Extension of the Incident Flux Response Expansion Method for Radiation Detection Modeling

The incident flux response expansion (iFLEX) method is an accurate and efficient tool to compute detector pulse height responses on-the-fly. Previously the incident angular flux at the detector window was expanded using discrete hemisphericalharmonics basis functions for a coupling to any S-N method to perform hybrid stochastic/deterministic S-N calculations for nuclear material detection systems. In this new work, continuous angular expansion functions are used to expand the incident flux at the detector window. The continuous choice eliminates the potential ray effects inherent in discrete ordinates methods while further improving accuracy of the pulse height spectra calculation. The on-the-fly computation speed is not affected because of the solution is attained by superposition of pre-computed response functions that are only detector type dependent. The main advantage in addition to the higher fidelity is iFLEX can now be coupled to any deterministic and stochastic methods whether the phase space is treated discretely or continuously. In this paper the method has been tested in benchmark problems with an incident flux representing a prompt gamma fission spectrum from a typical cargo with 1/3rd density water. It is found that the pulse height distribution predicted by the new iFLEX method agrees very well with that directly computed by continuous energy MCNP for the CsI detector. The discrepancies are within two standard deviations of the Monte Carlo reference (MCNP) solutions. The iFLEX method is found to be significantly (about 6 orders of magnitude) faster than the direct Monte Carlo method.