Beta Radiation in Thickness Control
In industries involving the production of materials with a specific thickness i.e. paper, plastic films, aluminium foil, steel etc beta radiations are used to measure and control the thickness. Strontium 90 is an example of a radioisotope used for this type of application. It is important to know the properties of the radioisotope when selecting its use. Although there are other radioisotopes which emit beta radiation their properties may not be best suited for use in a manufacturing environment. Strontium 90 has a half life of about 29 years; this means the radioactive source for the thickness controller does not need regular replacement. This is beneficial in a manufacturing environment as it means the processing line does not need to be interrupted regularly to replace the source. It also means that the user does not have to constantly worry about disposing of a radioactive material. An additional property of Strontium 90 is that the beta particles resulting from its decay are moderate in energy and this reduces the risk of exposure to any workers in its proximity.
The animation below shows how thickness control using a radioactive beta source is achieved in industry.
A radioactive source producing beta radiation is used to control the thickness in the manufacturing industry. In this example the thickness of aluminium foil is controlled by measuring the amount of beta radiation that passes through it. The detector is a Geiger counter and measures the intensity of the beta radiations passing through the material. The detector is integrated to a computer processor. The computer processor monitors the reading from the detector and compares it with the value of the required thickness (set point). It then determines the corrective action to take to maintain a set fitness by sending a signal to hydraulic arms attached to the rollers.
It is important to know the properties of a radioisotope when determining its use. The main properties to consider are:
The half life – A long half life in equipment which uses the radioisotope as a radiation source i.e. smoke detectors and thickness control. This reduces the frequency of replacement and waste disposal. Short half life radioisotopes for tracer use and radiotherapy.
The penetrating properties – In the use of thickness control, alpha rays would not be suitable as they are stopped by a thin sheet of paper. Gamma rays are the most penetrating of the radiations and would not be suitable in applications where thickness control is up to a few millimeters as they would all pass through. Beta rays are ideally suited as they have enough energy to pass through thin sheets of metal.
The energy of the beta emission – Radioisotopes can emit beta radiation at different energies varying from low, medium to high energies. It is important to know the energy of the emission so that people exposed to the source can take the correct level of precautions and protection.