An Absorbed Dose Water Calorimeter: Theory, Design, and Performance

Two calibrated thermistors sandwiched between two insulative polyethylene films were immersed in a 30 cm cube of water having a low thermal diffusivity. The product of the specific heat of water and temperature rise gave the combined effect of the local absorbed dose and any heat defect at a position along a temperature profile produced by cobalt-60 irradiation. The dose rate was near 18 mGy/s and exposure times were 3 min. The standard deviation for a daily set of measurements was about 0.6 percent. Calculations showed that conductive heat transfer produced a negligible effect at the position of measurement along the beam axis. Tests showed the absence of convection. Temperature drifts before irradiation were quickly controlled by changing the power dissipated in the water between two immersed electrodes. Reproducible measurements were obtained in distilled water supplies that had a wide range of impurities. Measurements, after saturating the water with nitrogen or oxygen, showed no difference. A difference of 0.6 percent would have been easily detectable. Tests with several chemicals added to water showed some unexpected results and changes in the measured absorbed dose rate versus accumulated dose. The measured absorbed dose rate in distilled water under the conditions described was 3.5 percent higher than that determined from measurements with a graphite calorimeter.