Simulated

I have been spending a very long time learning how to use MCNPX to simulate detectors, and an even longer time fiddling with my geometry to ensure what I modeled is actually what is there.  It is an iterative procedure; simulate, look at results, learning something about MCNPX, simulate, change a composition / geometry (mostly adding stuff, Martin's input was very back of the envelope), and continue.  Each simulation takes around 40 hours to run on a single processor, so I run it on the NE computing cluster with anywhere from 30 - 50 cores.  (The cluster only haves 150 cores, and I like to leave space for others to run their jobs.)

Using the (n,triton) reaction card (MT 105) I used an F4 (cell flux) tally modified by an FM card to calculate the interaction rate.  For the GS20 (a standard detector with published efficiency values) I also used an F1 (surface particle) tally to find the number of neutrons (of certian energy ranges) that cross the detector.

Results

For GS20 the simulated interaction rate CPS is  431, the simulated reaction rate from efficiency is 423 cps, and the observed count rate is 428 cps.  Less than a 1% difference.  For the PS based LiF loaded film, I calculated a simulated interaction rates of 108 cps, the measured is 96 cps, a 13% error.  However, this is to be expected since the PS film is opaque, and light generated may be quenched or pushed below the LLD setting, so our measurement may not record all of the light generated.