Crystal counter

(nucleonics) A particle detector in which the sensitive material is a dielectric (nonconducting) crystal mounted between two metallic electrodes.

A device, more correctly described as a crystal detector, that detects ionizing radiation of all types and is adaptable to measuring neutrons. The sensitive element is a single crystal with a dc resistance normally higher than 10¹² ohms. The crystals are small and are cut or grown to volumes ranging from less than 1 mm³ to approximately 200 mm³.

Crystal detectors fall into two categories: Certain crystals act as thermoluminescent detectors, of which lithium fluoride (LiF), lithium borate (Li₂B₄O₇), and calcium sulfate (CaSO₄) are among the best known. Other crystals, for example, cadmium telluride (CdTe) and mercury iodide (HgI₂), act as conduction detectors, delivering either pulses or a dc signal, depending upon the associated electronic circuitry. See also Ionization chamber.

Diamond is a unique crystal that functions as a thermoluminescent detector or, if suitable contacts are made, as a conduction detector. The efficiency of the diamond detector in the thermoluminescent or conduction mode is strongly dependent on the impurity atoms included within the crystal lattice, with nitrogen and boron playing dominant roles. Not all diamonds are good detectors; only the rare and expensive natural types IB or IIA are appropriate. Besides being stable and nontoxic, diamond has an additional attractive feature as a detector. As an allotrope of carbon, it has the atomic number Z = 6. Human soft tissue has an effective Z = 7.4, so that diamond is a close tissue-equivalent material, an essential characteristic for biological dosimetry, for example, in measurements in living organisms. See also Diamond.

Good crystal detectors are insulators and therefore have significant band-gap energies. A large band gap impedes the spontaneous excitation of charge carriers between the valence and conduction bands, thus lowering leakage currents and movement of charge carriers to trapping centers. Room temperature devices are consequently possible. See also Band theory of solids; Electric insulator; Traps in solids.

In thermoluminescent detectors, the crystal is heated at a controlled rate on a metal tray by means of an electric current. The photon emission from the crystal is monitored by a photomultiplier the output of which is directed toward an appropriate recording device. The result is a “glow curve”, the area of which correlates with the number of traps depopulated, which are in turn directly related to the radiation-field intensity. The integrated light output therefore becomes a direct measure of the total radiation dose. See also Dosimeter; Photomultiplier.

In a conduction detector, a charged particle entering the crystal transfers its kinetic energy to the bulk of the crystal by creating charge carriers (electron-hole pairs). A photon of sufficient energy interacts with the crystal atoms, losing all or part of its energy through the photoelectric effect, the Compton effect, or pair production. In each of these processes, electrons are either liberated or created, and they in turn have their energy dissipated in the bulk of the crystal by creating charge carriers. When the carrier pair is created, the individual carriers move under the influence of the electric field toward the oppositely charged contacts. On arriving at the contacts, the charges can be measured at the output point either as a dc or as a pulse signal, depending upon the circuitry. It is, however, necessary for full efficiency of the counting system that both types of carriers are collected equally. See also Compton effect; Electron-positron pair production; Gamma-ray detectors; Photoemission.

In the thermoluminescent mode the crystals measure the total dose of the applied radiation, whereas in the conduction mode they measure the instantaneous dose rate; in both cases it is ultimately the crystal itself that limits the sensitivity and resolution of the system. Present methods of synthesis for crystals permit the detection of radiation fields down to nearly background values (0.1 microgray/h or 10⁻⁵ rad/h) even with crystals as small as 1 mm³. Small crystals make detectors possible that are capable of very high spatial resolution. This feature is important in electron radiation therapy. See also Particle detector; Radiation biology; Radiology.