THE EFFECT OF HUMIDITY ON THE RADON COUNTING EFFICIENCY OF INTEGRATED INSTRUMENTS

Four commercially available integrated radon concentration measurement instruments, produced by the same manufacturer in the period between 1996 to 1998, have been tested to study the effect of absolute humidity on the radon counting efficiency. All four devices use high efficiency dRAM cells semiconductor sensor chips for alpha particle detection. The chips are located inside a measurement chamber, which is separated from the ambient air by a filter. Ambient air is continuously or quasi-continuously pumped through this filter into the chamber, in order to prevent the input of radon decay products and other solid isotopes along with the incoming air stream. Any radon inside the chamber decays to 218Po, which becomes positively charged by ionisation effects. The 218Po progeny is being forced to the chip detectors by a high voltage electrical field. The alpha particles emitted by 218Po penetrate the detector and they are registered as electrical pulses. Single channel analysis processing ensures that pulses of different heights, due to other radon and/or thoron daughters present, are not registered. It is known that absolute humidity highly affects the charged fraction of the radon progeny; therefore, a humidity effect on the radon counting efficiency of such instruments is reasonably expected. Simultaneous tests for all four instruments were performed in the Radon Calibration Facility of our Laboratory (8.5 m3 air tight chamber) within a relative humidity (φ ) range between 30 to 90% at 20-25 ° C, with radon concentrations between 0.35 to 4.53 kBqm-3 . No significant differences between the indications of the humidity sensors of the Facility (external) and the radon instruments (internal) were observed. However, the internal sensors registered temperatures about 2-4 °C higher than the external. The absolute humidity (ω ) is evaluated by assuming steam as an ideal gas by the formula : ω = φ (18 pg)/(29pair), where φ is the relative humidity, pg the saturation pressure of the steam at the prevailing temperature and pair the ambient pressure. It is apparent that the absolute humidity has to be calculated using environmental values indicated by the instruments internal sensors when available. Figure 1 presents the humidity effect on the radon counting efficiency for one of the instruments tested. Such graphs were also plotted for each of the other three instruments. It is concluded that there seems to exist a linear correlation between absolute humidity and radon counting efficiency with a 0.7 correlation coefficient in the range experimented. However, it is believed that for an absolute humidity range wider than the one examined, the radon counting efficiency is dependent rather on the square of the absolute humidity.