Radiolytic ozone yield G(O3) from 210Po alpha-particle radiation in air

Abstract

Radioactive materials ionize air surrounding them and produce molecules such as ozone and nitrogen oxides. Concentrations of these species above background can serve as a marker for radioactive materials and provide a means for stand-off detection of nuclear materials. Ozone is one of the primary radiolytic products in air and has a lower background level than oxides of nitrogen, so it is a molecule of interest for this study. Actinides of interest include 239Pu and 235U, which are strong alpha emitters which have a high linear energy transfer in air. Ozone yield, saturation, and decay time are dependent on creation and destruction processes which change with charge density, ozone density, radiation dose rate, and type of radiation, motivating measurements of ozone yield and saturation level from alpha radiation with dose rates more typical of actinide sources. As a stand in for Pu and U, studies were performed on ozone yield in air from the alpha particle emitter 210Po. The sample was enclosed in a 0.96 ± 0.01 L Teflon cylinder and both air flow and build-up measurements were performed, with air flow of ~1 L/min and build up times to 3966 min corresponding with 2640 Gy. Build up measurements showed an ozone concentration increase at short times (t ≤ 10 min) with a yield of G(O3) = 7.7 ± 0.3 molecules/100 eV. Air flow measurements showed a radiation chemical yield for ozone of G(O3) = 7.6 ± 0.2 molecules/100 eV. These yields are larger than in previous, very old alpha particle studies but consistent with low rate gamma and electron beam studies. To use ozone as a secondary marker of special nuclear materials the saturation level in still air is important. The extracted ozone saturation level in the cylinder is 1.35 ppm with a half life of 74 ± 3 min.