Ionization chamber response to a sudden change of ambient temperature in air, water, and polystyrene phantom

We have studied the time course of current collected with an ionization chamber and electrometer when the chamber was suddenly inserted in an air, water, or polystyrene medium whose temperature was typically 20 degrees C above or below room temperature. The results were analyzed in terms of a temperature time constant tau--the time required for a 63% change of the ionization current. It was found that for a Farmer-type chamber, tau for the in-air measurements ranges from 5-10 min, several times longer than that for the in-water measurements (2-3 min). These tau values were confirmed with in-cavity temperature measurements carried out with a small thermocouple.