Monte Carlo calculations of spectra and interaction probabilities for photons in liquid scintillators for use in the standardization of radionuclides.

The probabilities for the interaction of photons, Pint, in three different liquid scintillators (toluene, Ultima Gold, and Hionic Fluor) were calculated as a function of energy for three different geometries using the Monte Carlo codes PENELOPE and EGSnrcMP. The agreement in Pint values between the two codes in all of the geometries and scintillators for energies above 100 keV was within +/-0.5%. The agreement was better than +/-4% for energies between 20 and 100 keV. No substantive differences in the normalized absorbed energy spectra were observed for initial photon energies in the range of 20-1000 keV. The influence of scintillator composition on the simulation results was investigated in the context of a possible conflict in the published composition data for Hionic Fluor. A difference in interaction probability of about 27% was observed at 20 keV when an alternate composition having lower amounts of S and P is used. At 100 keV, the differences were within the calculational uncertainties.