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. Monte Carlo N-Particle version 4C 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 fourth order polynomial.