Estimation of Photofractions of Very Large NaI(Tl) Scintillators

The photofraction, i.e., the fraction of incident photons absorbed, and the Compton electron distribution of a very large NaI(Tl) scintillator are investigated. They are estimated from the measurement of energy spectrum of photons backscattered from the front face of a scintillator that is nearly semi-infinite for ⁶⁰Co and ¹³⁷Cs gamma rays. The estimated photofraction was 0.79 for ⁶⁰Co gamma rays and 0.85 for ¹³⁷Cs gamma rays. The Compton electron distribution obtained was compared with the results of Monte Carlo calculations. They show fairly good agreement.