ASTM E705-08

Refer to Practice E 261 for a general discussion of the determination of fast-neutron fluence rate with fission detectors.

²³⁷Np is available as metal foil, wire, or oxide powder. For further information, see Guide E 844. It is usually encapsulated in a suitable container to prevent loss of, and contamination by, the ²³⁷Np and its fission products.

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

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 E 320).

¹⁴⁰Ba-¹⁴⁰La is chosen frequently for short irradiations (see Test Method E 393).

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

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

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.

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 E 844).

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

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.

The ²³⁷Np fission neutron spectrum-averaged cross section in several benchmark neutron fields are given in Table 3 of Practice E 261. Sources for the latest recommended cross sections are given in Guide E 1018. 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.

Note 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 E 1018 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)

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 (1.6781 d) in transient equilibrium.

TABLE 2 Recommended Fission Yields for Certain Fission Products^A

^A Special issue on Evaluated Nuclear Data File ENDF/B-VII.0.” Nuclear Data Sheets, J.K. Tull Editor. Vol. 107 December 2006. Data available on the ENDF/B-VII website at URL:http://www.nndc.bnl.gov/exfor/endf00.htm.

^B All yield data given as a %; RC represents a cumulative yield; RI represents an independent yield.

FIG. 1 ENDF/B-VI Cross Section Versus Energy for the ²³⁷Np(n,f)F.P. Reaction

1.1 This test method covers procedures for measuring reaction rates by assaying a fission product (F.P.) from the fission reaction ²³⁷Np(n,f)F.P.

1.2 The reaction is useful for measuring neutrons with energies from approximately 0.7 to 6 MeV and for irradiation times up to 30 to 40 years.

1.3 Equivalent fission neutron fluence rates as defined in Practice E 261 can be determined.

1.4 Detailed procedures for other fast-neutron detectors are referenced in Practice E 261.

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.