Science Buddies: "Ask an Expert"

I built the radon detector described on this website. At first it seemed to perform as expected. However, over the four week period of data collection, the initial voltage readings (before bringing radon samples close to the detector) gradually increased from about 20 mV to 300 mV. In addition the accuracy of predicting radon values has become less reliable over time. I am trying to troubleshoot the reason for the initial voltage steadily rising. The battery, resistor and transistor all seem to be functioning correctly based on voltage or resistance measurements. I have cleaned the detection system to remove dust and air in the ionization chamber, but the voltage readings remain high. Does anyone have suggestions about why the baseline voltage readings are changing? Thanks for any suggestions.