Answer to Question #2067 Submitted to "Ask the Experts"

Q

We are planning to devise a new radon chamber for environmental radon monitoring and to measure the radon emanation rate from soil. We came across some papers that state to avoid thoron entering into the chamber; glass membranes are widely being used. Our questions: (1) How does the glass membrane act as a shield for thoron entry and only radon enters through it? (2) What are all the physical properties of such a glass membrane? (3) Is permeability a factor for this shielding of thoron? If so, explain it.(4) Can't the alpha emitters of thoron daughters enter the glass membrane? (5) What are all the other materials (instead of the glass membrane) available to prevent thoron entering into the chamber? (6) Where can we order this glass membrane or other related materials?

A

Unfortunately, your multiple questions would require more time and effort to answer than we can ask of our volunteer experts. We hope the following is adequate for your needs: I'm not familiar with using a glass membrane as a thoron barrier, so I do not have any information related to the properties or availability of such a membrane. However, I am familiar with using various plastic barriers to prevent thoron entry. The basic idea is to create some barrier such that the time to penetrate that barrier is significant compared to the half-life of thoron, and not all that significant compared with the half-life of radon. Because the half-life of radon is 3.8+ days, and the half-life of thoron is about 55 seconds, it is easy to see that if one creates a barrier where it takes something like 10 or 15 minutes for the radon/thoron to diffuse/penetrate/travel through, then essentially all of the thoron will decay before it can get through the barrier, whereas most of the radon will make it through. One could do the exact same thing by pumping the gas through a long tube so that it takes at least 10 or 15 minutes for it to get through and then filtering the gas at the end. There are plastics that all radons (radon/thoron/actinon, etc.) can penetrate, such as polyethylene. I am not sure what plastics are being used right now for thoron barriers, but if I was designing one, I would look at polyethylene and vary the thickness until I found a proper thickness such that the penetration time was on the order of 10 to 15 minutes. I presume that others have done this already. Radon (any of the radons), being a noble gas (and to my understanding radon actually dissolves in polyethylene although I'm not sure exactly what that means in this case), will permeate through the polyethylene. Any decay products that are created along the way, due to the decay of the radon, being solids will not permeate through the plastic and therefore will not breach the barrier. Obviously, since the half-life of actinon is even shorter than that of thoron, any such barrier for thoron will also serve as a barrier for actinon. Philip Jenkins, CHP, PhD