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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]

We propose an improved algorithm of generating scattering matrices based on the Monte Carlo method. The new algorithm can greatly improve convergence compared to the traditional approach of the collision estimator. The formula for estimating statistical errors in the new algorithm is given. How the new algorithm benefits the convergence without investing large neutron samples is analyzed, and we also point out that with properly partitioned energy groups, the precision of scattering matrices can get close to that of total scattering cross sections. The new algorithm has been implemented in the neutron transport code NPTS and validated with a number of critical benchmark problems. [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]

The next-event-estimation method, though commonly employed in Monte Carlo simulations, will underestimate point flux with reflecting, white, or periodic surfaces. This paper proposed a modification on the traditional next event estimation method to improve the calculation of point flux with reflecting surfaces. The new method has been implemented in the Monte Carlo transport code, NPTS (Neutron Photon Transport System). Several numerical cases are presented, and the results show that the modified next event estimation method works well with reflecting surfaces. The point flux calculated by the modified method is independent of reflecting surfaces. [ABSTRACT FROM AUTHOR]

Monte Carlo programs are increasingly considered the most accurate tool for simulating particle transport problems. The neutron-photon transport simulation capabilities of domestic particle transport Monte Carlo softwares are relatively advanced, while the electron transport simulation functions have not been well developed yet. This paper independently developed the functions of electron transport and photon-electron transport in the CH (condensed history technique) scheme based on the NPTS (Neutron Photon Transport System) program, and implemented the multi-coupling transport simulation of neutrons, photons, and electrons, which can meet the needs of actual applications. Several test models were run on both NPTS and the popular programs such as EGS and RMC to verify the correctness of NPTS. In addition, an energy deposition tally is developed. It is shown both theoretically and experimentally that the energy deposition calculation by NPTS are more accurate than RMC. [ABSTRACT FROM AUTHOR]