Accurately measuring Rn-220 is crucial for evaluating natural radiation exposure. For the large volume of normal scintillation cell, the detection efficiencies for different energy of α particles decayed from Rn-222, Rn-220 and their progeny are varied. To address this, two systems of the low-pressure scintillation cell were designed to make the shortest range of α particles with minimum energy decayed from Rn-222 longer than the longest distance between any two points in the scintillation cell by reducing air pressure. Thus, the detection efficiencies of the low-pressure scintillation cell for α particles decayed from Rn-222, Rn-220 and their progeny are same. The detection efficiency of low-pressure scintillation cell for α particles can be obtained by the standard radon chamber, and then the calibration factor of Rn-220 can be determined by low-pressure scintillation. Several verification experiments were performed. The experimental results show that the detection efficiencies of Rn-222 and Rn-220 at the air pressure of 0.4P are very close and become saturated for lower values of air pressure. The calibration factor of Rn-220 calculated using the detection efficiency (0.76 ± 0.04) at the air pressure of 0.4P is 53.75 ± 2.96 Bq m-3 min-1 (k=1).

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