Nuclear sensors are uncommon measurement devices, partly because of the strict safety
regulations that govern their use, and partly because they are usually expensive. Some
very low-level radiation sources are now available that largely overcome the safety
268 Sensor technologies
problems, but measurements are then prone to contamination by background radiation.
The principle of operation of nuclear sensors is very similar to optical sensors in that
radiation is transmitted between a source and a detector through some medium in which
the magnitude of transmission is attenuated according to the value of the measured
variable. Caesium-137 is commonly used as a gamma-ray source and a sodium iodide
device is commonly used as a gamma-ray detector. The latter gives a voltage output
that is proportional to the radiation incident upon it. One current use of nuclear sensors
is in a non-invasive technique for measuring the level of liquid in storage tanks (see
Chapter 17). They are also used in mass flow rate measurement (see Chapter 16) and
in medical scanning applications (see Webster, 1998).