< div class=" SectionLevel2" > < p class=" subsec1 cdone2" > < span class=" Head3 cdone" > 1.1< / span> & #x00a0 This test method covers a suitable means of obtaining the thermal neutron fluence rate, or fluence, in well moderated nuclear reactor environments where the use of cadmium, as a thermal neutron shield as described in Test Method < span class=" Body-link1 cdone" > E262< / span> , is undesirable because of potential spectrum perturbations or of temperatures above the melting point of cadmium.

< / div> < div class=" SectionLevel2" > < p class=" subsec1 cdone2" > < span class=" Head3 cdone" > 1.2< / span> & #x00a0 This test method describes a means of measuring a Westcott neutron fluence rate (< span class=" Body-link1 cdone" > Note 1< / span> ) by activation of cobalt- and silver-foil monitors (See Terminology < span class=" Body-link1 cdone" > E170< / span> ). The reaction ⁵⁹Co(< span style=" font-style:italic font-weight:normal " class=" overriddenformat" > n,& #x03b3 < / span> )⁶⁰Co results in a well-defined gamma emitter having a half-life of 1925.28 days < span style=" font-weight:bold " class=" overriddenformat" > (< span class=" Body-link1 cdone" > 1< / span> )< / span> .< span style=" font-style:normal " > < span class=" footnote-link" > 2< / span> < / span> The reaction ¹⁰⁹Ag(n,& #x03b3 )^110mAg results in a nuclide with a complex decay scheme which is well known and having a half-life of 249.76 days < span style=" font-weight:bold " class=" overriddenformat" > (< span class=" Body-link1 cdone" > 1< / span> )< / span> . Both cobalt and silver are available either in very pure form or alloyed with other metals such as aluminum. A reference source of cobalt in aluminum alloy to serve as a neutron fluence rate monitor wire standard is available from the National Institute of Standards and Technology (NIST) as Standard Reference Material 953.< span style=" font-style:normal " > < span class=" footnote-link" > 3< / span> < / span> The competing activities from neutron activation of other isotopes are eliminated, for the most part, by waiting for the short-lived products to die out before counting. With suitable techniques, thermal neutron fluence rate in the range from 10⁹ cm^{& #x2212 2} & #x00b7 s^{& #x2212 1} to 3 & #x00d7 10¹⁵ cm^{& #x2212 2} & #x00b7 s^{& #x2212 1} can be measured. For this method to be applicable, the reactor must be well moderated and be well represented by a Maxwellian low-energy distribution and an (1/ < span style=" font-style:italic font-weight:normal " class=" overriddenformat" > E< / span> ) epithermal distribution. These conditions are usually met in positions surrounded by hydrogenous moderator without nearby strongly absorbing materials. Otherwise, the true spectrum must be calculated to obtain effective activation cross sections over all energies.

< div style=" text-align:left " class=" NoteLevel1" >

< span class=" note cdone" style=" text-transform: uppercase " > Note 1:< / span> & #x00a0 < span class=" note cdone" > Westcott fluence rate < img align=" absmiddle" src=" http:/ / www.astm.org/ HTTP/ IMAGES/ E0481-16_1.gif" alt=" Equation E0481-16_1" / > < / span>

< / div> < / div> < div class=" SectionLevel2" > < p class=" subsec1 cdone2" > < span class=" Head3 cdone" > 1.3< / span> & #x00a0 The values stated in SI units are to be regarded as the standard.

< / div> < div class=" SectionLevel2" > < p class=" caveat cdone2" > < span class=" Head3 cdone" > 1.4< / span> & #x00a0 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.

< / div>