Radon gas detector "Hack"
chris.nafis
Posts: 17
Radon gas is a leading cause of Lung cancer. Charcoal test kits only
give you a small window view to your Radon levels. Radon levels vary
over the year and
change with the weather. I've only been able to find
one inexpensive
electronic radon gas detector. Unfortunately it isn't
set up to collect data.
I wanted to work with the local schools to
collect radon and weather data to
correlate.
Here is a "hack" to collect radon data:
[URL="mhtml:{AFAF993D-4889-4CB0-955E-A1EC8B2A2850}mid://00000092/!x-usc:http://www.howmuchsnow.com/arduino/radon/"]http://www.howmuchsnow.com/arduino/radon/[/URL]
give you a small window view to your Radon levels. Radon levels vary
over the year and
change with the weather. I've only been able to find
one inexpensive
electronic radon gas detector. Unfortunately it isn't
set up to collect data.
I wanted to work with the local schools to
collect radon and weather data to
correlate.
Here is a "hack" to collect radon data:
[URL="mhtml:{AFAF993D-4889-4CB0-955E-A1EC8B2A2850}mid://00000092/!x-usc:http://www.howmuchsnow.com/arduino/radon/"]http://www.howmuchsnow.com/arduino/radon/[/URL]
Comments
http://www.howmuchsnow.com/arduino/radon/
It's a cool-looking project!
Any idea on what principle the sensor actually works? Geiger tube? Solid state sensor?
air there will be 2.2 radioactive disintegrations each minute. For
example, at 4 pCi/l there will be approximately 12,672 radioactive
disintegrations in one liter of air, during a 24-hour period.
If you want to see another cool detector look at: http://www.youtube.com/watch?v=HO8VDf_rCXo Spark Detector
I recall seeing an article in one of the electronics magazines (don,t recall which one or when) on building a radon detector using an aluminum soft drink can as a geiger tube style detector. The tab end of the can was removed, a small hole was drilled in the bottom of the can, and an anode wire (insulated from the can) ran through the center of the can. The anode wire was connected to a high voltage supply (500~700V) through a resistor (100K~1M??). When an alpha particle was emitted from an argon atom it produced a negative going pulse that incremented a counter. The count rate was proportional to the argon concentration.
Sorry I could not provide more info. Perhaps someone with a better memory than mine will recall and post the article and magazine .
Sounds like you now have a choice of radon detectors and all you need to do is decide on a microcontroller to count pulses over a sampling period and store them. May I suggest the propeller?
Hi Chris, I am trying to reproduce your project using the HS71512 detector. You have a great diagrams and photos, it is very helpful, thanks! Everything seems works fine, I am getting the radon readings and push them to Xively. The only problem is how to figure out the Long term vs Short term state. I was expecting the "Long Term low on Digit 4" will be LOW in Long-term mode, but seems it works in some different way (don't have oscilloscope to verify). Can you advice please how you catched the L/S state in Arduino code?
Thanks again!
Andriy
(rokhmanov AT yahoo com)
You've piqued my interest in radon. Looked at links and youtube videos. The 8kV demo was telling, it seemed to say distance was important.
I'm confused about how to measure (with the pop can detector) 1 liter of air of alpha events over time if the air volume examined is only what's inside of the pop cans? How would the item be calibrated?
1) Air volume, CFM
2) Humidity
3) Voltage between the can and the wire
4) length of the wire
5) cosmic rays
6) hits due to progeny-decay products. For example polonium-218 has a half life of 3 seconds whereas radon is about 3.8 days, each emitting an alpha particle and beta.
7) height above the floor
8) probably many more
It seems that once all the variables are accounted and compensated for, then reasonable results are possible. Even though various affects may be non-linear.
Edit: Doing more internet reading. It looks like getting solid radon measurements is definitely not an exact science. Even two expensive units placed side by side give different results, not to mention the various "radon measurement services", labs, kits and what not.
In the world of instrument calibration, for example a DVM, the unit will be compared against a standard, which is compared against an even more accurate standard, eventually traceable to NIST, (previously NBS). This insures a DVM will give a voltage reading within its accuracy specs that is trustworthy. My impression thus far is the radon monitoring industry is more like Dodge City.
If we were grading on a curve, the pop-can may give extremely accurate measurements;)
CFM is not a factor unless there is absolutely no air flow at all, which is highly unlikely. Any area of a building will have adequate air movement to refresh the air in the can.
Humidity and the voltage between the can and the wire will not affect the measurement as long as the voltage is not high enough to cause arcing. The circuit is measuring a pulse of a specific height range caused by the alpha particle.
Cosmic rays and beta particle are more energetic than alphas and can be filtered out by the pulse height produced.
Polonium-218 and most other radioactive materials are solids and very rare as contaminants in aluminum so their effect on the measurement would be small. It can also be compensated for by performing a long term count in open air and using that to do a background subtract on the basement measurement.
Radon is one of the densest substances that is a gas so the detector height above the floor could be an important factor in the measurement.
I think it would be a great project to come up with a reliable DIY radon detector, with a data logger, that anyone could put together.
I tried to understand the energy levels released by the various progeny as they decay but still unsure. It seemed to me that a Po-218 decay would show up on the detector as an event because it releases an alpha. But how much energy, the same as radon? Same question for the rest of the decay chain. That gets me to wondering if the "alpha clicks" picked up is actually the total of all the progeny plus the radon decays. Also I have to wonder if the newly formed solid Po218 remains airborne rather than dropping to the floor within a few seconds or minutes or hours.
It would be nice to understand if:
1) all alpha energies are the same, say within an order of magnitude (therefore a Hi-Lo range should be part of the electronic circuit)
2) all alphas are counted, not just the radon's, and if this number is assumed within the EPA max level of 4 pCi/l (affect scaling over time)
3) the solid progeny settling is slow enough such that it is moves out of the detector before it accumulates (may need to have a controlled air flow)
A fun home project.
http://everything2.com/title/decay+chain
http://excelphysics.com/blogs/news/8145029-bug-zapper-alpha-particle-detector
http://chambrebrouillard.wifeo.com/alpha.php
The non-gaseous decay products like Po are electrically charged and stick to particles in the air (smoke, dust) and to surfaces. They have to be accounted for in the instrument design and calibration. The third reference above shows the quick multiple decay of 220Rn and 216Po in a home-made cloud chamber.
Alpha decay energy is usually in the range of 1 to 10 MeV. For example, Americium isotopes used in smoke detectors emit alpha particles at around 5.4 MeV, similar to that of Radon. They start out with a velocity of about 5% the speed of light. The strong interaction with matter makes them very dangerous when ingested; breathed in the case of radon. The ion tracks wreak havoc in living tissue.
About calibration, your quickest bet is a co-location. You use a commercial instrument and compare its reading with yours, just like you outlined for the calibration of a multimeter, ultimately traceable to a trusted authority.
This is great info. The alpha MeV is ~proportional to the isotope half-life, the MeV is ~proportional to how far it travels and the number of ions it stirs up. In the cloud chamber it appears the 216Po trail is about twice as long as the 220Rn (half-life of 55 vs .15 seconds). I assume theres a relationship between the MeV and the alpha particle speed: the higher the MeV the greater the speed, somewhere around 5% the speed of light.
The quantum tunneling makes me wonder if specific isotopes have a signature characteristic distribution of energy level decays. For example the 5.4MeV is a center point for Americium. Higher and lower MeV actually occur with a characteristic distribution curve.
edit: just found a youtube that I think demonstrates this. Spectrum
at about 10.5min.
Also learned that two decay chains, 238U and 232Th, produce 222Rn and 220Rn, both with different half lives. Interesting too that alpha passing through mica looses energy (slows down?); guessing that the He alpha plus charge is attracted to micas outer atom electrons, shedding energy to the mica in the form of heat.
I believe my previous questions have been answered:
1) Shorter half lives will group events of interest toward the upper end of the MeV range.
2) Eliminate progeny, with a hepa filter possibly, to measure only radon.
3) Controlled air flow should move most progeny out of the area of the detector in order to focus exclusively on radon.
For rough co-location comparisons Im getting a Safety-Siren HS71512 detector. A Femto-Tech has more accurate and trustworthy results but cost $5,000. Since Im in a large city there might someone local to do a co-location test along side the Safety-Siren using higher end equipment. Another calibration possibility might be to build a cloud chamber and manually count tracks. In the 3rd link, one process was to fill a container with air, add a uranium source, wait 4 days, and move air with radon to the cloud chamber. It might be possible to generate a known radon-to-air mixture using this technique to use for rough verification. Thanks for your help, it's greatly appreciated.
Americium 241, half-life 432 years, has two main alpha emission energy peaks, 5.486 MeV (85.2%) and 5.443 MeV (12.8%), and it decays to Neptunium 237, half life over 2 million years. The decay leaves the Neptunium nucleus in an excited state, and it quickly emits a gamma ray, several quantized energy levels, but mostly at 60keV (85%).
The Bragg effect is evident in the cloud chamber. The alpha track is thin close to the source and becomes heavy at a point farther out, then it quickly fades out. The interaction with matter is stronger after it has lost some of its energy and is moving with a lower velocity. It generates ions and loses energy at a faster rate at the end of its travel. In smoke detectors, Americium is combined into a gold foil a few microns thick, and that is rolled into a sandwich with nickel or palladium crusts. So the alpha particles coming out are already slowed down when they hit the air.
Here is a site with lots to look at, both educational and arcane...
http://ie.lbl.gov/toi.html
This has an interesting display of radon activity at about 1:45:
Cloud Chamber, Berlin The dispersal is fun to watch.
I'm considering a project and welcome a close look before I invest a lot of time in it.
I think it’s possible to develop a reference or means for testing that arrives at a high level of certainty a given radon detector is verified accurate to within +/- 20% of the measured value.
The technique will involve using a cloud chamber to count the number radon alpha particles per minute generated by a uranium source. Regardless of the chamber volume, the count over time allows the uranium sample to become a reference source for calibration within any other known volume container.
The decay of one radon atom and it’s progeny create 3 alpha particles in 3.859 days. The next decay sequence to generate an alpha will not occur for 22.5 years, essentially making the atom appear stable for the near term. The decay rates for the progeny vary greatly but given a quantity of more than a few 100 atoms the overall effect will be to average the three decay times after 4 days have elapsed. The averaged half-life is 1.286 days for the three isotopes as seen on the attached spreadsheet.
Within the chamber the initial time to reach equilibrium should be the total of the three half-lives, or about 4 days. The next task will be counting the number of alpha trails during a fix period in order to yield the rate of alpha production from the uranium source. The trick will be to make the entire chamber visible for alpha tracks, not just the lower portions of the chamber. Also the rate of alpha production must be high enough to count a few hundred over a period of minutes but not so many that some are missed.
Since the rate of alpha production from the uranium source has been determined and remains constant, it should be possible to create a variety of volumes of air with known alpha densities by varying the container volume or the exposure time. Air with this fixed value should be reliable enough to verify a radon detector’s accuracy shortly thereafter.
A cloud chamber is needed that allows full observation and counting of all alpha tracks within the entire chamber that radon atoms will be dispersed to.
Any feedback appreciated.
Decay.pdf
Here is the decay scheme of 241Am.
The energy step from the ground state of 241Am down to the ground state of 237Np is shown as 5.54MeV, but only 0.39% of alpha emission events take that step in one leap. The most common step (85%) is the one of 5.48MeV, which is followed by a 0.060 MeV gamma ray to bring 237Np to the ground state. There are 6 observed alpha transition energies, followed by one or more gamma rays having certain quantized energies, probabilities, and metastable lifetimes. In all of these, the 241Am itself no longer exists. These are just excited states of the sup]237[/sup]Np nucleus. The diagram is pretty old, so I suppose the numbers may have been refined with modern techniques.
For example, a typical smoke detector contains 0.28 µgram of 241Am, which amounts to 1 microcurie with an activity of 37000 Bq (Becquerel). That means it emits 37000 alpha particles per second. If you put that directly in a cloud chamber without attenuation, all you will see is fog, not individual tracks.
Electrically, 37000 alpha particles per second amounts to a current of only about 12 femtoampere. However, there is an avalanche effect in air. The track of each alpha generates 160000 ion pairs, multiplied times 37000 per second, creates around 6E+9 ion pairs, which can support a current approaching 1 nA.
Your point about the aggregate Bq may be a real problem. My technique depends on the alpha source being consistent for use as a transfer standard, so it requires accurate initial measurability. The components of time and air volume can all be measured accurately, perhaps 1% or even 0.5%.
The Bq caveat is that accurate counting and controlling the output of the source are unknown at this point. My only idea is to use lower or higher CPM samples or break the source into smaller parts to reduce the alpha output. I can’t think of a viable way to just turn a dial and attenuate the output unless excess atoms are bled off into the open atmosphere. I’m sure any mechanical approach will hurt accuracy. So breaking it into pieces is easy and should eliminate one source of inaccuracy.
It may be possible to create an alpha detector to work within larger volumes of air versus using a cloud chamber to manually count. I’m sure a Prop could keep track of all the counts and averages over periods of time.
Since radium-226 has a half-life of 1,590 years it should be a stable radon/alpha generator with a mathematically predictable decrease in output.