Graphite Calorimetry for absdorbed dose measurements in heavy-ion beams

Graphite calorimetry for absorbed dose measurements in heavy-ion beamsM.Sakama(1)T.Kanai(1.2)A.Fukumura(2)(1)Tokyo Institute of Technology,Japan(2)National Institute of Radiological Sciences,Japan(m_sakama@nirs.go.jp/Phone:+91-43-251-2111)\nIn order to sophisticate the radiotherapy,high accuracy knowledge of the absorbed dose delivered to the patient is essential. The main methods of absolute doseimetryare indicated as follows;(a)Doseimetry by ion chamber(b)Fricke dosimetry and (c)Calorimetry. The calorimetry is most direct method of dosemetry due to direct measurement of energy deposit in principle and no requirement of information of radiation fields for the calibration. Many countries tend to adopt the calorimerty to determine the standard absorbed dose to water and become to be capable of deciding the absorbed dose in precision of about 0.6% for photon and electron beams. Despite the recent progress of particle therapy, the parameters such as w-value and stopping power ratio for ionization chambers in the particles is not obtained accurately. Therefore that causes uncertainty in determination of the absolute dose. For this reason, we developed a graphite calorimeter to obtain high precision absobed dose and reduce the uncertainty for various beams. When the absorbed dose of 1 Gy is irradiated to the senseitive volume, the temperature rise is abou 1.4 milliKelvins. The performance require the resolution of plus or minus 7 micro Kelvins to measure it in precision of plus or minus 0.5%. The stability within seberal micro Kelvins per minute is necessary to obtain measureable background.The miniature glass bead thermistors were embedded in the sensitive volume to perform active control of temperature. The resistance change of these themistors is approximately 0.68 Ohms and 488 micro Ohms at temperature rise of 1.4 milliKelvins and 1 micro Kelvins from 20 degrees, respectively. The temperature-measuring themistor is connected to a Wheatstone bridge and the resistance change is measured by lock-in amplifier and nanovoltmeter. Simulations and experiments have been performed for photon and various heavy-ion beams.Ｃｏｍｍｉｔｔｅｅ on Space Research 36th COSPAR Scientific Assembly 200