SPECTRUM OF THE RADIATION FROM A COBALT 60 TELETHERAPY UNIT

The spectrum of the degraded radiation in the beam from a 60Co therapy unit has been determined from measurements of the once-scattered radiation at various angles. The therapy unit contained a source 2·5 cm diameter and 0·65 cm thick in a standard capsule. The once-scattered spectra were measured with a scintillation spectrometer and corrected using theoretical response functions. Changing the size of the beam from the unit produced very little change in the spectrum indicating that there was little radiation scattered from the inside of the collimating system. For photon energies below 0·5 MeV the measured spectrum agrees well with the calculations of Cormack and Johns (1958) for a 60Co source 2·5 cm in diameter and 1·3 cm thick with no collimator but at higher energies the experimental curve falls considerably below the calculated curve. The spectrum is also in satisfactory agreement with spectra measured recently by Costrell.