Characterisation of the spectral distribution of scattered photons for the French national primary 60Co beam

The beam from a 60Co source is degraded due to photon scattering by the irradiation system and the source itself. The resulting gamma-ray spectrum includes a continuous spectral distribution of scattered photons up to 1 MeV. Since accurate dose measurements require precise knowledge of the flux density, the French national primary beam originating from the Colire teletherapy head has been experimentally characterised. A deconvolution process was carried out using a library of monoenergetic pulse-height spectra. The measurement procedure also generated the scattering shape and intensity relative to the main radiation for different source-to-detector distances. Using these data, the mean-energy values of the radiation beam have been calculated.