Significance and Use
3.1 This test method uses one monitor (cobalt) with a nearly 1/v absorption cross-section curve and a second monitor (silver) with a large resonance peak so that its resonance integral is large compared to the thermal cross section. The pertinent data for these two reactions are given in . The equations are based on the Westcott formalism (( and Practice , ) ) and determine a Westcott 2200 m/s neutron fluence rate nv0 and the Westcott epithermal index parameter . References and , , contain a general discussion of the two-reaction test method. In this test method, the absolute activities of both cobalt and silver monitors are determined. This differs from the test method in the references wherein only one absolute activity is determined.
3.2 The advantages of this test method are the elimination of three difficulties associated with the use of cadmium: (1) the perturbation of the field by the cadmium; (2) the inexact cadmium cut-off energy; (3) the low melting temperature of cadmium. In addition, the reactivity changes accompanying the rapid insertion and removal of cadmium may prohibit the use of the cadmium-ratio method. However, the self-shielding corrections remain important unless the concentrations of cobalt and silver are small. Studies indicate that the accuracy of the two-reaction method for determination of thermal neutron fluence is comparable to the cadmium-ratio method ().
3.3 The long half-lives of the two monitors permit the determination of fluence for long-term monitoring.
1.1 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 , is undesirable because of potential spectrum perturbations or of temperatures above the melting point of cadmium.
1.2 This test method describes a means of measuring a Westcott neutron fluence rate ( ) by activation of cobalt- and silver-foil monitors (See Terminology ). The reaction 59Co(n,γ )60Co results in a well-defined gamma emitter having a half-life of 1925.28 days (). The reaction 109Ag(n,γ)110mAg results in a nuclide with a complex decay scheme which is well known and having a half-life of 249.76 days (. 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. ) 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−2 · s−1 to 3 × 1015 cm−2 · s−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/E) 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.
Note 1: Westcott fluence rate
1.3 The values stated in SI units are to be regarded as the standard.
1.4 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.