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Thank you for your reply.
Over the Christmas break, I did some more analysis and found the following:
It seems that the observed result is effectively a sum of two (or more) Gaussians, with non-equal amplitudes. Physically, this effect is varied most significantly with different absorption lengths of the scintillator crystal, and/or different reflection parameters of the optical surface between the scintillator crystal and the crystal reflector.
* Large absorption lengths increase the probability that highly-scattered photons will ultimately be detected by the photocathode * Different reflection profiles & parameters vary the number of bounces likely taken by the scintillation photons within the material.
It therefore _seems_ that there is a low probability that the detected yield on the photocathode is equal to the expected mean (slightly lesser, due to wall absorption, perhaps), and a higher probability that some fraction are attenuated by the crystal following multiple reflection by the walls, leading to approximately two distributions.
It is very difficult to find physical values for the optical properties of the NaI(Tl) crystal, especially those which give values over a range of photon energies ~eV. I've used an absorption length of ~4m as a minimum, to minimise this effect.
I found that the photopeak distortion depends both upon energy and detector size, as well as the above parameters. I believe that both of these can be explained in terms of the optical behaviour outlined above.
To support this; producing a naive scintillation photon distribution from the product of the energy deposition array and scintillation yield factor, does not exhibit this distortion; it must be a consequence of the behaviour of the travel / absorption within the crystal.
Has anyone a good reference for NaI(Tl) material data, and/or experience with this issue?