The Physics of Spallation Processes
The Physics of Spallation Processes The Physics of Spallation Processes
Die Atomik ist ein sehr verzwicktes Theorem,und man kann ihr mit Hilfe der Algebrabeikommen, man muss dabei aber graduellvorgehen, denn sonst kann es passieren, dassman die ganze Nacht damit verbringt, einenkleinen Teil davon mit Rechenschiebern undKosinen und anderen ähnlichen Instrumentenzu beweisen, ohne zum Schluss an das zuglauben, was man bewiesen hat.(Flann O’Brien)
AbstractA recent renascence of interest for energetic proton induced production of neutrons originateslargely from the inception of projects for target stations of intense spallation neutronsources (like the planned European Spallation Source ESS, the SNS in the US and J-PARCin Japan), accelerator-driven nuclear reactors, nuclear waste transmutation, and also fromthe application for radioactive beams. The ultimative objective is that the essential highandintermediate energy nuclear data, required in the framework of such applicationswill be available in an energy range where currently almost no data exist. Althoughin this work the issue has been quite successfully addressed experimentally by varyingthe incident proton energy for various target materials and by covering a huge collectionof different target geometries—providing an exhaustive matrix of benchmark data—theoverriding challenge is to increase the predictive power of transport codes employed forvarious applications in particle physics. To scrutinize several of such codes, reaction crosssections, hadronic interaction lengths, average neutron multiplicities, neutron multiplicityand energy distributions, and the development of hadronic showers are here investigated.The problem of radiation-induced damage of window- and target-materials employed inspallation neutron sources due to embrittlement and blistering caused by helium gasproduction is expatiated in the current work. As for example production cross sectionmeasurements for light charged particles on thin targets point out that appreciable distinctionsexist not only for different experiments, but also within the models applied here.The performance and flexibility of program packages like HERMES, LCS or MCNPX andtheir validation by using experiments is demonstrated.Besides this application driven motivation for investigating GeV proton-induced spallationreactions, a more fundamental or nuclear physics aspect related to the excitationof heavy nuclei and the investigation of their subsequent de-excitation and fragmentationmodes will be presented. The exploration of hot excited nuclear matter implies theunderstanding of their formation under extreme conditions (temperature, pressure). Tothis 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 incoincidence with neutrons for light particle induced reactions on various targets. Theseobservables allow for a quantitative determination of the energy relaxation process. Thethermal excitation energy E ∗ transferred to the nucleus is found to be less than 30% of thetotal available energy (kinetic energy of projectiles + eventually annihilation energy incase of p), irrespective of the projectile type. Therefore exotic decay modes like multifragmentationare unlikely and the experimental abundance of IMFs can be fully explainedby statistical models, i.e. no evidence for multifragmentation up to E ∗ ≈ 1 GeV is foundand nuclei decay predominantly statistically, i.e. by evaporation. If multifragmentationis defined as a process that has 3 or more IMFs in the exit channel, an onset is foundat about 4 MeV/nucleon. Even at highest excitation energies as for example for the1.2 GeV p+Cu and p+Ag reactions the average IMF multiplicities restrain to values of 1and 2, respectively. In accordance to this phenomenon up to the highest E ∗ the excitedheavy nuclei are shown to survive as self bound objects as high fission probabilities clearlyindicate. For the 1.2 GeV p+Cu reaction the onset of vaporization is observed at about7.5 MeV/nucleon, with a total vaporization cross section of 3 mb.
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Die Atomik ist ein sehr verzwicktes <strong>The</strong>orem,und man kann ihr mit Hilfe der Algebrabeikommen, man muss dabei aber graduellvorgehen, denn sonst kann es passieren, dassman die ganze Nacht damit verbringt, einenkleinen Teil davon mit Rechenschiebern undKosinen und anderen ähnlichen Instrumentenzu beweisen, ohne zum Schluss an das zuglauben, was man bewiesen hat.(Flann O’Brien)
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