THE CALORIMETRIC DETERMINATION OF LOCAL ABSORBED DOSE
The development of a calorimetric technique for the determination of the intensity of radiation has been previously reported. This method has been further developed and applied to the measurement of locally absorbed dose. Its use for the determination of the absorbed dose produced in tissue-equivalent material by cobalt-60 gamma rays is reported here. The entire irradiated medium under study is contained in the calorimeter. At the point where the absorbed dose is desired, a small segment of tbe medium is thermally isolated by means of vacuum gaps which are sufficiently small to prevent any perturbation of the gamma- ray absorption or oi the secondary electron equilibrium. Quasi-adiabatic thermal control is maintained continuously. Imbedded in the absorbing segment is a thermistor which responds to temperature changes and whose output is electronically amplified and recorded. The temperature change has been calibrated in terms of the absorbed dose by means of the dissipation of a known amount of electrical energy. The measurements are reproducible with a standard deviation of less than plus or minus one per cent. The absorbing medium employed in the present investigation is fabricated of conducting plastic, and has as its constituents an equal number of carbon and hydrogen atoms. For cobalt- 60 gamma rays it is closely tissue-equivalent. The major correction is for the energy absorbed in the crystal-lattice which does not appear as thermal energy, but as potential energy of chemical configuration. The results of Lawton et al. on the radiolysis of polyethylene were used to calculate a 2.36 per cent correction for the absorbed energy which is net thermally degraded. In addition to the calorimetric determination of absorbed dose in tissue equivalent plastic, an alternative technique for the determination of this quantity, the Fricke Ferrous Sulfate Dosimeter, has been utilized. A G value of 15.5 molecules/100 ev was employed. A small teflon container was constructed, with the same dimensions as the plastic absorbing volume of the calorimeter. This was inserted in a polystyrene absorbing cylinder with dimensions equivalent to those of the calorimeter. The locally absorbed dose determined chemically and calorimetrically agreed to within 1.5 per cent. The local absorbed dose calorimeter is physically direct and experimentally precise. It permits the measurement of the locally absorbed energy with a technique which avoids some of the problems encountered in the use of cavity ionization measurements for this purpose. (auth)
- Research Organization:
- Sloan-Kettering Inst. for Cancer Research, New York
- NSA Number:
- NSA-12-014921
- OSTI ID:
- 4307756
- Report Number(s):
- A/CONF.15/P/744
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
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Biological and clinical dosimetry. Comprehensive eighteen-year report, July 1, 1964-June 30, 1982
THE DIRECT MEASUREMENT OF LOCAL ABSORBED DOSE BY CALORIMETRY. INVESTIGATION OF THE RELATION BETWEEN THE ENERGY ABSORBED IN A MEDIUM EXPOSED TO X-RAYS AND ELECTRONS, THE IONIZATION PRODUCED IN A GAS CAVITY CHAMBER IN THE SAME ABSORBING MEDIUM, AND THE OXIDATION OF FERROUS SULFATE IN THE FRICKE DOSIMETER. Annual Progress Report for July 1, 1959-June 30, 1960
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