Jun 17 2015
A Geiger counter, or a Geiger-Muller tube, is used for detecting and measuring alpha, beta and gamma radiation. It consists of a pair of electrodes with a high voltage running between them. These electrodes are surrounded by a gas, usually argon or helium.
Radiation entering the tube ionizes the gas. Electrons and ions are attracted to the electrodes, producing an electric current. The current pulses are counted by a scaler, with a 'count' obtained whenever the gas is ionized by radiation.
The device includes a tube, counter and power supply. The tube is normally cylindrical with a wire across the center. The counter and power supply have timer options and voltage controls.
Three types of Geiger counters are commonly used. These are:
- Gamma-ray counters for gamma rays, cosmic rays and x-ray measurement
- Photoelectric quantum computers that combine features of counters and photo-cells and are used for the measurement of weak ultraviolet light
- Particle counters for measurement of alpha, beta, and H particles.
Research
Hans Geiger and Ernest Rutherford of the University of Manchester engineered a technique in 1908 for the detection of alpha particles that were used afterward in the Geiger Muller tube. This counter could detect only alpha particles and was part of a larger experimental device. The basic ionization mechanism discovered between 1897 and 1901 was termed as the Townsend discharge, after its inventor John Sealy Townsend. In 1928, Geiger and Walther Muller designed the sealed Geiger tube that was capable of sensing more types of ionizing radiation and began to be used as a practical radiation sensor.
Modern Geiger counters make use of the halogen tube discovered by Sidney H. Liebson in 1947. This was an advancement of the earlier Geiger tube and was advantageous due to its low operating voltage and long life.
N. Veall, from the British Institute of Radiology, designed a Geiger Muller counter in 1948 of the jacketed tube type for measurement of beta rays and also designed a convenient way of mounting in a lead shield. The obtained counting rate when the counter is filled is proportional to the amount of radioactive substance in the solution.
Current Applications
The Geiger Muller counter is used for a number of applications including:
- Particle detection - The counter was initially used to detect alpha and beta particles, and the instrument is still used for this purpose. For low energy beta particles and alpha particles, the end window type of the GM tube is used since these particles have a restricted range even in free air, and are stopped by a solid material. The design of the end window is thin enough to enable particles to pass through with the least attenuation, and the normal window density is 1.5 - 2.0 mg/cm2. However, a Geiger counter with an end window probe is not capable of differentiating between alpha and beta particles. The pancake Geiger Muller detector is a modification of the end window probe type, and is typically used as a contamination monitor for alpha/beta rays. A thin-walled windowless tube is also capable of detecting high-energy beta particles.
- Gamma and X-ray detection - It is possible to use Geiger counters for the detection of gamma radiation and for this purpose, the windowless tube is used. Efficiency is just 1% because of low gamma interaction with the tube. For high-energy gamma radiation, this depends on the interaction of photon radiation with the material in the tube wall, normally 1 – 2 mm of chrome steel, for producing electrons that can enter and ionize the fill gas. This is essential as there is a low gas density in the tube, and most high-energy gamma photons pass through without being detected. For low energy photons, another technique is used, involving direct interaction with gas in a long thin-walled tube. This design offers an added gas volume, and hence a higher chance of particle interaction; however, it still allows low-energy photons to enter the gas through the thin wall.
- Neutron detection - A Geiger tube modification is used for neutron measurement, where the gas used is Helium 3 or boron trifluoride and a plastic moderator is used to slow down neutrons. An alpha particle is created in the detector, allowing neutron counting. The Geiger principle is also used in area gamma alarms, which are used for the protection of personnel and in interlock and process measurement applications. A Geiger tube acts as the sensor, but the processing electronics will be more advanced and reliable than a hand-held survey meter.
References