Radioactive Fallout

A question of semantics ...

On my 70th birthday. Scary. Damn it, that means that as from today I’m no longer bullet-proof against COVID-19.

In my book, Saving our Skins, I took pains to explain the difference between radiation and radioactivity. In doing so I recounted how a colleague, Forest Mims III, had shown me high readings from a Geiger counter during a 1993 flight from his home in USA to a UV conference we were both attending in Japan, near Hiroshima. His main interest with the Geiger counter was nothing to do with UV radiation or the conference. It was to measure any residual radioactivity remaining in Hiroshima fifty years after its nuclear bombing that heralded the end of WWII. But he’s famously curious, and to pique that curiosity he’d also taken measurements during his flight to Japan.

While the readings in Hiroshima were close to background levels, the readings he’d made with the same Geiger counter during the flight weren’t. They showed very high count rates when the plane was at cruise altitude.

He’s since lost that old flight data, but the plot below shows readings he made with the same instrument during a flight from Geneva to Atlanta the following year. Note the lower readings during a short stopover when the plane landed after two hours. The same message is clear. The higher you go, the greater the Geiger counter signal. The high readings at altitude are caused by cosmic rays - high energy protons and atomic nuclei - and the byproducts of their collisions with air molecules.

But, because these instruments are primarily used to measure radioactivity, I called their readings indicative of high “radioactivity”. That sparked some lively discussion among some of my colleagues.

One of my old 1970s physics classmates at the University of Canterbury, John Le Heron, first raised the question. He reminded me that that term radioactivity is reserved specifically for describing the products of nuclear decay, rather than just Geiger-counter readings. He’s undoubtedly correct because he spent the latter half of his career working in Vienna for the International Atomic Energy Association (IAEA). And Britannica agrees with him. The point he was making is that while Geiger counters are used to measure radioactivity, a high Geiger counter reading doesn’t necessarily mean high radioactivity. So their readings can be biased if used for that purpose.

I wondered if there’s a better term that’s still understandable to the layman to describe the readings, or whether radioactivity can still suffice in this case. One could argue that ultimately the stellar and cosmological cosmic-ray source includes nuclear processes (fission and fusion), so it may still qualify to be called radioactivity. But maybe that’s too long a bow to be drawn. The main products of radioactive decay differ from cosmic rays by additionally including helium nuclei and electrons (alpha and beta particles) as well as high energy photons (gamma rays).

The general consensus from our discussions is that it should be called ionizing radiation - because the high photon energies are sufficient to strip an electron from an atom (so ionizing it). Maybe another good reason to fly less. That’s much more energetic than the UV-B photons in sunlight that cause damage to our skin. They have enough energy to break only the weaker molecular bonds between atoms in molecules like DNA.

Note that the updated version of Saving our Skins that’s now available at Amazon includes this correction. A good example of peer review.

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