Impact of the Doppler Effect on the Spectra of Gamma Rays in Inelastic Scattering of Tagged Neutrons by Carbon and Nitrogen Nuclei

Abstract

The result of measurements by a device with tagged neutrons is the spectrum of gamma quanta during inelastic scattering of tagged neutrons on the nuclei of substance in elementary volumes (voxels). The object under study can be represented by a set of voxels. The elemental composition of an object can be determined by decomposing the original gamma spectrum into spectra emitted by the nuclei of individual chemical elements. The results of the analysis are very sensitive to the relative error σW in determining the energy of gamma rays: for σW ≈ 1%, the error in determining the relative abundance of some elements increases to 10%. This leads to the need to take into account the Doppler effect, which causes a shift and broadening of the peaks of the gamma spectrum depending on the angle between the directions of motion of tagged neutrons and detected gamma rays. This effect manifests itself most strongly for excited energy levels of nuclei with a short relaxation time, in particular, for the first level of the carbon nucleus (the gamma-quantum energy is 4.438 MeV) and the second level of nitrogen (2.313 MeV). In this paper we present the analytical estimates, the results of experimental angular dependences of the shift and broadening of the gamma spectrum peaks from carbon, nitrogen and oxygen nuclei, measured by a setup with tagged neutrons.