The effects of air humidity on ionisation chamber response

A study of the effect of air humidity on different ionisation chamber wall materials has verified earlier studies and extended our understanding of how humidity affects ionisation chamber measurements. Nylon and A-150 plastic walls swell as they absorb water from the air. This volume change can increase the ionisation response by up to 2.6%. Graphite chambers do not swell, but readily desorb and absorb water into and from the surrounding air, which makes it difficult to maintain dry air in a wet graphite chamber. Humid air has a different linear stopping power and W-bar/e-value than dry air. We observe approximately a 2% decrease in response-a value in agreement with estimates based on those effects. Polyethylene walls are unaffected by humid air. (author)