Science Buddies: "Ask an Expert"

Hi! We are doing the 'build your own radon detector' project on your site and we were wondering what a normal, low and high reading for this would be. While searching online, all we could find were picocuries per liter, which this detector does not measure. Is there a way to convert it? Or do you know what those levels would be for this detector?

Thanks!

Hello michelasnow,

There's no simple, fixed answer to what the reading from your meter represents in absolute radioactivity. The conversion from meter reading to picocuries will depend on several details of how your meter is built. For example, transistors have varying gains and are only guaranteed to be in a certain range. The sensitivity of your can and wire arrangement would have to be calibrated to find out its conversion efficiency from ionizing radiation amount to current.

So, what could you do to calibrate your meter?

Radioactive sources that contain very small amounts of radioactive material are sold legally to the general public. See http://unitednuclear.com/index.php?main ... ca8ac8be10 for example. You could buy a few of these sources and measure what reading you get from each one. There are several issues in what sources to get and how to present them to the meter, and other issues. Also, each source costs about $100. I don't recommend this method. If you can spend the money, you might get one source just so you can bring it close to the meter and see if its working.

A better way to calibrate the meter is to take Radon measurements in various places and compare these readings to known or estimated Radon levels.

Start by performing a Radon test outdoors. Run your collector outdoors and then take the reading. You can find information about what Radon levels are expected outdoors by doing a web search. Then run a test indoors on the ground floor; then run a test in the basement. Note how much the reading increases from outdoors to ground floor to basement and compare the values assuming that your meter reading is proportional to radioactivity. For example, if you find that the expected outdoor Radon level in your area is 0.4 Pci/L and the meter reading is R, and then the meter reading is 2R on the ground floor, you expect the Radon level there is 0.8 Pi/L.

To further calibrate your meter, you could take a reading in some location and at the same time expose a commercial test kit. Send off the kit for an assay and see how it compares to your reading.

Hope this helps. Get back to us with any other questions.

Good luck, WW

Our radon detector, per the instructions on your site (http://www.sciencebuddies.com/science-f ... j&from=TSW) includes a digital multimeter that reads in volts. Do you know how that would work-translating it to pic/l ?

michelasnow,

Yes, I assumed you were using a multimeter as described in https://www.sciencebuddies.org/science- ... p064.shtml.

When you assay the filter from the outdoors collection, you will get some voltage reading. If your outdoor Radon level is 0.4 Pci/L and you read 0.5V (just any number for an example) Then your meter has a sensitivity of 0.4 Pci/L divided by 0.5 V which equals 0.8 Pci/L per volt. Then if we assume the reading is proportional to the Radon level, the Radon level for any other reading, R, is just

Radon level = (0.8 Pci/L per volt) x R.

The assumption of proportionality will be pretty good as long as the transistor circuit doesn't saturate. This occurs when the voltage across the transistor remains greater than about a volt. You should be safely in this proportional region as long as the meter reads less than about seven volts if we allow a little for a weak battery.

Let us know how it goes, WW.

michelasnow,

One other point about calibrating your Radon detector. You should determine what reading you get when no sample is near the meter. This is a background level that has little or nothing to do with Radon, so it should be subtracted from any Radon reading to get just the part due to Radon.

After subtracting the always-present background from each reading, the remainder of the calibration method is as previously described.

WW