Developing a radon monitor with less influence of humidity based on electrostatic collection method and CR-39 detector

A new radon monitor with a 50 mL hemispherical internal cell (Hynu EIRM) is designed to develop a radon monitor with less influence of humidity based on electrostatic collection and CR-39. The collection efficiency of Po-218 ions in the Hynu EIRM while the cell-wall voltage is 2000 V is estimated by a model established previously. The collection efficiency of Po-218 ions under the relative humidity of 1% is 4.81% higher than that under the relative humidity of more than 10%. Several verification experiments are performed. The Hynu EIRMs are used to measure the Rn-222 concentration under three relative humidity ranges: 5%-10%, 60%-70% and 98%-100%. Results show that the largest difference of the calibration factors of the three experiments is 9.24%, which is a little larger than the difference (4.81%) estimated by the model. The reasons for this outcome may be that the simplified model underestimates the differences of the collection efficiency under different relative humidities and the statistical uncertainty in the experiments. The difference of the calibration factors of the Hynu EIRM at any relative humidity is less than 10%. This study is the first time that the influence of humidity on the detection efficiency of radon monitors based on electrostatic collection and CR-39 is reduced by decreasing the volume of the internal cell and increasing the cell-wall voltage.