Sealed radiation sources

The radioactive sources used in industrial devices, such as nucleonic gauges or machines for industrial radiography, as well as those used in medicine for external radiotherapy, are all sealed sources. A sealed radioactive source consists in radioactive material that is permanently sealed in a capsule or bonded in a solid form. The capsule is designed to prevent the release of radioactive material from encapsulation, both under normal use conditions and also under the event of a feasible accident. Most of the sealed radioactive sources are cylindrical, since it allows the source to be better sealed by a continuous weld, they provide a more uniform distribution of the radioactive material and an easy handling. They are usually rather small, with diameter from about 1 to 30 mm and few centimeters long. However, source size and design can be changed for specific applications, as shown in Figure 1. Sometimes they could be rectangular or in shape of ring.
Sealed sources are designed by considering that the radiation will not have any significant damaging effects on the capsule. The capsule material has to be selected so as the absorption of radiation is as low as possible, such as aluminium or zirconium, but when the activity is high, the material must have strength and corrosion resistance too. In this case, stainless steel, titanium and nickel based alloys are used. Moreover, when the decay leads to the production of gas, such as radon, the capsule must be able to resist the increased pressure.
In most practices, the bare sealed radioactive source is installed in a device that is designed either to allow the source to move safely out of the shielded device to where the radiation beam is used and to be returned to the shielded device after the operation is complete, or to allow a beam of radiation to be released from the device while maintaining shielding around the source. The device may have different design and size,

depending on its purpose and function.
In some situations, the bare sealed radioactive sources may be stored in a shielded container that is designed to reduce and control radiation in the area around the source (Figure 1). Such containers are typically extremely heavy, with most using lead as a shielding material within the container. They most often look like boxes, spheres, or barrels, but could be in a variety of shapes and sizes.

Figure 1 – Example of bare sealed radiation sources (left) and shielded container for transport (right).

Photo credit: IAEA, CC-BY-SA 2.0

Some of the radioisotopes commonly used to make sealed sources are reported in Table 1. The production process of such radioisotopes differs depending on the type of radiation emitted.

Tab. 1 – Radioisotopes used in sealed radioactive sources with the relative properties and application fields.

Gamma radiation sealed sources (such as ⁶⁰Co and ¹³⁷Cs) are produced by placing reactor-irradiated metal pellets or cylinders into steel capsules, then

sealed by welding. By changing the number of pellets, the overall activity of the source can be controlled. The activities of such sources are extremely high, in the range of TBq.
Beta-emitting sealed sources are produced by chemical processing of ¹³⁷Cs extracted from fission mixtures to ¹³⁷CsCl. Then, a vitrification process of ¹³⁷Cs-sodium borosilicate glass allows the embedding of these particles into glass beads of 5-6 mm diameter that are sealed into metal capsules. In brachytherapy, ¹³⁷Cs sources, sealed in stainless-steel capsules, have activities in the range of MBq, while the highest activity Cesium irradiators may have activity of 8 PBq.
Alpha-emitting sealed sources are produced by extracting the given radionuclide from fission mixtures and deposited onto foils. Deposition techniques include sputtering, ion plating, spray and chemical vapor deposition. The thickness of these coatings, deposited on wires, rods, seeds and films, is in the range from 0.1 to few nm. Alpha emitting radionuclides, such as ²⁴¹Am, are used to produce neutron sources thanks to the encapsulation of a target material made of light elements (such as Li, Be, B) inside the source capsule. The activity of an alpha source may be up to several hundreds of GBq.
Radioactive sources, materials and machines that produce ionizing radiation are well regulated in most countries. Radioactive sources should be safely managed and securely protected during their useful life, production, transport, storage and disposal. Sources and devices must have labels to indicate the presence of radioactive materials and to facilitate their identification. Their transport requires certified transport containers and packaging and is performed by proper carriers. Personnel working with sealed radioactive sources are required to have proper authorization, specialized training and a proper support in order to deal with possible accidents.