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Re: [Phys-l] Geiger counter



We buy the cotton pads from the supermarket - their intended use is for wiping makeup off users' faces - and they are around 7 cm in diameter and 5 to 10 mm thick, uncompressed. They fit nicely over the end of the tube of the vacuum cleaner, with a metal gauze to stop them disappearing down the tube. After filtering for 5 minutes we put the pad on the RM80 geiger tube and the count rate jumps to around 10x the background, from around 20 per minute to 200, very consistently, every time we do it, year after year. Definitely beta, not gamma, as verified by shielding with a thin aluminium plate. We're sitting on top of the huge limestone mass of the Karst here near Trieste, so the geology is not specially favourable to radon emission, although supposedly one can get local hot spots where a deep fissure vents radon from under the limestone, but checks carried out some years ago by the state health service, measuring radon levels by counting the alpha pits in the bit of plastic exposed to air for a few months, showed nothing unusual. What we do for our lab is to get the decay curve of the radiation from the cotton pad and show that it is consistent with a mix of Pb 214 and Bi 214, which have half lives of around 30 min and 20 min respectively.

Could it be that the difference between your observations and those of the RM80 users is due to the fact that the RM80 has a much bigger diameter than the typical GM tube, so that it's capable of catching many more betas?

Mark
UWC Adriatic
Trieste, Italy

On 30/03/2011 21:12, Edmiston, Mike wrote:
I'm surprised by the various accounts of using a vacuum sweeper to pull air through a cotton pad or filter, then observing significant radioactivity with a GM-tube detector. From my experience, this does not make any sense unless your radon level is extremely high, or perhaps you have some different type of contamination.

For over 15 years my students and I have been measuring radon in homes in our area as a public service. We use a NaI scintillation system for observing the gamma decays of the 214-Pb and 214-Bi. Since the NaI scintillation system is hooked to a pulse-height analyzer, we see the individual identifiable photo peaks from these nuclides. We calibrate on three prominent 214-Pb gamma emissions (242 keV, 295 keV, and 352 keV) plus two of the prominent 214-Bi gamma emissions (609 keV and 1120 keV). There is no mistake we are looking at radon daughters since we identify all these gamma emissions from a single sample.

The NaI crystal (2-inch cylinder, 2-inches tall) is way more efficient at detecting these gammas than a GM-tube can ever hope to do. When we bring back samples from a home in which the radon is fairly high (20 to 50 picocuries per liter of air) the NaI system easily sees these radon daughters and allows us to calculate the radon level. But the same samples placed in front of our Bicron end-window GM-tube counter barely registers anything above background in these same samples that show sufficient radon-daughter activity in the NaI system to tell us the home owner has a problem.

We know our science building has radon levels about the same as outdoors in this area, which is about 2 picocuries/liter. This morning I used a sweeper to pull air through a cotton filter for over an hour. After collecting the sample and counting it, I could not detect noticeable activity above background in the GM detector. We generally count our home samples in the NaI system for 10 minutes. A ten-minute count of the cotton filter showed about 80 counts (in 10 minutes) above background, which is about 8 cpm above background, in a very sensitive NaI scintillation system. (The 80 counts above background are the total counts observed above background specifically in the peaks of the 5 gamma energies listed above.

At the Aware Electronics site there is mention of someone pulling air through a filter, and observing hundreds of counts per minute with the GM-tube detector. If this is true, I do not want to live there.

FYI, we trap the radon and daughters in activated charcoal canisters, but we don't collect passively. We use calibrated air pumps that pull air through two charcoal canisters in series. The first can generally traps 95 to 99% of the activity unless the sample site has very high activity, or very high humidity.

In our experience we can't tell much of anything from GM-tube counting. The levels are too low and the efficiency of the GM-tube is too low. It's possible that people reporting detectable radioactivity are seeing the betas rather than the gammas. The GM detector has much higher efficiency for the betas. The problem I have is that we don't have that same experience even in areas where the radon is high enough to be considered a health hazard. So I don't understand what people say they are seeing.

BTW, in the earthquake in Japan I haven't seen any 137-Cesium or any 131-Iodine. I would not easily see the 131-Iodine in my background because its 364-keV gamma would overlap with the352-keV gamma from 214-lead. However, I know the background peak I see in that area is 214-lead and not 131-iodine because I see the other lead gammas in the correct proportion as well as the 609 and 1120 keV peaks from 214-Bi. So my background is dominated by radon daughters, and also 40-potassium. My counter is in a 3-inch lead shield, but it sits next to a poured concrete wall. Even when I leave the shield open, I only detect radon daughters and 40-potassium.

Michael D. Edmiston, PhD.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
Bluffton, OH 45817
Office 419-358-3270
Cell 419-230-9657

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Mark Sylvester
Sent: Tuesday, March 29, 2011 2:52 PM
To: phys-l@carnot.physics.buffalo.edu
Subject: Re: [Phys-l] Geiger counter

I recommend the more expensive RM80, which is a wide diameter tube.
Also the "newer" software for windows (for years there was just the old DOS version) AND the device for adapting the output to a usb port so you can do the counting on a "new" computer (i.e. one that doesn't have a serial port). We've used the RM80 for years, Pasco and Vernier notwithstanding, to monitor the surprisingly high beta countrate that you get from a cotton pad after drawing air through it with a vacuum cleaner for a few minutes... presumably due to ionised bismuth and lead isotopes in the radon 222 decay series, that get stuck to the cellulose molecules... well described on the site. It's become one of the standard labs that we do.

Mark.
UWC Adriatic, Trieste, Italy.

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