Radon Plate-out and the Effects of Airflow and Electric Charge for Dark Matter Experiments

Abstract

The Cryogenic Dark Matter Search (SuperCDMS) is an international collaboration designed to search for and detect dark matter particles, which make up ~85% of the matter in the universe. The plate-out, or deposition of naturally occurring radioactive decay byproducts onto surfaces, can create backgrounds that interfere with dark matter detection experiments. In the first series of these experiments, we analyze the amount of radon progeny, 214Pb and 214Bi, that plate-out on polycarbonate samples while controlling factors such as electric charge and airflow. These samples are exposed to radon-spiked nitrogen gas in a polycarbonate wind tunnel to simulate plate-out conditions in a controlled environment. To determine radon progeny plate-out rates for each trial, the initial activity of radon progeny is calculated from the measurements of an Ortec alpha counter. In previous iterations of the experiment, we observed static charge buildup on surfaces, especially polycarbonate. This charge buildup was reduced by the implementation of an electric field source in the wind tunnel, yielding more consistent polycarbonate trials. After neutralizing the electric charge on polycarbonate, the second series of the experiment compares normalized radon daughter plate-out for polycarbonate and copper samples. Copper measurements demonstrated a positive correlation between air speed and radon daughter plate-out rate from speeds of 0 to 60 ft/min, stabilizing at speeds between 0 to 60 ft/min. Acrylic measurements demonstrated no observable relation between air speed and normalized plate-out rates. Results from both results deviate from the linear correlation of air speed and plate-out rate predicted by the Jacobi plate-out model [1]