Prompt Gamma Shielding of Neutron Guides from Ray-Tracing Monte Carlo Simulations
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rompt Gamma Shielding of Neutron Guides from Ray-Tracing Monte Carlo Simulations R. Kolevatova,* a
Department for Neutron Materials Characterization, Institute for Energy Technology, Kjeller, NO-2027 Norway *e-mail: [email protected] Received July 5, 2019; revised July 30, 2019; accepted August 8, 2019
Abstract—This communication provides a brief overview of an efficient, fast, and computationally inexpensive method for evaluating the contribution of prompt gamma rays to dose rate around shielded neutron guides. Parameterized probabilities for neutron absorption in the materials of the neutron guide coatings during reflection were implemented in McStas ray-tracing simulation software together with analytical formula and tabular data necessary for a numerical evaluation of a dose equivalent. This allows evaluating the dose rate beyond the lateral shielding upon simulation of the guide ray-tracing. Keywords: neutron guides, shielding, ray-tracing Monte Carlo DOI: 10.1134/S1027451020070241
INTRODUCTION Neutron transport in the guide from the source to the sample position is accompanied by the emission of high energy photons resulting from capture of neutrons in the materials of the guide coating. This prompt gamma-ray contribution dominates the dose rate at large distances from the source and beyond the line of sight. Its amount varies along the guide depending on the transported flux, spectrum and divergence, as well as m-value of the guide coating. Thus, in principle, guide shielding design should go along with optimization of the guide system, since the shielding costs are comparable to the costs of neutron optics. Several Monte Carlo transport codes, traditionally used for shielding calculations, also implement neutron reflection from supermirrors [1, 2]. However, they either require custom libraries with limited availability for users, or suffer from serious shortcomings in the implementation of supermirror physics [3], as well as lower rates of neutron transport through the neutron guides compared to the dedicated ray-tracing packages. Implementing evaluation of neutron capture rates using guide supermirror coatings and shielding parameters assessment directly in the raytracing packages used for design and optimization of the neutron optics would thus be an optimal solution. NEUTRON ABSORPTION IN THE GUIDE COATING The neutron absorption probabilities for guide coating materials (Ni and Ti) were calculated in [3, 4] for commercially produced non-polarizing supermir-
rors. Universal parameterizations for the capture rates in Ni and Ti per incident neutron were obtained for the momentum transfer q at reflection exceeding a critical value of qcNi = 0.022 Å −1 . The capture per incident neutron was found to grow linearly with momentum transfer (Eqs. (32)–(34) in [3]) up to the coating cutoff, q = mqcNi , where transmission regime takes over and the growth changes to 1/q scaling (Eqs. (35)– (37) in [1]). However, when a specular reflection occurs with a small momentum transfer, q < qcNi , a neutron doesn’t pe
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