Cobalt as a radiation source: The basic properties of Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal. Pure cobalt is not found in nature, but compounds of cobalt are common. Small amounts of it are found in most rocks, soil, plants, and animals. It is a ferromagnetic metal. The General properties of Cobalt include:
Atomic No. = 27
Atomic Mass = 58.9332 amu
Melting Point = 1495 °C
Boiling Point = 2870 °C
Density (at 20 °C) = 8.91 gm/cc
Allotropic Forms: (i) HCP stable at temperature less than 400 °C known as a-Co
(ii) FCC stable at temperature more than 400 °C known as b-Co
In powder form it is pyrophoric in nature (attacked by air and water). Oxidation is parabolic in nature.
The major use of cobalt metal continues to be in super alloys followed by its use in hard materials, pigments, magnets, batteries, orthopedic alloys, catalysts and coloring agents, etc. Besides this, extensive usage of its radioisotope Co-60, as a Gamma radiation source for a variety of industrial and medical applications, Self Powered Neutron Detectors (SPND’s), flux mapping, fuel management, etc. makes it a material of
Cobalt-60 is an isotope of naturally stable Cobalt-59.An isotope is a form of an element that has different number of neutrons. The unstable isotopes are generally radioactive, and are made artificially or by the decay of larger elements.Cobalt-60 emits γ-rays which is used in industrial, medicinal radiography and for the sterilization of the food and surgical instruments. In medical field, it is used to kill the cancerous cells in the radiation therapy.Cobalt-60 can be inserted internally by implanting the radioisotope on or near the cancerous cells. It can also be given externally by using the gamma rays of cobalt-60 for harder to reach areas by using teletherapy machine. This typically requires cobalt-60 pellets of 1mm in diameter and 1mm in length with density of about 95% of the theoretical value. Industrial use of Cobalt radioisotopes involves Non Destructive Testing (NDT) of samples having cracks and internal flaws. The largest use of cobalt is in super alloys, which are used to make parts of gas turbine, aircraft engines etc. Cobalt is also an important ingredient in other materials like cemented carbides, tool steels, magnetic materials, paint pigments, catalysts and artificial γ-ray sources.
Cobalt-60 is the gamma emitter and it is obtained from Cobalt-59 by bombarding with neutrons. Channels are there in the reactor for inserting slugs and the pellets inside the nuclear reactor. A normal nuclear fission reactor takes three full power reactor years for the conversion of the total cobalt-59 to cobalt-60. These radioactive pellets are then nickel coated and kept under the water when they are not in use, to prevent the radiation hazards.
In nut shell, the main uses for cobalt are:
§ hard materials, orthopedic alloys, magnets,
§ pigments, batteries, catalysts and coloring agents,
§ Radiation source for medical radiation therapy,
§ Sterilization of medical equipment,
§ Radiation source for medical radiotherapy,
§ Radioactive source for levelling devices and thickness gauges,
§ As a radioactive source for food and blood irradiation.