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Monte Carlo simulation has become an effective method for solving different physical problems, which is especially advantageous in the particle transport problems with complex geometric models. Charged particle transport simulation programs based on Monte Carlo method are widely used in medical therapy, radiation protection, accelerator design and other fields. Although the domestic neutron and photon transport simulation programs are well developed, there are few programs being able to do charged particle transport simulation. In addition, the energy range for programs based on the intranuclear cascade model and evaporative fission model is usually high, above several tens of MeV. For medium and low energy proton transport, the accuracy of nuclear reaction simulations based on nuclear databases is better than that of models. The biggest problem with Monte Carlo methods for simulating charged particle transport calculations is that the number of collisions of charged particles is too frequent. Therefore, this paper was based on the condensed history method to simulate the proton transport. In this study, with the application of key physical model of charged particle transport, such as the continuous slowing down approximation theory, multiple scattering theory and energy straggling model, a framework of heavy charged particles transport simulation program was established. Based on nuclear database of ENDF/B-Ⅶ.0 proton-nucleus interactions, the proton transport simulation function was implemented on the NPTS (neutron-photon transport simulation program). The extended NPTS program supported the simulation of proton transport problems in the energy range from 1 keV to 150 MeV and multi-coupling transport calculations of protons, neutrons, photons and electrons. A comparison of the stopping power of protons, deuterons and alpha particles in common materials with SRIM was carried out by the NPTS program, which proved the correctness of NPTS in calculating the stopping power of charged particles. The numerical results of neutron yield, average energy and angle for the thick target model agree well with GEANT4, MCNP6 and experimental results. In order to fully verify the program, a proton treatment chamber model with larger spatial dimensions and more complex geometry was used for the study. The neutron flux distribution inside the treatment chamber obtained by NPTS was basically consistent with GEANT4. The accuracy of the NPTS simulations of proton transport and multiparticle coupled transport was verified by several test models. Moreover, the proton energy and continuous slowing down approximation range curves obtained from the NPTS calculations can be used to quickly estimate the target thickness at maximum neutron yield, providing a favourable reference for BNCT (boron neutron capture therapy) target design. This paper effectively extends the application of the NPTS program in accelerator target design, shielding analysis, and radiation protection. The research effectively improves the application value of NPTS program, as well as enriching the research method of medium and low energy proton transport programs in domestic.

Pinhole imaging is an important test technique to obtain information on the spatial distribution of the radiation field in the target region and has been widely used in nuclear physics and inertial confinement fusion (ICF). Coded-aperture, able to maintain good resolution as well as enhancing signal strength, has become a more frequently used method than pinhole imaging in experimental studies. Thus, implementing Monte Carlo simulations of coded-aperture imaging will improve coded-aperture design, image reconstruction, and other related works. However, the current international mainstream Monte Carlo transport simulation programs do not possess the ability to directly simulate coded-aperture imaging. This paper develops a relatively complete coded-aperture imaging simulation function on the Neutron Photon Transport System code based on the next-event estimation method. With the application of Monte Carlo simulation techniques, such as variance reduction and rejection sampling, it is capable of simulating coded-aperture accurately, flexibly, and efficiently, including problems of multiple shapes and even irregular geometry. The results are consistent with combined pinhole imaging, and the computational efficiency has been improved significantly. [ABSTRACT FROM AUTHOR]

• Next event estimation for neutron transport in moving objects. • Directional emission probabilities based on nuclear data in different frames. • The effect of the motion of the pellet material on the pinhole imaging. The next event estimation is one of the important algorithms employed in Monte Carlo simulations for particle transport. It finds wide applications in point flux, image flux, and variance reduction techniques. With the advancement of transport simulations for moving objects, the next event estimation for moving target nuclei has yet to be realized. The next event estimation requires the calculation of directional emission probabilities based on the outgoing distribution provided by nuclear data. When the target nucleus is in motion, determining the outgoing direction in the coordinate system prescribed by nuclear data and calculating the emission probability in that direction becomes more intricate. In this study, regarding the outgoing distributions in different coordinate systems, we present a systematic approach to calculate the next event estimation for moving targets. The accuracy of this method is verified through the energy and angle distributions of surface current tallies in the test model. Applying the new method, we conducted pinhole imaging simulations for a moving radiator, quantitatively comparing the differences in simulation results with and without considering motion. [ABSTRACT FROM AUTHOR]

• A monte carlo simulation method is proposed specifically for thick pinhole imaging. • Thick pinhole imaging algorithm is improved by more than 10 times in efficiency. • A biased sampling method is designed for calculating the direct contributions. • The convergence of the scatter contributions is significantly accelerated. • The computational efficiency is independent of the imaging distance. Thick pinhole imaging, as an important diagnostic tool, is widely used in the research of inertial confinement fusion (ICF). Particles for pinhole imaging in this kind of applications have high penetrating power, which requires high attenuation material to be used for the shielding of the imaging system. Monte Carlo simulation is an effective approach for theoretical studies of thick pinhole imaging, due to the complex interactions of neutrons or photons within the materials. However, existing Monte Carlo programs can hardly support high-precision simulations in a satisfactory efficiency for thick pinhole imaging. In this paper, a thick pinhole imaging simulation method is proposed and has been implemented in the Monte Carlo code NPTS. The effect on the statistical variances of flux image tally caused by the scores from different emission events and different imaging trajectories has been considered sufficiently. The new method realizes the variance reduction of the image tally by reducing score fluctuations of each particle. The correctness of this method is verified by the calculations of a typical model. Compared to the tally of Flux Image Radiograph (FIR), more than tenfold increase in efficiency has been performed by the new method. And more importantly, the efficiency is no longer affected by the distance between sources and the pinhole shielding. [ABSTRACT FROM AUTHOR]