Research On Mechanism Of The Coalescence And Emission In Nucleon-induced Spallation Reactions

Because of wider and wider applications in materials science, biology, space sci-ence, surgery and other fields, the spallation reaction has been lucubrated both in ex-periment and theory. Especially, the requirement of high intensity neutrons source and nuclear transmutation or new concepts for nuclear energy production not only strongly boosted spallation neutron source research, but also triggered spallation neutron source construction. The data used in these applications can not be provided all by experi-ments. When experimental data are not available, theoretical model calculations have to be employed. There are many theoretical models to study spallation reactions, each one has its advantages but also has its shortages. The most of models can not describe the emission of light charged particles quiet well. So a theoretical model which can reproduce existent data and with strong prediction power is very important.In our previous works, the Improved Quantum Molecular Dynamics (ImQMD05) model plus statistical decay model has been used to investigate the spallation reaction-s. The model can reproduce the experimental data for the charge, mass and isotope distributions of products, the spectra and double differential cross sections of single nucleons. But it can not exactly describe the light charged particles (LCPs) emitted in the pre-equilibrium process, i.e.2H,3H,3He, and4He. The main motivation of this work is to further improve the present ImQMD05model, then it can be used to provide the spallation data of LCPs.From the knowledge about nuclear structures and reactions, people know that the LCP is formed in the surface of nuclear. So an phenomenological mechanism called surface coalescence and emission is introduced into ImQMD model:After induced nu-cleon touching the target nuclei, based on the geometric space, a low-density region surrounding the compound nuclei is defined. This region is a sphere shell which center locates at center of compound nuclei, with inner radius R0and with thickness Do. Any fast nucleon passing the shell to leave the compound nuclei is taken as leading nucleon. If the nucleon(s) nearby the leading nucleon satisfy the condition Rim x Pim≤ho and Rim>1fm, where Rim and P-im are Jacobian coordinates and Jacobian momentum of nucleons, these nucleon can be considered as one LCP. If this LCP has kinetic energy high enough to tunnel through the Coulomb barrier, it is emitted from mother nuclei in the direction of its c.m. momentum. With surface coalescence mechanism intro-duced into ImQMD model, the description on the DDXs of LCPs is greatly improved. By systematic comparison between calculation results and partial experimental data of nucleon-induced reactions, the parameters in the surface coalescence model are deter-mined. Then with the fixed parameters, chosen once for all, the prediction power of the model is tested by the nucleon-induced reactions on various targets, such as16O,27A1,56Fe,58Ni,63Cu,120Sn,197Au,208Pb,209Bi etc with beam energies from62to1200MeV. The experimental data of spectra and double differential cross sections for LCPs can be reproduced very well. It means that our model has good prediction power.Then the ImQMD model with surface coalescence mechanism considered is used to study the heavy-ion collisions. We hope to solve the problem that calculations over-estimate the hydrogen yields and underestimate the helium yields. Because of the multi-fragmentation, the method to choose the leading nucleon by geometric space can not be used in the heavy-ion collision. By analysing the spallation data, one can see that only the nucleons with local densities below0.167ρ0can be the leading nucleons. From the calculation results, we find that the hydrogen yields are depressed and the helium yields are enhanced, but the experimental data still can not be reproduced very well. More investigates are needed.