5.1 Refer to Practice E261 for a general discussion of the determination of fast-neutron fluence rate with fission detectors.

5.2 ²³⁷Np is available as metal foil, wire, or oxide powder. For further information, see Guide E844. It is usually encapsulated in a suitable container to prevent loss of, and contamination by, the  ²³⁷Np and its fission products.⁴

5.3 One or more fission products can be assayed. Pertinent data for relevant fission products are given in Table 1 and Table 2.

TABLE 1 Recommended Nuclear Parameters for Certain Fission Products

^A The lightface numbers in parentheses are the magnitude of plus or minus uncertainties in the last digit(s) listed.^B With  ^137mBa (2.552 min) in equilibrium.^C Probability of daughter  ¹⁴⁰La decay.^D With  ¹⁴⁰La (1.67855 d) in transient equilibrium.TABLE 2 Recommended Fission Yields for Certain Fission Products^A

^A The JEFF-3.1/3.1.1 radioactive decay data and fission yields sub-libraries, JEFF Report 20, OECD 2009, Nuclear Energy Agency.^B All yield data given as a %; RC represents a cumulative yield; RI represents an independent yield.

5.3.1 ¹³⁷Cs-^137mBa is chosen frequently for long irradiations. Radioactive products ¹³⁴Cs and  ¹³⁶Cs may be present, which can interfere with the counting of the 0.662 MeV  ¹³⁷Cs-^137mBa gamma ray (see Test Methods E320).

5.3.2 ¹⁴⁰Ba-¹⁴⁰La is chosen frequently for short irradiations (see Test Method E393).

5.3.3 ⁹⁵Zr can be counted directly, following chemical separation, or with its daughter ⁹⁵Nb, using a high-resolution gamma detector system.

5.3.4 ¹⁴⁴Ce is a high-yield fission product applicable to 2- to 3-year irradiations.

5.4 It is necessary to surround the ²³⁷Np monitor with a thermal neutron absorber to minimize fission product production from trace quantities of fissionable nuclides in the ²³⁷Np target and from  ²³⁸Np and  ²³⁸Pu from (n,γ) reactions in the   ²³⁷Np material. Assay of  ²³⁸Pu and   ²³⁹Pu concentration is recommended when a significant contribution is expected.

5.4.1 Fission product production in a light-water reactor by neutron activation products  ²³⁸Np and   ²³⁸Pu has been calculated to be insignificant (1.2 %), compared to that from  ²³⁷Np(n,f), for an irradiation period of 12 years at a fast neutron (E > 1 MeV) fluence rate of 1 × 10¹¹ cm⁻² ·s⁻¹, provided the  ²³⁷Np is shielded from thermal neutrons (see Fig. 2 of Guide E844).

5.4.2 Fission product production from photonuclear reactions, that is, (γ,f) reactions, while negligible near-power and research reactor cores, can be large for deep-water penetrations (1).⁵

5.5 Good agreement between neutron fluence measured by  ²³⁷Np fission and the  ⁵⁴Fe(n,p) ⁵⁴Mn reaction has been demonstrated (2). The reaction  ²³⁷Np(n,f) F.P. is useful since it is responsive to a broader range of neutron energies than most threshold detectors.

5.6 The  ²³⁷Np fission neutron spectrum-averaged cross section in several benchmark neutron fields are given in Table 3 of Practice E261. Sources for the latest recommended cross sections are given in Guide E1018. In the case of the  ²³⁷Np(n,f)F.P. reaction, the recommended cross section source is the ENDF/B-VI cross section (MAT = 9346) revision 1 (3). Fig. 1 shows a plot of the recommended cross section versus neutron energy for the fast-neutron reaction   ²³⁷Np(n,f)F.P.

FIG. 1 ENDF/B-VI Cross Section Versus Energy for the ²³⁷Np(n,f)F.P. ReactionNote 1-The data are taken from the Evaluated Nuclear Data file, ENDF/B-VI, rather than the later ENDF/B-VII. This is in accordance with Guide E1018 Guide for Application of ASTM Evaluated Cross Section Data File, 6.1. since the later ENDF/B-VII data files do not include covariance information. For more details see Section H of (10)