Neudep: a GPU-based Monte Carlo transport program, coupling full physical reaction models of neutrons, photons, electrons/positrons.

Monte Carlo (MC) simulations are considered the gold standard for calculating radiation dose in complex radiation fields. However, these simulations often require substantial computational resources. Based on our team's existing graphics processing unit (GPU) modules for photons and electrons/positrons, this research developed neutron GPU physics modules including elastic scattering, inelastic scattering, radiative capture, and fission. These were integrated into the Neudep (GPU-based NEUtron-photon-electron/positron coupled Dose Estimation Program). This program enables coupled multi-particle transport of neutrons, photons, and electrons/positrons across broad energy ranges and incorporates comprehensive physics for all particle interactions. During neutron interactions, photons and secondary neutrons are produced. These photons undergo various physical processes: the photoelectric effect, Compton scattering, and pair production, generating photoelectrons, Compton electrons, and recoil electron-positron pairs, respectively. The associated electron interactions include bremsstrahlung, ionisation, and multiple scattering. Bremsstrahlung, in particular, gives rise to secondary photons. Additionally, positron annihilation results in the production of secondary photons. All these secondary particles are stored in a memory stack and are transported only after the primary neutron transport process is completed. The Neudep program was validated for accuracy and tested for computational efficiency using both a homogeneous Water Phantom and the Chinese adult male voxel model (CRAM). The results indicate that the energy deposition discrepancies between Neudep and the reference MC code are less than 2%, with neutron incident energies of 3 MeV showing deviations of less than 0.5%. Organ dose differences generally remain within 5%. While maintaining computational accuracy, the Neudep program efficiently simulates 1 million neutrons in just 2 s. Additionally, the transport time for 10 million neutrons through a complex human model can be reduced to under 1 min. Neudep can reduce computation times by 78-5000 times compared to traditional central processing unit-based MC software. This tool demonstrates tremendous potential for rapid and accurate dose calculations.