A response function methodology to calculate induced fission neutron flux distribution in an active interrogation system (The 12th International Conference on Radiation Shielding (ICRS-12) and the 17th Topical Meeting of the Radiation Protection and Shielding Division of ANS (RPSD-2012)) -- (Deterministic Methods and Applications)

A hybrid deterministic and Monte Carlo radiation transport methodology is being developed for modeling an active interrogation system for detection of special nuclear material (SNM) in a container. This methodology includes four steps: i) (n, γ) interaction and subcritical multiplication in cargo; ii) (n, γ) interaction due to fission neutrons throughout the cargo; iii) gamma transport to the detector window; iv) detection of gamma-rays by the detector. This methodology enables accurate determination of neutron/gamma fluxes in real times for comparison with measurements, and examination of various materials compositions. In this paper, we will address the first step of the methodology. We have developed a response-function formulation to calculate the subcritical multiplication and the neutron flux distribution. Using the MCNP Monte Carlo code, the response coefficients are pre-calculated for various cargo materials and SNM combinations. We have demonstrated that the new response function methodology yields fission neutron rates which are in excellent agreement with the full Monte Carlo calculation predictions (within 10% for all sampled locations including those near the wall), while achieving significant speedups of several orders of magnitude.