Monte Carlo Calculation of the Effect of Subterranean Perturbations on Reflected X Rays

Monte Carlo calculations for monoenergetic x rays (70 to 200 keV) normally incident on soil have been performed to determine changes in the reflected radiation due to material perturbations at depths of 1 and 1.6 mean-free-paths. The accuracy of the calculations was achieved using a scheme of correlated sampling in which a “complete correlation” is maintained in the sense that identical sampled event sequences are used for the perturbed and unperturbed problem. A somewhat general discussion of correlated sampling is included with emphasis on the mathematical reasons for employing the method. It is shown for a special case that a naive use of correlated sampling may lead to results drastically inferior to those obtained by independent sampling. The Monte Carlo calculations are compared with two-dimensional discrete ordinates results and agreement is very good. Output quantities are reported in terms of energy-dependent fluence and current. The calculated results demonstrate the feasibility of the approach to albedo sensitivity problems in general.