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Spectral fluence of photoneutrons generated in the head of the radiotherapeutic linac Varian 2100 C/D was measured by means of the Bonner spheres spectrometer whose active detector of thermal neutrons was replaced by a track detector, i.e. a sandwich of four CR-39s with the boron radiator inserted between them. Measurements with different collimator settings showed that the fluence of photoneutrons was higher for the more open collimator.

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Photoneutron spectra around the treatment bed of a Varian Clinac 2100C machine were measured using a Bonner sphere spectrometer. To overcome problems with pulse pile-up and detection of non-neutron-induced events, the active detector of thermal neutrons normally used at the centre of the spheres was replaced by a sandwich of four CR-39 track detectors interleaved with 10B radiators. Track densities measured for the CR-39 detectors in Bonner spheres were used for the unfolding of neutron spectra. Neutron fluence and ambient dose equivalent for the whole energy range and partial energy intervals were derived from the neutron spectra.

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