Influence of the nuclear level density on the odd-even staggering in Fe56+p spallation at energies from 300 to 1500 MeV/nucleon

Abstract

Background: Special attention has been paid to study the shell effect and odd-even staggering (OES) in the nuclear spallation.Purpose: In this paper, we investigate the influence of the nuclear level density on the OES in the $^{56}\mathrm{Fe}+p$ spallations at energies from 300 to 1500 MeV/nucleon.Method: The isospin-dependent quantum molecular dynamics (IQMD) model is applied to produce the highly excited and equilibrium remnants, which is then de-excited using the statistical model gemini. The excitation energy of the heaviest hot fragments is applied to match the IQMD model with the gemini model. In the gemini model, the statistical description of the evaporation are based on the Hauser-Feshbach formalism, in which level density prescriptions are applied.Results: By investigating the OES of the excited pre-fragments, it is found that the OES originates at the end of the decay process when the excitation energy is close to the nucleon-emission threshold energy, i.e., the smaller value of the neutron separation energy and proton separation energy. The strong influence of level density on the OES is noticed. Two types of the nuclear level densities, the discrepancy of which is only about 7% near the nucleon emission threshold energy, are used in the model. However, the calculated values of the OES differ by the factor of 3 for the relevant nuclei.Conclusions: It is suggested that, although the particle-separation energies play a key role in determining the OES, the level density at excitation energy lower than the particle-separation energies should be taken into consideration