Angular distribution of neutron spectral fluence around phantom irradiated with high energy protons

Extended Bonner Spheres spectrometer was used to measure the angular distribution of neutron spectral fluence around NYLON6 phantom irradiated with pencil beam of 100, 150 and 200 MeV protons at the Proton Therapy Center Praha. Measurements were supplemented by a calculation of neutron spectral fluences at different depths of the phantom. The calculation of neutron spectral fluence at different depth of the phantom demonstrated that the majority of high energy neutrons was generated at the beginning of the proton trajectory in the phantom and the neutron yield decreased with increasing depth, with a minimum at the depth corresponding to the Bragg peak. Therefore, attention should be paid not only to the tissue behind the irradiated volume, but also to the preceding tissue. However, the neutron spectral fluence in the vicinity of the treated tissue can only be determined by calculation, mainly due to the dimensions of the neutron spectroscopic instrumentation. This paper presents a suitable technique and experimental conditions to acquire reliable data necessary for the proper determination of neutron spectral fluence. From the measured spectral fluences, the neutron fluence in whole-range and partial energy intervals were determined together with the corresponding ambient dose equivalents at measurement positions. The obtained results indicate that high energy neutrons predominate at the direction of the proton beam and more neutrons are generated by higher energy protons.