Scintillators and scintillation detectors

The scintillation process involves the conversion of high-energy photons into visible light via interaction with a scintillating material, and consists of the following steps:

1) a photon incident on the scintillator creates an energetic electron, either by Compton scatter or by photoelectric absorption.

2) as the electron passes through the scintillator, it loses energy and excites other electrons in the process.

3) these excited electrons decay back to their ground state, giving off light as they do so.

In a scintillation detector, the scintillator is optically coupled to a photomultiplier tube (PMT) which then generates an electrical signal in response to light incident upon its face. There are several variations on this theme which are used in PET - for example, a 2D array of crystals may be coupled to 4 PMTs (the block-detector, Casey and Nutt, 1986), an array of PMTs may be coupled to a single planar crystal (the Anger camera,Anger, 1958), or an array of crystals may be coupled to a multi-channel PMT (Cherry et al 1997).



When real scintillation detectors are exposed to mono-energetic photons, the energy measured is not that of the electron generated by the initial interaction, but rather the total energy deposited by the photon in the detector. This distinction is important because photons initially interacting by Compton scatter may subsequently be involved in further interactions within the detector. In a sufficiently large detector, most Compton-scattered photons will eventually deposit all their energy, and most events will register in the photon energy peak. Under these circumstances, this feature of the energy distribution is better described by the term "full-energy peak" rather than "photo-peak".

http://depts.washington.edu/nucmed/IRL/pet_intro/intro_src/section5.html