While the studies on the photon buildup factors for point gamma sources have been widely made, studies for linear gamma sources, for which the spent nuclear fuel rods and/or brachytherapy seeds could be a relatively good approximation, have become extremely scarce. Buildup factors for linear gamma sources are computed here by means of the MCNPX Monte Carlo code. The exposure buildup factors were calculated for isotropic linear gamma sources with different lengths (1, 10, and 100 cm) and also for different energies (0.1, 0.5, 1, 5, and 10 MeV) in a cylindrical concrete shield up to depths of 10 mean free paths. A number of 45 simulations were done to reach the appropriate results for the linear gamma sources and 5 more simulations were performed for point gamma sources to be compared with the linear sources of photons at the same energies. For validation, we calculated the total mass attenuation coefficients, mass energy absorption coefficients, and uncollided photon flux for some photon energy in concrete and we showed that the simulation results have a good agreement with NIST-XCOM database and theory. Finally, comparisons are shown for the exposure buildup factors of the linear source in terms of the gamma-rays energy, of the length of the linear source and of the cylindrical shield, which can be useful in the design of a suitable shield.

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In the present study, photoneutron production using a 5 MeV electron linear accelerator (LINAC) was investigated based on new photoneutron cross-sections (IAEA/PD-2019.2) from the International Atomic Energy Agency (IAEA). A detailed reconsideration of a past study has been done in order to make a photoneutron source based on a heavy water target. In this regard, new cross-sections of deuterium and beryllium were added to the library of the MCNP6.1 Monte Carlo code for the simulation of photoneutron production. In addition, the FLUKA4-3.0 Monte Carlo code was employed for comparison and data validation. Moreover, the obtained results from those two codes (MCNP and FLUKA) were compared and discussed with the theory and the previous study. Finally, it is concluded that the simulation of photoneutron production through an electron LINAC can have relatively different results based on the photoneutron cross-sections and even on the Monte Carlo code used. Therefore, a special attention should be paid to the difference between photoneutron cross-sections as well as the difference in Monte Carlo codes to maintain reliability.

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