A Real-time Scintillation Crystal Identification Method and Its FPGA Implementation

A common method for gamma ray detection uses a scintillation detector consisting of crystals optically coupled to a photomultiplier tube (PMT). Scintillators are widely used in medical radiation fields such as CT scanners, gamma cameras, positron emission tomography (PET) scanners [1] - [5]. Scintillation crystals respond to absorption of gamma rays by emitting light pulses. Light pulses are characterized by special characteristics of crystals such as decay time constant.

The scintillation counter consists of scintillation crystals (usually sodium iodide doped with antimony) attached to a photomultiplier. The crystal generates a set of flicker for each absorbed photon, the number of which is proportional to the photon energy. It is converted to a pulse from a photomultiplier that is proportional to the photon energy. The crystal must be protected with a relatively thick aluminum / germanium foil window, which limits the use of the detector at wavelengths below 0.25 nm. The scintillation counter is typically connected in series with an airflow proportional counter: the latter is provided with an exit window opposite the entrance to which the scintillation counter is connected. This arrangement is particularly useful for sequential spectrometers.

In the ideal world, scintillation crystals exhibit characteristics such as uniform blocking capability, transparency by photoelectric absorption and gamma ray, transparency to scintillation photons, and high conversion efficiency (gamma to scintillation photons). In practice, however, the blocking ability of the crystals changes, ultimately resulting in incomplete sensitivity and "hot" and "cold" spots on the image. As mentioned above, this change in count is a manifestation of non-uniformity. Furthermore, the light output of the scintillation crystal shows a mismatch. This is due to the change in crystal doping chemistry which acts as the activation center of the luminescence phenomenon. (In NaI crystals, doping is done with ruthenium). Finally, the image heterogeneity is also derived from the change in the proportion of the light collected by the photomultiplier.