Science Buddies: "Ask an Expert"

Urgent! How will I measure the picocuries per liter of radon in the air? I built the ionization chamber and radon daughter collector for the Radon Detector Project and I am confused on how to measure the amount of radon. My science teacher told me I would count the number of "blips," or changes in # on the digital multimeter. This is the only way I know of to convert to picocuries per liter of air. Otherwise I would be measuring the charge output and correlating that to the amount of radon, which would be very difficult. The thing is, the radon daughter collector is going to collect the radon and THEN it will be connected to the ionization chamber. So, I'm assuming I can't count blips for an hour, which is how long I'm going to test, since the collecting has to happen first. The sci buddies website doesn't mention how you actually measure the radon at all, but it does say that the meter reading will settle out to a significantly higher charge. So, I'm assuming I won't be counting the # of blips for how long I'm measuring? Please help, I am so confused and I hardly have any time left! Sorry for sounding desperate and pushy! Thank you in advance! I will probably respond to anything someone says by the way, just so you know. Thanks again! If this sounds confusing even if you are experienced with ionization chambers or have done the project, please feel free to send a message below, it would be greatly appreciated!

If you want to see the project: https://www.sciencebuddies.org/science- ... p064.shtml

You won't be measuring the actual amount of radon as this isn't a calibrated device or experiment. What you will be doing is measuring the relative increase in radiation as you move the device demo one place to another.

To read more in detail of how this works, you might look at http://madscientisthut.com/wordpress/da ... n-chamber/ or http://www.techlib.com/science/ion.html ... %20Version, or simply search for "darlington transistor radiation". What happens is that as radiation passes through this device, atoms lose their ions and the positive charged atoms are attracted to the transistor, causing it to make the multimeter read a higher voltage. There are no "blips" as you might find in other detectors like a Geiger counter. More voltage on the meter = more radon (or other radioactive material) in the air.

The EPA's website offers some suggestions on how to interpret long term chemical Radon tests. You won't have time to do one of these in your house as a control, but I'd recommend you find that information on their site. It will give you some background data on the amount of Radon often present in homes. It might help you with the part of the procedure where you're asked to estimate the level based on your multimeter readings. Good luck with your project!

Thanks so much for your help! This method should be fine, since I am just comparing the amount of radon found in construction sites to in a normal area. So the charge will indicate the amount of radon in the air? And then I will record that, and if the charge is higher in the next location the radiation will be higher.

Also, does this ionization chamber differentiate between, say, other radioactive elements? Or if it does, this won't have much of an impact on the total voltage, right?

Thanks!

**Also, I know some of the bare basics of how to use a digital multimeter, but do you happen to know how to set it to 2 volts, like it says on my link? I'm a bit confused, because I don't know why you would set it to a certain number of volts if you're determining the voltage output.

It sounds like you have a plan. I'm not a nuclear or chemical engineer but I'll guess that any ionizing radioactive particles the filter gathers will affect your readings. Hopefully your area isn't too radioactive!!

Different meters all get set differently so I can't tell you how your specific meter works. Generally you set it to a function, like DC volts, and select a range. Some are autoranging and will automatically set themselves based on the voltage you apply. Some of the fancier ones and most of the wimpiest ones also have the ability (or requirement) that you set the range manually.

Think of the range this way: If you wanted to measure 1 1/2 cups of water. You could use a one gallon measure and the 1.5 cups would barely cover the bottom. Would that be easy to measure accurately? If you had a two cup measure that would be better since you'd be filling it up mostly to the top. But what if you only had a one cup measure? You couldn't measure at all since the "range" of a one cup measure is only form 0 to 1 cup. With the meter, setting it to a 2 volt scale means it can only read up to 2 volts but it will read those lower voltages more accurately than if you set it to a 20 or 200 volt scale. The usual way of reading voltages with a manual scale meter is to start at the highest scale, say 200 volts. If that reads less than 20, switch to the 20 volt scale. If that reads less than 2 volts, switch to the 2 volt scale. If you meter goes lower, say 200 mV and the 2V scale says it's less than 200mV, switch to the lower scale. You simply get more accuracy *if* your voltage is within the range. The instructions simply say to start at 2V since the expected voltages will be less than 1 volt and most inexpensive meters have 2V as their lowest range.

Thanks!

After building the ionization chamber correctly and the voltage meter is set on DC volts 2 volts (correctly) and we are getting a 000 reading - 001 reading, is that the reading we should be getting before testing for radon (WHEN TESTING AGAINST THE CAN, NO RADON WAS TESTED YET). If this isn't correct, what would be the approximate reading before testing for radon?

Thanks to anyone who answers. My project is due on the 15th and this really is urgent, considering I need to test at least 3 households. I would have started testing today if it hadn't been for this issue. Any help would be greatly appreciated!

I haven't built one of these myself so I can't say for sure, but elsewhere on the web where people have built similar devices, a significant radiation collection can read up to 500mV or 1/2 of a volt. In an area with little or no radiation I'd expect a low number near 0. Does your meter have a lower scale, perhaps a 200mV scale? Look for the decimal point on your meter. I'll guess it's actually saying ".001" which is one millivolt or 1 mV. I don't know from personal experience what a normal background level should display. On http://www.techlib.com/science/ionchamber.htm I found a short discussion which said:

"The chamber is finished! Connect it to a digital voltmeter set to the 1 or 2 volt scale. The reading should drop to a few tens of millivolts, depending on the room temperature. In the cool laboratory, the background reading is 19 millivolts. Taping an older thoriated lantern mantle to the foil window increases the voltage reading to 116 mV! Remember, most newer lantern mantles don't contain thorium."

So I'm hoping that someone else here has actually built one and can answer your question. Unfortunately it's not easy (for obvious reasons) to find a radioactive source to test your device with.

"What happens is that as radiation passes through this device, atoms lose their ions and the positive charged atoms are attracted to the transistor, causing it to make the multimeter read a higher voltage."

Wouldn't the negative electrons be attracted to the transistor (and the middle wire), since there's a positive charge from the battery being carried through the wire?

Also, how exactly is the voltage created? I read that the voltage was the pressure caused by pushing the electrons. But then how does placing one multimeter lead on the electronic shielding at the top of the chamber, and another on the side of the chamber measure that?

Also, I'm pretty sure I've got this down, but the electrons attract to the center wire, and the positive ions attract to the outside of the chamber, right?

One more thing. Is the outside of the chamber truly "negative." Wouldn't it be neutral in this case. All the websites say its negative. But isn't it just neutrally charged, but there's still a difference in charges between the center wire and the outside.

Thanks for any responses.

The way this is wired, the positive terminal of the 9V battery is connected to the metal can. The *negative* ions are attracted to the can while the positive ions will end up at the transistor. The voltage is "created" by the 9V battery. When the ions collect on the transistor, that allows the transistor to conduct a wee bit of current - more when there are more ions. The transistor and battery together form an amplifer that allows a cheap multimeter to make a meaningful measurement based on the minute charge in the positive ions.

"but the electrons attract to the center wire, and the positive ions attract to the outside of the chamber, right? "

I think you have that backwards. Look at the schematic and you'll see that the positive 9V battery terminal is connected to the chamber can.

Thanks. I understand the charges of the chamber now. But what I'm confused about is how the voltage is conducted. You aren't measuring the voltage from the battery, obviously. You say a bit of current is created- but current is measured in amps. So how is it that we know the voltage? Is it because of the pressure of the ions against the transistor's wire? Thank you.

Amplifiers in general (including iPods and such) work by taking small amounts of current and using them to control larger amounts. Ity's kind of like twisting the knob of a faucet and controlling the flow of water. In this case, the tiny current generated by the ions attracted to the transistor lead allow the transistor to conduct current from the battery. It's this energy from the battery going through the transistor circuit that we're measuring on the meter. It's a magnified representation of the ion energy that you're seeing and not the actual ionization itself.