この度の放射線障害防止法の改定は,国際放射線防護委員会(ICRP)の1977年年勧告を法令に取り入れることを主な目的の一つとして行われた。法令改定の結果,放射線被曝を実効線量当量及び組織線量当量により規制し,外部放射線被曝は1cm線量当量,3mm線量当量,および70μm線量当量(以下1cm線量当量等という),また,内部被曝は摂取量を測定評価する事により管理する体系が導入された。しかし,導入された新しい概念と体系とには,科学的又は論理的な問題点が少なくなく,運用上の混乱を引き起こしている。当物理部会は,放射線計測に関する専門学会の立場から,これ等の問題点を指摘し,関係諸機関が速やかに適切な対策を講ずることを希望する。

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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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