ASTM E266-11

Refer to Guide E844 for the selection, irradiation, and quality control of neutron dosimeters.

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

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

²⁴Na has a half-life of 14.9574 h (2) and emits gamma rays with energies of 1.368626 and 2.754007 MeV (2).

Fig. 1 shows a plot of cross section versus neutron energy for the fast-neutron reaction ²⁷Al(n,α)²⁴Na (3) along with a comparison to the current experimental database (4). This figure is for illustrative purposes only to indicate the range of response of the ²⁷Al(n,α) reaction. Refer to Guide E1018 for descriptions of recommended tabulated dosimetry cross sections.

Two competing activities, ²⁸Al and ²⁷Mg, 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 2 days (for longer irradiations, 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 and health practices and determine the applicability of regulatory limitations prior to use.