Why is argon gas used in a Geiger Muller tube?

Answer 1

I'm assuming you understand the general workings of a GM counter to begin with. Basically, all of these ionizations occur, and positive and negative ions collect at the cathode and anode, respectively. The problem is that once the anode is full, the voltage stops. The molecules will recombine, but the electrons will enter a high-energy shell (instead of K, where there are openings). When the electrons de-excite, they release massive amounts of radiation in the form of x-rays. These x-rays then recirculate and start the "avalanche" all over. Halogen is used because it donates electrons to the positive ions so that they don't recombine with their own electrons. Additionally, halogens are typically good absorbers of UV radiation. Alcohol (organic gas) can also be used but the gas is expendable and runs out quickly. Halogens are used because they can recombine after the molecules dissociate.

Answer 2

Argon is electrically complete, in the outer electron shell, making it electro-chemically stable. Placing an electric potential across argon, if not too high, will not result in electric current flow due to ionization.

However, in the presence of ionizing radiation, such as gamma or beta, electrons can be "knocked off" the inner wall of the GM tube by that radiation. Those electrons are drawn towards the center, more positive electrode. Along the way, the electrons can interact with other electrons in the argon, "knocking them off" as well. This leads to a cascade effect and, by the time the electrons reach the positive electrode, they can induce a measurable pulse of energy, said pulse representing the initiation ionizing event from the radiation interaction.

This creates a radiation detector, one of several possible designs.

That ends the answer to the specific question but, for completeness sake, there is more...

Depending on the voltage between the outer case and the inner electrode, the GM tube can operate in linear (proportional) mode, or in avalanche mode.

In linear mode, the cascade effect is incomplete, and the pulse seen at the inner electrode is proportional to the energy of the initiating ionizing event. This allows the user to not only quantify the level of radiation, but also to qualify it, by understanding its energy spectrum. Often, in this configuration, there is a quenching gas mixed in with the argon (or neon, etc.), a Penning Mixture, which reduces the cascade effect.

In cascade mode, however, the voltage is higher, and the cascade effect is complete. Each ionizing event results in a pulse, but the amplitude of the pulse is no longer proportional to the initiating ionizing event. The downside is the loss of qualification of the energy spectrum. The upside is the increase in sensitivity this affords. The choice is simply one of intended use - either we attempt to measure dose rate, which is energy dependent, or we measure contamination, which is not.

Answer 3

Argon gas is used in a Geiger Muller tube because it is inert, which means it does not easily react with other substances that could interfere with the detection of radiation. Additionally, argon gas is efficient at transmitting ionization produced by radiation, allowing for accurate measurements of radiation levels.