THE INTENSITY AND SPECTRAL DISTRIBUTION OF SCATTERED RADIATION FROM COBALT-60 SOURCES.

Abstract

It was previously shown (1) that the scattered intensity at the center of the water-shielded Co60 sources at the National Bureau of Standards amounts to about 21 per cent of the total and that its spectral distribution is approximately the same as that of singly scattered photons. We have since made approximate computations to allow for differences between the model and the actual sources and find an appreciable intensity of multiply scattered photons. The spectral intensity can be represented as three components with fractional intensities and energies as follows: A. Primary photons 0.8026 1.17 Mev and 1.33 Mev B. Singly scattered photons 0.1758 0.21 Mev to 1.33 Mev C. Multiply scattered photons 0.0216 0 to 1.33 Mev Although the multiply scattered component, C, is not large, some knowledge of its energy is desirable for accurate absorbed dose computations. To obtain that knowledge, two spherical cavity chambers were constructed of aluminum and copper. These and the previously used graphite-walled chamber (1) were calibrated in terms of exposure, using the Bureau's x-ray and γ-ray standards. The normalized sensitivities of the chambers are shown in Figure 1. The currents produced in them by exposure at the center of the water-shielded sources were measured. Both the aluminum- and copper-walled chambers showed excess response relative to the graphite-walled chamber if it was assumed that the spectrum consisted of only components A and B. As the relative sensitivity of the aluminum chamber differs from that of the graphite chamber only below about 200 kev, there must be an appreciable intensity of photons below 200 kev energy. The measurements just described are, alone, clearly not sufficient to determine the shape of the multiply scattered photon spectrum. We have chosen to represent it as a single energy, assigning to it the fractional intensity 0.0216. The measured current ratio of the copper and carbon chambers was then used to determine the proper energy to be assigned to the multiply scattered component. This energy was found to be 105 kev which is near the peak of response of the copper chamber. With the use of the spectrum thus determined, the computed current ratio of the aluminum and graphite chambers was within 0.3 per cent of the measured ratio. This close agreement we take to indicate that our representation of the spectrum is adequate for accurate absorbed dose computations in materials of atomic number as high as 29.