Determination of the γ-ray component of a neutron beam for medical irradiation applications at the HFR in Petten

The spectrum of γ-rays contaminating the epithermal neutron beam for BNCT at the HFR in Petten has been measured with a HPGe-detector. These measurements show that the main γ-ray component is in the energy range between 3 and 7 MeV. This γ-ray component contributes to the dose in healthy tissue at patient irradiations. Knowledge of the energy spectrum of the beam is important for correct treatment planning. Furthermore, it gives an insight into the origin of these γ-rays, it determines the influence of irradiation experiments in the reactor core on the γ-ray component and this contribution may be reduced. The measurements were feasible without causing damage to the detector by the high neutron intensity of 3.8×108 n/cm2 s. There are two ways to do this. The first is to measure directly in the beam, removing most of the neutrons by scattering in polythene. The second way is to measure the Compton scattered γ-rays at an angle of 90°, after they have scattered in a slab of polythene. In this report, direct measurements of the beam γ-rays and a preliminary analysis of the energy distribution are presented. Furthermore, the origin of its components is determined. In a next stage, such measurements in combination with MCNP simulations allow the reconstruction of the energy-dependent γ-ray fluence rate of the beam.