Proportional detectors. gamma rays

Alpha-particle detector Beta-particle detector Gamma-ray detector proportional counters silicon (Si) diode with spectrometer proportional counters Geiger-Muller counters liquid scintillation (LS) counters thallium-activated sodium iodide (Nal(Tl) detector with spectrometer germanium (Ge) detector with spectrometer... 

The properties of a scintillation material required for good detectors are transparency, availability in large size, and large light output proportional to gamma ray energy. Relatively few materials have good properties for detectors. Thallium activated Nal and Csl crystals are commonly used, as well as a wide variety of plastics. Both Nal and Csl require an activator such as Thallium for proper operation. Nal is the dominant... 

We would expect a more efficient, larger detector to have a larger background count rate. The proportion of gamma-rays striking the detector that end up on the Compton continuum is related to the peak-to-Compton ratio (P/C). Figure 13.1 shows how this varies with relative efficiency. The data in this figure were derived... 

Gas-filled detectors are used, for the most part, to measure alpha and beta particles, neutrons, and gamma rays. The detectors operate in the ionization, proportional, and G-M regions with an arrangement most sensitive to the type of radiation being measured. Neutron detectors utilize ionization chambers or proportional counters of appropriate design. Compensated ion chambers, BF3 counters, fission counters, and proton recoil counters are examples of neutron detectors. 

Figure 10. Detector efficiency as a function of gamma-ray energy for an 8-cm.3 (Ge(Li)) detector (samples in aluminum cans). The efficiency of the 18-cm.s detector increases in proportion to the surface area...

Gas-filled detectors are used for X-rays or low energy gamma rays. These include ionization chambers, proportional counters and Geiger-Miiller counters. Scintillation detectors are used in conjunction with a photomultiplier tube to convert the scintillation light pulse into an electric pulse. Solid crystal scintillators such as Csl or Nal are commonly used, as well as plastics and various liquids. 

Semiconductor detectors, made from single crystals of very pure germanium or silicon, are the highest performance detector type. The superior resolution of these detectors has revolutionized data-gathering for X-ray and gamma-ray measurements. The comparison of the pulse resolving ability of the three types of X-ray detectors scintillator, gas proportional and Si(Li) is shown in Fig. 5.18. 

Fe beta particles are counted with a proportional detector or its gamma rays are analyzed with a Ge detector and spectrometer. The sample is then measured for Fe content with a thin Ge detector and spectrometer or xenon-filled X-ray proportional detector with a thin (e.g., 140 mg cm ) beryllium absorber. The Fe count rate is adjusted for background, the Fe contribution, self-absorption in the plated sample, and the chemical yield, and converted to the disintegration rate. The activity of both radioisotopes is corrected for radioactive decay from the sampling date. 

The reverse— gamma-ray background in beta-particle detectors—also occurs due to electron-producing interactions of gamma rays in the walls of the gas-filled proportional and G-M detectors. The magnitude of the detection efficiency for energetic gamma rays, i.e., above 0.1 MeV, typically is about 1% of that for beta particles (Knoll 1989). 

An important point in measurements is that conversion electrons are counted with the usual end-window beta-particle detectors and not with gamma-ray spectrometers. The decay fraction counted under these conditions with a beta particle detector is 1.094. The K Auger electrons may also be counted with beta particles and conversion electrons in an internal proportional counter. In a Ge detector and spectrometer, in addition to the gamma ray, the various K X rays are counted. 

Gamma and x back-scattering techniques are based upon the Compton photon back-scatter effect. Collimated low-energy gamma rays or X-rays are emitted and beamed at the inspected material. The rays become scattered back toward the detector in direct proportion to the mass of the material in front of the probe. A. scintillation cry.stal detector is used to convert the back-scattered protons into an electrical signal that can be related to material thickness, provided that the material density is constant. Back-scattered. x-rays can be detected by an ariay of scintillation detectors, and the position of the detector relative to the sample relates to different depths in that sample [84]. 

Most of the radioisotopes used as isotopic labels in activation analysis decay with beta (positron and negatron) radiations and/or gamma rays. By convention, beta-emitting radionuclides are usually measured by gas-filled or gas-flow proportional counters or Geiger counters. Sometimes, liquid scintillation counters are used to complete a beta-ray measurement. The more conventional method for gamma-ray measurements involves the use of a gamma-ray spectrometer equipped with either a scintillation or solid-state detector. Stevenson (918) discusses the characteristics of radioactive decay and gives details on the methods and instruments used to detect emitted radiations. 

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