Scintillation counters are basically made up of the following main components: a scintillation material (crystal), a photodetector (photomultiplier in this case) which is used to count flashes (scintillations) from the crystal and an electronic pulse forming and pulse height discriminating circuit. The scintillation crystal is typically sodium iodide.
Operation of Scintillation Counter
The energy from ionizing radiation which collides with the crystal lattice is converted, in part, to a flash of light (secondary photons), the intensity of which is proportional to the energy of the absorbed radiation. The intensity of the flashes or scintillations is very weak and must be converted to electrical pulses by the photomultiplier.
The photomultipliers used in scintillation counters have flat, transparent photocathodes. Photons from the scintillation crystal strike the photomultiplier photocathode and cause the release of a number of electrons, proportional to the intensity of the flash, the intensity of which is proportional to the energy of the ionizing particle. The few photo-electrons released at the photocathode are amplified by the dynode electrodes of the photomultiplier to form a large output current pulse. The current amplification of a photomultiplier (anode current/photocathode current) depends on the supply voltage used, the tube’s geometry and so forth, and can range from 102 to 5 x 106. The overall photosensitivity of photomultipliers can range from 0.01 to 2 x 104 A/lumen. The photomultiplier current amplification increases proportional to the square root of the supply voltage. With liquid nitrogen cooling to reduce noise, photomultipliers can resolve scintillations as small as 2 x 10-16 lumen. In general the response time of a photomultiplier is < 10 ns, hence such tubes should be capable of counting as fast as 107 cps. The output pulses from photomultiplier circuits are often reshaped to make them sharper. An example of such a sharpening circuit, utilizes a shorted delay line to cancel the long tail on the primary output pulse, making the pulse narrow.
Application of Scintillation Counters in Medical Imaging
Scintillation systems are extensively utilized in nuclear medicine in conjunction with gamma (ϒ)-emitting isotopes to image tissues with cancerous growth. For instance the presence of a tumour generally implies that more of the radioisotope (given to the subject per the physician guideline) will be taken up than by normal tissue, and this can be detected by imaging systems employing scintillation counters.
Planar arrays of scintillation crystals and their associated photomultipliers are called gamma cameras and are used in nuclear medicine to visualize parts of the body that have selectively absorbed a radioisotope. This type of gamma camera provides relatively coarse resolution because of the physical size of the scintillation crystals and photomultipliers.
Related articles:
- The Instrumentation for the Medical Use of Radioisotopes Diagnosis
- Ultrasound Scanning Techniques
- Positron Emission Tomography (PET)
- Magnetic Resonance Imaging (MRI)
- Radionuclide Imaging Techniques
- Optical Tomographic Imaging
