360 Chapter 6. Scintillation Detectors and Photodetectors
few parts per million level can substantially reduce its transparency to the ultraviolet
scintillation photons (4). This implies that a liquid xenon based detector can only
be reliably operated if the purity of liquid xenon at all times is maintained. This
points to the operational difficulty of such detectors since it involves not only regular
purification but also continuous monitoring. Certainly for small volume detectors
in which the highly purified liquid xenon can be kept in an air tight container, the
contamination from the environment is not an issue.
The scintillation mechanism in xenon is a multi-step process as described below.
1.Ionization:This is the first step in which the incident radiation ionizes the
xenon atoms through
γ+Xe→Xe++e, (6.3.1)
where aγ-ray photon is shown to knock an electron off of the xenon atom.
2.Molecule Formation: The xenon ion created during the ionization process
attracts a neutral xenon atom to form a xenon molecular ion.Xe++Xe→Xe+ 2 (6.3.2)3.Electron Capture:The xenon molecular ion captures a free electron, which
not only dissociates the xenon atoms but also leaves on of the atoms in a highly
excited state.
Xe+ 2 +e→Xe+Xe∗∗ (6.3.3)4.De-excitation:The highly excited xenon atom releases some of its energy to
the liquid in the form of heat, which does not completely de-excites it.Xe∗∗→Xe∗+ heat (6.3.4)Here the (∗) represents the excited state.5.Excited Molecule Formation:The excited xenon atom attracts a neutral
xenon atom to form a xenon molecule in an excited state.Xe∗+Xe→Xe∗ 2 (6.3.5)6.Radiative Decay:This is the last step of the scintillation process. Here, the
excited xenon molecule produced in the earlier step dissociates into two neutral
atoms and as a result emits a scintillation photon in the ultraviolet region of
the electromagnetic spectrum.Xe∗ 2 → 2 Xe+γuv (6.3.6)6.4 Transfer of Scintillation Photons
The scintillators we studied in the previous sections produce photons of certain wave-
lengths that normally fall within the visible region of the electromagnetic spectrum.
To detect these photons, photodetectors are used. We will study different types
of photodetectors later on in the Chapter. First we will see how the scintillation
photons are transferred from the scintillator to the photodetector.