A Clinac-4 therapy accelerator has been modified to function also as a computed tomography (CT) scanner. We report on measurements made to calibrate the beam. 104 Bismuth-germanate silicon-photodiode detectors were placed in a circular arc of radius 180 cm, centered on the source, in the arrangement of a third-generation CT scanner. With the beam collimated to a fan section and just filling the detectors, the signal S(chi), obtained with thickness chi of plastic phantom, closely followed the relationship S(chi)/S(0) = exp [- (alpha chi + beta chi 2)], where coefficients alpha and beta varied systematically over the extent of the beam. This result agrees with results of similar work by others. It may be explained in terms of the spectral changes that are known to be an inherent property of such beams. Expressions relating alpha and beta to the source spectral density are derived; also estimates of the source spectral density are calculated using the method of Laplace inversion. It is shown that scattered radiation arising from the fan beam within the scattering medium does not contribute significantly to the values of alpha and beta.