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What is a recombination ionization chamber?

  1. Apr 4, 2012 #1
    I have been under the impression that recombination is a bad thing, not a good thing. And you increase your voltage on an ion chamber until recombination is eliminated. I also understand that recombination is a bigger problem for neutron and alpha radiation because its ionization trail is much denser (and leaves a column of ions), and since the ionization happens so densely, the ions and electrons are very close together such that their electric field attraction is much greater, and so the electric field of the ion chamber must be much higher to overcome this dense column from recombining.

    So, I am curious what a recombination chamber is. Does it use recombination for some measurement purpose? How could recombination be a desired effect in an ion chamber?

    Edit: I tried researching online, but the only article I could find that looked helpful required a purchase, and it was expensive with no guarantee of answering my question.
     
  2. jcsd
  3. Apr 5, 2012 #2
    Recombination in ion chambers is generally a bad thing that adversely affects their operation. There are several causes of recombination in ion chambers and proportional counters

    1) Dense track (high dE/dx) recombination
    2) Volume recombination
    3) Electronegative gases or impurities
    4) Pulsed radiation operation; e.g., like pulsed neutron BF3 chamber

    Raising HV generally reduces recombination loss, but often not possible in proportional chambers.

    You might review this
    http://books.google.com/books?id=WN...CEYQ6AEwAQ#v=onepage&q=boag dosimetry&f=false

    or read chapter on ionization chambers by Boag in this book by Attix

    http://books.google.com/books?id=r6BFAAAAYAAJ&dq=boag dosimetry&source=gbs_similarbooks
     
  4. Apr 5, 2012 #3
    Hi Bob S,
    thanks so much for your help.

    I understand what you have said. I have come across a special chamber called a "recombination chamber". I actually have a better idea what it is after doing a better google search. This article explains:

    http://www.irpa.net/irpa9/cdrom/VOL.4/V4_92.PDF

    Its used to determine quality factors of unknown radiation fields by finding the "slope" of recombination from different voltage levels.
     
  5. Apr 5, 2012 #4
    Interesting paper. It utilizes the collection inefficiency to get the quality factor (QF). Their gas is not TE (tissue equivalent; too much hydrogen). Because my interest is more in radiation protection around pulsed radiation sources, I would prefer measuring correct absorbed dose, with no dependence on either pulse duty factor or QF.

    I am not sure what their Fig, 1 plot is measuring; QF for neutrons is maximum between 1 and 10 MeV.

    I would have more confidence in method (1) than method (2). Their circuit diagrams are very incomplete.
     
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