Cobalt-60 (Co-60) is a synthetic radioactive isotope that does not occur naturally. It is manufactured by bombarding the stable form, Cobalt-59, with neutrons inside a nuclear reactor, a process known as neutron activation. Because it emits highly penetrating radiation, Co-60 has been a valuable tool in medicine and heavy industry for decades. Understanding its radioactivity requires examining the rate of its decay and the powerful energy of the radiation it releases.
Defining Cobalt-60’s Radioactivity
The term “radioactivity” for Co-60 describes the physical process of its unstable nucleus shedding excess energy and matter. This process begins with beta-minus decay, where a neutron converts into a proton, an electron (the beta particle), and an antineutrino. This decay event transforms the parent Co-60 atom into an excited state of Nickel-60 (Ni-60).
The emitted beta particle is low-energy and typically absorbed within the metal source capsule itself, posing little external threat. The immediate danger arises from the subsequent step, where the newly formed Ni-60 nucleus instantly relaxes from its excited state. This relaxation releases the remaining energy in the form of high-energy electromagnetic waves known as gamma rays.
The rate of decay is defined by Co-60’s half-life, which is approximately 5.27 years. This means that after about five years, a given quantity of Co-60 will have only half of its original radioactivity remaining. The relatively short half-life ensures that sources remain intensely radioactive for a practical period of use before needing replacement. This consistent, predictable decay rate makes Co-60 a reliable source for industrial and medical applications, but sources retain substantial radioactivity for over a decade, necessitating careful long-term management and storage.
Quantifying Gamma Energy and Penetration Power
The severity of Co-60’s radioactivity is primarily due to the high energy of the gamma rays it emits. Each decay event releases two distinct, high-energy gamma photons, with energies of 1.17 million electron volts (MeV) and 1.33 MeV, averaging a mean gamma energy of 1.25 MeV.
These high-energy photons are classified as ionizing radiation because they possess enough energy to knock electrons out of atoms in the material or tissue they strike. This ionization causes cellular damage, making the radiation biologically hazardous. The high MeV energy level gives these gamma rays immense penetrating power, allowing them to pass through significant thicknesses of dense material, including human tissue.
In contrast, the beta particles released during the initial decay are low energy and typically stopped by the source’s encapsulation. The external hazard from Co-60 is almost entirely due to the penetrating gamma rays, requiring substantial shielding for safe handling. Stopping Co-60 gamma rays requires thick barriers of high-density materials like lead or concrete. The biological hazard is quantified using dose measurements such as the Sievert (Sv), which accounts for the energy absorbed by tissue. Mishandling a large industrial source can deliver a dose quickly enough to cause acute radiation sickness or death within a short exposure time.
Common Uses and Exposure Risks
Co-60 is widely used because its powerful, penetrating gamma rays are effective for treating materials or tissues at a distance. In medicine, it is known for its use in teletherapy, where a large external source directs a beam of radiation to treat various cancers. Modern applications include the Gamma Knife, a system that uses numerous small Co-60 sources focused on treating brain tumors with precision.
In industry, the isotope is a primary source for sterilization and non-destructive testing. Its applications include:
- Sterilization of single-use medical devices, such as syringes and gloves.
- Food irradiation to extend shelf life by killing microbes.
- Industrial radiography for inspecting welds and castings for flaws.
- Gauges used to measure material thickness or liquid levels.
Exposure risks for the general public are tied to accidental events involving these sealed sources. Incidents often occur when industrial or medical sources are lost, improperly disposed of, or inadvertently mixed with scrap metal destined for recycling. The accidental melting of contaminated scrap metal has led to widespread, unintended exposure when the material is incorporated into consumer products or construction materials. Safety precautions emphasize the principles of time, distance, and shielding to minimize exposure. Increasing the distance from the source dramatically reduces the dose received. Shielding, typically thick concrete bunkers or lead containers, is designed to absorb the gamma rays and prevent public access.