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 determination of fast-neutron fluence rate with threshold detectors.

5.3 Pure aluminum in the form of foil or wire is readily available and easily handled. ²⁷Al has an abundance of 100 % (1).3

5.4 ²⁴Na has a half-life of 14.958 (2)4 h (2) and emits gamma rays with energies of 1.368630 (5) and 2.754049 (13) MeV (2).

5.5 Fig. 1 shows a plot of the International Reactor Dosimetry and Fusion File (IRDFF-II) cross section (3, 4) versus neutron energy for the fast-neutron reaction ²⁷Al(n,α) ²⁴Na (3) along with a comparison to the current experimental database (5, 6). While the RRDF-2008 and IRDFF-1.05 cross sections extend from threshold up to 60 MeV, due to considerations of the available validation data, the energy region over which this standard recommends use of this cross section for reactor dosimetry applications only extends from threshold at ~4.25 MeV up to 20 MeV. This figure is for illustrative purposes and is used to indicate the range of response of the ²⁷Al(n,α) reaction. Refer to Guide E1018 for recommended sources for the tabulated dosimetry cross sections.

FIG. 1 ²⁷Al(n,α)²⁴Na Cross Section, from IRDFF-II Library, with EXFOR Experimental Data

5.6 Two competing activities, ²⁸Al (2.25 (2) minute half-life) and ²⁷Mg (9.458 (12) minute half-life), are formed in the reactions ²⁷Al(n,γ)²⁸Al and ²⁷Al(n,p)²⁷Mg, respectively, but these can be eliminated by waiting 2 h before counting.

1.1 This test method covers procedures measuring reaction rates by the activation reaction ²⁷Al(n,α)²⁴Na.

1.2 This activation reaction is useful for measuring neutrons with energies above approximately 6.5 MeV and for irradiation times up to about two days (for longer irradiations, or when there are significant variations in reactor power during the irradiation, see Practice E261).

1.3 With suitable techniques, fission-neutron fluence rates above 10⁶ cm⁻²·s⁻¹ can be determined.

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

1.5 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.

1.6 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.