Standard Test Method for Measuring Reaction Rates by Analysis of Barium-140 From Fission Dosimeters

Importancia y uso:

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.

(A) Thermal = 0.0253 eV19.(B) Fast = 0.4 MeV.(C) From JEF-3.1.1 (Ref (1, 2)), except as noted. Uncertainties in percent of given value.(D) From ENDF/B-VIII.0 (Ref (3)). Not available in JEF-3.1.1. Uncertainties in percent of given value.

5.5 The half-life of ¹⁴⁰Ba is 12.752 days. Its daughter ¹⁴⁰La has a half-life of 1.6781 days (4).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.

Subcomité:

E10.05

Referida por:

E0720-23, E0721-22, E2005-21, E0798-24, E1854-19, E0704-19, E0705-18, E0261-16R21, E1005-21, E0944-19

Volúmen:

12.02

Número ICS:

17.240 (Radiation measurements)

Palabras clave:

Barium-140; fission dosimeter; fission product; fission reaction rates;