Hans Geiger and Walther Muller introduced a particle detector which was later named as GM tube. Geiger Muller counter (GM counter) is a device that is used to detect and measure the incoming ionizing radiations (alpha, beta, gamma or cosmic rays). It is useful to tell the range of coming radiations. This particle detector is a gas filled counter. This is also termed as Geiger counter or Geiger tube. A Geiger Muller counter can count the incoming particles at the rate up to 10,000 per second.
GM counter is useful for the detection and measurement of beta and gamma radiations, not alpha rays. The reason is that alpha particles (Radon- 222) are more massive so they have short range and have high ionization rate. Comparatively, beta (Sr-94) and gamma rays (Co-60) have long range.
The main difference b/w GM counter and a proportional counter is in the formation of avalanche and amplification. Avalanche is formed only at a single point in proportional counter whereas in GM counter it is formed in central wire (anode). Secondly, amplification in GM counter is independent of initial ionization that is caused by ionizing particle.
GM Counter Construction
Radioactive source is placed in a lead box, with a cavity to allow the escape of radiations into GM tube.
GM tube consists of a hollow tube that has an interior of metal. This inside metallic coating behaves like a cathode, which is further connected to negative terminal of battery.
A wire, made of tungsten, is stretched along the axis of tube, which behaves as anode. The anode is connected to the positive terminal of battery through a resistor R.
Firstly the GM tube is evacuated and then filled with 90% of an inert gas (argon, helium etc) and 10% quenching gas (ethyl alcohol). Pressure of this mixture of gases is maintained very low (about 0.1 Pa), to which a very high external voltage is applied (about 1000 V). It should be kept in mind that the applied potential is less than the ionizing potential of the quenching gas.
One end of the tube has a thin mica window to allow the entry of ionizing radiation and the other end is closed.
An amplifier is attached to this tube that can amplify the signal from 5 V to 50 V, which is further connected to a counter (also called scalar).
Principle and working of GM counter
Basic working principle of GM counter can be understood as follows. When an ionizing radiation passes through a gas, it produces ions (depending upon its ionization power of incident radiation) by striking with gas molecules. It causes production of electron-hole pairs. Now at the same time two event start. First is when holes start moving towards cathode and the other is when electrons move toward anode (central wire). The holes that are moving toward cathode can cause the emission of unwanted electrons that can disturb the pulse, so those holes are quenched by adding quenching gas. This quenching gas captures those holes and becomes neutral. On the other hand, the primary electrons strike with other gas molecules, causing production of secondary electrons which further continue the multiplication of electrons. Finally, when this avalanche of electrons reach anode, then a very small pulse is detected that is further amplified by amplifier.
Only if the applied potential difference is strong enough then ions will produce a secondary ion avalanche whose total effect will be proportional to the energy associated with the primary ionizing event.
The size of final detected pulse is dependent only on triggering off of ionization due to ionizing particle but is independent of the energy of this particle.
A high energy particle entering through the mica window will cause one or more of the argon atoms to ionize. The electrons and ions of argon thus produced cause other argon atoms to ionize in a cascade effect. The result of this one event is sudden, massive electrical discharge that causes a current pulse.
Limitations of GM counter
- GM tube cannot discriminate b/w the energy of radiation or their type as output pulse is always of same amplitude.
- This tube cannot measure the high radiation rate because each ionization event is followed by dead time.
Plateau graph of Geiger counter
The tube stops working below a certain voltage. Several hundred volts are possible. The number of pulses is proportional to the voltage after that. The proportional region is the name given to this area.
If the applied voltage is increased any more, a point will be reached where the count rate will remain constant over a specific area. The plateau region, also known as the Geiger region, is located in this area. The Geiger Muller activity takes place in this area.
Beyond the plateau, the applied electric field is so strong that a constant discharge occurs in the tube, causing the count rate to rapidly increase. It is not necessary for an ionization event to occur, so the tube should not be used in this region.
Around 500 particles per second can be counted by the Geiger Muller counter. Particles that pass through the GM counter during the dead time will not be counted. The time it takes for the tube to recover between counts, or the time when the GM tube is unable to count any particles, is referred to as dead time. The tube takes about 200 seconds to recover.
If a large number of particles reach the GM tube at once, the tube will not have enough time to recover, and several particles will be missed.
The ratio of observed counts per second to the amount of ionizing particles entering the counter per second is the counter’s effectiveness. The capacity of the GM counter to count is known as counting efficiency.
Applications of Geiger counter
Geiger counters are used to detect radioactivity in a variety of ways. Here are a few examples.
- In the process of mineral prospecting, the GM tube is used to identify radioactive rocks and minerals. For fire responders, the GM tube is used to make an initial assessment of radiation risk.
- To search for radioactivity levels in the air around a nuclear power plant.
- To see if there is any risk in the event of a nuclear explosion or a spill of radioactive coolant.
- To search for radioactive pollution in your workplace’s clothes and shoes.
- In the scrap metal manufacturing industry, radiation monitoring is used.
- In a medical facility, to scan for potential leakage or X-ray exposure.
- To look for contamination in places where depleted uranium ammunition was used.
- In the jewelry industry, to look for irradiated gemstones.
- To measure iodine 131 levels in cancer patients undergoing radiation therapy.
- To determine whether food has been contaminated with radioactivity.
- To double-check materials in your area of anthropology or archaeology.
- To test for radioactivity in metal products in your home or workplace that may have been manufactured from recycled radioactive materials.