[in Japanese]

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Dose distributions of phoroneutrons, which were emitted from the gantry of a 10MV electron linac used as an X-ray source for radiotherapy were measured by means of CR-39 plastic track detector. The typical density of etch pits produced in the plastic detector placed at the isocenter of the X-ray field (10cm x 10 cm) was 491 mm2 for the X-ray dose of 100 Gy at the same position.
This etch pit density correspon d s to neutron dose of 2.7 cSu.

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The differential w-values were measured for proton, deuteron,3He ion, neutron and 60Co gamma-ray beams. The measurements were made by eight ionization chambers with different shapes and sizes. The gases investigated were air, argon, nitrogen, carbon dioxide, methane and methane based tissue-quivalent gas, and the density of the gases at 0 C,101.3 kpa is 1.293,1.784,1.250,1.977 0.717 and 1.142 mg/cm³, respectively. The tank gases were used of pure grade.
The differential w-values were determined from the Bragg-Gray cavity theory. For the 70MeV proton beams, we estimated the w-value of 35.3,27.0,36.5,34.3,27.9 and 30.4 eV in the gases mentioned above, respectively. Our results agree with those recommended in ICRU report 31. Similar results were also obtained for the other particle beams. The use of these w-values for the evaluation of abosrbed dose with ionization chambers for proton beams gave good agreement with a tissue-equivalent calorimeter. The uncertainties of the w-values are estimatedto be 4.3%.

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