The FiR 1 photon beam model adjustment according to in-air spectrum measurements with the Mg(Ar) ionization chamber

• In order to verify the photon beam model of the FiR 1 BNCT facility in Finland, the photon beam spectrum is determined experimentally through ionization chamber measurements with multiple build-up caps of different thickness.
• Large discrepancy is found between free in air measurements with the ionization chamber and the Monte Carlo calculations indicating that the initial beam model needs to be adjusted.
• The unfolded spectrum and photon beam intensity may be applicable as a more correct photon beam model.

The mixed neutron–photon beam of FiR 1 reactor is used for boron–neutron capture therapy (BNCT) in Finland. A beam model has been defined for patient treatment planning and dosimetric calculations. The neutron beam model has been validated with an activation foil measurements. The photon beam model has not been thoroughly validated against measurements, due to the fact that the beam photon dose rate is low, at most only 2% of the total weighted patient dose at FiR 1. However, improvement of the photon dose detection accuracy is worthwhile, since the beam photon dose is of concern in the beam dosimetry. In this study, we have performed ionization chamber measurements with multiple build-up caps of different thickness to adjust the calculated photon spectrum of a FiR 1 beam model.