Scintillation detector resolution

The excellent, high-resolution y- and X-ray spectra which can be obtained from semiconductor detectors make the detectors very important in modern instruments. A typical spectrum is shown in Figure 10.11(b) which may be compared with the much broader peaks from a scintillation detector (Figure 10.11(a)). The spectra are not immune from the problem of Compton scattering (p. 461) but a good quality modem detector will have a photopeak to Compton peak ratio of 50 1 or better. Computer-aided spectrum analysis also serves to reduce the interference from the Compton effect. 

It is also interesting to note that the limitations of most neutron systems lie not in the neutron source(s) but rather in the (gamma ray) detectors and subsequent data acquisition system. The obvious need is for development of detectors with higher rate capability and improved energy resolution. All modern digital data processing techniques have allowed significant improvement in the performance of conventional scintillation detectors with respect to rate and pileup rejection. 

An obvious extension to the CSCT technique as described so far is to replace the scintillator detectors with room temperature semiconductors, thus markedly improving the energy and thus the momentum transfer resolution [41],... 

These have been used for detecting gamma rays with high positional resolution in 2D-ACAR experiments (see 7.2) [20], although—principally for reasons of availability, lower complexity and more straightforward maintenance—scintillation detectors are more common. Very briefly, a spark chamber is modified to detect gamma radiation with positional... 

As already mentioned, area (or position sensitive) detectors were first developed for neutron crystallography, since neutrons are scarce and expensive and it is important both to shorten the experiment and to use aU neutrons scattered by a sample in various directions, rather than waste all but one reflection at a time. Besides 2D scintillation detectors, there are banana ID multiwire detector and 2D detectors with two mutually perpendicular sets of parallel wires. The latter design is usefiil for time-of-ffight experiments because of ideal time resolution once an ioiuzation discharge induces current in one or more wires, we instantly know both the place and time of its arrival. 

For the measurement of y emitters in solids Nal(Tl) scintillation detectors or Ge detectors are most suitable, depending upon whether high counting efficiency or high energy resolution is required. For comparison, the spectra of Co taken with a Nal(Tl) scintillation detector and with a Ge(Li) detector are plotted in Fig. 7.16. 

Scintillation detectors with Nal(Tl) crystals may also be used for y spectrometry. Because Nal(Tl) crystals can be made in larger size than Ge crystals and because the atomic number of I is larger than that of Ge, the internal counting efficiency of Nal(Tl) detectors for y rays is higher than that of Ge crystals, as already discussed in section 7.5 (Fig. 7.16). On the other hand, the energy resolution is appreciably lower (5 to 7% for y energies of the order of 100 keV). Scintillation detectors are operated in a way similar to that used with Ge detectors, but without cooling. 

The patients are positioned inside a ring of about 50 to 100 scintillation detectors, and the ring is rotated and moved in a programmed maimer. As in SPET, the results are evaluated by computer software to give a three-dimensional picture of the distribution of the radionuclide in the organ of interest. The resolution is also of the order of 1 mm. 

Many types of detectors, such as Geiger-Miiller counters, proportional counters and scintillation detectors, are used for charged particle detection. The selection is made on the basis of resolution and range of particle in the gas or scintillator. In some cases, the particles are not completely stopped within the detector for an energy measurement, but deposit only a portion of their energy. This is related to the relative ionization of the particle and can be used to identify different kinds of particles. 

These detectors are made of semiconducting materials. In these detectors, solid-state electrodes are made from Li doped with Si or Ge. The resolution is approximately 1-2 keV for 1 MeV y-Rays and sometimes provides a greater than 10-fold improvement over Nal (Tl) scintillation detectors, described below. These are commercially available and more often used in research-grade instruments. 

In gas-filled as well as scintillation detectors, the observed count rate is typically less than the actual decay rate of the radionuclide. The efficiency of detection may differ from particle to particle under identical conditions using the same type of detector. The factors that affect the efficiency of detection are operating voltage, resolving time, geometry of the instrument used in relation to the position of the sample with respect to the detector, scaler, energy resolution, absorption by cells, and sometimes constituents of the sample itself. 

Detector size One factor that greatly affects the spatial resolution is the intrinsic resolution of the scintillation detectors used in the PET scanner. For multidetector PET scanners, the intrinsic resolution (Rf) is related to the detector size d. R, is normally given by d/2 on the scanner axis at midposition between the two detectors and by d at the face of either detector. Thus it is best at the center of the FOV and deteriorates toward the edge of the FOV. For a 6-mm detector, the Ri value is mm at the center of the FOV and 6mm toward the edge of the FOV. For continuous single detectors, however, the intrinsic resolution depends on the number of photons detected, not on the size of the detector, and is determined by the full width at half maximum of the photopeak. 

For measuring Tc and other gamma emitters, a Nal(Tl) scintillation detector is used. The resolution of the scanner is dependent on the width of the slit-collimator, the distance between chromatogram and detector, and the window settings on the scaler. Artificial results may be obtained if the peaks are not symmetrical and comparable. 

Figure 9.6 shows a typical NAA spectrum obtained with a multichannel analyzer equipped with scintillation (upper curve) or semiconductor (lower curve) detectors. Each peak can be ascribed to a certain y-energy, which in most cases identifies the nuclide. A number of nuclides can be identified simultaneously with semiconductor detectors, but with Nal(TI) scintillation detectors the poor resolution limits simultaneous multi-element analysis. 

NaI Tl) scintillation-detector. The energy resolution of the detector is measured by the width (W) of the full-energy peak (FEP) at one-half the maximum height H/2) of the FEP. 

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