Counting efficiency relative

The lithium drifted detectors provided greater resolution but at a higher cost and lesser counting efficiency relative to the Nal(Tl) detectors. Additionally, the Ge(Li) detector must be kept at liquid nitrogen temperature at all times to prevent lithium ions from drifting in the germanium to destroy the detector. 

Picer et al. [49] described a method for measuring the radioactivity of labelled DDT contaminated sediments in relatively large volumes of water, using a liquid scintillation spectrometer. Various marine sediments, limestone and quartz in sea water were investigated. External standard ratios and counting efficiencies of the systems investigated were obtained, as was the relation of efficiency factor to external standard ratios for each system studied. 

Inaccurate calibration of relative counting efficiencies. In the use of a dual sample transfer system where both sample and standard are counted simultaneously with different counters it is important that the relative efficiencies of the two counting systems be accurately determined and that these efficiencies do not vary with time. These errors may be partly compensated for by irradiating the same sample and standard several times and alternately reversing the sample and standard counting positions. 

The counting efficiency for the shown system approaches 52% for radionuclides with high maximum beta-particle energies (see Fig. 2A.2). This value exceeds the 39.6% based on the geometry of a 2.2-cm-dia. sample on a filter relative to the detector window. The counting efficiency exceeds the... 

Calculate the ratios of the counting efficiencies for the various energies at each sample volume relative to those for the 480-mL volume in the cylindrical container to observe the effect of sample volume on efficiency. 

This experiment examines the count rate as a function of sample thickness. All other variables are held constant (except for a small change in source-detector distance). As the sample becomes thicker, more of the beta particles are absorbed in the sample itself. This is called self-absorption, and is shown in Figure 4.1. In thin samples, self-absorption is relatively small or negligible, but in thick samples it is measurable and must be considered when calculating the counting efficiency. 

Determine counting efficiency of the proportional detector in Step 5 for three 3,000-s periods to measure alpha particles and beta particles. Record in Data Table 7.2. Also perform overnight count (50,000 s) for alpha-particle spectral analysis of the planchet to identify the uranium isotopes and any other radionuclides and to determine their relative amounts from their alpha-particle energy spectra and record results in Data Table 7.2. Count alpha- and beta-particle background in proportional counter and alpha-particle spectral background in spectrometer for at least the same periods. 

In real life , humans and animals can be exposed to some toxicants both pre- and postnatally. Many organic xenobiotics have the potential to bioaccumulate within exposed individuals, possibly affecting future generations by way of genetic and epigenetic effects. However, reproductive endpoints, such as conception rates and sperm counts, are relatively insensitive, and subtle, toxicant-induced changes in reproductive efficiency can be overlooked or missed (Evans, 2007). 

Unfortunately the counting efficiency of the system was relatively poor, 0.2% for tritium and 17% for carbon. However, the advantage of this method is that due to the cell being packed with beads, it would have little flow resistance and limited peak dispersion and thus if used in conjunction with suitable low dispersion connecting tubes, it could be used with relatively high efficiency columns. As a consequence, many modem commercial radioactivity detectors are designed on the same principle, but with more efficient scintillators and more efficiently designed sensors. 

The main advantage of liquid scintillation counting is the relatively high counting efficiency, which can amount to about 90 to 100%. 

As indicated in section 7.1, measurements of relative and absolute activities are to be distinguished. For determination of relative activities, the overall counting efficiency Tj in eq. (7.3) must be constant, but it need not be known, whereas ri must be known exactly for determination of absolute activities. The overall counting efficiency can either be calculated or be determined by calibration. In both cases all factors in eq. (7.3) must be considered. 

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. 

The assay for Sr and Sr together will require that standards be used to set up counting efficiencies in two regions of interest. Backgrounds for these regions of interest would also be required and the contribution of the Sr (1.49 MeV beta) to the Sr (0.546 MeV beta) count will be required. With this information, a single count of a mixture would yield information on the amount of both isotopes, the accuracy depending on the total count and the relative quantities of the two nuclides. 

End-window proportional counters with thin windows that separate sample from detector have the alpha-particle counting efficiency shown in Table 8.1 for thin samples on planchets. The lesser counter efficiency relative to the internal detector is due to less than 27t geometry and attenuation in sample, air space, and window. 

Once accepted, the instiument must be calibrated for counting efficiency and, if a spectrometer, for energy response. For the former, the radionuclide standards must be prepared by the national calibration facility— NIST in the United States—or by another facility in a manner traceable to NIST (ANSI/IEEE 1995). Standards used for calibration may be supplied as a point source, an extended source of the same geometric configuration as the samples that will be counted, or as a sealed solution which is converted by the user to the desired form (see Section 8.3). A certificate that contains all appropriate information described in Section 11.2.6 must accompany all standards. The typical relative standard uncertainty of radionuclide standards is in the range of 1-2%. 

Instead of the above-cited control chart to test the hypothesis that the measured values are randomly distributed, a tolerance chart may be established to compare periodic measurements with established acceptance limits (see ANSI N42.2, Appendix A). In this system, an acceptable relative standard deviation—say 2% for the standard source count and 10% for background count— is selected and horizontal lines that represent these values are drawn as upper and lower acceptance limits. These limits should be wider than the statistically based limits if they were narrower, they would be exceeded frequently. They need not be symmetrical on the positive and negative side. Their purpose is to keep the counting efficiency and background count rate within limits that give acceptable results even if the instrument and its output are subject to nonrandom fluctuations. 

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