Radiation monitoring method

The Aware Electronics fan filter attachment is a small fan which draws air through a foam filter positioned in front of the Alpha window on the RM’s geiger tube.

As dust particles are collected in the filter the radioactive decay is detected by the RM.

Most airborne radioactivity is the result of naturally occurring Uranium and Thorium decay series. Known as Radium B, RaB or Pb-214 (Lead 214) has a half life of 26.8 minutes and it’s decay product Radium C, RaC or Bi-214 (Bismuth 214) has a half life of 19.9 minutes. As RaB decays it produces RaC and they are found in “secular equilibrium” in proportion to their half lives ie: close to RaB = 57.39% and RaC = 42.61% of the total contribution. The other Radon product which might be collected is the Rn-220 daughter, Pb-212 with a half life of 10.64 hours which is the result of the Thorium decay series and usually contributes only a small amount, if any, to the overall reading.

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I installed the RM-fan filter in the lower level of the house with a new filter attached. After 1 hour I started the fan (see above graph 10:08 to 11:08 am January 4th) and ran it for 2 hours until 1:08pm. The CPM increased to over double the previous reading. Indicating the presence of radioactive particles collected on the filter. After stopping the fan, the readings diminished over the next few hours to return to the original background reading before the fan was turned on. This is because the RaB and RaC have an effective half life of about 40 minutes. (half of the radon collected decayed in the first 40 minutes then half of that again in the next 40 minutes and so forth until the readings are not noticably different from the background). I’ve noticed it takes about 10 half lives for this to happen.

When the fan was started again at 21:08 (9:08pm) you can see the sharp increase at first then it levels off but still increases more gradually until about 10am January 5th when readings start to decrease. The sharp increase was caused by radon collecting on the filter and when the decay of RaB and RaC + RaC from RaB decay was about the same as the collection of new Radon on the filter, the increase slowed down. The peak at around 10am 5th January shows that the influx of Radon was less than loss due to decay. Then when I shut the fan off again at 1:08pm the influx was zero and the decrease followed the decay curve of the material on the filter.

I left the fan off until 21:08 that day when I restarted it, see the sharp increase again in the above graph.

At this point I left the fan running for several days. The up and down variations are due to varying levels of Radon in the basement of the house.
Filtering air continuously at equilibrium gives a much more sensitive reading of the levels of radioactive particulate dust in the air. Although RaB and RaC are significant gamma emitters, the filter also allows the RM to detect some of the Beta emitted through the Alpha window on the geiger tube. I guess some of the Alpha produced by the Polonium-214 product of RaC might get detected as well.

On January 10th (on the right side of the above graph) there was an upswing in collected radiation levels. This graph ends at that point due to a power outage at 12 noon.

When the power was back on, I re-started the monitor and left the fan running. The levels remained high but started to decline later in the day.

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I left the fan running for the next few days and noticed a cyclical type variation in levels. Then on the 14th levels started to rise even higher again. So at 21:00 I stopped the fan to record the decay produced.

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The question is “What radioactive decay produced the curve evident in the above graph?”
Since it’s most likely Radon, we start by trying to fit a Radon decay curve to a curve fitted to the actual data.
Using an excellent math program called Simfit “Simulation, fitting, statistics and plotting”, I used the exfit module to get a “fair” fit to the data.

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Then using information from Aware electronics and the Equations of Radioactive Decay in a spreadsheet I generated ideal decay curves for RaB, RaC+C’ and Pb212, adjusting the starting percentages to best approximate the fitted curve.

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There is a pretty good fit for 65% RaB with 0.9% Pb212.
Aware electronics provide programs for graphing (AW-Graph) and (Makdecay) for generating the Radon decay curves. They are both excellent programs that can also be used for this decay analysis.

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