Scintillation detectors detection efficiency

Detectors. Two general types of detectors are used in x-ray medical imaging scintillation and gas ionisation. Scintillation detectors are used for both conventional projection and computerized tomographic imaging. Ionization detectors have been used only in CT appHcations. All detectors used in detection of x-ray radiation must be linear and have a maximum efficiency at the wavelength of the x-ray photon to be detected. 

Radioisotope detection of P, 14C, and Tc was reported by Kaniansky et al. (7,8) for isotachophoresis. In their work, isotachophoretic separations were performed using fluorinated ethylene-propylene copolymer capillary tubing (300 pm internal diameter) and either a Geiger-Mueller tube or a plastic scintillator/photomultiplier tube combination to detect emitted fi particles. One of their reported detection schemes involved passing the radiolabeled sample components directly through a plastic scintillator. Detector efficiency for 14C-labeled molecules was reported to be 13-15%, and a minimum detection limit of 0.44 nCi was reported for a 212 nL cell volume. 

Problem 3.4 Positrons in a 5mm-diameter beam are detected in two different ways (a) directly by a channeltron (lOmm-diameter mouth) placed at the end of an evacuated beam line, and (b) indirectly via annihilation radiation from the channeltron by a 75mm-diameter, 75mm-long Nal(Tl) crystal on a PM detector mounted outside the vacuum system directly to one side of the channeltron. The distance from the channeltron to the front face of the scintillator is 100mm, and the intrinsic detection efficiency of the... 

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. 

He and (as BF3) are used in gas tubes and Li is used in scintillator detectors. For vibrational spectroscopy gas tubes are most commonly used. At research facilities, the use of BF3 is disfavoured on grounds of low detection efficiency and safety. 

A disadvantage of the coincidence scintillation cameras is that they have low sensitivity due to low detection efficiency of Nal(Tl) crystal for 511-keV photons, which results in a longer acquisition time. To improve the sensitivity, thicker detectors of sizes 1.6-2.5 cm have been used in some cameras, but even then, coincidence photopeak efficiency is only 3-4%. This increase in crystal thickness, however, compromises the spatial resolution of the system in SPECT mode. Fast electronics and pulse shaping are implemented in modern systems to improve the sensitivity. Also, there is a significant camera dead time and pulse pileups due to relatively increased single count rates in the absence of a collimator in PET mode. Low coincidence count rates due to low... 

Modern PET scanners can have 10,000-35,000 detectors arranged in blocks and coupled to several hundred PM tubes. Because of the variations in the gain of PM tubes, location of the detector in the block, and the physical variation of the detector, the detection efficiency of a detector pair varies from pair to pair, resulting in nonuniformity of the raw data. This effect in dedicated PET systems is similar to that encountered in conventional scintillation cameras used for SPECT and PET studies. The method of correction for this effect is termed the normalization. Normalization of the acquired data is accomplished by exposing uniformly all detector pairs to a 511-keV photon source (e.g., 68Ge source), without a subject in the field of view. Data are collected for all detector pairs in both 2D and 3D modes, and normalization... 

The sensitivity of a PET scanner is defined as the number of counts per unit time detected by the device for each unit of activity present in a source. It is normally expressed in counts per second per microcurie (or megabecquerel) (cps/pCi or cps/kBq). Sensitivity depends on the geometric efficiency, detection efficiency, PHA window settings, and the dead time of the system. The detection efficiency of a detector depends on the scintillation decay time, density, atomic number, and thickness of the detector material that have been discussed in Chap. 2. Also, the effect of PHA window setting on detection efficiency has been discussed in Chap. 2. The effect of the dead time on detection efficiency has been described in Chap. 3. In the section below, only the effects of geometric efficiency and other related factors will be discussed. 

A flow-through scintillation detector equipped with a lithium glass solid scintillator flow cell is used to detect the eluted "Tc. The glass scintillator enables an absolute detection efficiency of 55% and is stable in the 8 mol/l nitric acid medium for pertechnetate elution. 

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