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Name: Shakeel A.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 4/22/2004


Question:
Why do we use inert gas (principal gas) say neon inside the Geiger tube rather than any other gas? If the radiation enter the Geiger tube, it will first ionize neon or bromine (quenching agent)?


Replies:
Quenched tubes are a slightly subtle system. If I talk it through, perhaps the explanation you seek will be included.

The radiation particle streaks through the gas in the tube. It ionizes a number of gas atoms and molecules along its path, making positive ions and free electrons, which have no particular velocity. This is only a smallish number of electrons, maybe 100, maybe fewer, a little too small to be measured reliably with a cheap transistor amplifier circuit. Also, a large fraction of these free charges will try to recombine and never make a net current between the electrodes of the Geiger tube. With a low voltage between the electrodes, it could take milliseconds to collect the small remaining charge. A very small charge pulse, delivered slowly, is doubly difficult to pick out of thermal noise in the electronics.

So we decide to amplify and speed up the initial trail of ionization by means of avalanche multiplication. We place a higher voltage across the electrodes, almost high enough to make a spark even with no radiation. Then, when radiation makes free electrons, they "fall" hard towards the positive electrode. Due to the gas population, on the way they periodically bump into another gas atom or molecule. By that time they have accumulated enough falling speed to knock another free electron out of that atom. This continues as an "avalanche" chain reaction, which can multiply one loose electron into briskly moving millions. And this good-sized current starts up in nanoseconds.

Later on we change our mind about all that. We have started a big current breakdown which has no reason to stop. Once the current is big enough to detect, then we want it to stop so we can start watching for another radiation particle. We could stop the current by allowing a capacitor to finish discharging so the current stops, then waiting until the free electrons and ions, now stagnant, all recombine. This is how xenon flash lamps in cameras and simple neon-bulb blinkers work all the time. But these take microseconds to discharge, a fraction of a millisecond to finish recombining, and then more milliseconds for the capacitor to charge again, so it is a little slow for us. We would prefer a tube that works from 1 pulse per second to more than 10,000 pulses per second.

Using a quenching species happens to fill this need for us. The avalanche floods the vicinity with ionizing current. A large percentage of the available Br2 is split into Br atoms, Then there are enough electron-hungry atoms, to reduce the average mean-free-path enough, so that the average impact energy is too low to ionize the next atom, then the electrons are trapped on massive, low-mobility atoms until they drift around and find a positive ion to recombine with, and the last tail of the current pulse is stopped. There is something about the quenching agent absorbing UV photons, too, so they do not start other avalanche trees. I do not have a feel for that.

To do good avalanche multiplication you want several things:

- a permanent gas, so the population does not change with temperature outside the tube.

- no "electron affinity", so electrons never get captured without multiplying. They should always either bounce off with creation of another free electron, or bounce off clean but get to try again farther "downhill". This eliminates 100% O2, F2, Cl2, Br2, which can carry an added electron, and also easily break into atoms which are very hungry to become negative ions.

- no permanent chemical changes caused by this electric spark through the gas. This confines us to elemental monatomic or diatomic gasses.

Other than the inert gasses, that leaves 100% H2 and 100% N2. Maybe they would work. Hydrogen is notoriously tough to keep in some tubes, and we want ours to have a thin window to let in low-energy particles, and we want it to last unchanged for years. Not sure if it would self-quench well, being relatively electropositive and light in the nucleus too. Being the lowest-numbered element, it would have the worst possible radiation sensitivity. I suppose 100% N2 gas might just be useable. Because all the avalanche gas is also quenching gas, maybe it would quench the current sooner than Ne with small percentage Br2. And so the current pulse might be some factors of 10 smaller in total charge. Maybe it would be reluctant to show a distinct "avalanche now / quench later" behavior. But someone should try it. I did see some mention of a tube with air inside (at low pressure, i.e. 50 Torr). N2 would be the avalanche gas, and O2 would be the quenching agent.

In your Ne/Br2 Geiger tube, the radiation particle ionizes nearby neon and bromine indiscriminately. True, Br2 has a higher ionization cross-section than Ne, but it is not enough more to matter. Most of the ions are from neon, just because the percentage of bromine is very low, maybe 1%. Once an electron is freed, it does not matter which species it came from. It still helps the avalanche grow. The left-behind Br2+ is electron-hungry, but until the current pulse is large, there are very few of them, maybe 1% of 1% of the total gas population density. Too few to change the average free-falling distance of the electrons and stop the avalanche. As to whether every Ne+ will find the nearest Br2 and transfer its [+] charge there, I do not know. If the process takes a little time, it will not suppress the avalanche. It may even be a normal part of activating the quenching with an appropriate (microseconds) time delay.

Electronic circuitry has come a very long way since Geiger tubes became the popular and practical way to look for radiation. I wonder if one could make about as good a detector today, using only a pure inert gas, medium instead of high voltage, amplifiers with sensitive field-effect transistors, and active electronics to briefly stop and restart the voltage after each pulse is detected. It might actually be easier that way, today. Maybe it would increase battery life. Then I imagine I would choose Xenon, because its high atomic number would maximize the density of the initial ionization trail. Xenon might be excluded from consideration in Geiger tubes, because Bromine quenching gas was included, and XeBr2 can form and cause variable amounts of early quenching. Perhaps Neon was chosen because it is the heaviest inert gas that cannot make any halides.

Weird how these periodic-table games narrow down fast.

Read this: http://fusor.net/board/view.php?bn=fusor_neutrons&key=1020359169.

Here, Richard Hull describes Geiger tubes very well, touches your question of other gasses, and sounds like he has made electronic substitutes for chemical quenching.

Jim Swenson



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