Significance and Use
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 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.
5.5 The half-life of 140Ba is 12.752 days. Its daughter 140La has a half-life of 1.6781 days.3 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.
5.6 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 140 La.