Influence of the low-energy backscattering spectrum on the calibration of thin alpha-particle sources

Abstract

The activity of thin alpha-particle sources with negligible self-absorption can be absolutely determined using 2π counting geometry systems, correcting for backscattering from the source backing. This procedure requires to perform an extrapolation to zero energy in the experimental counting rate, due to the existence of events produced by recoiling daughters and electronic noise in the low-energy region. In addition, the Monte Carlo simulation has proven that the energy distribution of the backscattered alpha particles is not flat, showing a ‘‘peak” at the low-energy region which is due to multiple scattering in the backing. In this work, we demonstrate that a significant part of the backscattered particles included in this “peak” are not contained in the extrapolated counting rate, which implies an underestimation of the source activity if the total backscattering coefficient is considered. We applied MC simulation with the well-known code SRIM to evaluate this underestimation depending on the energy of alpha emitters and on the backing. Our results show that the underestimation in the activity can be even close to 1 % in the case of backings with high atomic numbers, which can be considered as significative in metrological measurements for the standardization of alpha-particle sources. In addition, a new procedure is proposed here to correct for this effect, which includes only the simulation of alpha particles in the source backing, without attending to the source substrate. This new procedure is finally applied to some sources from the literature, where the backscattering coefficients were obtained experimentally, providing satisfactory results.