1998 年 18 巻 2 号 p. 230-241
The cavity-gas calibration factor Ngas for a Markus parallel-plate ionization chamber was determined by comparing with a calibrated Farmer chamber using three different calibration methods as recommended in the AAPM's TG-39 protocol. These methods are: (a) high-energy electron beam calibration, (b) photon beam in-phantom calibration, and (c) 60Co in-air calibration. Ngas with the photon beam in-phantom calibration was determined by 60Co,4, and 10 MV photons using solid water and MixDp phantoms. The Ngas values determined by the photon beam in -phantom and 60Co in-air calibrations were about 1% higher than those determined by the high-energy electron beam, while the difference in Ngas values between the in-air and in pha ntom calibrations with a 60Co beam was not observed. The replacement correction factor Prepl,pp for the Markus chamber was also measured as a function of nominal electron energies from 4 to 15MeV by comparing with the Farmer chamber whose Prepl,cyl was obtained from data in the JARP protocol. Resultant Prepl,pp value decreased in the energy region below 10 MeV.
