8. Measurement of Absorbed Dose by Solid Calorimeter

Kaname Omata, Takeshi Hiraoka, Suoh Sakata, Akifumi Fukumura and Mitsue Takeshita

Keywords: calorimetry, dosimetry, carbon calorimeter, absorbed dose, radiation detector

To determine an absorbed dose for proton and heavy ion beams, a quasi adiabatic solid calorimeter with a carbon absorber was designed and constructed. The absorber was a graphite block with 2.5cm diameter and 4mm thickness.

For the purpose of heat insulation the absorber was covered with a carbon jacket which was fixed inside an acrylic vacuum chamber, and then installed inside a Styrofoam box which was evacuated by a rotary pump for several hours before the measurement. The absorber contained three thermistors, one was used as a detector for the absorbed energy which appears as a temperature rise and the other two were electrical temperature calibrators. DC bridge was used for measuring change of resistance of the thermistors installed inside the absorber. The electrical calibrations without irradiation, done between every real measurement with irradiation, determine the calibration factors which convert the change of the potential difference of the bridge caused by the heat due to irradiations to the energy delivered to the absorber. Tolerance for radiation damage of the thermistors was checked by irradiation of 1kGy delivered to five thermistors of the same type as used for the carbon calorimeter in a Ne-20 beam of 123.8MeV/u with LET of 62.4keV/mm and no change was recognized in the response of temperature versus resistance of the thermistors before and after irradiation.

The variance of coefficient of the calibration factors acquired above was less than 0.4%, which means the calibrations can be done with good reproducibility. The measurement showed even the dose rate of 0.5Gy/min caused a change of 10 V per minute in the potential difference of the bridge. It was confirmed the system had a sufficient reliability to make measurements of absorbed dose in a particle beam for dose rate larger than 0.5Gy/min.

Voltage output of the bridge was amplified by a nano-voltmeter, and data of 7-digit precision were acquired through the GPIB interface and analyzed by means of personal computers. The method allowed the change of the potential difference of the balanced bridge to be acquired as digital data and to be processed statistically. It reduced uncertainty of measured values and it made objective evaluation of the results possible. It should be noted measurements can be carried out with better precision than before even under conditions with much noise induced electromagnetically. Although it is difficult to handle the thermistors, for example with respect to the thickness of the lead wires because the thermistors themselves are very small, they are useful probes with respect to characteristics of temperature response and toughness against irradiation. The whole circuit sysytem for measurements was digitized, measured values were acquired by means of personal computers and an attempt was made to have automatic processing of acquisition and analysis of data as far as possible, for example in the data analysis a calculation code for absorbed dose was prepared. As a result of the automatic processing, there was a drastic improvement in analysis of data and carrying out measurements through a big cut in labor needed for these tasks.

Publications:
Omata, K., Hiraoka, T., Sakata, S., Fukumura, A., Takeshita, M.: Jpn. J. Med. Phys. 18, 327-332, 1998.