Natural Radioactivity

This is just a brief intro to the subject. Natural airborne radioactivity was an important aspect of my work in the late 90s on one of the verification technologies for the Comprehensive (Nuclear) Test Ban Treaty, or CTBT. The gamma-ray spectroscopy, which is one of my specialties, that was done for the CTBT “radionuclide” verification technology was strongly affected by natural airborne radioactivity.

Logo for the Prototype International Data Center, where I worked on one of the verification technologies for the Comprehensive (Nuclear) Test Ban Treaty. The operational Data Center is now in Vienna, Austria.

Natural radioactivity is a part of life on this planet; like it or not, it is pretty much unavoidable unless you live on a raft in the middle of an ocean. Most soils contain uranium, the predominant isotope of which, U-238, has a very long half-life and so is still around today. Another element in soils is thorium, whose long-lived isotope Th-232 is also important. The amount of these elements in soil varies in different parts of the world and even within a country.

Half-Life

(Since this is often misunderstood, let's pause here to go over it. The half-life is the time needed for half of however much radioactive material exists now to decay, i.e., emit its radiation. So, if at noon on Monday there is one unit of material, and its half-life is one day, then at noon Tuesday there will be 1/2 unit left, by noon Wednesday 1/4 left, Thursday 1/8 left, and so on. Note that two half-lives elapsed does not mean it's all gone; in practice we usually consider eight half-lives to be enough time to reduce the amount of the original radioactive material to practically zero. Also, that material does not disappear; it emits its radiation and is transformed into another element/isotope, and the total mass of material stays essentially the same, since the mass of the emitted radiations is very small.)

Potassium-40 (K-40)

In addition to uranium and thorium, another important naturally-occurring element is potassium, whose isotope K-40 also has a very long half-life. The abundance of K-40 in potassium is only about 0.01% so there's very little K-40 in everyday foods; nothing to be concerned about. Although, in my teen years I drank a lot of milk, and one day in 1965 I had a "whole-body count" done before starting a summer job at the Armed Forces Radiobiology Research Institute (AFRRI). This was a gamma-ray spectrum analysis of whatever radioactivity was already inside me, before starting the job. The peak for K-40 in my gamma spectrum was enormous. (I don't recall that any fission products showed up- see my story about radioactive rain in 1965, which I think happened only a few weeks before this.)

Decay Chains

U-238 is at the top of a "decay chain" of several elements, one decaying (transforming) into another by the emission of either "alpha" or "beta" particles, in a defined sequence. These particles are not a hazard to us, since this radioactive decay is occurring in the soil and those particles have little penetrating power. At one point in this decay-chain process, radium is formed. It has a half-life of about 1600 years, so it would not still be around if it were not being replenished by the decay chain of uranium. There is another pathway that also produces radium, via a similar decay series, from Th-232. From the uranium chain we get Ra-226, from thorium we get Ra-224.

Radon

These isotopes of radium then decay into their respective radon isotopes, Rn-222 and Rn-220, which are usually called "Radon" and "Thoron" (together, RnTn) even though the latter is not a real element. Yes, this is the radon you have heard about. The important point is that radon is of course a noble gas; it does not combine chemically with anything in the soil. That chemical binding would keep it in the ground. Instead, it works its way up to the surface of the earth and into the air. The rate at which it does this is far from constant; it varies with soil and weather conditions and even with the time of day.

Being a noble gas, radon by itself would not be a huge problem in everyday life, but its decay products are themselves radioactive and they decay into a sequence of elements/isotopes that tend to attach to fine particulate matter in the air. These decay products do pose more of a problem for us, in some circumstances, especially if they accumulate in a building, since we can then inhale them over long periods of time. Once inhaled these fine particles tend to stay in the lungs, far longer than just the radon gas by itself would. So, it is desirable to try to minimize radon and its accompanying decay products in our buildings, especially homes; this has become more of an issue with "tighter" energy-efficient designs.

Air Sample Gamma-Ray Spectrum

To illustrate what the RnTn decay products on particulates in the air look like, so to speak, the high-resolution gamma-ray spectrum shown at the top of this story is from a CTBT air sample. This was just outdoor air, which was pulled through a large filter at a high flow rate, for about 24 hours. The filter collects particulates; it was removed, squished down to a manageable size, then placed on a detector and analyzed with a gamma-ray spectroscopy system.

Notice all the peaks, each of which corresponds to a gamma ray at a specific energy; those energies in turn can be identified with particular isotopes. Actually this filter had been allowed to sit for four hours before analysis, so that most of the shorter-lived natural activity has decayed away. Without that delay there would be quite a few more peaks in the spectrum.

There are no man-made isotopes indicated in this spectrum. Those are what we are looking for, in the CTBT context, and the natural airborne radioactivity peaks represent an interference in that detection process. The main peaks for several natural isotopes are labeled; Be-7 comes from cosmic-ray interactions in the atmosphere, rather than from soil. While the peaks in this spectrum are somewhat exaggerated (large), due to the sampling and analysis conditions, it is probably a bit unexpected and surprising to see any gamma-ray peaks at all in ordinary, everyday air.

But the simple reality is that every one of us has been breathing this stuff, to one extent or another, since we took our first breath on Earth. You are breathing it now- doesn't seem to be hurting anything, does it? This natural airborne radioactivity is just that; natural. It's part of the deal we signed when we bought this planet.