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The gas-filled capacitor is used to calculate the total number of ionizing particles that it contains. Such capacitors are known as Geiger counter. Under the influence of these ionizing particles that go through the gas volume the capacitor breaks. There’s also some additional feeding, which usually constitutes about 300V with the help of which the discharge quenching is provided.
There are several types of Geiger counters that are commonly known as self-quenching and non-self-quenching. The type varies depending on the structure of the gas and the volume. The thickness of material in its wall also plays an important role in defining the counter’s type.
The counters of 400V are widely spread and used in the radio technology of Russian and the former USSR:
- «SBM-20» is extremely thin almost like a pen, SBM-21 reminds a cigarette filter by its size and has a steel case, which would be perfect for gamma and beta rays.
- «SI-8B» is used to measure the beta rays thanks to its mica opening in its body.
Thanks to its high sensitivity, wide range of radiation sorts registration, simplicity in use and relatively cheapness, Geiger Muller counter has become one of the most popular counters in the industry. This device got its name by Geiger, the inventor of the device. Later Muller, who was working in this field, improved the device, which event added his name to the counter.
The structure of Geiger counter is simple. Its core is the metal or glass tube. The metal fiber is fixed with the axis of cylinder. Thanks to such arrangement the fiber plays the role of the anode and the tube constitutes the cathode. The rarefied gas, such as argon or neon, establishes the pressure between the anode and cathode of about 1500V.
Collision ionizations is practically the base for the counter operation. The counter has the next sequenced of operation:
- Radioactive isotope releases the gamma-quantum
- Gamma-quantum are pushed away by the electrons from the counter’s walls.
- Positive ions appear due to the beating out of electrons from the atoms in the process of the atoms colliding
- Collision ionization starts in the result of electric field appeared in the result of the speeding up of the electrons by cathode and anodes. This event causes the huge mass of ions, which allows to increase the current in the counter.
- The pressure appears on the resistance R
- To register the particle, which is contained in it, the mass of charge needs to be extinguished.
- The voltage drops that causes the pressure between anode and cathode to decrease and the discharge to stop. The counter operation can be started.
The counters are very efficient in their usage. Not all gamma-ray photons are registered by the counter and not all of them produce the secondary electrons. This happens because the gamma rays interact with the substances rarely, so the secondary electrons not registered by the counter are absorbed by the device.
Thickness of walls, material and gamma radiation influence on the efficiency of the counter. The greater the nuclear number Z the better is the efficiency. The same holds true for the thickness. The walls need to be of the sufficient thickness as if the walls are too thick, this won’t allow secondary electrons to pass in operating volume of the counter and this won’t cause the impulse current occurrence.
The counter also has a big disadvantage which is reflected when the Geiger counter doesn’t allow to identify particles and measure its energy. Scintillation counters don’t have such disadvantages.