Room: Exhibit Hall
Purpose: In order to reduce the uncertainty of absorbed dose to water in high-energy electron beams, the National Metrology Institute of Japan is developing a measurement technique by a calorimetric method. Absorbed dose to water in high-energy electron beams was determined by a compact graphite calorimeter in a water phantom.
Methods: The compact graphite calorimeter was positioned at the reference point in a water phantom. Absorbed dose to graphite determined using a temperature rise of the calorimeter core during the beam irradiation. We measured the absorbed dose to graphite for 9, 12, 15 and 18 MeV high-energy electron beams from a clinical linac (Precise, Elekta Inc.). The absorbed dose was converted from graphite to water using an EGS5 Monte-Carlo code. Several Farmer-type ionization chambers were calibrated with absorbed dose to water in high-energy electron beams determined by the calorimeter.
Results: The absorbed dose to water in 9, 12, 15 and 18 MeV electron beams were determined with a relative expanded uncertainty of 1.2 % (k=2). The dominant factor of the uncertainty is the long-term reproducibility of the absorbed dose measurement by the calorimeter: 0.4 % (k=1). The relative uncertainty of the conversion factor were estimated as 0.3 % (k=1). The present beam quality conversion factors were about 1% larger than those of TRS-398, TG-51 and the calculation results by Muir, although these differences were smaller than the present uncertainty.
Conclusion: The absorbed dose to water in high-energy electron beam was measured with the relative expanded uncertainty of 1.2 % (k=2). Our results are in agreement with those in other publications.