Radiation Sensors Beyond the Geiger Counter
The science of detecting nuclear radiation has advanced dramatically during the past three decades. Spurred by security and safety concerns, numerous research laboratories and defense contractors have developed and refined the ways that we detect and measure ionizing radiation. And it’s not just for safety and security applications where detecting radioactivity is important. Radiation detection sensors are used for a variety of purposes including radioactive dating measurements, medical diagnosis, and mining applications.
Most of us are familiar with the concept of the Geiger-Mueller tube used in the traditional Geiger counter, however there are other types of sensors which are employed to detect and measure radioactivity.
Solid State Nuclear Radiation Sensors
The need to miniaturize radiation detection devices has led researchers to develop the solid state detectors. There small profile sensors are actually semiconductors which convert nuclear radiation into an electrical current just like the Geiger counter tube does. However, unlike the proportional Geiger counter tube, it does not use gas but rather uses an element such as Germanium. A variety of sensors have been developed with utilize similar materials such as silicon and cadmium zinc telluride.
Beyond their obvious advantage of reducing the required device footprint, the main advantage of these sensors is their energy resolution, which is extremely high. They are excellent at measuring the radiation energy with great precision down to atmospheric levels and below. However, unlike the Geiger Mueller tube, whose production costs have dropped steadily for the past half-century, solid state detectors are rather expensive and require costly integration into more complex circuit designs. That is one reason this type of radiation detectors are used more in military security and nuclear power industry applications.
An ionization chamber is a classic radiation detector using a method of detecting radiation that goes back to the early days of atomic particle research. The design is simple and straightforward; a metal cylinder is used containing an internal electrode which runs down the axis. The cylinder chamber is occupied by a gas, which could be could be an exotic gas like xenon or krypton, or a simple gas such as dry air, depending on the application.
To detect a radiation particle, a high voltage is applied. Radiation which enters the chamber ionizes the gas causing electrons to attach themselves to neutral molecules and in doing so, form negative ions. This allows an electrical current to flow. The current which flows is measure and calibrated to display the imputed radiation dose rate.
Although an advantage as a radiation detection sensor is that it is inexpensive and simple to build, it is a crude design concept and not suited for advanced applications. Moreover, it produces an electrical current which is extremely weak and therefore should be amplified.
A proportional counter is similar to an ionization chamber. The main difference is in the amount of voltage applied, which is much higher. As a result, when a radiation particle gets into the counter a gas molecule is ionized, the higher voltage accelerates the freed electron(s), which cause them to ionize additional molecules. This causes a higher current to be produced which is proportional to the radiation’s energy, hence the name of the detector. The output current is in the form of a single pulse. If this pulse is measured, the incident radiation’s energy can be determined.
The Geiger counter has come a long way since it was first invented in 1908. A Geiger counter works just like the proportional counter and ionization chamber, the difference being that the voltage produced is higher still. The intensity of the radiation field is measured by the number of pulses per second. Some Geiger counters display exposure rates although most Geiger counters do not distinguish between radiation of different energies. Because of this, all the current pulses produced have roughly the equal level.
Some recent improvements have included the addition of alarm and control circuitry to further process and interpret the pulse data. Also, many newer models feature size reduction due to a decreases in the size of the Geiger-Mueller tube itself, as well as enhancements to the power circuitry allowing much longer operating time between battery change.
A special crystal used for scintillation detectors is often sodium iodide or some similar material which is capable of converting the radiation energy into light. A solid or liquid material is used for the scintillation radiation detectors whose atoms are easily excited by incoming radiation. These excited atoms emit visible light when they return to their ground state. A photo multiplier tube is used for detecting this light and changes it to electrical current and amplifies it at the same time. The scintillation detectors are more receptive than the Geiger counter, mainly because of the detecting medium which is of higher density.
The advantages of crystals are that they usually are very efficient mainly for the higher energy gamma rays. Scintillation detectors can be very sensitive. Their main disadvantage is their price as they are all quite expensive.
Personnel dosimetry film badges are used to measure radiation exposure due to X-rays, beta particles and gamma rays. The detector consists of a piece of radiation sensitive film packaged in a light and vapor proof envelope.
A special film coated with two different emulsions is used. When exposed to radiation, it fogs. Personnel who work in nuclear industries wear them and after some time, they are sent to a laboratory where the amount of radiation they were subjected to is calculated using the badges. More and more these days we see companies abandon the use of badges and move to belt-mounted Geiger counters due their lower price and compact design.