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1.1 This test method covers the quantitative determination of the fraction of ²³⁵U in uranium using measurement of the 185.7 keV gamma-ray produced during the decay of ²³⁵U.

1.2 This test method is applicable to items containing homogeneous uranium-bearing materials of known chemical composition in which the compound is considered infinitely thick with respect to 185.7 keV gamma-rays.

1.3 This test method can be used for the entire range of ²³⁵U fraction as a weight percent, from depleted (0.2 % ²³⁵U) to highly enriched (97.5 % ²³⁵U).

1.4 Measurement of items that have not reached secular equilibrium between ²³⁸U and ²³⁴Th may not produce the stated bias when low-resolution detectors are used with the computational method listed in Annex A2.

1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.6 This standard may involve hazardous materials, operations, and equipment. 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 and health practices and determine the applicability of regulatory limitations prior to use.

5.1 The enrichment meter principle provides a nondestructive measurement of the ²³⁵U fraction of uranium-bearing items. Sampling is not required and no waste is generated, minimizing exposure to hazardous materials and resulting in reduced sampling error.

5.2 This method relies on a fixed and controlled geometry. The uranium-bearing materials in the measured items and calibration reference materials used for calibration must fill the detector field of view.

5.3 Use of a low resolution detector (for example, NaI detector) to measure uranium with ²³⁵U fraction approximately 10 % which is contained in a thin-walled container can provide a rapid (typically 100 s), easily portable measurement system with precision of 0.6 % and bias of less than 1 %.

5.4 Use of a high resolution detector (for example, high-purity germanium) can provide measurement with a precision better than 0.2 % and a bias less than 1 % within a 300-s measurement time when measuring uranium with ²³⁵U fraction in the range of 0.711 % or above which is contained in thin-walled containers.

5.5 In order to obtain optimum results using this method, the chemical composition of the item must be well known, the container wall must permit transmission of the 185.7 keV gamma-ray, and the uranium-bearing material within the item must be infinitely thick with respect to the 185.7 keV gamma-ray. All items must be in identical containers or must have a known container wall thickness and composition.

5.6 Items to be measured must be homogeneous with respect to both ²³⁵U fraction and chemical composition.

5.7 When measuring items, using low-resolution detectors, in thin-walled containers that have not reached secular equilibrium (more than about 120 days after processing), either the method should not be used, additional corrections should be made to account for the age of the uranium, or high-resolution measurements should be performed.

5.8 The method is often used as a enrichment verification technique.