The determination of isotopic composition by gamma-ray spectrometry is a nondestructive technique and when used with other nondestructive techniques, such as calorimetry (Test Method C 1458) or neutron counting (Test Methods C 1207C 1493C 1500), can provide a totally nondestructive plutonium assay necessary for material accountancy and safeguards needs.

Since gamma-ray spectrometry systems are typically automated, the routine use of the test method is fast, reliable, and is not labor intensive. Since the test method is nondestructive, requiring no sample preparation, it does not create waste disposal problems.

This test method assumes that the isotopic composition of plutonium in the sample being measured is homogeneous (see see 7.2.4 and (5)).

The ²⁴²Pu abundance is not measured by this test method and must be estimated from isotopic correlation techniques, stream averages, historical information, or other measurement techniques.

A daughter product of ²⁴¹Pu is ²⁴¹Am. The ²⁴¹Am/²³⁹Pu atom ratio can also be determined by means of this test method (assuming a homogeneous isotopic distribution of plutonium and ²⁴¹Am) and is necessary for the correct interpretation of a calorimetric heat measurement.

The isotopic composition of a given batch or sample of plutonium is an attribute of that sample and, once determined, can be used in subsequent inventory measurements to verify the identity of a sample within the measurement uncertainties.

The method can also measure the ratio of other gamma emiting isotopes to plutonium assuming they have the same spatial distribution as the plutonium in the sample. Some of these “other” gamma-emitting isotopes include isotopes of uranium, neptium, curium, cesium, and other fission products. (The same methods of this standard can be used to measure the isotopic composition of uranium in samples containing only uranium (4–6)).

1.1 This test method is applicable to the determination of isotopic abundances in isotopically homogeneous Pu-bearing materials. This test method may be applicable to other plutonium-bearing materials, some of which may require modifications to the described test method.

1.2 The procedure is applicable to sample sizes ranging from a few tenths of a gram up to the maximum sample weight allowed by criticality limits.

1.3 Because ²⁴² Pu has no useful gamma-ray signature, its isotopic abundance is not determined. Isotopic correlation techniques may be used to estimate its relative abundance (Refs 1, 2).

1.4 This test method has been demonstrated in routine use for isotopic abundances ranging from 94 to 70% ²³⁹ Pu. This test method has also been employed for isotopic abundances outside this range.

1.5 The values stated in SI units are to be regarded as the standard.

1.6 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.