Goldenbaum, F. (Corresponding author)FZJ*

2004
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 3-89336-346-7

Abstract: A recent renascence of interest for energetic proton induced production of neutrons originates largely from the inception of projects for target stations of intense spallation neutron sources (like the planned European Spallation Source ESS, the SNS in the US and J-PARC in Japan), accelerator-driven nuclear reactors, nuclear waste transmutation, and also from the application for radioactive beams. The ultimative objective is that the essential highand intermediate energy nuclear data, required in the framework of such applications will be available in an energy range where currently almost no data exist. Although in this work the issue has been quite successfully addressed experimentally by varying the incident proton energy for various target materials and by covering a huge collection of different target geometries-providing an exhaustive matrix of benchmark data-the overriding challenge is to increase the predictive power of transport codes employed for various applications in particle physics. To scrutinize several of such codes, reaction cross sections, hadronic interaction lengths, average neutron multiplicities, neutron multiplicity and energy distributions, and the development of hadronic showers are here investigated. The problem of radiation-induced damage of window- and target-materials employed in spallation neutron sources due to embrittlement and blistering caused by helium gas production is expatiated in the current work. As for example production cross section measurements for light charged particles on thin targets point out that appreciable distinctions exist not only for different experiments, but also within the models applied here. The performance and flexibility of program packages like HERMES, LCS or MCNPX and their validation by using experiments is demonstrated. Besides this application driven motivation for investigating GeV proton-induced spallation reactions, a more fundamental or nuclear physics aspect related to the excitation of heavy nuclei and the investigation of their subsequent de-excitation and fragmentation modes will be presented. The exploration of hot excited nuclear matter implies the understanding of their formation under extreme conditions (temperature, pressure). To this the transition of an ensemble of nucleons to thermal equilibrium has to be analyzed. As experimental observables the energy spectra of high-energetic charged particles like p, d, t, $^{3}$He, $^{4}$He, IMF(intermediate mass fragments), FF(fission fragments) are studied in coincidence with neutrons for light particle induced reactions on various targets. These observables allow for a quantitative determination of the energy relaxation process. The thermal excitation energy E* transferred to the nucleus is found to be less than 30% of the total available energy (kinetic energy of projectiles + eventually annihilation energy in case of $\overline{p}$), irrespective of the projectile type. Therefore exotic decay modes like multifragmentation are unlikely and the experimental abundance of IMFs can be fully explained by statistical models, i.e. no evidence for multifragmentation up to E* $\approx$ 1 GeV is found and nuclei decay predominantly statistically, i.e. by evaporation. If multifragmentation is defined as a process that has 3 or more IMFs in the exit channel, an onset is found at about 4 MeV/nucleon. Even at highest excitation energies as for example for the 1.2 GeV $\overline{p}$+Cu and $\overline{p}$+Ag reactions the average IMF multiplicities restrain to values of 1 and 2, respectively. In accordance to this phenomenon up to the highest E* the excited heavy nuclei are shown to survive as self bound objects as high fission probabilities clearly indicate. For the 1.2 GeV $\overline{p}$+Cu reaction the onset of vaporization is observed at about 7.5 MeV/nucleon, with a total vaporization cross section of 3 mb.

Keyword(s): physics

1. Experimentelle Hadronstruktur (IKP-1)