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Development of a Portable Graphite Calorimetry System for the High Energy Photon Beam

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B Kim

J. P. Chung , I. J. Kim , J. H. Kim , K. Y. Park , C. Y. Yi , B. C. Kim*, Korea Research Institute of Standards and Science, Yuseong-gu, Daejeon


SU-I-GPD-T-556 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: A portable graphite calorimetry system has the advantage of the absolute measurement of an absorbed dose in spite of paying a price on the uncertainty evaluation. It can be used in the off-site calibration service for the clinical radiotherapy equipment and the high dose irradiator. Minimizing the control system is the most challenging part. We resolved it by using the combination of a commercial resistance bridge and a temperature controller.

Methods: A Domen-type graphite calorimeter was constructed. The temperature calibration of the graphite calorimeter was carried out by using a calibrated SPRT in a controlled water bath. Model 372 and 350 by Lake Shore Cryogenics were used to minimize the size of control system. Model 372 was used to measure the tiny temperature rising of the core and to control the temperature of the jacket. The temperature of the shield was kept constantly by using Model 350. Heat transfer study was carried out using COMSOL Multiphysics®. And Monte Carlo simulation was carried out to determine the dose conversion factor from graphite to water.

Results: The temperature of the graphite calorimeter was converted from the resistance of thermistors using Steinhart-Hart equation. The temperature resolution of Model 372 was about 0.01 mK in the water thermostat and we could measure the temperature of the core in a sub mK resolution. Model 350 controlled the shield temperature in a mK range. From the heat transfer study, we found that an absorbed dose can be overestimated by 0.035 % for the graphite calorimeter. The dose conversion factor from graphite to water was evaluated as 1.7356 for the KRISS 6 MV photon energy.

Conclusion: A portable graphite calorimetry system has been build using the combination of commercial bridge and controller. And the graphite calorimeter was characterized to determine the water absorbed dose.

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