Current techniques in the handling and distribution of cobalt 60 radiation sources.

Abstract

One of the most important early uses of gamma-ray sources was in radium therapy. With the advent of nuclear reactors and pile-produced isotopes it became quickly apparent that a major step in radiation treatment could be made by replacing radium with much more intense sources, artificially produced. It is not surprising that some general techniques of production and handling should develop around the special requirements of the first of these high-intensity multicurie sources. The characteristics of a source suitable for radiation therapy are high intensity, small size, reasonably long half-life, and sufficiently high-energy gamma-ray emission (1). Of the various gamma-ray emitters which can be produced in a reactor, cobalt 60, with gamma-ray energies of 1.17 and 1.33 MEV, has appeared to be most suitable and has been most used. The high intensity required for a therapy source together with the need of a small geometrical size suggest the use of material of high specific activity. High intensity allows treatment from a distance great enough for good depth-dose characteristics and small size results in a desirably small penumbra. Specific activity is proportional to the neutron activation cross section and to the neutron flux. For cobalt, this cross section is relatively large, 34 barns, but the maximum specific activity is still limited by the neutron flux available. Not until high-flux reactors such as Canada's heavy-water reactor, NRX, began operating was it possible to produce the high specific activity, and therefore the high-intensity sources, suitable for radiotherapy. With NRX, for example, in a standard production position, specific activities of 35 curies per gram may be producedin less than eighteen months. With this specific activity, a source containing approximately 2,000 curies of cobalt 60 can be made which will deliver a dose of 50 r per minute in air at a treatment distance of 80 cm. The absorption of neutrons in the target material—cobalt metal in this instance—produces other effects which must be taken into account. In a reactor, a mass of neutron-absorbing material will lower the neutron flux in the vicinity; in addition, the outer layers of the mass will shield the inner layers from the neutrons. Each of these two phenomena, “flux depression” and “self-shielding,” , will lower the effective neutron flux and thus the specific activity produced (1). In the production of cobalt 60 in large quantities in NRX, it is estimated that the specific activity is lowered by 50 per cent, due primarily to flux depression. To reduce self-shielding, the Canadian approach has been to use the cobalt in the form of small cylindrical pellets 1.0 mm. in diameter by 1.0 mm. long. These pellets can be held during neutron irradiation in thin (approximately 3 mm.) cylindrical shells and then, after removal from the reactor, be transferred into a source container suitable for use in therapy machines.