Monte Carlo Optimization of Coincidence Prompt Gamma-Ray Neutron Activation Analysis

Prompt Gamma-Ray Neutron Activation Analysis (PGNAA) is a non-destructive, rapid on-line method for determination of the elemental composition of bulk coal and other granular solid samples. However, PGNAA has an inherently large background due to the neutron source, natural radioactivity and non-sample response. Introducing the gamma-gamma coincidence technology into PGNAA could highly suppress all interference and eliminate some entirely. To overcome or alleviate the main drawback of coincidence PGNAA, which is a low coincidence counting rate, the system has to be optimized. This optimization at present relies on Monte Carlo simulation. The CEARCPG specific purpose code is the first and only code that is now available to simulate the normal and coincidence spectra of PGNAA. The MCNP5 code has been used in parameter study, dose rate and other suitable parts of this work.;To achieve a better accuracy and efficiency, the detector response function (DRF) used in CEARCPG has been updated with DRFs that generated by a new code named CEARDRFs. CEARDRFs can generate relatively accurate detector response functions for cylindrical or rectangular NaI and BGO detectors in speed that are hundreds of times faster than MCNP5. CEARCPG has also been modified to execute on Linux clusters. Other improvements have been made to the CEARCPG code, including parallel and differential operator (DO) features for the application of the Monte Carlo Library Least Squares approach.;With all the improvements to Monte Carlo simulation codes, the coincidence PGNAA could be optimized. For lab size samples, the paraffin moderator has been optimized for thickness. Through rearranging detectors or using different shape detector, the ratio of improvement can be up to 66.5 and 223.7 for single and coincidence response respectively compared to the original coincidence arrangement. There is also higher efficiency for higher energy gamma-rays for certain cases. For large size sample, the self-moderation of the bulk coal is sufficient. A new detector choice, a plastic scintillation detector, has been investigated to utilize its large volume and excellent time resolution. The simulated 2D coincidence spectra show the feasibility of using the plastic detector as a trigger to another detector that has better energy resolution. The ratio of improvement can be up to 2.5 and 6.2 for single and coincidence responses respectively compared with placing two 6”x6” cylindrical detectors in the opposite of neutron source.;Among all the interferences, the fission gamma-ray source remains the major one while the interference from the analyzer structural material also contributes significantly. Q-value projection on the 2D spectra could further suppress the interference. The MCLLS analysis on the Q-value projected spectra shows better accuracy than using the total coincidence spectra.;With proper shielding, the dose rate around the analyzer is pretty low.;The funding of this work has been provided by the Center for Engineering Application of Radioisotopes (CEAR) at North Carolina State University (NCSU).