Radon Daughter Plate-out Measurements at SNOLAB for Polyethylene and Copper

Polyethylene and copper samples were exposed to the underground air at SNOLAB for approximately three months while several environmental factors were monitored. Predictions of the radon-daughter plate-out rate are compared to the resulting surface activities, obtained from high-sensitivity measurements of alpha emissivity using the XIA UltraLo-1800 spectrometer at SMU. From these measurements, we determine an average ${}^{210}$Pb plate-out rate of 249 and 423 atoms/day/cm${}^2$ for polyethylene and copper, respectively, when exposed to radon activity of 135 Bq/m${}^3$ at SNOLAB. A time-dependent model of alpha activity is discussed for these materials placed in similar environmental conditions.