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On the other hand, there are several great advantages to the Ion Chamber ...
For one thing, my 440 RF/C has a low range of 1 mr/H full scale, the older 440 is 3 mr/H full scale. The greatest differences in Ion Chamber has to do with two major factors - pressure and size.
As for pressure, yes, it GENERALLY involves exotic gases and high pressures ... that being said, there is another side to the coin - vacuum, or at least reduced pressures.
To avoid the entire morass of pressure and gasses one can, of course, increase the size of the chamber - a one gallon paint can at one atomosphere can become an extremely sensitive ion chamber - though size isn't the only way to increase sensitivity. Creating and using the capacitive effect can increase the sensitivity of an ion chamber by an order of magnitude....
Even the lowly little CDV 710's, 15's, and 20's can make for very interesting experiments - but generally at the expensive of having to "turn back the clock" in one technological sense. The failure to reach ultimate sensitivity in most units is not the nature of the chamber, but rather the electronics - solid state components, by their very nature and constuction, CREATE noise - it can't be helped, the junctions themselves are a source of low and even high level noise that cause the designer to have to reach compromises to eliminate noise or at least bring the signal above the noise floor.
Using vacuum tubes, especially electrometer types can prevent this problem and greatly enhance performance. In an age when we consider the vacuum tube to be a relic of the past and relegate it to the closet along with the dinosaurs - we often have to return to our "roots" to take a step forward.
A bit of a conundrum, yes, but true.
The Ion Chambers, at least in my opinion, are much too often over-looked by the home experimenter - his only experience being poorly or under engineered civil defense units ....
Jim RabidWolf
Uncle Rabid ( http://www.unclerabid.com )
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----- Original Message -----From: GEOelectronicsSent: Sunday, October 01, 2006 7:55 AMSubject: [GeigerCounterEnthusiasts] Ion Chamber type detector Factoids
1 ATM ( atmospheric pressure) Ion Chambers, even of excellent modern design reach their low end at about 3 mR/H full scale. This is with every conceivable improvement, far beyond my meager efforts. Their chief advantage is smooth response to a very wide range of energies, again something that is achieved only with superlative design and execution. A GM tube of any description and all sodium iodide scintillators have a big swing in their energy response, some of which can be corrected for at the expense of ultimate sensitivity ( energy compensated GM tubes are wrapped in lead or cadmium). Basically the wrap cuts down the lower energies, making the overall response from the tube more linear.Energy response is the parameter that would make one isotope read say, 1 mR/H when another isotope of the same field strength would read 10 mR/H.Very sensitive ion-chambers are made with high pressure inside, as much as 10X atmosphere or about 150 pounds per square inch. These offer smooth energy response and extreme sensitivity down to the microR/H. Usually these are coupled with sophisticated computerized electronics, especially in the environmental monitoring mode. Handheld units are likewise very complex and needless to say expensive to the extreme. Earlier, much simpler versions would include the RADECTOR series, but their pressure is lower and sensitivity is of a lesser level too.None of these would be fodder for the home lab constructor in my opinion, as the shell material is of exotic nature, and 150 PSI lab gas is nothing to be played around with.Far safer are the gas flow proportional detectors. These are very sensitive and give an output that is proportional to the number of radiation events, making them ideal for true quantification studies. Their design is simplicity itself, and many examples can be purchased for peanuts on ePay. Electronics are slightly more advanced than the ordinary handheld, but again many units by Ludlum especially are already rated to run these detectors. The only reason they are not much more universally used is the fact that they take a special gas called P10, 10% methane, 90 percent Argon- not to mention special regulators and gas flow monitors. Nonetheless, commercial systems are available and can sometimes be had used at Home lab prices. I have managed to put together a complete gas flow system, comprising of Matheson regulator, Ludlum gas flow monitors, hoses with QC fittings, and an array of different detectors.....all at modest cost but over a long period of time. No gas has been purchased yet so I can't report on results, but this is a definite upcoming event - funds and time allowing. So many projects, so little time................. and far less $$$$$ Geo
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