Radionuclides measuring instruments

Quality assurance of radiopharmaceutical preparation and use is obviously a very important topic because of its direct impact on patient diagnosis, treatment and health (see, e.g. Abreu 1996). Reference materials play only a small - but nevertheless important -role in this process, mainly in the area of calibration of radioactivity-measuring instruments. The materials of interest are all pure chemical containing calibrated activities of selected radionuclides used commonly in nuclear medicine (e.g. Co, Ga, I,... 

At present, there are no widely distributed certified reference materials containing all of the radionuclides in the uranium and thorium decay series. Such reference materials are needed to calibrate instruments that make radionuclide measurements and to compare analytical results from different laboratories. The most critical need is for reference materials in the 235U decay series 231Pa, 227Ac, and 223Ra. 

All of the radionuclide measurements were performed by direct instrumental analysis of the air filters. Most of the measurements were... 

The certified values of the produced standards are linked to the national standards by the Primary Laboratory of the Bureau National de Metrologie . This traceability is achieved through the utilisation of measuring instruments which are periodically calibrated standards provided by the Primary Laboratory. At international level, trace-ability is established for a certain number of radionuclides, with national laboratories with which the Primary Laboratory performs direct comparisons or indirect comparisons through the International Reference System of the Bureau International des Poids et Mesures . An official calibration certificate containing all necessary information is provided with each standard. 

Estimation of the detection limit of the measurement instrument in the time period available for counting, linked with the required detectable concentration and the expected concentration of the radionuclide to be determined, guides selection of sample size for the analysis. Calculation of the minimum detectable activity is discussed in Section 10.4.2. Additional documents of interest are Altshuler and Pasternack (1963), Pasternack and Harley (1971), and Currie (1968). The terms minimum detectable concentration and lower limit of detection also have widespread use. A document that addresses these and other topics pertinent to radiation monitoring and measurement was developed by a committee of the Health Physics Society (EPA 1980a). 

The radioanalytical chemist who is responsible for such monitoring selects or designs procedures that meet the client s specifications of sample type, list of radionuclides, measurement reliability and sensitivity, and response time. The analyst also considers the limits imposed by prescribed sample size, solution volume appropriate for chemical separation, and radiation detection instruments at hand. Potentially applicable procedures are selected for these criteria by literature review and then evaluated in a methods development and testing process. A chemical separation procedure can be devised either by selecting the most applicable published method and introducing any needed modifications or by combining pertinent separation steps. 

The radiation detection systems employed in radioanalytical chemistry laboratories have changed considerably over the past sixty years, with significant improvement realized since the early 1980s. Advancements in the areas of material science, electronics, and computer technology have contributed to the development of more sensitive, reliable, and user-friendly laboratory instruments. The four primary radiation measurement systems considered to be necessary for the modern radionuclide measurement laboratory are gas-flow proportional counters, liquid scintillation (LS) counters. Si alpha-particle spectrometer systems, and Ge gamma-ray spectrometer systems. These four systems are the tools used to identify and measure most forms of nuclear radiation. 

Finally, the measuring instrument with which the activity is detected must be selected with a view to the type of rays and the radiation energy emitted by the radionuclides. If the radionuclides to be detected are not known, this selection process is restrictive from the outset, unless different measurement systems are used consecutively for the same sample. 

Depending on the type of radiation emitted, at least three different types of measuring instruments will be required in order to be able to carry out total activity measurements for all radionuclides or radionuclide groups ... 

Operational intervention levels are not restricted to power-reactor emergencies. They can be calculated for all other types of activities based on the radionuclides present, the exposure pathways and the types of measurement instruments available. 

Detailed records of the measurements of radiation dose rates (including locations, times and instruments) and related information on the calibration of instruments should therefore be maintained. Similarly, detailed information about measurements of radionuclides in airborne and water-borne discharges should also be maintained. This includes information on discharge points, sampling periods, radioanalytical procedure(s) and instruments used and related data on instrument calibration. The details of measurements of the discharge flow rates that are correlated with the radionuclide measurements should also be retained, together with appropriate calibration data. 

One of the limitations of the portable field survey instruments in the measurement of americium is that their quantitative accuracy depends on how well the lateral and vertical distribution of americium in the soil compares with the calibration parameters used. These methods can provide a rapid assessment of americium levels on or below surfaces in a particular environment however, laboratory-based analyses of samples procured from these environmental surfaces must be performed in order to ensure accurate quantification of americium (and other radionuclides). This is due, in part, to the strong self absorption of the 59.5 keV gamma-ray by environmental media, such as soil. Consequently, the uncertainty in the depth distribution of americium and the density of the environmental media may contribute to a >30% error in the field survey measurements. Currently, refinements in calibration strategies are being developed to improve both the precision and accuracy (10%) of gamma-ray spectroscopy measurements of americium within contaminated soils (Fong and Alvarez 1997). 

In terms of atomic spectrometry, NAA is a method combining excitation by nuclear reaction with delayed de-excitation of the radioactive atoms produced by emission of ionising radiation (fi, y, X-ray). Measurement of delayed particles or radiations from the decay of a radioactive product of a neutron-induced nuclear reaction is known as simple or delayed-gamma NAA, and may be purely instrumental (INAA). The y-ray energies are characteristic of specific indicator radionuclides, and their intensities are proportional to the amounts of the various target nuclides in the sample. NAA can thus... 

Instrumental layouts and developments in AMS are reviewed by Kutschera.195 Today AMS is the most powerful, sensitive and selective mass spectrometric technique for measuring long-lived radionuclides at the level of natural isotopic abundances (10-16 to 10-12). Accelerator mass spectrometry (AMS) allows uranium isotope ratio measurements with an abundance sensitivity for 236U in the range of l(rlo-10 l2.l98J"... 

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. 

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