Sunday, January 06, 2008

Fun With a Geiger Counter

A surprising number of household objects are radioactive, and you can verify this with a cheap geiger counter, available on ebay for less than $100. (But be sure you get a geiger counter, not a radiation survey meter. The latter is good only after a nuclear attack, and is not sensitive enough for the experiment I describe here.)

One of the most surprising, at least to me, is water softener pellets -- more precisely, the kind that are made of postassium chloride (KCl). A 20 kg bag (below) sells for about $10 at your local supermarket or hardware store.


Here's a picture of my smallest geiger counter in an empty tupperware container. As you can see, it's registering 12 microRoentgen per hour. Probably most of this background radiation comes from cosmic rays or the smoke detector in my study.


Now I load up the tupperware container with about 1 kilogram of potassium chloride pellets, and try again:


Now the geiger counter is registering 40 microRoentgen per hour. It's not very radioactive, but it is about 2.3 times background.

Why are these water softener pellets radioactive? Surprisingly, it's just due to the potassium content. About 1 in every 8500 potassium atoms is K-40, a radioactive isotope of potassium (and the one that is used in potassium-argon dating). The half-life of K-40 is about 1.3 billion years, which means that potassium-argon dating can be used to date very old rocks (see Dalrymple, The Age of the Earth). K-40 emits both beta particles and gamma rays..

Now, your body also contains potassium, about 140g worth for the average person. So are people radioactive? Yes, slightly. According to this table, potassium-40 accounts for most of the self-irradiation of the body, with carbon-14 a close second. Altogether, about 8000 atoms a second are decomposing inside your body, and this can be measured with a sensitive detector.

Other radioactive items you might find in your house include smoke detectors (some use Americium-241), mantles for gas camping lights (some use Thorium oxide to make the light brighter, although this is less common now), and vaseline glass (uranium is added to the glass to get the yellow color).

17 comments:

Anonymous said...

Try eating a bunch of bananas and then do your reading ...

Jeffrey Shallit said...

Anonymous:

According to one of the pages I cited, "The human body maintains relatively tight homeostatic control over potassium levels. This means that the consumption of foods containing large amounts of potassium will not increase the body’s potassium content. As such, eating foods like bananas does not increase your annual radiation dose. "

Bayesian Bouffant, FCD said...

A physics professor I know tells of working his way through a large antique store with a Geiger counter, looking for Vaseline glass. A couple dozen other customers got interested and tagged along. "Back off man, I'm a scientist."

Anatoly said...

Does that geiger counter count any beta particles or just the ones from potassium? I feel the urge of seeing how radioactive my friends are in their carbon decay rates (though technically it should be the same).

Jeffrey Shallit said...

Yes, I think it should also count betas. The plastic case has some airholes right next to the Geiger-Mueller tube, so the betas should be able to get through there.

Jason Spaceman said...

A couple of years ago I worked at the Darlington nuclear generating station here in Ontario. We had to take a two day radiation safety course before we were allowed to work inside the plant.

I remember the course instructor brought in a counter and a box of household items; he would take them out and hold the counter next to them, and you could hear the familiar 'clicking' sound as the counter would detect small amounts of radiation. The items consisted of everything from clay pots and coffee cups (thorium?) to old alarm clocks with the glow-in-the-dark numbers on them (radium). It was interesting stuff.

Mister Troll said...

Since when can you measure dose rates with a Geiger counter?

I just tried looking that up on the ol' interweb, and apparently you can get calibrated Geiger counters. Never heard of 'em before. I assume they are very inaccurate.

Mark said...

Shallow ground water in some areas of Maryland has elevated radioactivity due to the presence of radium. A possible reason for higher concentrations of radium in these places might be disposal of water softening backflush water into the ground via septic-tank systems (the water softeners may have exchanged radium along with calcium etc.).

Mister Troll said...

Doh! Roentgen: exposure, not dose. I should reflect and/or read more carefully before posting.

A quick calculation of the dose rate in bulk KCl suggests it's about 0.5 mrad/hr from beta and 0.15 mrad/hr from gamma. Fairly hefty, if I got the numbers right. Obviously the dose rate in the *water* is vastly less.

Olorin said...

I have a couple of vacuum tubes from a WWII radar transmitter. The electrodes are sealed with uranium glass, which is mildly radioactive. I have seen a picture of one of them (type 327) glowing in the dark.

lannejhang said...

A major source of natural radioactivity often forgotten is radon, a gas that can be found in badly ventilated cellars build on rocks containing elevated amounts of uranium and thorium, of which radon is a decay product. It slowly diffuses from the soil into the atmosphere, cellars and also mines. People with prolongated exposure to radon carry high risks of lung cancer.

AndreasB said...

In some old photographic lenses (1940s to 1960somethings I believe) Thorium was used in the glass to increase the refractive index and keep dispersion low. This also leads to browning of the lenses after decades of irradiation.

Apparantly lenses like these were also used in some rare telescope and microscope eyepieces, where people would hold their eyes close for extendend periods of time.

Elijah said...

you say altogether 8000 atoms per second, is this altogether potassium, or is this altogether potassium and carbon?

Dont lose sight of this question just because I go on to say .....
I am trying to figure out why Day 7 of Creation is 7000 years (6000 +1000) starting with Adam, but potassium argon is saying the six days of Creation before Adam are each 88 million years. Either day 7 doesnt have to be the same length as these large days of creation preparation, or somehow 100,000 years are coming up as 1.3 billion years.

Jeffrey Shallit said...

Elijah:

The 8000 figure is for all radionuclides inside your body.

But of course this figure has nothing to do with imaginary characters such as Adam.

Jerry Davis said...

Jeffrey, interesting article. I am looking to buy a geiger counter just to keep up with radiation in our drinking water. I'd like one that connectes to my PC so I can download data for charting purposes. Do you recommend one that is rugged enough for daily use, and that offers the data collection component?

Jerry Davis
www.jerrywdavis.com

ChrisVande said...

I would like to comment on the statement made regarding the initial reading of your Geiger counter. None of the background can be related to the radioactive source in the smoke detector. Smoke detectors use Am-241 which decays by alpha emission which can travel a few inches in air if unobstructed. This is a great post. The same effect can be seen with NO SALT salt substitute.

Jeffrey Shallit said...

ChrisVande:

Both my geiger counter and my scintillation counter have no problem detecting radioactivity from my smoke detector.

Quoting from Wikipedia: "Americium-241 decays to 237Np emitting alpha particles of 5 different energies, mostly at 5.486 MeV (85.2%) and 5.443 MeV (12.8%). Because many of the resulting states are metastable, they also emit gamma-rays with the discrete energies between 26.3 and 158.5 keV.[89]"