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How does a dose rate meter determine dose with only a GM tool?

  1. Jun 11, 2009 #1
    I'm talking about one of these:

    http://www.eldan.biz/emall/shops/1903/45983-tb-Automess-dose%20rate%20meters.jpg [Broken]

    The use a Geiger-Muller gas chamber which can only detect when A UNIT of gamma or beta radiation (or alpha if you have a really tiny window) goes through, but it says nothing about the energy or the origin of the pulse.

    So, all it measures is (compensating for the dead time) counts/sec.

    Yet the output is in units of uSv/h or mR/h.

    From what i know, one needs to know the source constant h to go from cps to dose, using:

    D = h*A / r^2

    And to determine the source constant h, one needs to know the energy and type of incoming radiation. The GM-counter obviously does not have this information, so how does it calculate the dose rate?
     
    Last edited by a moderator: May 4, 2017
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  3. Jun 11, 2009 #2

    QuantumPion

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    How do you know it is a gas tube operating in the geiger-muller region? It could be a proportional counter, which is a gas chamber that operates at a lower voltage and can determine particle energies. Judging by the small size of that device however, it could also be a solid state scintillation detector.
     
  4. Jun 11, 2009 #3
    Neither a Geiger-Muller counter or a proportional chamber operating in a direct counting rate mode can give a reading proportional to REM (rad equivalent man, tissue dose rate). A proportional counter that creates a signal proortional to charge and followed by a current to frequency converter can, however. The best is probably an ionization counter with tissue equivalent walls and gas. A solid state ionization chamber (back biased Si diode) gives Rad, but not REM. Silicon is not tissue equivalent.
     
  5. Jun 11, 2009 #4
    Well, i'm reading about it and it says that most are equipped with a GM-counter. I've seen one and it displays uSv/h, hence the question...


    Okay, but given the large thickness of the wall, alphas can't be measured anyway, and assuming there are no high energy protons or backscatter protons from neutrons, the radiation weight factor is 1 (beta&gammas), so that rad = REM?


    By the way, i've read that an ionization chamber is VERY sensitive to noise and for practical purposes, especially a hand-held dose rate meter, not handy? I think a proportional counter would make more sense.


    One more question about that, however. Just knowing the energy is not enough. Well, it's possible if the computer inside has a list to "recognize" source of radiation from a database of common energies and determine the source constant h from that. But if that's not an option, then u/p (mu over roh, can't find the symbols) has to be known as well, to go to dose rate. Correct?
     
  6. Jun 11, 2009 #5
    Rads are REMs except when neutrons are around, and the rad dose has to be multiplied by a quality factor QF.
    The gain of a proportional counter is very sensitive to voltage, so re-calibration is very important. I once made a REM-equivalent ionization chamber system (1 liter volume) with a charge-to-pulse digitizer (~100 pC per pulse), that has been in use for many years.
    Do you mean high LET or linear energy transfer (e.g., dense ionization tracks)? I have not seen any like that. Sort of like re-arranging the deck chairs on the Titanic. I remember being in a sudden high radiation area, and one of the guys was trying to recalibrate his meter in a radiation environment. Being approximately right and confident of the reading is better than trying to be too precise and not getting any reading.
     
  7. Jun 11, 2009 #6

    QuantumPion

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    ion chambers are used for very high radiation areas because they have lower sensitivity, but do not saturate as easily. They work just fine as hand-held meters, I have an old victoreen civil defense one at home.

    I've never seen a portable computerized dosimeter that analyzes particle energies to tell you what the source is but I guess such a device would be possible to make.

    * actually after a little googleing I found that such portable gamma spectrometers do exist but are a bit pricey :)
     
    Last edited: Jun 11, 2009
  8. Jun 11, 2009 #7

    Thanks for the replies, but i'm a bit confused now.

    I'm not talking about high LET.

    Exactly how does a proportional counter go from a count rate + energy to a dose rate? What formule is used? I only took a course in dutch, and the formula depends on whether it's betas or gammas we're talking about.
     
  9. Jun 11, 2009 #8
    Lennox:
    The definition of 1 rad is 100 ergs per gram. If you use a gas like ethene (C2H4) which is tissue equivalent, in an ion chamber, there are about 34 eV per electron ion pair. Because the ethene mass is 28 grams per 22.41 liters at STP, you can calculate the charge yield per rad in 1 liter of ethene. You could also use air or argon, but it is not tissue equivalent.
    See Eqn (5) in (pdf is about 1.7 MB):
    http://ab-div-bdi-bl-blm.web.cern.c...ectors/Literature/schaefer_biw02_tutorial.pdf
    If you get 700 picocoulombs per cm3 per rad (argon gas), then 1 millirad/hr is 700 pC/hr in a 1 liter chamber. If you use an integrating charge-to-pulse (current to frequency) converter with a calibration of 700/60 pC = 11.66 pC/count, then you will get 1 pulse per minute.
    Bob S
     
  10. Jun 12, 2009 #9
    I see. So a gas ionisation chamber is design-able with tissue-equivalent gas, where the amount of charge collected per time unit can, if calibrated, be converted directly to dose rate?

    I did not know detecting charges in the order of pC's was seperable from noise.
     
  11. Jun 12, 2009 #10
    See formula (5) in
    http://lss.fnal.gov/archive/2001/pub/Pub-01-337.pdf
    As long as the gas ionization constant (e.g., air is 34 eV per ion pair) and the gas density (grams per liter) are known, the picoCoulombs per rad can be calculated. A charge digitizer can then be built to get a digital pulse output with a direct calibration in rads or Gy.
    FermiLab (Fermi National Accerator Lab) near Chicago has been using an ionization chamber type area radiation monitor since 1971. The present design uses an electrometer amplifier with a 2 femtoamp input bias current made by National (LM6041) in a digitizer circuit with a sensitivity of 0.25 microGy (25 microrads) per pulse. See
    http://lss.fnal.gov/archive/2001/pub/Pub-01-337.pdf
     
    Last edited: Jun 12, 2009
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