Improving the sensitivity of fast neutron inelastic scatter analysis to iron using associated particle collimation

Abstract Purpose The purpose of this study is twofold; first, a prompt gamma-ray neutron activation analysis (PGNAA) system will be constructed and employed in order to find the approximate sensitivity to elemental iron upon spectroscopic analysis . Concurrently, a Monte Carlo (MC) simulation model of the same PGNAA system will be defined such that the neutron source and resulting gamma-ray emissions are temporally collimated using the neutron associated particle (AP) technique. The experimental and simulated gamma-ray spectra may then be compared – taking into account the technological parameters and physical limitations of the PGNAA system – to determine how the sensitivity of the system may be improved due to photon signal discrimination inherent to the application of AP timing. Methods The presented PGNAA system utilized a deuterium–deuterium (DD) neutron generator with an isotropic flux on the order of 1E9/s and a 2” x 2” thallium-doped sodium iodide (NaI(Tl)) detector integrated with a Canberra Digital Spectrum Analyzer (DSA-1000). The accompanying MC model was defined in MCNP by an isotropic 2.5 MeV DD neutron source, iron as the target element, the components of the neutron generator, the supporting apparatus and shielding materials, and finally, by a volume of sodium iodide analogous to the experimental detector to serve as the spectroscopic focal point. The MCNP f8 pulse height tally – in combination with Gaussian energy broadening – was used to generate the simulated gamma-ray spectra, while collimation settings within MCNP enabled the simulation of time-filtered spectra with significant background reduction as would be possible upon employing the AP technique. Results The experimental PGNAA system provided only macroscopic sensitivity to elemental iron, as confirmed by the unfiltered MCNP simulations. However, upon incorporation of AP collimation, the apparent iron sensitivity increased by a factor of nearly 8, with an accompanying increase in signal-to-noise ratio (SNR) of over 700% in the spectral region of interest. Even with modest detection capabilities in a rudimentary PGNAA system, AP coincident time gating allowed for a simulated lower limit of detection (LLD) for iron Conclusions The sensitivity to elements such as iron offered by AP-gated PGNAA lends itself to potential applications in material characterization in homeland security and most intriguingly, to noninvasive tissue analysis where elemental disease signatures may be defined and explored as a means to novel diagnostic and interventional pathways. Incorporation of associated particle imaging transducers and electronics in forthcoming sealed tube neutron generator designs will not only allow for the experimental determination of elemental detection limits on a case-by-case basis but the capability to spatially orient quantified elemental distributions in three dimensions. Though preliminary, the results of this study indicate the efficacy of AP collimation with regard to increasing SNR and provide an important basis for refining future PGNAA systems.