Escape peak to photopeak ratios in NaI(Tl) crystals

The ratio, R, of the iodine K x-ray escape-peak intensity to that of photopeak for various sizes of thin and thick NaI(T1) scintillation crystals has been calculated for gamma-rays varying in energy from 35 to 200 keV for the case of cone geometry using Axel's formula for escape probability and by Monte Carlo method. The theoretical results are tested experimentally using NaI(Tl) crystals for 59.54 keV gamma-rays from a 241Am radioactive source. The ratios calculated for Axel's formula using the revised photon cross sections and fluorescence yields are lower compared to experimental values. The discrepancy between the experimental values and those calculated using Axel's formula are maximum for the thin crystal. The escape ratios calculated using Axel's formula are independent of crystal thickness and depend only on incident energy, whereas the experimental escape ratios depend both on thickness and on incident energy. The iodine K x-rays generated in a small crystal tend to escape with more frequency than x-rays generated in a larger crystal. Monte Carlo calculations agree closely with experiment. (author)