The measurement of the output spectrum of a cobalt 60 teletherapy unit

Abstract

The output spectrum of radiation emerging from a Coteletherapy unit contains a certain amount of low energy radiation in addition to the primary Co60 -rays with energies of 1.17 and 1.33 Mev. Since the beam strength is approximately 2 x 109 photons/cm2/sec at 80 cm from the source, direct measurement of this spectrum was not practicable. The spectral distribution of single scattered radiation from this beam, at each of a number of scattering angles, was therefore measured. This scattered spectrum was then converted to the output spectrum by the application of the appropriate scattering equations.

The measured spectrum was corrected for crystal and photomultiplier tube distortions by means of a correction matrix. This matrix was formulated from information contained in a paper by Berger and Doggett on the response function of NaI(Tl) crystals, and was constructed such that its intervals were proportional to the square root of the photon energy. This provided more detail in the low energy region of the matrix.

The curves of the output spectrum obtained from various scattering angles were essentially the same, and the mean spectral curve compared favorably with the theoretical data derived by Cormack and Johns.