Technical note: 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 ) 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 is well known and located at the center of the inner surface of the entrance window. For cylindrical ICs, P is shifted from the chamber's center toward the beam source. According to IAEA TRS-398, this shift can be calculated as 0.75·r for light ions with r 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 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 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 for carbon ions, 0.88·r , and 0.84·r , which results in a difference of only 0.1 mm. The difference to the recommended value of 0.75·r is 0.4 and 0.3 mm, respectively, which is larger than the positioning uncertainty. In the proton beam, a P of 0.92·r was obtained.

Conclusions: The P 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.