MCNP Analysis and Optimization of a Triple Crystal Phoswich Detector

Researchers at the University of Missouri-Columbia have designed a triple crystal phoswich detector that allows for simultaneous detection of alpha, beta, and gamma radiation. A ZnS:Ag layer detects alpha particles, a CaF2:Eu scintillator preferentially interacts with beta particles, and a NaI:Tl cell is used for gamma detection. The detector output is digitally collected, processed, and analyzed by a personal computer using custom software. MCNP simulations of this detector found that the phoswich design has inherent minimum energy limits of 250 keV Emax for beta particles and 50 keV for gamma-rays. For a 2.54 cm thick NaI:Tl crystal, intrinsic gamma efficiency for photons ranges from a maximum of 80% at 100 keV to 26% for 2 MeV photons. Mischaracterized gamma events in the CaF2:Eu crystal above 175 keV can be corrected by subtracting 26 +/- 4% of the total number of counts in the NaI:Tl crystal from the CaF2:Eu response. Beta induced events in the NaI:Tl crystal primarily result from Bremsstrahlung interactions and can be estimated by multiplying the CaF2:Eu energy spectrum by a normalizing factor