Radioactivity counting efficiency

Tajima and co-workers [108] determined the surface excess of sodium dode-cyl sulfate by means of the radioactivity method, using tritiated surfactant of specific activity 9.16 Ci/mol. The area of solution exposed to the detector was 37.50 cm. In a particular experiment, it was found that with 1.0 x 10" Af surfactant the surface count rate was 17.0 x 10 counts per minute. Separate calibration showed that of this count was 14.5 X 10 came from underlying solution, the rest being surface excess. It was also determined that the counting efficiency for surface material was 1.1%. Calculate F for this solution. 

Assay of Radioactive Compounds. The radioactive samples were counted on steel planchets in a Nuclear Chicago Model D-47 low-background gas-flow counting chamber with an absolute counting efficiency (estimated by comparison with a standard) of about 20%. 

The chemical and radiochemical purity of the labeled compound must be ascertained prior to use. In practice a value of 95% or greater is usually acceptable. The desired specific activity of the administered radioactive compound depends on the dose to be used as well as the species studied. Doses of 14C on the order of 5 pCi/kg for the dog and 20 pCi/kg for the rat have been found adequate in most studies, while doses of 3H are usually two to three times higher owing to lower counting efficiency of this isotope. 

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. 

A lot of ready-to-use scintillation cocktails are available. Besides counting efficiency, chemical, biologic, and environmental hazards should be reflected when choosing a cocktail. Since vials and their contents are radioactive waste, number and volume of vials should be as small as possible. 

The specific radioactivity of the labelled pectic polysaccharides thus obtained was about 8000 cpm/mg of polysaccharides, counting efficiency of 3H being 30%. Of the total radioactivity incorporated, almost 50% was localized in the galacturonic acid... 

Outputs from the LSC must be corrected for background radiation and counting efficiency. The parts-per-million (ppm) equivalent to the radioactive chemical in a biological, soil or water sample can then be determined by accounting for aliquot size, specific activity, and other correction factors specific for the sample. Partitioning characteristics and chromatographic data can be treated in a like manner. Although many... 

The counting efficiency (e) of the proportional detector is calculated as the ratio of the net count rate, in s, to the activity (A), in Bq, of this standard radionuclide solution. The net count rate is the standard s gross count rate (RG) minus the detector s background count rate (RB). The reported disintegration rate (A) is the product of the radionuclide concentration, in Bq L 1, and the amount of counted sample, in L, adjusted for the radioactive decay of the radionuclide between standardization and measurement. Equation 2A.1 is the general form of this equation. 

In this experiment, the Ge detector with spectrometer is calibrated for its efficiency, s, with a standard that emits a set of gamma rays at energies that span the range of interest, usually from a few keV to 1.5 MeV. The counting efficiency is calculated from the ratio of the net count rate to the reported disintegration rate at each full-energy peak in the spectrum. A correction for radioactive decay is needed. 

Essential to reporting amounts of radioactivity in samples is the ability of the radiochemist to convert observed quantities of radioactivity (e.g., count per second) to an absolute scale (e.g., disintegration per second). When the sample radioactivity is measured with a radiation counter, the factor that converts the observed count rate to the absolute scale is the counting efficiency. The counting efficiency is defined as the ratio of the net count rate to the disintegration rate. 

Compare the activity reported for the tracer solution with the activity obtained with the proportional counter and the alpha-particle spectrometer based on their respective counting efficiency (s) values, adjusted for sample volume and radioactive decay. Discuss whether the differences in activity are significant and decide which values are more reliable and should be associated with the tracer solution for subsequent measurements of plutonium. 

Sr on the basis of the 90Y ingrowth. Count the two filters with 89Sr to calculate the counting efficiency and observe the radioactive decay of the radionuclide. Record the gross count rates and background count rate in Data Table 13.4. 

Step 6. Transfer with a pipette a predetermined quantity of sample to a counting container calibrated for counting efficiency. Close and label it. Swipe the outside of the container and count swipe to check for radioactive contamination. If no radioactive contamination is detected, proceed to Step 7. If contamination is found, thoroughly clean container with a detergent solution and ethanol until no contamination remains. To assure cleanliness, seal the counting container in a plastic bag. 

Meanwhile, prepare the HBSS/citrated plasma (see Subheading 2.2.4.) and five tubes (see Note 5) labeled T, S1 S2, S3 (each filled with 990 pL HBSS/BSA) and T10 (filled with 900 pL HBSS/BSA). These tubes will be used to assess the labeling efficiency (Sj-S3) and to convert radioactive counts/minute into number of inIn-labeled eosinophils (T10). 

OF THE RADIOACTIVITY CONCENTRATION Radioactivity measurements are carried out by the liquid scintillation counting procedure in -spectrometers using an external standard device which permitted the counting efficiency to be determined by the channel ratio method (explained for instance by Dyer (1980)). 

Radioactivity can be measured by a variety of methods producing values which relate to Bq (i.e. one disintegration per second). Due to inherent limitations of certain instruments which cannot measure each and every disintegration in a sample (i.e. efficiency <100%), often the measure of decay is recorded as radioactive counts per unit time-typically counts per minute (or cpm). In most cases these instmments will additionally correct for background... 

Radioactive measurements usually detect only a proportion of the decay events, and the number of observed counts per minute (cpm) is related to the disintegrations per minute (dpm) by the counting efficiency ... 

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. 

Three methods have evolved to ascertain the degree of efficiency loss both within the instrument and as a result of quenching. These techniques are termed (1) internal standardization, (2) channels ratio quench correction, and (3) external standard channels ratio quench correction. Determination of counting efficiency by internal standardization may be performed in two steps. The sample is first accurately counted followed by the addition of a precisely known quantity of radioactivity to the vial (50,000-80,000 dpm C or 100,000-150,000 cpm H). It is important for the amount of added radioactivity to be considerably larger than that originally present in the vial. The sample is then counted a second time. The first count is the sample cpm and the second count is the sample cpm + (efficiency)(standard dpm). That is. 

The activity (disintegration rate) A as defined in section 4.2 is a property of radioactive matter and can be measured by various devices which give a certain counting rate I, which depends on the activity A, the overall counting efficiency rj of the device and the background counting rate /q ... 

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