Numerical calculation of efficiency and calibration factors for CR39 detectors, in the detection of alpha particle emitted by radon gas and its progenies. A software was employed for the latest version of the Monte Carlo code together with SRIM2013. The results showed that, the initial energy influence is one of the important parameters for the alpha detectors. Three methods have been used to calculate the calibration factor for CR-39 detector, and it is found that, they are consistent. The optimum radius of the cylindrical can is 3.5 cm for 1.5 cm x 1.5 cm CR39 detector. It was found that the calibration factors calculated by mean critical angle and Monte-Carlo methods are very closed, for both radon and thoron.
