Saturday, October 1, 2011

Homemade Geiger Counter - Part 1 -- The basics

My Blog has been kind of quiet lately. the Reason is that I have been focused on building a Geiger counter. So here is what I have learnt.

Geiger basics. A Geiger counter is basically a simple event counting device. A Geiger tube connects to an amp, noise filter, comparator circuit and some form of counting system.

The Geiger tube is basically a outer housing, containing gas and a cathode and anode. The housing can be metal or even ultra thin foils, The gas is normally some inert gas and the cathode/anode can be made using anything from glass to metals.

The Geiger tube is charged to high voltage, this excites the gas inside the tube. In this state no current is yet flowing and the tube appears as an open circuit. When radiation is incident on the tube it must first travel through the outer housing, and strike one of the excited gases particles. This collision will tear free one or more electrons and these with then bump into other atoms and tear free more electrons and thus creating an avalanche effect.

Note carefully the limitations of the device,
1) the radiation must travel through the tubes outer housing and strike a gas atom. So the outer housing of the tube will absorb some particles without causing a detectable effect. Also particles can travel right thought the tube without causing an event. The size and thickness of the tube also effects how sensitive it is to radiation. But designers of the tube need to consider mechanical damage from shocks and contact with other items. Most directional tubes have a very thin wall made of mica that is very delicate. In summary there are a large range of tubes with different sensitivity to the various forms of radiations.

2) The main incident particle transfers energy to a single electron knocking it off the atom, this electron then hits another, these 2 electrons then knock more free and so on until there are no more electrons to be knocked free, this is the avalanche effect. This sea of freed electrons flows out the cathode, and eventually the gas recharges to high energy state. Now if the radiation was to knock a second electron free or second unconnected event was to hit the gases in the tube at the same time the tube is discharging then there is no way to tell the difference. This period is the tubes dead time. A Geiger counter that is taken into a high radiation area will suddenly go silent this is not broken it is because there are so many events hitting the tube that avalanche will never stop and the comparator will never detect the switch between Geiger's tubes quiescent state and the avalanche discharge state

3) This is a very simple device all it does is count events. Using it for anything other than the number counts requires some level of interpretation.

What these limitations mean is that what caused the event that was counted cannot be determined. If we assume your measuring air and it was an alpha particle then it is unlikely to penetrate skin and is therefore a lower dose. If it was gamma it will likely pass through your body and the wall behind u, thus giving it a bit more potential to cause some real damage to you. (Its not that simple but sufficient to highlight my point). So the conversion of counts to dosage is highly dependent on the type of tube your using, and the procedure that you used to measure it, what is causing the count (IE the spectrum of radioactive substances your measuring), and what the interaction of the substance is with the living being being dosed. In summary the devices dose conversion calibration needs to understood rather carefully, and the device used correctly.

Now Geiger counters don't necessarily need to be made with a a Geiger tube. A cheaper an less accurate design is to apply a similar circuit to a standard solar panel or LDR that is covered to prevent the entry of normal light but allow gamma radiation to still be incident on the panel or junction. These designs will measure radiation however, solar panel's and LDRs are not designed for radiation readings and so these designs have even more drawbacks than a standard Geiger counter.

Unfortunately post fukushima, Geiger counter prices sky rocketed and supplies became rarefied. These cheaper devices where affordable and available in a market with buyers who were desperate to get their hands on something/anything that can read the radiation. As a result these cheaper devices have been sold en-mass in japan and few customers realize the difference between what they have purchased and what the real item is. Keep in mind that the device does work so its technically not illegal to sell it as a radiation measurement instrument... Its just that if the customer is expecting a real device then they are getting hoodwinked by their own limited understanding. I have tried explaining this to Joe average before only it never seems to work. The discussion seems to always work out like explaining the difference between 24 Carat Gold and 12 Carat Gold to a person who only wants to understand that Gold is yellow.

Also note that Geiger tubes are far from the ultimate device to measure radiation a scintillator crystal is capable of producing varying colors and intensity of light as radiation strikes it, devices using these crystals are a level(and cost) far beyond the humble Geiger counter.

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