Answer to Question #12829 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

The following question was answered by an expert in the appropriate field:

Q

How do I estimate 3He detector counts in Monte Carlo N-Particle (MCNP) transport code? Which tally should I use, and how do I calculate the detector response function?

A

A radiation detector is calibrated by correlating the fundamental output of that detector against an appropriate and well-known radiation field parameter—in your case, the number of 3He(n,p)3H reactions occurring in the active region of your 3H detector due to a given neutron fluence. Based on your question, I assume that you have a basic programming knowledge of the MCNP code and that you can simulate your neutron source and detector geometry.

After simulating your source and problem geometries, use an MCNP F4:n tally to calculate the average neutron fluence per simulated source-neutron (neutrons cm-2 per simulated source-neutron) within the active detection-region of your 3H detector. You next convert this neutron fluence into a 3He(n,p)3H reaction rate by using an FM card which takes the form of "FMn C M R." Here, n is your tally number (e.g., FM4), C is the atomic density (in atoms barn-1 cm-1) of the material comprising the active-detector region of your detector, M is the material number (m) that you used to specify the material inside the active detector region (Mm for your active-detector cell), and R is the ENDF reaction number. In your case, R = 102 (see Appendix G of the MCNP6 manual) will determine the (n,p) reaction rate in your F4 tally cell per simulated source-neutron.

Your detector response, therefore, is found by multiplying your tally output by your neutron fluence rate. That said, because MCNP is a radiation transport code, it will not simulate the electronic noise of your detector or the conversion efficiency of an (n,p) reaction into a measured count; these parameters would be determined experimentally.

I hope that this helps.

David Medich, PhD

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