Intermediate-mass-fragment Production in Spallation Reactions

1. M. Valentina Ricciardi Aleksandra Kelić Karl-Heinz Schmidt GSI Darmstadt Germany Intermediate-mass-fragment Production in Spallation Reactions INTRODUCTION The precise knowledge of production cross-sections of every nuclide produced in proton- or neutron-induced reactions up to the GeV regime is of extreme importance for the design of accelerator-driven system (ADS), and for many other technical applications. Nuclear reaction codes based on the statistical model have high predictive power thanks to the high physics content. They are relatively fast and represent the best tool to provide nuclear data. During the last ten years a large body of experimental data concerning spallation residues was obtained in a devoted experimental campaign realized at GSI, Darmstadt [1]. Parallel to the experimental campaign, the statistical-model code ABLA, which describes the statistical deexcitation of a heated nucleus, was developed. Here we report on the progress in the development of the ABLA code concerning the production of of intermediate-mass fragments (IMFs). The range of emitted fragments was extended above Z=2. Two mechanisms were implemented. At low excitation energies IMFs are emitted as evaporated nuclei. If the excitation energy of the system exceeds a given threshold, IMFs are formed in the simultaneous break-up of the system, modelled according to a power-law distribution, which is suggested by several theoretical models. The improved code was benchmarked on a large amount of experimental data, among which the high-precision data measured at GSI. EVAPORATION OF IMFs The evaporation of light nuclei with Z>2 and below the Businaro-Gallone maximum of the mass-asymmetry dependent barrier was implemented in ABLA. The statistical weight for the emission of these fragments is calculated on the basis of the detailed-balance principle. The decay width as a function of the excitation energy depends on the inverse cross section, on the level densities of the two final nuclei (the evaporated IMF and the remaining daughter nucleus) and on the level density of the compound nucleus above the ground state. The barrier is calculated using the fusion nuclear potential of Bass [2]. The figures show the comparison of our calculation (lines) with the experimental data (dots), taken during the GSI campaign, for the reaction 1 GeV protons on 238U. The Z distribution, the mean N/Z-ratio and width of the isotopic distributions are shown. The results are very good. BREAK-UP PRODUCTION OF IMFs If the excitation energy acquired during the short collision stage overcomes the freeze-out temperature of 5.5 MeV, the nucleus enters the region of spinodal instabilities and breaks-up in pieces. The IMFs, which are produced simultaneously, turn apart due to the repulsive Coulomb force among them. In the ABLA code the mass of the break-up IMF is sampled from an exponential distribution (“power law”) with a slope parameter fixed to 2, a value which is found experimentally to be rather universal. Its charge is sampled from a Gaussian distribution, whose mean value is determined from the A/Z-ratio and the width from the symmetry term of the nuclear equation of state and from the nuclear temperature [3]. The figure show the comparison of our calculation (lines) with the measured isotopic distributions from the reaction 1 GeV p on 56Fe [1]. The results are very good. [2] R. Bass, Nuclear Reactions with Heavy Ions (Springer, Berlin, 1980) [3] A. R. Raduta and F. Gulminelli, Phys. Rev. C 75 (2007) 024605. REFERENCES [1] www.gsi.de/charms/data.htm