Investigation of the Response of a Neutron Moisture Meter Using a Multigroup, Two-Dimensional Diffusion Theory Code

摘要

A multigroup diffusion code has been used to predict the count rate from a neutron moisture meter for a range of values of soil water content omega , thermal neutron absorption cross section Ssub(a) (defined as sigma sub(a)/rho) of the soil matrix and soil matrix density rho. Two dimensions adequately approximated the geometry of the source, detector and soil surrounding the detector. Seven energy groups, the data for which were condensed from 128 group data set over the neutron energy spectrum appropriate to the soil-water mixture under study, proved adequate to describe neutron slowing-down and diffusion. The soil-water mixture was an SiO sub 2 implies water mixture, with the absorption cross section of SiO sub 2 increased to cover the range of sigma sub(a) required. The response to changes in matrix density is, in general, linear but the response to changes in water content is not linear over the range of parameter values investigated. Tabular results are presented which allow interpolation of the response for a particular omega , Ssub(a) and rho. It is shown that R( omega , Ssub(a), rho) rho M(Ssub(a)) + C( omega ) is a crude representation of the response over a very limited range of variation of omega , and Ssub(a). As the response is a slowly varying function of rho, Ssub(a) and omega , a polynomial fit will provide a better estimate of the response for values of rho, Ssub(a) and omega not tabulated. (ERA citation 11:014514)