Simulation of He-3 Neutron Detectors with DRiFT, a Detector Response Function Toolkit

Gas detectors, particularly those that utilize helium-3, are important tools for the detection of neutrons across a number of applications. The MCNP
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Monte Carlo transport code has the capacity to model neutrons’ transport to and interaction in such a gas detector, but it has no capability to simulate a detector’s response to that radiation. DRiFT, a Detector Response Function Toolkit, post-processes the output of MCNP
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to simulate a realistic gas detector response. Using DRiFT, individual detection events are simulated to determine the contributions of wall effects, where the resultant protons and tritons from the neutron capture reaction deposit energy in the detector wall rather than the active volume, and the effects of end-tube inactive regions, where the detector is insensitive to energy deposition. This allows a realistic detector response to be calculated, and representative pulse shapes and detector energy spectra to be generated. Measurements of a californium source were taken in the Epithermal Neutron Multiplicity Detector (ENMC) to serve as a point of comparison for MCNP
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simulations with and without DRiFT. The neutron count rate was measured with the source at different vertical positions throughout the ENMC instrument well, corresponding to different locations along the He-3 detectors’ active volumes. MCNP
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and DRiFT were then utilized to simulate the detector’s response. Using DRiFT with MCNP
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increased the fidelity of the simulations over MCNP
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alone by allowing for a more realistic treatment of the end regions of the He-3 tubes. This demonstrates the value of DRiFT as a tool for high fidelity modelling of gas detector response to neutrons.