When a gamma-ray photon travels within a detector, the detector output signal depends on the location of the interactions inside the detector and the size of the detector. 

If the incident gamma-ray energy is above 1022 keV pair production events can occur, which depending on the size of the detector mean that single or double escape peaks may arise along with the full energy peak, the Compton continuum and the pair production peak at 511 keV.

Investigate the effect of the size of the detector by selecting each of the options below to show the interactions in the detector and the resulting energy spectrum:

 

A small detector is one so small that only one interaction can take place within it. Only the photoelectric effect will produce full energy absorption. Compton scattering events will produce the Compton continuum. Pair production will give rise to the double escape peak due to both gamma-rays escaping.

 

A large detector is one in which we can ignore the surface of the detector. Various successive photoelectric absorption, Compton scattering and pair production interactions will occur. The result is complete absorption of the gamma-ray and a single gamma-ray peak, referred to as the full energy peak.

 

Within a real detector the interaction outcome is not as simple to predict as the small or large detector case. Compton scattering may be followed by other Compton scatterings before the gamma-ray photon escapes from the detector. Also, pair production may be followed by the loss of only one annihilation gamma-ray, resulting in a single escape peak as well as a double escape peak.