EQUILIBRATION OF AIR-TEMPERATURE INSIDE THE THIMBLE OF A FARMER-TYPE ION-CHAMBER

Ionization chambers are frequently moved from one environment to anoth er, sometimes with significant differences in temperature between the chamber and measurement phantom. To obtain reliable ionization data, t he temperature of the air in the chamber must be allowed to equilibrat e with the measuring phantom. The air temperature inside a thimble of a Farmer-type ion chamber was measured as a function of time for vario us phantom materials (air, water, and plastic). Equilibration rates fo r the various conditions are presented. Heat-diffusion theory is prese nted to explain the characteristics of the measured data. Waiting time s for temperature equilibration down to 10% of the initial temperature difference ranges from 1 to 18 min, depending on the phantom material and use of bare or covered thimble. Radiation measurements confirm th e temperature data. (C) 1998 American Association of Physicists in Med icine. [S0094-2405(98)00104-7].