Developing a radon monitor for simultaneous measurement of Rn-222 and Rn-220 with less influence of humidity based on electrostatic collection and CR-39 detector

Based on electrostatic collection and CR-39, a novel radon monitor (Hynu EIRM-II) was designed to reduce the effect of humidity for simultaneous measurement of Rn-222 and Rn-220 with two 50 mL hemispherical internal cells connected in a serial configuration. A 10 cm tube connects two internal cells to avoid entry of Rn-220 into the second one. While the relative humidity in the internal cell of Hynu EIRM-II is 1% or above 10%, the difference of Po-216 collection efficiency estimated by an improved model is 8.36% at a cell-wall voltage of 2000 V. The simultaneous measurement experiments of Rn-222 and Rn-220 concentrations were performed under three relative humidity ranges of 5%-10%, 80%-88% and 99%-100%. These experimental data show that the maximum differences of the calibration factors for Rn-222 and Rn-220 are 10.13% and 14.42%, respectively. The detection efficiency of Hynu EIRM-II was less affected by humidity. The measurement data of Rn-222 and Rn-220 by the Hynu EIRM-II can provide references for further improvement of radon monitors to accurately measure radon and thoron concentration.