For $\gamma$ ray peaks the spectral "background" in the region of a peak:
- the pulses related to radiation from other sources(i.e., the background radiation);
- the pulses from higher energy $\gamma$ rays from the source being measured;
- 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.