Cobalt 60 protection design.

Abstract

The recent report by the National Committee on Radiation Protection, published as Bureau of Standards Handbook 54 (1) includes basic information on cobalt 60 teletherapy shielding. So far as experimental attenuation data for the primary and 90° scattered radiation are concerned, the Handbook is particularly complete; data based on experience with completed installations, however, are limited, since cobalt teletherapy is a rather recent development. It is the purpose of the present report to give this supplementary information. The study includes surveys of completed installations, the design of which was based on experimental attenuation measurements (2–4). In Figure 1 are shown schematically the three types of radiation which should be considered in protection design; the useful beam, scattered radiation, and leakage radiation. Since the intensity and energy of the scattered radiation vary with angle of scatter, size of field, etc., its attenuation presents the more difficult problem. The structural shielding requirements of teletherapy installations vary widely and depend on a number of factors, the most important of which are as follows: 1. Degree of occupancy of nearby regions. 2. Orientation of useful beam. 3. Type of equipment. 4. Weekly work load in r/week at 1 meter (in air). 1. Degree of Occupancy: Handbook 54 is the first protection code to give occupancy factors for areas of various types. This permits the designer to adjust the barrier thickness to meet actual requirements. A stairway, for instance, is assumed to be occupied not more than one-sixteenth of the time by the same personnel. Since an occupancy factor of 1/16 reduces the required thickness by four half-value layers, a primary concrete barrier is reduced by 10 inches. It is obvious, therefore, that significant savings can be gained by proper location of the teletherapy room with respect to occupancy of surrounding regions and by orientation of the equipment so that the beam is directed toward an outside wall. Where there is a possibility of a future increase in the degree of occupancy, it is more economical to provide for this originally. A corner room on the bottom floor, preferably below ground, usually requires minimum shielding. This does not necessarily mean minimum expense, since excavation may cost more than the required additional shielding. In some instances, it is more practical to locate the telecobalt room on an upper floor in order that it be close to existing x-ray therapy facilities. In such cases efficient shielding design is essential. This is not merely a matter of cost. Often the weight of the shielding determines whether an upper-floor location is feasible. 2. Orientation of Useful Beam: The most effective way of reducing the shielding cost and weight is by limiting orientation of the useful beam to actual requirements. Such restriction permits the use of secondary barriers of less thickness for all areas exposed only to scattered and leakage radiation.