Energy Spectrum of Fast Neutrons Produced by 130 MeV Protons in Water

In proton radiation therapy nuclear reactions of highenergy protons hitting beam line components (e.g. collimators or other beam shaping devices) lead to the production of secondary neutrons which may result in significant neutron dose equivalent values (typically of the order of mSv per treatment Gy), depending on the treatment techniques [1]. For scanning devices the neutron dose contribution was found to be very small [2]. Nevertheless, secondary neutrons are also (inevitably) produced in the patient tissue itself and it is interesting to study their characteristics and to compare them with simulations. Neutron spectral fluences were established in earlier work by Yan et al [3] with unfolding techniques from Bonner sphere measurements. It was found that the biggest dose contributions come from fast neutrons in the range of about 0.1 to >100 MeV.