The Study On The Mechanism Of Proton-induced Spallation Reactions And The Nucleon-nucleon In-medium Effective Interaction And Cross Sections

As two inputs of microscopic transport theoretical model, the nucleon-nucleon in-medium effective interaction and cross sections are of importance for the descriptionon the intermediate energy nuclear reaction. The equation of state for nuclear matter,especially for asymmetric nuclear matter, and the in-medium nucleon-nucleon crosssections are still not well determined so far though a lot of efforts have been made boththeoretically and experimentally. In this thesis, we try to work on these subjects byusing the intermediate nucleon-induced spallation reaction.It is important to study the spallation reactions both on applications and basic re-search. Comparing with the temperature in the heavy ion collisions, the temperaturein the intermediate energy spallation reaction is lower when the incident energy is notvery high. So we can extract the information of the equation of state for nuclear matterwhich is more close to the zero-temperature case than the one from HIC. Intermediateenergy proton-induced spallation reactions with the targets ¹⁶O, ²⁷Al, ⁵⁶Fe, ²⁰⁸Pb, etc.are studied by the improved quantum molecular dynamics model (ImQMD) incorpo-rated the statistical decay model. The influences of the different Skyrme interactions(SkP, SkM^*, Sâ…¢, SkT6, SLy7) on the mechanism of spallation reaction and the doubledifferential cross sections of emitted neutrons are studied. It is found that the differentSkyrme interactions influence the low energy part of spectrum of emitted neutrons moreobviously. This effect decreases as incident energy increases. The double differentialcross sections of emitted neutrons are found to be in good agreement with experimen-tal data when the Skyrme parameter set SkP is adopted. The double differential crosssections of proton andαand the cross sections of spallation productions are also pro-vided by our calculations. Because there is no adjustable parameter, our model hasgood prediction power. That is very significative for the applications of the spallationreactions.The five sets of different zero-temperature isospin, energy and density dependentin-medium nucleon-nucleon (NN) elastic scattering cross sections provided by G. Q. Li(in PRC48, 1702; PRC49, 566), C. Fuchs (in PRC64, 024003), M. Kohno (in PRC57,3495) and Q. F. Li (in PRC62, 014606) calculated with microscopic many-body theorymethods are tested by comparing with the data of proton-induced reaction cross sections on the target ¹²C, ²⁷Al, ⁴⁰Ca and ⁹⁰Zr at energy rang 80-200 MeV. The calculations arebase on the improved quantum molecular dynamics model. We find that the reactioncross sectionsσ_R is very sensitive to the in-medium NN cross sections and can be takenas a good candidate of the probes for in-medium NN cross sections. The comparisonindicates that the five sets of in-medium NN cross sections reasonably describe thesuppression effect of the medium correction on NN cross sections whenρ＜0.5ρ₀,but provide too weak suppression effect on NN cross sections when 0.5ρ₀＜ρ＜ρ₀.This conclusion is obtained by the investigation of the difference in the distribution ofthe local densities where the collisions of the incident proton and the nucleons in targettake place in inelastic events between reactions p+¹²C and other three reactions.The isospin effects resulting from both the density dependence of the symme-try energy and the isospin dependent nucleon-nucleon cross sections, are studied withnucleon-induced reactions on isotopes of tin ^112-132Sn and their correspondingβ-stableisobar based on the improved quantum molecular dynamics model with free nucleon-nucleon cross sections. We find that: 1. The calculated reaction cross sectionsσ_R withfree NN cross section overestimate the experimental data, but the slope that theσ_R de-creases with the incident energy increases and increases with the target mass increasescan be described correctly. 2. In the case of proton onβ-stable nuclei, the relationsbetween reaction cross section ((σ_R/Ï€)^1/2) and target mass (A^1/3) obtained from cal-culations are found to be in good agreement with the experiential formula extractedfrom experimental data forβ-stable nuclei. While in the case of proton on isotopesof Sn, there is obvious departure from the experiential formula, a strong isospin effectis observed. 3. Theσ_R is influenced by the form of the density dependence of thesymmetry energy in two aspects: Firstly, the different density dependences of the sym-metry energy provide different thicknesses of neutron skin which leads to differentσ_Rbecause the difference betweenσ_pp,nn andσ_pn which are rather different at low energy.Secondly, the direct influence of the different density dependence of the symmetry en-ergy on the motion of the incident nucleon. In the case of proton-induced reactions,the softer symmetry energy provides stronger attraction effect on the incident proton,which enhance the probability of collisions between incident proton and target and thusenhance theσ_R. While in the case of neutron-induced reactions, the symmetry en-ergy weakens the attraction of the mean-field on the incident neutron, theσ_R is decided mostly by the thickness of neutron skin. The stiffer symmetry energy provides a thickerneutron skin, then a lowerσ_R we get.