ASTM E393-19

5.1 Refer to Guide E844 for the selection, irradiation, and quality control of neutron dosimeters.

5.2 Refer to Practice E261 for a general discussion of the measurement of neutron fluence rate and fluence. The neutron spectrum must be known in order to measure neutron fluence rates with a single detector. Also it is noted that cross sections are continuously being reevaluated. The latest recommended cross sections and details on how they can be obtained are discussed in Guide E1018.

5.3 The reaction rate of a detector nuclide of known cross section, when combined with information about the neutron spectrum, permits the determination of the magnitude of the fluence rate impinging on the detector. Furthermore, if results from other detectors are available, the neutron spectrum can be defined more accurately. The techniques for fluence rate and fluence determinations are explained in Practice E261.

5.4 ¹⁴⁰Ba is a radioactive nuclide formed as a result of fission. Although it is formed in fission of any heavy atom, the relative yield will differ. Table 1 gives recommended cumulative fission yields for ¹⁴⁰Ba production and direct (independent) fission yields for the daughter product ¹⁴⁰La. The independent fission yields for ¹⁴⁰La are relatively low compared to the ¹⁴⁰Ba cumulative fission yield and will not significantly affect the accuracy of the nondestructive procedure and need not be considered.

1.1 This test method describes two procedures for the measurement of reaction rates by determining the amount of the fission product ¹⁴⁰Ba produced by the non-threshold reactions ²³⁵U(n,f), ²⁴¹Am(n,f), and ²³⁹Pu(n,f), and by the threshold reactions ²³⁸U(n,f), ²³⁷Np(n,f), and ²³²Th(n,f).

1.2 These reactions produce many fission products, among which is ¹⁴⁰Ba, having a half-life of 12.752 days. ¹⁴⁰Ba emits gamma rays of several energies; however, these are not easily detected in the presence of other fission products. Competing activity from other fission products requires that a chemical separation be employed or that the ¹⁴⁰Ba activity be determined indirectly by counting its daughter product ¹⁴⁰La. This test method describes both procedure (1.3 With suitable techniques, fission neutron fluence rates can be measured in the range from 10⁷ n (neutrons) · cm⁻² · s⁻¹ to approximately 10¹⁵ n · cm⁻² · s⁻¹.

1.4 The measurement of time-integrated reaction rates with fission dosimeters by ¹⁴⁰Ba analysis is limited by the half-life of ¹⁴⁰Ba to irradiation times up to about six weeks.

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 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.

1.7 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.