Simulation of the Effects of Small Angle Scattering When Applying the Enrichment Meter Principle

The 235U enrichment of uranium items which are thick compared to the attenuation length of the 185.7 keV gamma ray can be determined simply from the intensity of the line observed using a spectrometer calibrated against reference material measured with the same field of view. Corrections are needed to compensate for the difference in the attenuation coefficient between the assay item and the calibration item and also in the difference in the attenuation through the container wall. Traditionally these corrections have been calculated using simple analytical formulae based on Beer’s exponential law for uncollided flux. However, the actual transport of gamma rays in the uranium matrix and container wall is more complex. In particular gamma rays, that scatter, suffering only a slight energy loss may enter the detector and be registered within the full energy peak which has a finite energy resolution. The effect of small angle and multiple scattering and its potential impact on enrichment meter measurements using scintillators has not to the authors knowledge been studied systematically from first principles. In this paper, we apply the general purpose Monte Carlo code MCNPTM to simulate the behavior of 235 U emissions under measurement conditions representative of field measurements using the IMCN equipment. The contribution to the peak, both shape and intensity, is studied systematically as a function of wall thickness. The potential magnitude of the bias introduced by ignoring this effect over a working dynamic range is discussed along with options for correction for the effect within the framework of the traditional Region of Interest spectral analysis. Areas of follow-up work needed to further elucidate these effects are outlined.