In the previous post, I described false starts with off the shelf radon detectors. Radon is radioactive, and anyone who has seen a movie or two knows that the good guys have Geiger counters that make noises when there is radioactivity. So of course, the solution is to get a geiger counter.
The first one I tried was one made by Mighty Ohm. Not knowing any better, I got one that had an SBM-20 tube. This tube detects beta, and gamma particles, but not alpha. Nice geiger counter, great kit, great to put it together and get it working, but let’s skip forward. I need one that’ll measure not just beta, and gamma, but also alpha particles.
Recall that when Radon decays, it releases an alpha particle. See the picture below.
I had two choices, get another kit, or get something that was pre-built. I chose the GMC-600+ from GQ Electronics. It comes pre-assembled, and pre-calibrated, it detects alpha, beta, and gamma particles, and reviews gave it a good battery life. Most importantly, it was available on Amazon with two day delivery. So I ordered one, and waited. After a false start with the first one (had a line of dead pixels), the second one has proved to be really good.
It also has a USB port on which it appears as a simple serial port device, and you can read, and write from it directly. They also give you some software (I didn’t try it, it was Windows only). I wrote some software based on their documented protocol, and it worked quite easily. GQ Electronics makes some interesting hardware, they clearly are not software people. But, I do like their Geiger counter, and I’ll open source the software I’ve written.
The GMC-600+ uses an LND-7317 tube. As shown on its specification page, it can detect alpha, beta and gamma particles. I found that to convert CPM to uSv/h for this tube, one must divide by 350. I’m not really sure why this is, but for now, I’m using this number and moving forward.
On two different days, I conducted the following experiment. I placed the geiger counter inside the air-conditioning duct, right next to the filter (inside a ziploc bag).
I then ran the circulating fans for two hours, and then shut them off.
On 9/26 the fans were run between 12:30 and 14:30 (local time). On 10/9 the fans were run between 13:45 and 15:45 (local time). Here are the results from the GMC-600+. Note, I converted CPM to mSv/year.
In the test on 10/09, the counter was placed in the a/c duct many hours before I started to run the fans. The background radiation is about 1 msV/year in both tests. On 9/26 the peak was just over 3 msV/year, on 10/09 it went just over 1.5 msV/year.
In both cases, the radiation level dropped by 50% (over background) in about 50 minutes.
If you look at the radioactive decay chart above, from Po218 to Pb210 takes ~50 minutes. It sure looks like the dust in the filter is radioactive, and has a decay characteristic that could be related to the decay from Po218 to Pb210! Lots of fun and interesting math to follow in the next blog post.
WARNING: You can’t just add half life (times) to get effective decay rates, and half life. A half life is an exponential decay curve, and mere addition is meaningless. It has been a while since I studied Bateman’s equations, but in the simplest form, Bateman’s assumes a chain of decay beginning with all particles of the first type in the chain. That’s not what I’m dealing with here – at the time when the fan goes off, there are a collection of particles on the filter, each with its own decay (half life), and fraction. The effective half life is more complex than Bateman.