ESTRO 2024 - Abstract Book

S3387

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

The electrons can be further differentiated into electrons produced outside M e-out , and inside M e-in , of the sensitive volume. To differentiate the particles species, simulations were performed with particle filters for protons and electrons. The deposited energy of the electrons that are produced inside of the sensitive volume were simulated by increasing the production cut of electrons outside of the sensitive volume to 1000 km. The difference between the electron contributions from the two simulations, with normal and increased production cut, represents the contribution from the electrons outside of the sensitive volume.

Results:

The results for different chamber components are presented in figure 1(a). Within the simulation uncertainty, all curves show the same magnetic field dependence. The factors M 0 /M B start to increase from B = 0 to a maximum at around B = 0.35 T, after which the factors decrease again. Figure 1(b) presents the contribution of individual particle species. The factors for protons (red closed circles) fluctuate with a maximum deviation of 0.3 % around unity. Both, the electrons produced outside and inside of the sensitive volume demonstrated a strong magnetic field dependence. While the ratio of the electrons from outside (grey close circles) increases with increasing magnetic field strength, the ratio of the electrons from inside (green close circles) increases slightly up to about B = 0.3 T and then decreases with further increasing magnetic field strength. Combining both contributions of electrons leads to the observed behavior of the total contributions of all electrons regardless of their origins (blue close circles), with an increasing portion before the maximum and a decreasing portion after the maximum.

Figure 1. (a) Simulated ratios M 0 /M B of the Roos chamber evaluated for the complete chamber, without the chamber wall and with sensitive volume + guard ring without wall. (b) Simulated ratios M 0 /M B for the total deposited energy, protons, electrons total, electrons from inside and electrons from outside.

Conclusion:

It can be concluded that the magnetic field dependence of the Roos chamber is mainly caused by the presence of the air volume itself. The contribution of the protons and fragments towards the magnetic field dependent ratio M 0 /M B