Standard Active Last Updated: Dec 31, 2018
ASTM E705-18

Standard Test Method for Measuring Reaction Rates by Radioactivation of Neptunium-237

Significance and Use

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 15 and Table 2.

(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 (1.67850 d) in transient equilibrium.
(E) Primary reference for half-life, gamma energy, and gamma emission probability is Ref (1) when data is available. Note this reference is to the BIPM data that was recommended at the time of the recommended fission yields were set, that is, as of 2009, and not to the latest Vol 8 data that was published in 2016.
(A) The JEFF-3.1/3.1.1 radioactive decay data and fission yields sub-libraries, JEFF Report 20, OECD 2009, Nuclear Energy Agency (2).
(B) All yield data given as a %; RC represents a cumulative yield; RI represents an independent yield.
(C) The neutron energy represents a generic “fast neutron” spectrum and has been characterized in the JEFF 3.1.1 fission yield library as having an average neutron energy of 0.4 MeV.

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 0.661657 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 (3).

5.5 This dosimetry reaction is important in the area of reactor retrospective dosimetry (4, 5). Good agreement between neutron fluence measured by  237Np fission and the  54Fe(n,p) 54Mn reaction has been demonstrated (6, 7). The reaction  237Np(n,f) F.P. is useful since it is responsive to a broader range of neutron energies than most threshold detectors.

5.5.1 Fig. 1 shows the energy-dependent cross section for this dosimetry reaction. The figure shows that, while it is not strictly a threshold detector, because of its sensitivity in the greater than 0.1 MeV neutron energy range it can function as a detector with good sensitivity in the fast neutron region. In the fast fission 252Cf spontaneous fission benchmark field, ~1 % of the 237Np fission dosimeter response comes from neutrons with an energy less than 0.1 MeV. In the cavity of a fast burst 235U reactor, ~5 % of the 237Np ifssion dosimeter response comes from neutrons with an energy less than 0.1 MeV. In the cavity of a well-moderated pool-type research reactor ~50 % of the fission response from the 237Np(n,f) reaction comes from energies less than 0.1 MeV. The importance of this low neutron energy sensitivity should be determined based on the aplication.

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 Russian Reactor Dosimetry File, RRDF (8). This recommended cross section is identical, for energies up to 20 MeV, to what is found in the latest International Atomic Energy (IAEA) International Reactor Dosimetry and Fusion File, IRDFF-1.05 (9) . 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 RRDF/IRDFF-1.05 Cross Section Versus Energy for the 237Np(n,f)F.P. Reaction

 RRDF/IRDFF-1.05 Cross Section Versus Energy for the Np(n,f)F.P. Reaction RRDF/IRDFF-1.05 Cross Section Versus Energy for the Np(n,f)F.P. Reaction

Scope

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 90 years, provided that the analysis methods described in Practice E261 are followed. If dosimeters are analyzed after irradiation periods longer than 90 years, the information inferred about the fluence during irradiation periods more than 90 years before the end of the irradiation should not be relied upon without supporting data from dosimeters withdrawn earlier.

1.3 Equivalent fission neutron fluence rates as defined in Practice E261 can be determined.

1.4 Detailed procedures for other fast-neutron detectors are referenced in Practice E261.

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

1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

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Details
Book of Standards Volume: 12.02
Developed by Subcommittee: E10.05
Pages: 6
DOI: 10.1520/E0705-18
ICS Code: 17.240; 27.120.30