EPA and Alpha, Beta, Gamma

Submitted by Anonymous on Tue, 2011-05-17 22:18

Berkeley Radiological Air and Water Monitoring Forum

Just a quick question.....

Watching the EPA graphs....

If the beta counts are consistently elevated for a time (no spikes), and assuming that the equipment is working right and calibrated correctly, will the increased beta counts always correlate with an increased gamma count?

Are there any particles that will increase the beta count, yet not increase the gamma count? Any beta emitters that will increase alpha and not gamma?

I've looked on a couple of websites, and they list some pure alpha emitters, yet others say that there are no pure alpha emitters.

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Alpha, Beta, and Gamma

Submitted by bandstra on Mon, 2011-06-06 09:57.

Let me divide this question into two pieces: (1) In the EPA RadNet graphs, should beta count rates and gamma count rates correlate? (2) In nuclear physics in general, are there any correlations between alpha, beta, and gamma radiation?

(1) Assuming that there are no instrumental effects, the spikes in the EPA graphs would be caused by naturally-occurring radioactive isotopes, mostly from the decay chain of Radon-222. Taken together, the chain emits alpha, beta, and gamma radiation. This is because the chain contains radioactive isotopes that exhibit all three kinds of radiation:

Rn-222 alpha-decays (with hardly any gammas) to
Polonium-218, which alpha-decays (with no gammas) to
Lead-214, which beta-decays (with lots of gammas) to
Bismuth-214, which beta-decays (with lots of gammas) to
Polonium-214, which alpha-decays (with hardly any gammas) to
Lead-210

Lead-210 has a 22-year half-life, so the short-term variability in the radioactivity is determined by these first 5 isotopes. Because of Lead-214 and Bismuth-214, which emit both gammas and betas, I would expect a correlation between the gross beta counts and the gamma counts. I just pulled some data for May 2011 for Denver, CO and highlighted the correlated spikes between the gamma counts and beta counts:

So you can see there's a pretty good correlation there. I've tried some other stations but have had difficulties seeing as tight a correlation, but most spikes I have seen happen in both beta and gammas.

(2) Almost all naturally-occurring alpha emitters are "pure" alpha emitters in the sense that some gamma-rays are emitted, but the probability of emission is very tiny (much less than 1%). For example:

Polonium-210 (natural)
Uranium-238 (natural)
Thorium-232 (natural)

Some manmade alpha emitters are also "pure" in this sense (i.e., very few gammas), such as:

Plutonium-239 (manmade)

There are also some "pure" alpha emitters in that they don't emit gammas at all. Here is one example:

Polonium-218 (natural)

Some alpha emitters also emit lots of gamma-rays, such as:

Americium-214 (manmade)

There are pure beta emitters. These actually do not emit any other radiation besides gammas:

Carbon-14 (natural)
Strontium-90 (fission product) and its direct decay product,
Yttrium-90

Usually, however, beta emitters also emit quite a lot of gamma-rays, such as:

Potassium-40 (natural)
Cesium-137 (fission product)

There are also some interesting cases, such as this naturally-occurring isotope that can undergo both alpha and beta decay, and both types of decay also emit gamma rays:

Bismuth-212 (natural, Radon-220 decay chain)

But again, because radon decay products will be responsible for a lot of the varying natural background, the beta and gamma count rates should be correlated. Mark [BRAWM Team Member]

Variation due to local weather conditions

Submitted by bandstra on Mon, 2011-06-06 16:28.

I left off one crucial piece of information here — the reason the radon decay products vary so much is due to the local weather conditions. Radon gas seeps up out of the ground from the decay of naturally-occurring uranium in the soil (U-238 in particular). It is a heavy gas, and so it prefers to stay near the ground. The atmospheric pressure and temperature can greatly affect its concentration. Here's a paragraph about this from the UNSCEAR 2000 Report Vol I. Annex B, paragraph 120:

Concentrations of radon in the outdoor environment are affected not only by the magnitude of the exhalation rates in the general area but also by atmospheric mixing phenomena. Solar heating during the daytime tends to induce some turbulence, so that radon is more readily transported upwards and away from the ground. At night and in the early morning hours, atmospheric (temperature) inversion conditions are often found, which tend to trap the radon closer to the ground. This means outdoor radon concentrations can vary diurnally by a factor of as much as ten. There are also seasonal variations related to the effects of precipitation or to changes in prevailing winds. These effects must be taken into account when interpreting the available measurements, many of which are daytime samples.

So there can be huge variations in radon concentration (and therefore the concentration of its daughters) over a single day, just due to temperature effects. This can explain the large spikes that many people are noticing, many of which happen over the course of a day or so. As that report indicates, precipitation will also be a huge factor. Rain "washes out" or "scrubs" particles from the air, including the radon progenies. So during periods of rain, the activity in the air can quickly decrease from the washing out of the decay products, while the activity measured on the ground can increase. I'm not sure how the beta counters for RadNET are placed, but periods of precipitation could show large fluctuations as well. Mark [BRAWM Team Member]