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We are looking at designing a Compton camera for 662KeV photons and have been told that "the fraction of Compton interactions to total interactions in the photopeak is usually higher than fraction in an un-doped Sodium Iodide crystal".

I expected the converse to be true - Thallium has a high Z and photoelectric absorption has a higher Z dependence than Compton interactions. I have been doing some reading and it appears that the Thallium acts as an activator in the Sodium iodide crystal; the benefits of the crystal being transparent to the scintillation radiation is obvious but I cannot see how this affects the interaction probabilities of Compton to photoelectric interactions.

Any help would be very much appreciated!

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    $\begingroup$ My guess: when the scintillation yield is lower in undoped NaI, the lowest-energy part of the Compton background is below the detection threshold. $\endgroup$ – Pieter Feb 4 '18 at 11:57
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The doping of the NaI crystal with thallium improves the scintillation efficiency by improving the light emission due to the improved recombination by light emission of electrons and holes at the dopant site. Thallium in small concentrations in the NaI crystal is a so-called scintillation activator The effect of thallium doping is not related to any direct interaction of thallium with the gamma-rays. Here you'll find an article which describes the effect of thallium doping in NaI scintillators for gamma-ray detection.

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