Effect of a Boron Screen on the Sensitivity of Neutron Fission Chambers

Abstract

It is suggested that absorbing screens with ¹⁰B be used to maintain constant sensitivity under prolonged irradiation of fission chambers with natural uranium. The transmission factor T _α(E) of boron screens with various thicknesses (δ = 0.1–2 ge/cm²) for a wide neutron energy range and attenuation of a spectrum of the type ϕ_e/E are estimated. The group and average group constants of the transmission factor of boron are calculated for neutron fluxes in 25 energy groups of the neutron cross sections library.

The contribution of ²³⁸U and ²³⁵U to the signal of a fission chamber with natural uranium is analyzed as a function of the boron screen thickness. ²³⁹Pu accumulation and ²³⁸U burnup are estimated using ²³⁸U group capture cross sections, ²³⁸U and ²³⁹Pu fission cross sections, and the group values T _α(E)ΔE/E obtained by the authors. It is shown that in the absence of a boron screen for thermal-neutron fluence 10¹⁷ cm^–2 the sensitivity of a fission chamber with natural uranium increases as a result of the formation of ²³⁹Pu. A boron screen with δ = 1 g/cm² makes it possible to maintain the sensitivity of the fission chamber constant up to thermal-neutron fluence 5·10²² cm^–2.