Radionuclides detectors samples

Radioactive contamination on the detector, sample holder, and other components Radionuclides in the counting room walls, floor, and air... 

Compute the amount of a radionuclide necessary to perform an experiment with a sample count rate of 1000 cpm, a detector efficiency of 33%, a sample aliquot for counting consisting of 10% of the total isolated sample and where the percent incorporation of the nuclide into the total isolated sample was 0.5%. 

The ability to measure an added radioactivity in food depends on the type of radiation (jfl-ray, positron, 7-ray), the energy of the radiation, the concentration of the radioactivity, the chemical characteristics of the radioactive element, and the chemical characteristics of the food. In other words, the measurement capability depends on the detector sensitivity for the specific radionuclides involved and on the background radioactivity in the chemically separated sample to be counted. 

An example of a SI separation system is shown in Figure 9.2.44 The column is followed by a diverter valve so that the column effluent can be sent to waste during sample load and wash steps. This approach prevents matrix components and high-activity interferences from fouling the detector. During the elution of the radionuclide... 

Errors in timing. For the determination of elements yielding short half-life indicator radionuclides (such as in the determination of oxygen via 7.3 sec16N), accurate timing is extremely important. For these cases electronic scaler timers are to be preferred over electromechanical types of timers. Errors due to variable detector dead-time must also be considered when the gross activities of the sample and the standard differ appreciably and the indicator radionuclide is short-lived. 

The method selected to prepare a radionuclide for counting depends on the skills and preferences of the analyst, available detectors, and conditions associated with the radioactive analyte, accompanying radionuclides, and the sample matrix. All aspects have to be considered to obtain a measurement that meets reliability and sensitivity specifications for radionuclide identification and detection. In some cases, the analyst has many options in others, choice is restricted by circumstances such as small samples, low radionuclide concentration, half-life considerations, or unavailability of certain detector types. 

The detection systems first must be calibrated for counting efficiency to permit conversion of the sample count rate to the disintegration rate. These systems are monitored periodically for their stability and performance by measuring the count rates of reliable radionuclide sources and the radiation background. Records are maintained for each instrument to comply with quality assurance specifications. Graphs of count rates recorded at frequent intervals for periods of months or years provide a visual record of detector and background stability and indicate deviations from the norm. 

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... 

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. 

Count the sample for a sufficient time period and frequency to collect reliable data for determining the gamma-ray energy, the count rate for characteristic gamma-ray energy peaks, and the rate of decay of that peak. Also count the empty container with the Ge detector and spectrometer to determine whether any radionuclides remain sorbed on container walls and to estimate the fractional loss by sorption for each radionuclide of interest. Note that the counting efficiency for radionuclides retained on the container must be estimated. 

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