Peak Efficiency of NaI(Tl) Crystals for Gamma Rays from 0.150 to 7.5 Mev

The peak efficiencies of cylindrical 1-1/2 X 1-inch, 3 X 3-inch and 3 X 3-inch bevelled crystals have been determined by the multiplication of the theoretical total efficiency for a particular energy gamma ray by the experimentally determined ratio of the areas under the full energy peak and under the total pulse height spectrum for the gamma ray. Care was taken to eliminate spurious effects in the pulse height spectrum such as scattering, bremsstrahlung, and other interfering gamma radiation from the source, etc., in the experimental measurements. The peak efficiencies have also been determined at certain energies by ß-γ coincidence experiments and yield the same results within 5 per cent for energies to 2.76 mev. The intensities of gamma rays determined in this way have been compared to the disintegration rates obtained from 4π ß-counting techniques from sources with known decay schemes, and the results agreed within the limits given above. The extension of these efficiencies to crystals of different composition or to crystals with the same shapes but different dimensions will be discussed.