ESTRO 2024 - Abstract Book

S3382

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

As shown in figure 1, the ratios M 0 /M B for both chambers depend on the magnetic field strength and on the initial energy of the proton beam. A clear distinction between positive and negative magnetic fields can also be observed for both chambers that can be attributed to the direction of the resulting Lorentz force with respect to the chamber’s axis. Comparing both chambers, the ratio of the smaller PinPoint chamber exhibits stronger magnetic field dependence than that of the Farmer chamber. For the Farmer, M 0 /M B varies within ±0.5%, whereas for the PinPoint chamber, M0/MB varies within ±1.5%, for the range of magnetic fields between -1 T to +1 T.

Figure 1.Ratios of M 0 /M B for the Farmer (left) and PinPoint 3D (right) chamber as a function of magnetic field for four initial proton energies.

Comparing the ratios derived from the sensitive volume and the complete air volume for E = 152 MeV (figure 2), their magnitudes decrease by a maximum of 0.13% and 0.31% using the complete air volume for the Farmer and PinPoint chamber, respectively. Replacing the chamber stem by water in the model also largely removes the asymmetric behavior between positive and negative magnetic field. In general, the variation of the ratio M 0 /M B is reduced when the chamber geometry is more symmetrical, where the chamber’s stem, especially for the PinPoint chamber, contributed strongly to the magnetic field dependence.