Radon concentration dependence of alpha radioactivity measurement using ionized air transportation

We are developing an alpha radioactivity measurement apparatus for uranium contaminated waste with large and complex surfaces (alpha-clearance monitor). This monitor uses the alpha radioactivity measurement method using ionized air transportation. Our goal of the monitor is to measure 4 Bq of alpha radioactivity for a 10 minutes measurement (5.7 Bq at present). A problem in achieving the goal is fluctuation in the background (BG) ion current. This fluctuation is predicted to be mainly due to radon inside the monitor. Therefore we developed a method to measure the radon concentration inside the monitor without an additional radon detector which disturbs airflow. And as a preliminary study, we determined the radon contribution to the BG current using this method. In addition, ion mobility and ion recombination coefficient, which are the other factors influencing the detection limit, were measured and evaluated under various environmental conditions (temperature, humidity and aerosol density).