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Alpha detection, Radon vs. Thoron

By George W. Dowell

Radon gas collection is one of the few "separations" that can be legally done in the home lab, and is relatively easy.

There are a number of Radon isotopes, all coming from decay of different Radium isotopes. The two of major interest to us in the home lab are 1) Rn-220, also called Thoron, from Radium 224, which is in the Thorium-232 decay chain, and 2) Rn-222 the one commonly called RADON when we are referring to Radon in homes. Rn-222 comes from the Radium 226 in the Uranium-238 decay chain. Radon is a gas, heavier than air and sometimes is called "radium emanation".

Collection can be accomplished by adsorption onto activated charcoal , or air pumping. see: http://www.p2pays.org/ref/17/radon/pubs/devprot3.html

Once the sample is captured, you have pure Radon for a time, depending on the half-life of the isotope.

Our goal is to count Alpha particles coming from the Radon, and time the half-life. Using a Lucas Cell makes it easy. A Lucas Cell consists of a container, whose inside walls are coated with Zinc Sulfide, at one end is a clear window so the scintillations can be observed and counted by a photomultiplier tube in a dark enclosure, the other end contains one or two valves for the introduction of air. ( Photos in FILES section) Cells with one valve must be first pumped to a vacuum with a pump,even a simple hand vac pump will do, then an air sample is "grabbed" by releasing the valve until the air is sucked inside, then closing the valve again. Cells with two valves use a simple air pump to circulate the air, until a sample is collected, when both valves are closed.

Once the air and Radon are inside the Cell, you must count the CPM being created by the Alphas right away. For Radon-222, the counts will remain steady over a period of time, hours at least. The reason for this is the long half-life ( 3.8 days) of the Radon itself, and also the high energy of the Alphas from the decay daughters, Po-218 and Po-214. Over time, the energy of the Alphas will shift from 5.49 of Rn-222 to 6.02 of Po218 to 7.69 of Po-214. See the decay scheme at: http://www.oversight.state.id.us/ov_library/Contaminant_Fact_Sheets/ DecaySeries_FactSheet_ANL.pdf

For Thoron, it is more dramatic, and far better for lab demonstration purposes. I use a "cow" for collection, which is just a large kitchen funnel with a coffee filter in the bottom. In the bowl of the funnel is place the uranium ore ( for Radon) or thorium mantles ( for Thoron). On the spigot is a tube going to an air pump, which sucks the air over the samples, and delivers it via another inline filter ( to keep any solids out) to the Lucas Cell or other detector ( pancake tubes work, but be careful not to contaminate them!).

Thoron collected in this way gives a great reading on the meter, and begins to die away in mere seconds as the half-life of Rn-220 is only 56 seconds. The Polonium-216 daughter decays in less than a second, so within a few minutes the experiment is over. If not flushed, the daughters will again start producing Alphas in about 12 hours, from the further decay into Po-212.

I hope this gives you some ideas for experiments, and reminds you that simple equipment can do real science.

Lucas cells can be made from any suitable container, and lined with ZnS powder or one of my large ZnS screens.



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