Based on the Geant4 program-the package for simulating particle transportation in materials, simulations of the irradiation by neutrons with 1 MeV energy in zirconium were conducted The two adjacent elastic collisions between injected neutron and target atoms produce numerous primary knock-on atoms (PKA). It is found that the average distance of adjacent collisions is 29.47 mm, and the kinetic energy of most PKAs ranges from 1 keV to 15 keV. The damaged area induced by the PKAs is in nanometer scale, which is far less than the distance between the two PKAs. According to the fact that, the subsequent cascade collisions caused by the two PKAs can be considered as a set of independent processes, it is reasonable to study the cascade collisions of the PKAs by means of molecular dynamics method. The cascade collision progress of PKAs with different energies was performed, and the number of interstitial atoms and the size of the damaged regions in the material were extracted. Through the combination of Monte Carlo method and molecular dynamics simulation, a complete physical picture of the primary damage caused by the 1 MeV neutrons in the zirconium was obtained.

It was given that the characteristics of the fluence distribution and the energy spectrum structure of 4MV photons on the Phase Space (PhSp) plane and a method to analyzing the characteristics. Methods: After the PhSp file of 4 MV photons was acquired by the method of Monte Carlo (MC) calculation, the photons recorded by PhSp file were grouped based on the energy bin, and it was analyzed that the spatial distribution and energy spectrum structure of the photons. The photons in each energy group were continually grouped to sub-files according to momentum bin, and the primary and scattered photons could be separated according to the character of the fluence distribution of the photons in the sub-files. Results: The energy of 4 MV beam is a continuous spectrum. The energy constituent on a pixel at different distances from the center point is different, and the average energy on the center axis of the field is the highest; The photons with 0-1.0 MeV had 42.6% of all; that with energy more than 3.0 MeV had 11.7%; greater than 4 MeV, just 1.5%. The primary and scattered photons were easy collected according to the distribution characteristics of sub-groups. Conclusions:The work to acquire and analyze the PhSp file of the 4 MV beam is significant. 4 MV, 6 MV, 8 MV, 10 MV and 15 MV energy beams basically cover the beams of radiotherapy, and a database of the energy beams could be built for the MC related research of other scholars.