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

The spectrum of gamma-rays contaminating the epithermal neutron beam for BN CT at the HFR in Petten has been measured with a HPGe-detector. These measu rements show that the main gamma-ray component is in the energy range betwe en 3 and 7 MeV. This gamma-ray component contributes to the dose in healthy tissue at patient irradiations. Knowledge of the energy spectrum of the be am is important for correct treatment planning. Furthermore, it gives an in sight into the origin of these gamma-rays, it determines the influence of i rradiation experiments in the reactor core on the gamma-ray component and t his contribution may be reduced. The measurements were feasible without cau sing damage to the detector by the high neutron intensity of 3.8 x 10(8) n/ cm(2) 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 se cond way is to measure the Compton scattered gamma-rays at an angle of 90 d egrees, after they have scattered in a slab of polythene. In this report, d irect measurements of the beam gamma-rays and a preliminary analysis of the energy distribution are presented. Furthermore, the origin of its componen ts is determined. In a next stage, such measurements in combination with MC NP simulations allow the reconstruction of the energy-dependent gamma-ray f luence rate of the beam. (C) 2000 Published by Elsevier Science B.V. All ri ghts reserved.