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Chapter 15, Quantification of Radionuclides

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Chapter 15, Quantification of Radionuclides 15 Quantification of Radionuclides Portions of this chapter have been extracted, with permission, from D 3648-95-Standard Practice for the Measurement of Radioactivity, copyright American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy of the complete standard may be purchased from ASTM (tel: 610-832-9585, fax: 610-832-9555, e-mail: [email protected], website: www.astm.org). 15.1 Introduction This chapter presents descriptions of counting techniques, instrument calibration, source preparations, and the instrumentation associated with these techniques, which will help determine what radioanalytical measurement methods best suit a given need. This chapter also describes radioanalytical methods based on nuclear-decay emissions and special techniques specific to the element being analyzed. For example, samples containing a single radionuclide of high purity, sufficient energy, and ample activity may only require a simple detector system. In this case, the associated investigation techniques may offer no complications other than those related to calibration and reproducibility. At the other extreme, samples may require quantitative identification of many radionuclides for which the laboratory may need to prepare unique calibration sources. In the latter case, specialized instruments are available. Typically, a radiochemical laboratory routinely will encounter samples that require a level of information between these two extremes. A number of methods and techniques employed to separate and purify radionuclides contained in laboratory samples, particularly in environmental samples, are described in Chapter 14 (Separa- tion Techniques), and sample dissolution is discussed in Chapter 13 (Sample Dissolution). This chapter focuses on the instruments used to detect the radiations from the isolated radionuclides or the atoms from the separations and purification processes. A typical laboratory may be equipped with the following nuclear counting instrumentation: Gas proportional detectors for alpha and beta-particle counting (GP); Sodium iodide or high resolution germanium detectors for gamma detection Contents and spectrometry [NaI(Tl) and HPGe]; 15.1 Introduction ......................... 15-1 15.2 Instrument Calibrations ................ 15-2 Low-energy gamma- or X-ray detectors 15.3 Methods of Source Preparation .......... 15-8 [HPGe or Si(Li)]; 15.4 Alpha Detection Methods ............. 15-18 15.5 Beta Detection Methods .............. 15-46 Solid-state detectors for alpha spectrometry 15.6 Gamma Detection Methods ............ 15-68 15.7 Specialized Analytical Techniques ...... 15-94 (HPGe); and 15.8 References ........................ 15-101 JULY 2004 15-1 MARLAP Quantification of Radionuclides Liquid scintillation counters suitable for both alpha- or beta-emitting radionuclides (LSC and Photon Electron Rejecting Alpha Liquid ScintillationPERALS®). It may also have the following equipment, which rely on atom- or ion-counting techniques, molecular methods of analysis, or gamma-ray spectrometry: Kinetic Phosphorimeter Analysis (KPA) Mass Spectrometric Analyses Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) Thermal Ionization Mass Spectrometry (TIMS) Accelerator Mass Spectrometry (AMS) Neutron Activation 15.2 Instrument Calibrations In this chapter, the term test source is used to describe the radioactive material prepared to be introduced into a measurement instrument, and laboratory sample is used to identify the material collected for analysis. Thus, a test source is prepared from laboratory sample material for the purpose of determining its radioactive constituents. Calibration source means that the prepared source is for calibrating instruments. The goal of calibration- or test-source preparations is to maximize detection capability while minimizing the introduction of bias and uncertainty into the measurement process. To achieve this goal, calibration sources should be prepared in a manner that provide comparability to test sources with respect to geometry, composition, and distribution of the test-source material within a container or on a source mount. This section will provide an overview of the need for calibration and test-source-correspondence congruence, analyte homogeneity within the source, corrections for self-absorption and scattering of the emitted radiations, and estimation of calibration uncertainty. Specific information and guidance relative to these topics can be found in the subsequent sections of this chapter and in Chapters 13, 14, 19, and 20. Proper instrument calibrations are essential for the proper identification and quantification of radionuclides in samples. It is important to initially calibrate the instruments with calibration sources that are traceable to a national standards body. Once calibrated, the continuing validity of calibrations should be checked on a periodic basis (Chapter 18, Laboratory Quality Control) as specified in a laboratorys quality manual. This is usually done by counting a check source or some secondary calibration source in an instrument and comparing the results to those previously obtained when the instrument was known to be in calibration. The frequency and other aspects of calibrations and verifications may be specified in project planning documents and laboratory quality documents (Chapter 4, Project Plan Documents) and in analytical statements of work MARLAP 15-2 JULY 2004 Quantification of Radionuclides (Chapter 5, Obtaining Laboratory Services). Section 18.5.6 (Summary Guidance on Instrument Calibration, Background, and Quality Control) within Chapter18 provide guidance on the frequency of instrument calibration and quality controls checks when requirements are not specified in a statement of work. 15.2.1 Calibration Standards Instrument calibration should be performed as needed only with sources traceable to a national standards body such as the National Institute of Science and Technology (NIST) in the United States (ANSI N42.23). Calibrations of instruments should be made using certified reference materials of known and documented value and stated uncertainty. These certified reference materials may be supplied by: NIST (www.nist.gov) and the New Brunswick Laboratory (www.nbl.doe.gov) directly; A calibration-source supplier whose measurement capabilities or manufacturing processes are tested periodically by NIST (complies with ANSI N42.22); International Atomic Energy Agency (www.www.iaea.org/programmes/aqcs/main_database. htm); Other national standards bodies such as the National Physics Laboratory (NPL) of the United Kingdom (www.npl.co.uk/) and Physikalisch-Technische bundesanstalt (PTB) of Germany (www.ptb.de/); or A calibration-source supplier who documents derived materials with stated uncertainty, and whose values has been verified with analytical and measurement systems that have been tested periodically through an unbroken chain of comparisons to the national standards. The sections on alpha, beta, and gamma-ray detection methods have subsections (15.4, 15.5, and 15.6) that list the nuclides commonly used for instrument calibrations. 15.2.2 Congruence of Calibration and Test-Source Geometry For nuclear-decay emission analyses, instrument calibrations generally are performed to establish the detector counting efficiency of an instrument. The detector counting efficiency establishes the rate of detected events registered in the detector(s) of a counting system compared to the particle- or photon-emission rate of the source. Counting efficiencies are specific to the radionuclide (emission type or energy), the geometrical relationship between the source and detector, and a number of characteristics of the source material, especially those that affect the absorption and scattering of the radiation. It is common practice to have several different calibrations on a given detector in order to accommodate a number of radionuclides, source-to-detector distances, and JULY 2004 15-3 MARLAP Quantification of Radionuclides counting containers that a laboratory will be required to employ in order to meet project analytical requirements and the variety of media encountered. Where the efficiency of the detector varies with energy, it may be necessary to perform the calibration at a number of energies and to establish an efficiency curve that covers the range of energies to be encountered. Some radiation detection instruments require other types of calibrations. These are discussed under specific instrument calibrations. Generic issues that govern the conduct of calibrations are discussed below, and specific instrument and test-source considerations are provided in the appropriate sections in this chapter. To assure that the instrument calibration is unbiased, calibration sources should be prepared and counted in a manner that assures that they are virtually identical to the test sources in all respects that could affect the counting efficiency determination (ANSI N42.23). The geometry, including the size and shape of the calibration source and counting container (beaker, planchet, vial, etc.) and source-to-detector distance and alignment, should be controlled. Backscatter, scattering, and self-absorption present during test-source counting should be duplicated in the calibration process. The density of the calibration source material should be consistent with that of the test sources. When possible, counting efficiency calibrations should be performed using the radionuclide whose activity is to be determined
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