A walk-in type radon calibration chamber was designed to provide controlled environment for calibration of instrument and evaluation of methodology in Radiation Monitoring Center of Taiwan. However, it takes time to adjust radon concentration in the chamber to meet the requirements of different situations. A mathematical model, based on a set of differential equations, was established to predict radon level in the chamber and to assist the operator to arrange the calibration schedules. The measured results by Lucas cell method were coincided with the theoretical model calculation. The differences between measured data and calculated ones are less than 3% at steady state. Transient state measurements, for example, injection or flush out procedure, caused larger bias that was due to timing lag property of Lucas cell. Radon level can be controlled by regulating injection rate or ventilation rate. Ventilation control method is more practical than injection control.
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