The annihilation radiation exhibits conspicuous features in the low-energy segment of the orbital gamma spectrum, which contains a substantial amount of geological information about the lunar surface. The fluence rate can reflect the element composition, density, maturity, and other characteristics directly.

In order to further clarify the primary source and influencing mechanism of annihilation radiation on the lunar surface.

A quantitative model for the annihilation radiation characteristic peak of orbital gamma spectrometers was established. The gamma rays induced by protons of varying energies were simulated using GEANT4 to further understand the primary source and mechanism of annihilation radiation on the lunar surface. The data from the "Chang'e-1" high-energy particle detector (CE1-HPD) was used as the input term, and the annihilation radiation characteristic information induced by 4~400 MeV protons galactic cosmic rays interacting with five typical rocks was calculated. After subtracting the 0.511 MeV characteristic peak collected by the "Chang'e-1" gamma spectrometer (CE1-GRS) from natural radioactivity, the results were compared with the annihilation radiation fluence rate induced by 4~400 MeV protons galactic cosmic rays.

The results indicate that the rock's composition have a negligible effect on the annihilation radiation. The probability of cascading shower generating annihilation radiation is directly proportional to the incident proton energy. Additionally, the contribution of 4~400 MeV protons to the annihilation radiation present in the orbital gamma spectrum is relatively low, only (1.97±0.66)×10^-4 .

The established model has proven to be accurate in reflecting the related characteristics of the gamma radiation field on the lunar surface and can be used for quantitative analysis of annihilation radiation. The results indicate that the contribution of 4~400 MeV protons galactic cosmic rays to the annihilation radiation present in the orbital gamma spectrum is minimal.