Purpose

The accurate knowledge of the effective point of measurement (Peff) 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 (IC), Peff is well known and located at the center of the inner surface of the entrance window. For cylindrical ICs, Peff is shifted from the chamber's center towards the beam source. According to IAEA TRS-398, this shift can be calculated as 0.75·rcyl for light ions with rcyl being the radius of the cavity. For proton beams and in absence of a dose gradient, no shift is recommended. We have experimentally determined Peff 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 Peff 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 Peff for carbon ions, 0.88·rcyl and 0.84·rcyl, which results in a difference of only 0.1 mm. The difference to the recommended value of 0.75·rcyl is 0.4 and 0.3 mm respectively, which is larger than the positioning uncertainty. In the proton beam, a Peff of 0.92·rcyl was obtained.

Conclusions

The Peff 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.

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