What is a sealed source?

A sealed radioactive source is an item (typically a capsule) containing radioactive material which is designed to prevent the radioactive material from being released. These sources are used in a range of industrial applications in both fixed and portable nuclear gauges.

Radioactive material releases energy in the form of radiation when it undergoes nuclear decay. During this process there are four main types of radiation that may be emitted: alpha, beta, gamma and neutron. These radiations are classed as ‘ionising’ because they are able to strip electrons from other atoms and molecules.

Work involving sealed sources in England and Wales will potentially be subject to both the Ionising Radiations Regulations 2017 and the Environmental Permitting Regulations 2016.

Nuclear GAUGES

Sealed radioactive sources are used in 3 types of nuclear gauge:

  • Transmission - radiation is transmitted through a material and partially absorbed as it passes through.

  • Backscatter - radiation is scattered off the surface of a material.

  • Reactive - radiation produces fluorescent X-rays which are used for material identification.

Consulting an RPA

Where the Ionising Radiations Regulations apply employers are legally required to consult an RPA. At Northern RPA we have years of experience helping employers work safety and legally with sealed sources. If you have any questions, or are interested in appointing us as your RPA, please telephone us on 0161 456 9662 or email us for advice.

Common uses of sealed sources


Portable density & moisture gauges

These nuclear gauges contain two sealed radioactive sources: Cs-137 (gamma source) and Am-241/Be (neutron source).

Density measurements can be made in either transmission mode or backscatter mode. In transmission mode the Cs-137 source rod is inserted into the test material and the density is calculated by measuring how much radiation is absorbed before reaching the radiation detector in the base of the gauge. In backscatter mode, the Cs-137 source rod is kept in the base of the gauge and gamma rays are reflected off the surface of the test material before being detected.

When taking moisture measurements, the mixed Am-241/Be source in the base of the gauge is used in backscatter mode. The source emits neutrons which are reflected back by hydrogen in water molecules before reaching a neutron detector in the base of the gauge.

Flow rate gauges

Flow rate gauges are frequently used for industrial process control to determine the mass of a material moving along a conveyor or flowing through a pipe. A radioactive source is mounted at one side of the pipe/conveyor and emits radiation towards a detector mounted on the opposite side. Passing material will absorb some of the radiation which allows the flow rate can be calculated. Gamma emitters such as Cs-137 are commonly used.

thickness gauges

Radiation is transmitted though a material of known density before reaching a radiation detector on the other side. By measuring how much radiation passes through the material, the thickness can be calculated. Common sources include:

  • Cs-137 or Co-60 - gamma sources, used for measuring the thickness of dense materials such as metal

  • Kr-85 - beta source, used for measuring the thickness of paper or card

Industrial Radiography

A non-destructive testing technique for inspecting the integrity of welding joints, aircraft components, pipes etc. Commonly used gamma sources include Cs-137, Co-60 and Ir-192. Ideally, radiography should be performed in a shielded enclosure but open beam radiography is conducted where this is not practicable. Due to the risk associated with this work radiographers are frequently designated as classified radiation workers.