Proportional Detector (Counter) Noreen Marwat NORI, Isb • These detectors operate in region III of Fig. 8.3, where there is a built-in amplification (~106) of the primary ionization through the production of secondary ionization. • A sufficient amount of current is produced by a single ray for it to be counted, and the current is directly proportional to the energy of the radiation. • Hence, a proportional detector, unlike the ionization chamber, can be used to count individual events and to determine the energy of the radiation • Proportional counters require a sufficient amount of expertise in their construction and use. Their stability, with time and voltage fluctuations, is not as good as that of the ionization chambers. • Proportional counters are rarely used in nuclear medicine. Geiger-Mueller Detector (Counter) • The operating voltage of GM detectors is in the region IV. • The incoming particle in this case causes a discharge in the gas, and the amount of the current produced is more or less independent of the energy of the radiation and of the voltage. • Once the discharge is established in the gas by a ray, how does one stop the process so that the detector is ready for the next ray? This is accomplished chemically. Small amounts of halogens or their organic compounds are introduced in the gas as impurities. • These impurities, known as chemical quenchers, absorb the ultraviolet light produced during the discharge and the energy from the secondary ion pairs. Absorption of energy by the quencher molecules causes the discharge to stop but leads to their dissociation. • However, within a short time, most quencher molecules are able to recombine to form the original molecule. Therefore, there is little depletion of the quencher. It takes from about 50 to 200 µsec to quench the discharge. During this time, the GM counter does not respond to another radiation; therefore, this is approximately the dead time of the detector and is nonparalyzable. • The maximum usable count rate for a typical GM counter is 60,000 counts/min. GM detectors are the most sensitive of the gas-filled detectors. Scintillation Detectors (Counters) • A variety of substances known as phosphors scintillate (produce light) under the influence of high-energy radiation. • This property is used for the detection of radiation by instruments known as scintillation detectors • for the detection of the light generated in a material, it should be able to be transmitted out of the material itself. • In liquids, it poses no serious problem. • For solids, only single crystals or glasses can be used because in powders (microcrystals), light is absorbed and scattered at the boundaries of the microcrystals, thereby causing significant and variable loss of light before its detection. • Two other types of substances, thermoluminescent and photoluminescent, are also used as radiation detectors. • These substances do not produce light immediately after interacting with radiation but can store some of the energy given by the radiation for a long time Scintillator • The number of scintillating substances (scintillators) is large. Anthracene, naphthalene, various plastics, alkali halide crystals doped with or without impurities such as NaI(Tl) or CsF, lead tungstate, bismuth germanate, and aromatic compounds such as terphenyl can be used as scintillators. Mechanism of Light Production • Because the mechanism of light production in a substance under the influence of radiation is complex and not well understood, there are no theoretical laws to predict the behavior of a given substance in this regard. Briefly, a γ-ray loses its energy in a scintillator through the photoelectric, Compton, or pair-production mechanisms. The electrons thus generated then lose their energy within short distances through ionization and excitation of the scintillator molecules • The ion pairs thus produced then combine among themselves or with other atoms or molecules of the scintillator and produce certain excited states that, during their subsequent decay, emit light. The nature and quantity of these excited states determine the amount, color, and phosphorescent decay time of light. Properties of Scintillator • A scintillator is primarily characterized by • intrinsic efficiency, • amount of light produced per unit of absorbed energy (light conversion efficiency), • and the time in which emission of light takes place (phosphorescent decay time). • The intrinsic efficiency of a scintillator for a γ-ray of a given energy depends on its linear attenuation coefficient, which in turn depends on the atomic number and the density of the material • The amount of light produced per unit of absorbed energy, which in effect determines the energy resolution of these detectors, and the phosphorescent decay time, which is an important parameter for the dead time of these detectors, vary from substance to substance • Light in a scintillator is produced in a very small volume, mainly determined by the range of photoelectrons or Compton recoil electrons produced in the scintillator. • For energies of x- or γ-rays less than 1 MeV, this range does not exceed more than a millimeter in a NaI(Tl) crystal or other scintillators used in PET. From this small volume of production, light travels in all directions. • The direction of most of the light toward the PM tube is achieved by coating the outside surface of the scintillator, except the side facing the PM tube, with a light reflector such as magnesium oxide. The coupling of the crystal assembly to the PM tube is also important because of possible light loss at the interface of the crystal and PM tube. This loss is generally minimized by the use of optical grease. Scintillator for Scintillation Camera, NaI(Tl) • Of all the known scintillators, sodium iodide crystals doped with small amounts of thallium, NaI(Tl), are most widely used in nuclear medicine, particularly in the scintillation camera. Its moderate density (d = 3.67 g/cm3) and effective atomic number (Zeff = 45) make it very efficient for the detection of x- or γ-rays in the energy range of 30-500 keV • The amount of light produced per unit of absorbed energy in a NaI(Tl) crystal is one of the highest, despite the fact that sodium iodide crystals without thallium doping do not produce much light. The presence of small amounts of thallium (one part in 106) enhances the light output by a factor of 10 or more. The phosphorescent decay time, which eventually determines the dead time of a scintillation detector, is ~0.23 µsec and is adequate for the amounts of radioactivities presently used in nuclear medicine. In addition, the technology required to grow these crystals in large sizes and various shapes is well advanced, making their use more economical compared with many other scintillators. Scintillators for PET • Since in PET, the energy of the radiation to be detected is very high, 511 keV as opposed to 140 keV for 99mTc, • NaI(Tl) is not a good choice as a scintillator material as its detection efficiency drops off rapidly with the energy of γ ray. • Instead, another scintillator, Bismuth Germanate, commonly known as BGO, has been the scintillator of choice. • • BGO has a big advantage over NaI(Tl) in the detection efficiency at 511 keV. However, its light output/keV of absorbed energy, which determines its energy resolution, is very low (1/7 that of NaI) and therefore it has poor energy resolution compared to a NaI(Tl) detector. • Poor energy resolution means poor scatter rejection Also, its phosphorent decay time is slightly longer than NaI making its dead time slightly longer than that of the NaI(Tl) detector Associated Electronics PM Tube • The amount of light produced in NaI(Tl) crystals or any other scintillator is quite small relative to that which the human eye can easily detect • A PM tube is a light-sensitive device that converts light into measurable electronic pulses. It consists of a photocathode facing the window through which light enters, a series of metallic electrodes known as dynodes arranged in a special geometric pattern, and an anode—all enclosed in vacuo in a glass tube. When the light photon hits the photocathode, it produces an electron of low energy (0.1-1 eV) through photoelectric interaction. This photoelectron is then accelerated toward a dynode by the application of a voltage (between 50 and 100 V) to that dynode. As a result of this acceleration, the electron acquires sufficient kinetic energy (50-100 eV) to produce a number of secondary electrons when it collides with the dynode Preamplifier • The electric pulses arriving at the anode of a PM tube are small (microampere) and therefore require amplification to several volts before they can be further analyzed or processed. The output of a PM tube, however, cannot be directly fed into an amplifier because of the wide difference in the output impedance (an important electronic parameter) of a PM tube and the input impedance of an amplifier that results in signal distortion and attenuation. A preamplifier is a device that solves the problem of impedance mismatch. It is located next to the PM tube because long connecting cables at this stage also attenuate the signal. Output of the preamplifier, however, can be sent many feet away from the detector assembly without attenuation. Linear Amplifier • A linear amplifier amplifies the incoming pulses in a direct proportion. The ratio between the amplitudes of the outgoing and incoming pulses is known as amplifier gain. The amplification can be changed by adjusting the gain control G specifically provided for this purpose. Pulse-Height Analyzer or Single- Channel Analyzer • A pulse-height analyzer PHA or single-channel analyzer SCA is an electronic device that accepts the pulses whose voltage (height) lies between a preselected range and rejects the pulses whose voltage lies outside this range. An SCA is provided with two controls known as lower level and window that determine the range of the pulses to be selected. For example, if pulses with voltages varying from 1 to 10 are being produced by a linear amplifier in response to an x- or γ-ray, to select pulses whose voltage lies between 5 and 6, it is necessary to set the lower level of SCA at 5 V and the window at 1 V. The SCA will produce an output pulse only when the input pulses lie in this range Multichannel Analyzer • A PHA or SCA selects only one range of pulse- height distribution. In a multichannel analyzer, there are many PHAs (1000 or more) that can simultaneously separate the pulses into multiple ranges; therefore, the whole spectrum of pulse-height distribution can be measured at one time. Scaler and Timer • These devices comprise an electronic counter used for counting the pulses coming from an amplifier or SCA either for a specific time interval or until a predetermined number of counts has been collected. In either case, both the number of counts and the time during which they are collected are obtained. Rate Meter • This is a device that, instead of giving the number of counts and the time period in which the counts were collected, yields directly the count rate (counts/min). With the ubiquitous use of computers, rate meters are now rarely used in nuclear medicine instruments.
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