Research on Monte Carlo simulation method for thick pinhole imaging

•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.