8/15/13 The Intensity and Spectral Distribution of Scattered Radiation from Cobalt-60 Sources

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doi: 10.1148/84.1.129January 1965 Radiology, 84, 129-130.

The Intensity and Spectral Distribution of Scattered Radiation from Cobalt-60

Sources

B. Petree, Ph.D. and J. C. Humphreys, M.S.

↵1 From the X-Ray Standards Section, National Bureau of Standards, Washington, D. C. Presented at the Fiftieth Annual Meeting of the

Radiological Society of North America, Nov. 29–Dec. 4, 1964.

This work is supported by the Division of Biology and Medicine of the United States Atomic Energy Commission.

Excerpt

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.

Copyrighted 1965 by The Radiological Society of North America, Inc.