Radionuclide Standards

Am, 90Sr, 239Pu, 240Pu, 137Cs, 129I, or 14C, which also lack solution standards. NIST has an active program to address the development of all of these radionuclide standards except 14C. 

Application of the CN2003 software to evaluate the suitability of proxy radionuclides for LSC calibration in the absence of certified radionuclide standards is also of considerable benefit. Comparison of predicted efficiency curves for confirms... 

Frequent calibration of the instruments with the use of appropriate standards is required to make suitable allowances for decreases in the efficiency of the instruments. Such calibration standards are available from the National Institute of Standards and Technology (NIST). Other sources traceable to NIST standards through active program of participation in comparison measurements also provide such standards. The United States Pharmacopeia (USP) also provides nuclear decay data for new calibration standard. USP 24 lists t j2, energy of photons, and number of photons per disintegrations, for the following radionuclide standards ... 

The counting efficiency of these detectors is calibrated with radionuclide standards or Monte Carlo simulation (Briesmeister 1990). Typically, the alpha-particle detector has the same efficiency for thin samples at all commonly encountered... 

For the first three applications, a radionuclide- and mass-specific counting efficiency musf be selected. For the fourth application, a thin sample—below 2.5 mg/cm for alpha-particle counting—should be prepared so that efficiency values are similar af commonly encountered energies. For counting beta particles, the sample should not exceed 10 mg/cm. An intermediate-energy (e.g., 0.6-0.8 MeV Pmax) radionuclide standard provides reasonable efficiency estimates except that the activity of a radionuclide that emits only low-energy beta particles will be underestimated. 

An aqueous sample may be added to the cocktail directly, after minor prior processing, or at the end of a radiochemical separation procedure. Direct addition is the equivalent of gross activity counting discussed in Section 7.2.4 except that some spectral analysis may be possible. Alpha particles can be differentiated from beta particles by deposited energy, pulse shape, and decay time. Self-absorption is of no concern. Quenching and luminescence, discussed in Section 8.3.2, often occur. Identification by maximum beta-particle energy is approximate, and requires comparison to radionuclide standards. 

Prepare and count a radionuclide standard for each radionuclide and source-detector configuration. 

Prepare and count selected radionuclide standards to determine curves of counting efficiency v. energy for each source-detector configuration. 

In the first two calibration approaches cited above, a certified radionuclide standard is measured to obtain the practical counting efficiency of the detector. 

The radionuclide standard must be accompanied by a certificate (see Section 11.2.6) with detailed descriptions of its chemical, physical, and radiological characteristics and the uncertainty of the reported disintegration rate. The uncertainty of calibration depends on the reported uncertainty of the standard, compounded by the uncertainty due to source preparation and measurement. 

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%. 

Written instructions and procedures must be prepared to control and document the receipt of equipment, supplies, and services that affect the quality of measurement. Examples are reference materials (e.g., radionuclide standards), reagents, supplies (e.g., graduated cylinders, pipettes, planchets), and computer software and hardware. Controls should ensure that only correct items are accepted. If specifications are not met or the material is otherwise unsuitable, the QA officer should be notified and the material returned. 

Standard radioactive Material (SRM) solutions are radioactive materials with accurately known radionuclide content and radioactive decay rate or rate of particle or y-ray emission. They are used primarily to calibrate radiation detection instruments and to prepare QC samples that test analytical accuracy. The supplier prepares radionuclide standard solutions in flame-sealed glass ampoules. Other standard radionuclides are in the form of point sources (usually on thin backing) or as solids in configurations that represent actual samples. 

Activity calibration of SRMs must be performed by the National Institutes of Standards and Technology (NIST) or be traceable to NIST. If the radionuclide standard source is purchased from a supplier other than NIST, its accompanying certificate must confirm that the standard is traceable to NIST. Traceability requires reporting an unbroken chain of comparisons to stated references. NIST traceability is discussed at htq) //ts.nist.gov/traceability/ (December 2005). A terminal date for use of a specific standard is appropriate when the radionuclide decays to a small fraction of its original concentration, an initially minor interference has become major, or chemical decomposition occurs with time. 

The QA effort for radiation detection instruments is directed toward correct installation, accurate calibration, and stable operation. For correct installation, the instrument must be shown to function as designed by the manufacturer and specified by the purchaser. Accurate calibration depends on reliable radionuclide standards handled correctly. Stable operation is demonstrated by comparing control parameters such as the count rate for a stable radiation source and the detector radiation background measured at selected intervals. 

For some radionuclides, only the direct measurement is needed, based on calibration with a radionuclide standard and reference to the measured sample mass. For other radionuclides, the isotope ratio to its stable element is needed. An example of a more complex situation is measurement of the 14C/12C isotope ratio of an environmental or archeological sample in comparison to the modern atmospheric CO2 value. This value must be adjusted for anthropogenic 14C produced by atmospheric testing of nuclear weapons and by other nuclear operations, and also for changes in atmospheric CO2 with cosmic-ray flux fluctuations over time. For an element such as Pu, which has no stable isotope, the total quantity is measured by isotope dilution mass spectrometry in which the sample is traced (or spiked) with 242Pu or 244Pu (see Section 17.3.3). 

Standards for these nuclides are available from radionuclide standard suppliers. 

Inspection of the scintillation spectra produced by measuring the radionuclide standard solutions singly showed the following main spectral regions for the radionuclides ... 

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