Standard Practice for Measuring Neutron Fluence Rates by Radioactivation of Cobalt and Silver

Importancia y uso:

3.1 This practice 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 its thermal cross section. The pertinent data for these two reactions are given in Table 1. The equations are based on the Westcott formalism ((2, 3) and Practice E261) and determine a Westcott 2200 m/s neutron fluence rate nv₀ and the Westcott epithermal index parameter . References (4-6) contain a general discussion of the two-reaction test method. In this practice, 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.

(A) The numbers in parentheses following given values are the uncertainty in the last digit(s) of the value; 0.729 (8) means 0.729 ± 0.008, 70.8(1) means 70.8 ± 0.1.(B) The decay constant, λ, is defined as ln(2) / t_1/2 with units of sec^–1, where t_1/2 is the nuclide half-life in seconds.(C) Calculated using Eq 10.(D) In Fig. 1, Θ = 4E_rkT/AΓ² = 0.2 corresponds to the value for ¹⁰⁹Ag for T = 293 K, ∑_r = N₀σ_r,max,T=0Kσ_r,max,T=0K = 31138.03 barn at 5.19 eV (13). The value of σ_r,max,T=0K = 31138.03 barns is calculated using the Breit-Wigner single-level resonance formula where the ¹⁰⁹Ag atomic mass is A = 108.9047558 amu (14), the ENDF/B-VIII.0 (MAT = 4731) (13) resonance parameters are: resonance total width Γ = 0.1427333 eV, formation neutron width Γ_n = 0.0127333 eV, and radiative/decay width Γ_γ = 0.13 eV, with a resonance spin J=1, and the statistical spin factor where s₁ = ¹/₂ and s₂ = ¹/₂ are the spins of the two particles (neutron and ¹⁰⁹Ag ground state (15)) forming resonance.

3.2 The advantages of this approach are the elimination of four 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; and (4) the potential for high dose-rate encountered when handling activated cadmium. In addition, the reactivity changes accompanying the rapid insertion and removal of cadmium may prohibit the use of the cadmium-ratio method. Self-shielding corrections are only important if the concentrations of cobalt and silver are large, but may be neglected for diluted alloys (<1 %). Studies indicate that the accuracy of the two-reaction method for determination of thermal neutron fluence is comparable to the cadmium-ratio method (16).

3.3 The long half-lives of the two monitors permit the determination of fluence for long-term monitoring.

Subcomité:

E10.05

Referida por:

E0262-17R24E01, E1854-19, E0261-16R21, E1005-21, E0944-19

Volúmen:

12.02

Número ICS:

17.240 (Radiation measurements), 27.120.30 (Fissile materials and nuclear fuel technology)

Palabras clave:

activation; cobalt; dosimetry; foil; silver; thermal neutron;