【国外标准】 Standard Test Method for Determination of Uranium Isotopic Composition by Gamma-Ray Spectrometry

4.1 The determination of uranium isotopic composition by gamma-ray spectrometry is a nondestructive technique and when used with other nondestructive techniques that quantify a single isotope, such as Test Methods C1133 (Segmented Gamma Scanning), C1221 (Solution Assay), C1455 (Holdup),and C1718 (Tomographic Gamma Scanning), can provide a wholly nondestructive assay of uranium mass necessary for material accountancy and safeguards needs. This method can be used with calorimetry (Test Method C1458) for kilogram quantities of high-enriched uranium and is also used to convert an Active-Well Coincidence Counter (4) measurement of 235U mass to total uranium mass.4.2 Because gamma-ray spectrometry systems are typically automated, the routine use of the test method is fast, reliable, and is not labor intensive. The test method is nondestructive, requires no sample preparation, and does not create waste disposal problems.4.3 The test method does not require that the system be calibrated to a specific geometry.4.4 The test method assumes that all uranium in the measured item has the same isotopic distribution. This is often termed isotopic homogeneity.4.5 The application of the test method does not depend upon the physical or chemical form of the material being analyzed.4.6 The 236U abundance is not measured by this test method and must be estimated from isotopic correlation techniques, stream averages, historical information, or other measurement techniques.4.7 The isotopic composition of a given item of uranium is an attribute of that item and, once determined, can be used in subsequent inventory measurements to verify the identity of an item within the measurement uncertainties.4.8 The method can also measure the ratio of other gamma-emitting isotopes in the measured item to uranium assuming they have the same spatial distribution as the uranium in the item. Some of these “other” gamma-emitting isotopes include daughter isotopes of uranium, cesium, and other fission products.4.9 The method can be applied to gamma and x rays in two overlapping energy regions, depending upon the nature of the measured item, its containment, and the characteristics of the detector used for data acquisition.4.9.1 60 keV to 250 keV—This energy range requires good energy resolution provided by planar or semi-planar HPGe detectors. The analysis methods must be capable of deconvoluting the x-ray peak line shapes from the gamma-ray peak shapes.4.9.2 120 keV to 1010 keV—This energy range generally requires higher efficiency detectors typified by larger coaxial detectors (> 25 % relative efficiency) or large semi-planar detectors (> 30 mm thick).4.10 Fig. 1 shows the decays that produce most of the prominent gamma and x rays that are measured in this analysis.(A) Energies and Branching Intensities from Ref (1).(B) Uncertainties in parentheses are absolute 1σ values.(C) Relative values from unweighted mean of plutonium decay data from Ref (1).1.1 This test method applies to the nondestructive determination of the isotopic abundances of uranium, typically 234U, 235U, 236U, and 238U, in isotopically homogeneous uranium-bearing materials using gamma spectrometry. The material is commonly inside a container and is measured without specimen preparation.1.2 This test method is applicable to items containing sub-gram quantities of uranium to the maximum uranium mass allowed by criticality considerations.1.3 Measurable gamma ray emissions from uranium cover the energy range from below 80 keV to above 1000 keV. K-X-ray emissions from the isotopes of uranium and their daughters are found in the energy region around 100 keV. This test method has been applied to all portions of this energy range.1.4 The isotopic abundance of 236U is usually not directly determined because its low-energy gamma rays are too weak (1)2 to be detected under normal measurement conditions. Isotopic correlation techniques have been used to estimate its relative abundance (2).1.5 This test method has been demonstrated in routine use for isotopic amount fraction (atom %) of 235U from 0.2 % to 97 %.1.6 This test method requires decay equilibrium (160 days for 99 %) between 238U and its 24.1 d half-life 234Th daughter. Corrections can be made if the date of chemical separation of the 234Th daughter is known.1.7 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.9 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.