超小型電離箱の飽和特性と極性効果

抄録

With increasing use of linear accelerators for photon beam radiosurgery, many kinds of very small ionization chambers have been commercially developed. Ion recombination loss and polarity effect are the most important factors to express chamber characteristics. To investigate the characteristics, one parallel plate chamber, six cylindrical and two hemispherical ionization chambers, which had nominal volumes between 0.1m1 and 3μl, were used.
Measurements were made using 10MV X-rays and 70MeV proton beams for saturation characteristics and the polarity effect. The polarity effect as a function of applied voltage was evaluated as the charge ratio for vertical irradiation of the 10MV X-ray beam. All the chambers showed that the effect was less than 5%, except for one chamber which was larger than 50%.
The experimental points of ion collection efficiency measured for all ionization chambers were plotted as a function of d²/V, where is the gap length and V, the applied voltage. If the ionization density generated in each chamber gas is identical, the experimental values are ideally expressed as one line. The actual values differed for the chambers because their shapes deviated from ideal cylindrical or spherical shapes. It was concluded that the ion recombination correction factor for very small ionization chambers must be determined experimentally for the actual beam to be used.