Mobility determination of lead isotopes in glass for retrospective radon measurements

In retrospective radon measurements, the 22-y half life of ²¹⁰Pb is used as an advantage. ²¹⁰Pb is often considered to be relatively immobile in glass after alpha recoil implanted by ²²²Rn progenies. The diffusion of ²¹⁰Pb could, however, lead to uncertain wrong retrospective radon exposure estimations if ²¹⁰Pb is mobile and can escape from glass, or lost as a result of cleaning-induced surface modification. This diffusion was studied by a radiotracer technique, where ²⁰⁹Pb was used as a tracer in a glass matrix for which the elemental composition is known. Using the ion guide isotope separator on-line technique, the ²⁰⁹Pb atoms were implanted into the glass with an energy of 39 keV. The diffusion profiles and the diffusion coefficients were determined after annealing at 470-620°C and serial sectioning by ion sputtering. In addition, the effect of surface cleaning on diffusion was tested. From the Arrhenius fit, the activation enthalpy (H) was determined, which is equal to 3.2 ± 0.2 eV, and also the pre-exponential factor D ₀, in the order of 20 m²s¹. This result confirms the assumption that over a time period of 50 y ²⁰⁹Pb (and ²¹⁰Pb) is effectively immobile in the glass. The boundary condition obtained from the measurements had the characteristic of a sink, implying loss of ²⁰⁹Pb in the topmost surface at high temperatures.