Efficiency scintillation detectors

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. 

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. 

Altria et al. reported the CE separation and detection of radiopharmaceuticals containing mTc, a 7 emitter with a 6-hour half-life (2, see also 10). Their design involved passing a capillary tube (= 2 cm long) through a solid block of scintillator material and detecting the light emitted as technetium-labeled sample zones traversed the detection volume. Unfortunately, detection limits and detector efficiency were not reported. 

Reagents. Aqueous ethanol solutions of the triethylammonium salts of adenosine-5 -[a-S2P]triphosphate (a- PATP), adenosine-5 -[7-S2P]triphosphate (7-S2P-ATP), thymidine-5 -[a-S2P]triphosphate (a- p-TTP), cytidine-5 -[q-S2P]triphosphate (a-S2P-CTP), and guanosine-5 -[a-S2P]triphosphate (a-S2P-GTP) were purchased from Amersham (Arlington Heights, IL). Radioactive sample concentrations reported for detector efficiency determination were adjusted from the manufacturer s specifications after subjecting several diluted aliquots of the stock solution to liquid scintillation counting. 

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. 

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. 

In both scintillator and gas detectors, the absorption of radiation causes excitation and ionization however with the scintillation process, the absorbed energy produces a flash of light, rather than a pulse of current. The principal types of scintillation detectors found in the clinical chemistry laboratory are the sodium iodide crystal scintillation detector and the organic liquid scintillation detector. Because of the crystal detector s relative ease of operation and economy of sample preparation, most clinical laboratory procedures have been developed to measure nucfides, such as which can be counted efficiently in a crystal detector. A liquid scintillation detector is used to measure pure (3-emitters, such as tritium or C. 

Liquid Scintillation Detector, This detector measures radioactivity by recording scintillations occurring within a transparent vial that contains the unknown sample and liquid scintillator. Because the radionuclide is intimately mixed with, or actually dissolved in, the fiquid scintillator, the technique is ideal for the pure -emitters, such as H, and Typical efficiencies for liquid scintillation counting in the absence of significant quenching are 60% for tritium and 90% for... 

Other Components and Techniques. Other components of a liquid scintillator detector include (1) electronics, (2) a photomultiplier tube, (3) a preamplifier, and (4) a pulse-height analyzer. Description of these components and discussion of relevant topics such as (1) efficiency of scintillation counting, (2) quenching, (3) counting statistics, (4) assay optimization, and (5) radiation safety can be found in an earlier edition of this textbook. ... 

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. 

Note that the sensitivity of a PET scanner increases as the square of the detector efficiency, which depends on the scintillation decay time and stopping power of the detector. This is why LSO, LYSO and GSO detectors are preferred to Nal(Tl) or BGO detectors (see Table 2.1). In 2D acquisitions, system sensitivity is compromised because of the use of septa between detector rings, whereas these septa are retracted or absent in 3D acquisition, and hence the sensitivity is increased by a factor of 4-8. However, in 3D mode, random and scatter coincidences increase significantly, the scatter fraction being 30—40% compared to 15-20% in 2D mode. The overall sensitivities of PET scanners for a small-volume source of activity are about 0.2-0.5% for 2D acquisition and about 2-10% for 3D acquisition, compared to 0.01-0.03% for SPECT studies (Cherry et al, 2003). The greater sensitivity of the PET scanner results from the absence of collimators in data acquisition. 

For routine determination of Tc in solutions a fast and convenient method was reported that uses the measurement of bremsstrahlung emitted by conversion of " Tc particles at the wall of a vial containing the Tc compound in a suitable solvent. A sample of 0.1-0.5 mg Tc in 5 ml solution was transferred to a vial and measured within a well-type y-ray Nal(TI) scintillation detector. The background was about 1000 counts/min, the counting efficiency 39.4 counts/min for 1 pg of Tc. The elemental composition of the sample and the solution density did not affect the determination [33],... 

Many factors affect a gamma radiation measurement (Knoll, 1979 Tsoulfanidis, 1983). The most important factors relevant to the CAPTF are the characteristics of the radioactive source, the interaction of gamma rays with matter, the position of the source relative to that of the detector, the efficiency of the scintillation detector, and the dead-time behavior of the whole measurement system. These factors are separately discussed in the following subsections. 

Radioactive tracer techniques have long been used to study particle motion in solids fluidization systems. The advantage of this technique is that the flow field is not disturbed by the measurement facility and, therefore, the measurement of the motion of the tracers represents the actual movement of particles in the system. The tracer particles are usually made of gamma-emitting radioisotopes, and their gamma radiation is measured directly by scintillation detectors. Factors that affect gamma radiation measurement were identified as the characteristics of the radiation source, interactions of gamma rays with matter, the tracer s position relative to the detector, detector efficiency, and dead time of the measurement system. 

A plastic scintillation detector was to be calibrated for absolute measurements of /3-radiation. For this purpose a 2.13 X 10 M TlCl3 solution was available with a specific activity of 13.93 iCi ml T1 emits iS-particles with 0.77 MeV. Of this solution 0.1 ml is evaporated over an area of exactly 0.1 cm on a platinum foil. The sample is counted in an evacuated vessel at a distaiKe of 15.3 cm from the detector, which has a sensitive area of 1.72 cm. The detector registers 2052 cpm with a background of 6 cpm. What is (a) the surface weight of the sample, (b) the backscattering factor, and (c) the detector efficiency for the particular /3 s ... 

---