Estimation of photoneutron intensities around radiotherapy linear accelerator 23-MV photon beam

•Fast neutron relative intensities around a high-energy (23 MV) linac was studied.•CR-39 nuclear track detectors were used to study the fast neutron variations.•Neutron fluence was decreased by increasing the distance from isocenter.•Photoneutron intensities and distributions at isocenter level were also determined.

CR-39 solid-state nuclear track detectors (SSNTDs) were used to study the variations of fast neutron relative intensities around a high-energy (23 MV) linear accelerator (Varian 21EX) photon beam. The variations were determined on the patient plane at 0, 50, 100, 150 and 200 cm from the isocenter of the photon beam. In addition, photoneutron intensities and distributions at isocenter level with field size of 40×40 cm2 at Source Axis Distance (SAD)=100 cm around 23 MV photon beam were also determined. The results showed that the photoneutron intensities decreased rapidly by increasing the distance from the center of the x-ray beam towards the periphery, for the open fields.