Permeability of various material to Radon gas

  1. Has anybody done testing on how well various materials can prevent the passage of Radon gas? For example, a container made of thin low density plastic will apparently gradually permeate out Radon, but a high density plastic container will not. Other materials I have in mind are rubber, electrical tape, sheet aluminium or silastic goo.

    I am asking this because I am making up containers to retain Radon emissions from sediments for the sake of measuring the Rn-222 activity and hence the sediment's parent Ra-226 activity when in eqilibrium.

    A further question: If I have a material that permeates less Radon than is produced from the contained sediment, will the Radon:
    1. gradually reach eqilibrium with Ra-226 (the parent) over a longer period of time
    2. reach eqilibrium at an activity lower than the parent Ra-226.
    3. not reach any equilibrium at all.
    4. none of the above!?

    Chris
     
  2. jcsd
  3. One avenue of enquiry could be the HPA in the UK, formerly the National Radiological Protection Board, as it has a specialist radon dept. There is not much detail on their website but they may have a contact who could advise you.

    http://www.hpa.org.uk/radiation/services/radon/index.htm
     
  4. Sorry to harp on about this one again but I need to know some opinions:

    Fact : If I have a Rn-222 emitting substance, i.e. soil, filled up in a gas tight container then it will take at most several half lives of Ra-222 to reach equilibrium with the parent Ra-226. The equilibration period is therefore about 3 weeks or so given the 3.8 day halflife, but it is also dependent on how much Rn-222 was lost to the atmosphere before putting the soil into the container.

    Question 1) If the container is mostly full of air, will this delay or suppress the equilibration of Ra-222?

    Question 2) If the container has a very tiny gas leak, what is the effect on Rn-222. Will it equilibrate at a different and lower level of activity, or will it eventually reach equilibrium after a longer period of time, or will it go nowhere!

    Regards

    Chris
     
  5. Ra-226 is the parent of Rn-222, not Ra-222, but that's probably just a typo.

    Question 1) The air shouldn't affect equilibrium, decay is a nuclear process and chemical processes with air shouldn't affect the Radium or Radon on that level.

    Question 2) If there is a leak, another decay constant is usually taken into the calculations that considers the amount of Rn-222 that leaves the system through the leak and that is added to the natural radioactive decay constant. This is your effective decay.
     
  6. Morbius

    Morbius 1,160
    Science Advisor

    Chris,

    No - the radioactivity of the parent nucleus, and hence the decay rate is UNAFFECTED
    by the presence of air or any other substance. The decay rate is determined by what
    is going on in the nucleus of the parent nuclide, and that's not suppressed by the
    presence of air in the chamber.

    An analogy of an atom, the nucleus is like a grape sitting in the Grand Canyon. How the
    grape decays is independent of whether there is another canyon in the next state over.

    It will equilibrate at a lower activity level. The source rate will be unaffected - that
    will be the same. However, there are now two loss mechanisms - decay and leakage.

    Since the source is unaffected, and the sum of the decay and leakage losses has to
    balance the source rate - the equilibrium amount of decay will be less if there is a leak.

    Therefore the equilibrium concentration will be less.

    Dr. Gregory Greenman
    Physicist
     
  7. theCandyman and Morbius

    My reason for questioning the effect of air in the container is because the Rn-222 produced in the soil will have another space to diffuse towards rather than staying insitu the soil. When Rn-222 decays shortly to daughter products my thoughts were that those products may attach to the container walls and lid or back to the surface layer of the soil rather than where they were inside the soil. Either way, detection of the daughter products (which are what I am measuring by their gammas to obtain the soil's Ra-226 activity) will be affected by their presence being ununiformly distributed. Could that not be so?

    Chris
     
  8. Chris

    I run a gamma counter for 210Pb dating among other things. Anways our sediment samples are freeze dried, put into plastic (HDPE?) test tubes, tamped down a bit (2cc seds which is 1-2 g ussually) and sealed with 1 cc of 30 min epoxy. And after waiting about 20 days for Equib of radon to be achieved we run the samples.

    For as I know what we do is pretty standard, and I have rerun old samples and never found an appreciable diff in our 214Bi activity which is easier/more reliable to measure than the 226Rd directly (we do that to).

    What are you planning on doing with your seds?

    Cheers,

    Johan
     
  9. Sorry 226Ra not 226Rd

    Cheers,

    Johan
     
  10. Johan

    I operate 8 gamma detectors for sediment tracing and dating. Since my sediment masses range from about 2 to 140 grams I have a variety of containers, but inevitably there is at least some empty space in what ever container is used as filling them to the top is not usual. Calibration for geometry is thus made compicated, but that is another story. Your method of sealing with epoxy inherently dodges the air space issue. I am required to reclaim the material afterwards for other form of analyses so I cannot entomb the sample too permanently. I measure Bi-214 and Pb-214 as proxies for Ra-226 and it is some of those nuclides that I fear could migrate from the soil to the container walls, via Rn-222 form, where the empty space is. That process may confound the measurement of those nuclides unless someone could shed some light on the issue. That is the subject of my original post!

    Regards

    Chris
     
  11. Chris,

    While I'm not going to be able to help you with your original question I'll just spitball a thought or two on it.
    1) Radon will escape from any soil matrix your testing to some degree.
    2) If you have an air tight lid (thick HDPE) on for 20+ days before and during sample run the radon and it's decay products will not escape the container.
    3) When it decays the resulting products will rain down on to your sediment sample. My main concern would be whether electro static interactions cause them to be attracted to the plastic container and then stick.

    So while we seal ours in with epoxy, we do sometimes cut them open to get the seds for other analyis. We probably recover 3/4 of the the original mass but that will vary with your container and sed load.

    My next suggestion I'm sure you thought of given your orginal question. Basically make some air tight gasket you could piston down into the container to seal in the seds in.

    Out of idle curiosity what are your count times and who manufactured your gama specs? We have 2 coaxil germainium detectors by Ortec and we usually run samples for 23 or 47 hrs. 210Pb, 214Bi 226Ra, 137Cs and 40K is what we look for.

    Its not often I get to talk to someone else running a Gamma spec so its nice to see what others are up to.

    Cheers,

    Johan
     
  12. Jonan

    Counting time for me is however long it takes to acheive reasonable counting statistics, usually more than 1000 counts per peak is required for about 5% uncertainty except for Cs-137 where the quantity may be tiny. About 1 to 2 days counting per sample is typical.

    I like the idea of a "piston" of some sort to take up air space and I had thought of it before. However, things will be simpler if it proves to be not necessary.

    I keep some contact with a few other gamma spec labs in Australia. I suppose that we should communicate via e-mail or message about futher gamma spec issues rather than using up forum space! :wink:

    Regards

    Chris
     
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