ASTM E393-08

Refer to Guide E 844 for the selection, irradiation, and quality control of neutron dosimeters.

Refer to Practice E 261 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 E 1018.

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 E 261.

¹⁴⁰Ba is a radioactive nuclide formed as a result of uranium fission. Although it is formed in fission of any heavy atom, the relative yield will differ. Recommended fission yields for ¹⁴⁰Ba production are given in Table 1. The direct (independent) fission yield of the daughter product ¹⁴⁰La, which is counted, is given in Table 2. These independent fission yields 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.

The half-life of ¹⁴⁰Ba is 12.752 days. Its daughter ¹⁴⁰La has a half-life of 1.6781 days. The comparatively long half-life of ¹⁴⁰Ba allows the counting to be delayed several weeks after irradiation in a high-neutron field. However, to achieve maximum sensitivity the daughter product ¹⁴⁰La should be counted five to six days after the irradiation during nondestructive analysis or five to six days after chemical separation if the latter technique is used. An alternative method after chemical separation is to count the ¹⁴⁰Ba directly.

Because of its 12.752 day half-life and substantial fission yield, ¹⁴⁰Ba is useful for irradiation times up to about six weeks in moderate intensity fields. The number of fissions produced should be approximately 10⁹ or greater for good counting statistics. Also, if the irradiation time is substantially longer than six weeks, the neutron fluence rate determined will apply mainly to the neutron field existing during the latter part of the irradiation. The ¹⁴⁰Ba decay constant and yield are known more accurately than those of many fission products, so it is sometimes used as a standard or base reaction with which other measurements can be normalized.

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 (a), the nondestructive determination of ¹⁴⁰Ba by the direct counting of ¹⁴⁰La several days after irradiation, and procedure (b), the chemical separation of ¹⁴⁰Ba and the subsequent counting of ¹⁴⁰Ba or its daughter ¹⁴⁰La.

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 and health practices and determine the applicability of regulatory limitations prior to use.