Experimental determination of the effective point of measurement of the PTW-31010 ionization chamber in proton and carbon ion beams

Purpose The accurate knowledge of the effective point of measurement (P-eff) is particularly important for measurements in proximity to high dose gradients such as in the distal fall-off of particle beams. For plane-parallel ionization chambers (ICs), P-eff is well known and located at the center of the inner surface of the entrance window. For cylindrical ICs, P-eff is shifted from the chamber's center toward the beam source. According to IAEA TRS-398, this shift can be calculated as 0.75 center dot r(cyl) for light ions with r(cyl) being the radius of the cavity. For proton beams and in absence of a dose gradient, no shift is recommended. We have experimentally determined P-eff for the 0.125 cc Semiflex IC in both proton and carbon ion beams. Methods The first method consisted of simultaneous irradiation of a plane-parallel IC and the Semiflex in a 4-cm wide spread-out Bragg peak. In the second method, a single-energy beam was used, and both ICs were positioned successively at the same measurement depths. For both approaches, the shift of the distal edge of the depth ionization distributions recorded by the two chambers at different reference points was used to calculate P-eff of the Semiflex. Both methods were applied in carbon ion beams, and only the latter was applied in proton beams. Results Both methods yielded a similar P-eff for carbon ions, 0.88 center dot r(cyl), and 0.84 center dot r(cyl), which results in a difference of only 0.1 mm. The difference to the recommended value of 0.75 center dot r(cyl) is 0.4 and 0.3 mm, respectively, which is larger than the positioning uncertainty. In the proton beam, a P-eff of 0.92 center dot r(cyl) was obtained. Conclusions The P-eff for the 0.125 cc Semiflex IC is shifted further from the cavity center as recommended by IAEA TRS-398 for light ions, with the shift for proton beams being even larger than for carbon ion beams.