Studies on the alpha-recoil implantation of 214Pb and 210Pb in glass surfaces,Implications for retrospective radon measurements

Abstract: Glass sheets are often used in radon surveys to estimate retrospective radon concentrations, as radon progenies are embedded in the upper surface layer. The implanted activity is influenced by a variety of different environmental conditions. In this work, experimental methods based on etching to determine the depth distribution of recoil-implanted 210Po in glass from radon decay in air were developed. By carefully controlling chemical concentrations and the exposure time during which the glass was etched, stepwise removal of the surface material was possible. Two different etching agents, diluted HF/HNO3 and NaOH were utilised, with very similar results. The depth profiles were compared with results from simulations using the SRIM computer program. Theoretically estimated distributions showed good agreement with experimental 210Pb (or 210Po) activity depth profiles obtained by sequential etching. The maximum depth of implanted activity was about 100 ± 20 nm, and activity located deeper than about 70 nm was associated with the second alpha decay from 214Po activity previously implanted in the glass matrix. However, there was no depth interval where the activity was solely due to a single decay from the surface. Furthermore, the influence of different types of particles (paraffin/stearin, salt and polystyrene) and different particle size distributions was investigated, in order to estimate the deposition velocity onto surfaces and the implantation probability in glass. Once at the glass surface, the recoil nucleus produced in the radioactive decay may, to a certain extent, implant either into the particle (deposited on the glass) or into the glass matrix. Two approaches were utilised: One investigation of the probability of implantation was performed in a wind tunnel in order to decrease influences from non-implanting radon progeny and the other was performed in a walk-in radon exposure chamber. The measured implantation fractions of (214Pb) 214Po in glass varied from 45% for activity unattached to aerosols to about 10% for aerosol attached activity. The deposition velocity varied by a factor of 10 to 100 between unattached (0.17-0.09 cm s-1) and attached (0.001-0.0009 cm s-1) radon progeny.