Cobalt-60

In Chapter 16 of the 4th edition of Intermediate Physics for Medicine and Biology, Russ Hobbie and I mention the radioactive isotope cobalt-60. Three times—in the captions of Figs. 16.13, 16.15, and 16.46—we show data obtained using ⁶⁰Co radiation. So, what is a cobalt-60 radiation source, and why is it important?

In Radiation Oncology: A Physicist’s-Eye View, Michael Goitein discusses this once-prevalent but now little-used tool for generating therapeutic photons.

Radioactive isotopes are one source of radiation, and the ⁶⁰Co therapy machine takes advantage of this. A highly active source of ⁶⁰Co is placed in a heavy lead shield which has an aperture through which the photons produced in the decay of ⁶⁰Co can escape to provide the therapeutic beam. The whole is then usually mounted on a rotating gantry so that the beam can be directed at the patient from any angle. ⁶⁰Co therapy machines are little used these days, except in areas of the world where the supply of electricity and/or repair service are problematic. I mention these machines because they are unusual in that their photon beam is near mono-energetic. It consists primarily of γ-rays of 1.17 and 1.33 MeV energy – which are close enough together that one can think of the radiation as consisting of 1.25 MeV primary photons. However, photons interacting with the shielding around the ⁶⁰Co source produce lower energy secondary photons which lower the effective energy of the beam somewhat.

The Gamma Knife is a device that uses hundreds of collimated cobalt sources to deliver radiation to a cancer from many directions. It was once state-of-the-art, but now has be largely superseded by other techniques. Most modern radiation sources are produced using a linear accelerator, and have energies over a range from a few up to ten MeV. However, cobalt sources are used still in many developing countries (see a recent point/counterpoint article debating if this is a good or bad situation).

Cobalt-60’s 5.3-year half-life makes it notorious as a candidate for a dirty bomb, in which radioactive fallout poses a greater risk than the explosion. Isotopes with much shorter half-lives decay away quickly and therefore produce intense but short-lived doses of radiation. Isotopes with much longer half-lives decay so slowly that they give off little radiation. ⁶⁰Co’s intermediate half-life means that it lasts long enough and produces enough radiation that it could contaminate a region for years, creating a Dr. Strangelove-like doomsday device.

Fortunately, dirty bombs remain hypothetical. However, cobalt sources have a real potential for causing radiation exposure if not handled properly. Here is an excerpt from an International Atomic Energy Agency (IAEA) report about one radiological accident.

A serious radiological accident occurred in Samut Prakarn, Thailand, in late January and early February 2000 when a disused 60 Co teletherapy head was partially dismantled, taken from an unsecured storage location and sold as scrap metal. Individuals who took the housing apart and later transported the device to a junkyard were exposed to radiation from the source. At the junkyard the device was further disassembled and the unrecognized source fell out, exposing workers there. The accident came to the attention of the relevant national authority when physicians who examined several individuals suspected the possibility of radiation exposure from an unsecured source and reported this suspicion. Altogether, ten people received high doses from the source. Three of those people, all workers at the junkyard, died within two months of the accident as a consequence of their exposure.