SummaryThe probability distribution of i<;mization energy in a sodium iodide crystal 2·5 em long by 2·5 em diameter due to the absorption of 18 MeV quanta has been calculated taking account of side escape of electrons from the crystal due to multiple scattering. The results confirm the conclusions of the authors' earlier work.
SummaryThe pulse-height distributions to be expected from scintillation counters using thallium-activated sodium iodide crystals have been investigated for y-radiation of energies 6, 12, and 18 MeV_ with two sizes of cylindrical crystal (2· 5 cm.long by 2· 5 cm. diameter and 7·5 cm. long by 4 cm. diameter). A Monte Carlo calculation of the partial shower process by which photons are absorbed in the crystals has given the probability distributions of ionization energy from homogeneous incident photons. The spreading of the distributions by statistical effects in the photomultiplier has been measured using light flashes of constant integrated intensity from a rotating mirror. The curves predicted by combining these two factors are compared with experimental pulse-height distributions for y-radiation from proton bombardment of fluorine, boron, and lithium.It is concluded that energy resolution above 10 MeV. is severely limited by broadening of peaks introduced by an appreciable and variable fraction of the energy of each incident photon escaping from the crystal as bremsstrahlung. This could be minimized by use of crystals with dimensions large compared with 8 cm., the maximum mean free path of bremsstrahlung photons in sodium iodide.
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