92,100Mo address these topics. The accurate determination of the bremsstrahlung cross section through angular distribution measurements is necessary if one is to extract reliable GDR parameters. Since GDR and nucleon-nucleon bremsstrahlung emission take place from different reference frames, their characteristic a₁(E)'s differ in an easily calculable way. Thus, the a₁(E) should allow an almost model independent separation of GDR and bremsstrahlung components.

Fig. 3.2-1 shows fold 4 -ray spectra obtained using the Seattle 10" x 15" NaI spectrometer for the reaction ¹⁸O+¹⁰⁰Mo at 9.4 MeV/u bombarding energy. The left panel shows our 90° spectrum with a simultaneous fit of both statistical (CASCADE) and bremsstrahlung components (solid line), and the bremsstrahlung component alone, modeled by a simple falling exponential (dashed line). The center panel shows the same data with the phase space from the density of nuclear states divided out in an approximate way to give a pseudo absorption spectrum.³ The right panel shows the measured a₁ coefficient and the predicted a₁ (solid line) from the 90° cross section fit. A surprising result of this preliminary analysis is the narrow value of 8.4±0.5 MeV deduced for the GDR width. Further work including a simultaneous fit to both cross section and a₁ coefficient, investigation of possible contributions from pre-equilibrium emission, as well as further measurements at higher excitation energy will take place in the coming year.

Fig. 3.2 -1. Fold 4 -ray spectra for ¹⁸O+¹⁰⁰Mo producing ¹¹⁸Sn at E^*= 145 MeV.