House return air filter high total CPM
I'm getting Total CPM of 1200 when I immediatley test my return air filter. The counts will then fall gradually over 24 hrs. to around 100cpm. Can you guys think of a reason for the fall? I've placed the filter in an airtight bag each time.
Thanks,
James
Answered in another thread
This issue was resolved in a follow-up thread: RETURN of the Return Air Filter
The further data from James matched radon progeny decay exactly.
Mark [BRAWM Team Member]
Take Measures
Given these measurements, which appear plausible ...
It would be prudent to 'Take Some Measures' to protect the family.
An internet search would probably give you some actions to consider.
Some will probably be bogus rip-offs. Some exorbitantly priced.
Some affordable and useful home remediation tricks, suitable for your climate.
Radionuclides are entering your home and accumulating inside. So sealing the floor/basement and ventilation to get rid of the stuff, seem to be preliminary prudent considerations to Rule-In or Rule-Out.
IMHO
If the filter is inside a
If the filter is inside a plasic bag when you are testing, you should only see gamma emissions. The plastic bag would block all alphas and the static nature of plastic would accumulate beta electrons.
To my knowledge the decay chain from Radon and progeny is mostly alpha with some beta. Little to no gamma is expected.
I wonder what is being monitored?
To be more specific, the half-life decay chain from Radon at equilibrium to lead 210 at equilibrium takes roughly 8 hours.
If your sample takes longer than 8 hours to decay to 100 CPM it probably isn't Radon progeny either.
I suggest BRAWM test your filter.
Right, I overlooked the
Right, I overlooked the plastic bag. Yes, you would only expect to detect gammas. But the radon progeny actually include some strong gamma emitters: Pb-214 (27 minutes) with strong gamma lines at 295 keV and 352 keV; and Bi-214 (20 minutes) with a strong gamma line at 609 keV. We see these lines all the time in our detectors as background. So these gammas could be what is being detected.
I agree that the decay time would definitely be a big clue.
Mark [BRAWM Team Member]
Probably radon daughters
Hi James, what kind of detector are you using? A standard Geiger counter? It sounds to me like you might be detecting radioactivity from Radon daughters that get trapped in your air filter (various isotopes of bismuth, lead, polonium, ...). For example, you might be detecting beta particles from Pb-214, which has a 30 minute half-life. You can read more about radon daughters here and here.
If you are interested in confirming this, you could record the count rate (CPM) at regular intervals -- say, every 10 minutes -- for a few hours or until the activity has died down a lot. Then you could plot the count rate and determine the half-lives of the isotopes that are dominating the activity.
You are definitely not detecting isotopes from Japan -- the levels are so low that it takes us a long time (days) with our shielded detectors to see anything in our air filters. Also, whatever you are detecting has such a short half-life that it would never have made it over from Japan in the first place.
Mark [BRAWM Team Member]
Hey Mark, I have a Radiation
Hey Mark,
I have a Radiation Alert Inspector Plus by SE International. I just ran a 10 Minute average and got 152CPM. All gamma since the filter is still sealed in the plastic bag? From 1200CPM to 152CPM in four hours. Does that seem to fit the radon daughters decay rate? We live in the Southern Rockies of New Mexico. Lots of granite.
Thanks Mark and Thanks Everyone for the awesome responses.
Very cool site,
James
This sounds consistent to me
This sounds consistent to me with a measurement of the daughters of Rn-222 decay. In a post above, I say that you're probably measuring the gammas from Pb-214 and Bi-214, which have 27 minute and 20 minute half-lives, respectively. The other radon progeny are not strong gamma emitters.
The Pb-214 and Bi-214 decays happen one after the other in the decay chain. The total activity of the two nuclides decays away roughly with a half-life of 30-40 minutes. The calculation is a little involved, but after 240 minutes I calculate a total activity that is about 1% of the initial activity.
This means that after 240 minutes, about 0.01*(1200-152) = 10 CPM would still be coming from the Pb-214 and Bi-214. We would probably need more data to be sure, but this timescale seems close to me.
Mark [BRAWM Team Member]
1200 -> ~150 in four hours
1200 -> ~150 in four hours seems a little quick, but my recollection of the progeny decay curve is that there is a very steep drop off in the middle at about 6 hours (averaging all progeny).
If you are just measuring bismuth 212 then that decay rate seems more appropriate.
I should add that if it is
I should add that if it is Bismuth 212 being measured, that it originates from Thorium decay and not Uranium.
Is the soil around your area rich in Thorium?
sorry, meant Iodine 128 or
sorry, meant Iodine 128 or 132, that have a very short half life, not Iodine 129 that I mention below.
Maybe some iodine isotope,
Maybe some iodine isotope, was it I129 that has a half life of 54 minutes, do not remember which kind but there is one iodine which has a very short half life. Where are you located?
Actually
Could someone please explain the type of filter, and the air sampling volume used?
These count rates indicate a higher than average Rn problem.