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truthrevealer
New Zealand
2 Posts |
 Posted - 06/20/2014 : 01:13:11
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| On this forum I read that all gm counters should give the same cpm.This does not make sense to me,since tubes differ in size and therefore more radiation/minute will pass through those which are larger (present a greater surface area);hence,higher cpm.I am assuming that a click means that a particle or ray has been detected.For my 320+ I read that it has been factory calibrated for inside and outside use.Is this calibration for microSv (for this tube I notice that 20cpm = 0.1 microSv)? |
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| Reply #1
Alchemy2
Canada
89 Posts |
Posted - 06/21/2014 : 03:59:13
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Truth,
You are correct: different tube geometries and sizes will result (or should) in slightly different CPMs. Each click or scintillation created by the GM tube is an ionization event that has been detected by your units "window" of energies and thus are read as CPMs integrated or summed over time, averaged per minute. And yes, the 320+ has 200 CPM = 1.0 uSv. I read on a review of GM tubes that the M4011 is closer generally to 151.5 = 1.0 uSv, and when I compare my 320+ to my dosimeter that I have (also with a M4011 tube in it) I get almost perfect agreement when using that calibration factor when averaged over time. My Dosimeter was calibrated to 137Cs. I use a 152:1 ratio or very close, for all three cal points on mine. Any time that you reset to defaults, you'd have to redo this.
So, I have been using that factor. However, really any number in the 150-200 range should be fairly close for low dosage rates. These units are more semi-quantitative unless calibrated with a radioactive standard. CPMs are resulting from approx. a 20% efficiency in the detector, which is typical for a GM tube like this. uSv/h is a dosage rate based on the amount of ionizing energy absorbed by a person, which varies widely with the intensity of the field, the nature and energy of the particle, etc.
Please forgive me if you hate math, but let's do an example: 90Sr (Strontium-90). It's a beta emitter with a ~0.55MeV emission, with a half-life of 28.8y. Forgetting activity decay over time for simplicity, if you had a 1uCi sample disk of this material, it behaves pretty close to a point source radiator with spherical symmetry. A full sphere has 4pi steradians (solid angle measurement). Imagine particles emanating in 3D as a spherical "bubble" about the source. Now we put our 320+ say 1cm from this source, centered over the disk. If we got for example, a reading (above background) of ~1500CPM off this source, then that is 1500 counts per min, or 25 events/second (25Bq) [25x60=1500]within the "window" of our tube appearing to look like a rectangle "slicing" this sphere. I'll use 8x1cm for this example, which is pretty close to the cross sectional area of the tube. If we take the detector to be around 1/8 steradian in geometry in this case (close enough, if we consider a radius of 8cm about the sample for this example area ratio) then the total counts that would have been seen in our detector if it could "wrap around the sample" would be: 1 / (8x4pi) = 1/32pi = ~0.099 0r very close to a 1:100 ratio, or 1%. This means our detector tube "sees" 1% of the emanations from this sample. So, given that, this means our 25CPS count needs to be corrected: 25x100 = 2500Bq.
We're not done. A 1uCi sample = 37000Bq. So our detector efficiency is: 2500/37000 = 6.75%
A range of energies for ionizing particles can also give the factors needed to convert into uSv/h dose rates. So, if my detector is seeing in it's "window" 1500CPM, and the actual equivalent dose rate for that was shown to be around 10uSv/h, then 150 = 1uSv/h. and so on.
I just arbitrarily picked the numbers, but the math is fairly sound. Given our tube shape and geometry, this means that a more uniform radiation field, such as those seen in contamination areas like Chernobyl, Fukashima, regions of extended NORM deposits, or a Uranium rich ore site, environmental detection of radiation levels above background, etc. and will be pretty much independent of the detector from the perspective of WHAT the unit sees. Knowing the emanation and energies from them, one can then use that ratio of CPM:dose rate/h as a good general indication of the exposure.
It's not a perfect science when using tubes of this type, because the response is variable somewhat over its range. Also an Xray, beta or gamma can trigger this tube count. Not knowing which, we can only get a general idea of relative radioactivity or ionizing radiation in the vicinity.
Hope that didn't spin you about too much, and clarified it a bit...
D |
The more I learn, the more I realize I do not know! |
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| Reply #2
Alchemy2
Canada
89 Posts |
Posted - 06/21/2014 : 04:15:40
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| BTW, before someone with a really good physics background jumps all over me for the detector geometry, technically I should treat the point sample cutting a 1cm radius sphere, and integrate the lengths of the tube outside this as a 1/r2 response, to be more accurate and get a "mean" geometry response. I leave it to those with that background to do this for us. I may later, but not everyone here understands integral calculus. Math for that can be pretty heavy for many.... |
The more I learn, the more I realize I do not know! |
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Count Rate depends on Tube?
