Geiger counters: what you need to know about the principle

After Japan discharged its nuclear effluent into the sea, the population grabbed not only salt, but also Geiger counters. Geiger counters are nuclear detectors that are able to detect different types of nuclear radiation, such as alpha particles, beta particles,radiation levels geiger counter gamma radiation, and in some cases neutrons, through a sort of indirect method. The principle on which this Geiger counter is based is very interesting, but before we go deeper, let's discuss the structure of this counter.

As shown in the figure above, this is the general structure of a Geiger counter. The structure consists mainly of a Geiger Miller tube, which is actually a hollow metal cylinder and contains some sort of gaseous medium inside. The gaseous medium usually consists of some kind of inert gas (e.g. argon) and other alkyds.

Now, this metal cylinder is connected to the negative terminal of a very high voltage battery, so it acts as a cathode. Through the center of the cylinder there is a metal electrode made of tungsten which is connected to a load resistor and again to the positive terminal of the same battery. The load resistor is also connected to an electronic device that detects the voltage drop that occurs across the resistor and counts it.

So how does this device count nuclear radiation? When nuclear radiation enters the gaseous medium in the Geiger-Miller tube, it causes ionization. What is ionization? When some external nuclear particle such as an alpha particle enters a material medium, it collides with the molecules of the material and transfers energy to it. The electrons in the outermost layers of these molecules or atoms absorb some of the energy, and if the energy is high enough,digital nitrate test these electrons will escape, resulting in a positive ion and a free electron.

We can see that the central tungsten rod acts as the anode and the metal cylinder as the cathode. When connected to a high-voltage battery, a strong electric field is generated between them. When these positive ions and free electrons are produced by some external nuclear particle, they are affected by the external electric field: the electrons are accelerated towards the anode, while the positive ions are accelerated towards the cathode.

What happens now is that once the electron is accelerated to a very high velocity, it will also collide with gaseous molecules located elsewhere, and this electron will be able to further induce secondary ionization. Thus, we will get a free electron induced ionization by a nuclear particle and an electron produced by the secondary ionization of that electron. But it doesn't end there, they continue to induce ionization, eventually creating a chain reaction or avalanche effect, which causes a lot of ionization along the path before it reaches the central electrode, a phenomenon known as a Townsend avalanche.

Once all these electrons reach the center, they are absorbed by the anode and then they will move along the circuit and cause a voltage drop across the load. The electronics on the load will detect this and count it. Now that these electrons have accomplished their mission,best water hardness test kit they will continue to move along the circuit and combine with the positive ions that have collected on the surface of the metal in order to produce neutral molecules, thus returning the entire device to its original state. Thus, the entire process caused by the presence of a nuclear particle will produce a count.

The process from the ionization caused by the nuclear particle, to the creation of the Townsend avalanche, to the recombination of the electrons with the positive ions, takes a little time. During this process, the nuclear detector is unable to detect any more external particles, so the detector is somewhat dead during this time. As long as this process is not complete, it cannot detect another nuclear particle, so this time period is called dead time. The dead time of a Geiger-Miller tube is usually about 200 to 400 microseconds.

In addition, when the electrons complete the circuit and recombine with the positive ions, the process may result in the emission of a photon. The energy of this photon may be high enough to be able to cause another avalanche effect as well. We do not want this to happen because the counting is already complete and we only want the counts associated with the external nuclear particles.

Therefore, in order to prevent this from happening, certain measures need to be taken, which are called quenching. One of these methods is chemical quenching, and what we use is to mix the gaseous medium with certain other organic compounds (e.g., alcohols). In this case, we have 90% argon and 10% alcohol. The effect of the alcohol is that whenever an electron recombines with a positive ion, instead of emitting excess energy in the form of a photon, the excess energy is usually released in the form of vibrations or spins that are absorbed by the nearby alcohol.

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