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I'm reading the paper "Analytical functions for fitting peaks from Ge semiconductor detectors" by R.G. Helmer and M.A. Lee.

For $\gamma$ ray peaks the spectral "background" in the region of a peak:

  1. the pulses related to radiation from other sources(i.e., the background radiation);
  2. the pulses from higher energy $\gamma$ rays from the source being measured;
  3. the pulses from the desired $\gamma$ ray, but for which enough energy is lost from the sensitive volume of the detector to put the count in the spectral distribution below the peak.

The third contribution should be represented by a step-like function.

FIRST QUESTION: I really don't understand why this contribution is considered as a background

Generally for peak shapes one can consider a gaussian with a tail on the left side. For the tail various analitycal expression are proposed.

SECOND QUESTION: I don't understand the physical origin of this tail it seems to me to be the same one that determines the appearance of the background contribution modeled by the step-function.

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    $\begingroup$ Gammas deposit energy across some volume (energetic electrons made by the gamma can travel far, creating more excited electrons along the way). If the entire path length is not inside the detection volume, you don't capture all the electrons and the pulse that comes out has less charge, thus indicating a lower energy gamma. The actual shape depends on the detector and experimental setup, but guessing it is a step function to first order might be OK. Or you can measure it. $\endgroup$ – Jon Custer Sep 25 at 12:36
  • $\begingroup$ @JonCuster Thank you so much for your answer what is the physical origin of the tail in the gaussian peak shapes? $\endgroup$ – Stefano Barone Sep 25 at 12:51
  • $\begingroup$ At least partly in the fact that the production and collection of charge, even if all in the detection volume, is a stochastic process - in addition to exciting electrons one also excites phonons which don't have charge. This adds a tail to the detected charge on the low energy side. $\endgroup$ – Jon Custer Sep 25 at 12:57

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