Measurement of radioactivity of radon decay products in rainwater with a handheld radiation detector

Posted 7/22/2011 11:00 pm

Many weeks ago someone posted this thread where someone measured radioactivity from a paper towel used to wipe down their car after a rain. This came up again recently with another person posting very similar videos in Canada. Out of curiosity, and wanting to refute the idea that this has anything to do with Fukushima, I set out to make a similar measurement.

(By the way, what allowed me to do this is that we had a rain shower on 6/28, and that sample was analyzed and showed no fission isotopes from Fukushima to very low levels.)

For these measurements, I used our lab's survey counter, a Ludlum Model 2241-3 with a Model 44-9 pancake probe sensitive to alphas, betas, and gammas. For the setup, I mounted the probe as shown in the pictures below and did not move it.

Testing rainwater directly

First, I tried to measure a 100 mL pan of rainwater, but it did not have an elevated count rate:

Measurement of 100 mL dish of rain (no elevated levels observed)

Testing a swipe of a metal surface

The radon progeny particles should stick to metal as the rain washes over, thus concentrating the particles. So while it was still raining, I wiped down a horizontal metal surface (area approximately 1 sq. meter) with a paper towel, folded it so it was about 3 inches square, and placed it in a ziploc bag. The paper towel was noticeably dirty, turning a grayish color. For a background, I took data without any sample. When the sample was placed underneath the probe, the plastic bag was nearly touching the probe's bottom surface (see pictures below). The radiation from the rainwater swipe sample was well above background:

Measurement of paper towel rain swipe (elevated levels measured)

What is the radiation made of?

Next I set about figuring out what kind of radiation was coming from the sample. The three kinds of radiation are shielded in different ways. Since I placed the sample inside a plastic bag, the radiation could not be alpha particles, which would be blocked by the plastic. If the radiation is primarily beta particles, then different thicknesses of metal sheets should lead to more and more attenuation. Also, thin sheets of metal will not attenuate gamma-rays very much. Because of time constraints I only made one measurement, using a single sheet of copper:

The number 350 keV is a rough approximation of the energy losses of beta particles passing through that thickness of copper. According to the decay data for Lead-214, the average beta energy is 223 keV. And according to the decay data for Bismuth-214, the average beta energy is 642 keV. The data show that a significant proportion of the radiation is made of low-energy betas, which is consistent with the Pb-214 and Bi-214 decay data.

The detector could be detecting gammas as well, but I did not do enough testing to determine this.

The decay of the radioactivity

I then proceeded to track the decay of the radioactive sample by taking measurements over 10 minute intervals for 3 hours. Here's a plot of the data, fitted with a model of the radon decay products Lead-214 and Bismuth-214, both strong beta and gamma emitters:

Once again, this model gives a perfect fit to the observed data. (Frequenters of the BRAWM forum may remember James's air filter measurements, which were also successfully modeled with the same radon decay chain model as these data.)

Not from Fukushima

Lastly, can we conclude that this is natural and not Fukushima fallout? The answer is yes, it must be natural, for multiple reasons:

1. This rain, when measured with our detector, did not show any radioactive isotopes from Fukushima to very low limits. Our germanium detectors have much better resolution than a handheld counter and can see radiation to very tiny levels.
2. If we assume that the radiation is due to a single isotope, this leads to a measured half-life of 32 minutes for these particular data. Since it takes about 7 days for the fallout to reach the West Coast, the amount of the isotope would have decayed away to only this fraction:

         2^{–(7 days)/(32 minutes)} = 2^–315 = 1.5×10^-95,

   which is practically zero.

3. Radon and its decay products are omnipresent, well understood, and well measured. In fact, radon and its decay products give us all about 50% of our background radiation dose. We would expect to see these decay products in rain water, and indeed we always see them in our germanium spectra.

The world we live in is naturally radioactive, and this measurement is a perfect demonstration of that fact. The fallout from Fukushima is so tiny that very specialized detectors are needed to see it above the intense natural background. Handheld detectors will not do the job.

Mark [BRAWM Team Member]

(See also my measurement of the radioactivity of potassium chloride for a similar measurement.)