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Cosmic Rays on a Plane!

I took an international flight recently, and did something I've intended to do for some time: monitor the background radiation flux as the plane changed altitudes. I brought along a QuartaRAD RADEX RD1706 Geiger-Müller counter which detects beta particles (high energy electrons) and photons in the x-ray and gamma ray spectra and displays a smoothed moving average of the radiation dose in microsieverts (μSv) per hour. The background radiation depends upon your local environment: areas with rocks such as granite which are rich in mildly radioactive uranium and thorium will have more background radiation than those with rocks such as limestone.

One important component of background radiation is cosmic rays caused by high energy particles striking the Earth's atmosphere. The atmosphere is an effective radiation shield and absorbs many of these particles before they reach sea level, but as you go to higher altitudes, fewer particles are absorbed and you experience a higher background radiation dose from cosmic rays. Background radiation at sea level is usually around 0.10 to 0.13 μSv/h. At Fourmilab, at an altitude of 806 metres above mean sea level, it usually runs around 0.16 μSv/h.

I waited until the flight was at cruising altitude before turning on the detector and placing it on my tray table near the window of my window seat. This was not a high-flyer: the plane was a Bombardier Q400 Dash 8 regional turboprop on a medium-range flight within Europe, with a cruising altitude of 7000 metres (the plane's service ceiling is 8229 metres, modest compared to the Boeing 747-8's ceiling of 13,000 m). My first reading was:

Radiation monitor: 1.24 μSv/h

Wow! 1.24 microsieverts per hour is almost ten times the usual reading near sea level. And this was inside the fuselage of an airplane cruising at a modest altitude.

About half way through the flight, we encountered moderately high turbulence (enough to turn on the seat belts sign, but nothing really scary), and the pilot in command requested a lower altitude to try to escape it. Air traffic control approved a descent to 6000 metres. During the descent, the background radiation level smoothly decreased. Here is part way down the slope.

Radiation monitor: 0.86 μSv/h

And now we're at at the new cruising altitude of 6000 m.

Radiation monitor: 0.67 μSv/h

Finally the plane began its descent for landing. Here are readings on the way down, with the last one on final approach over water shortly before touchdown on the runway on the coast.

Radiation monitor: 0.20 μSv/h

Radiation monitor: 0.13 μSv/h

Now the radiation level has fallen to that around sea level. But wait, there's more!

Radiation monitor: 0.07 μSv/h

This is at an altitude of just dozens of metres, still over water, seconds before touchdown. Background radiation is now around half the usual at sea level. (This wasn't a fluke—I got this reading on several consecutive measurement cycles.) But think about it: the contribution to background radiation from terrestrial sources (such as thorium and uranium in rocks) and cosmic rays are about the same. But in an airplane flying low over water, the terrestrial component is very small (since the sea has very few radioactive nuclides), so it's plausible that we'll see around half the background radiation in such a situation as on terra firma. Indeed, after landing, the background radiation while taxiing to the terminal went back up to around 0.13 μSv/h.

It would be interesting to repeat this experiment on an intercontinental flight at higher altitude and through higher latitudes, where the Earth's magnetic field provides less shielding against cosmic rays. But the unpleasantness of such journeys deters me from making them in anything less that the most exigent circumstances. There is no original science to be done here: extensive monitoring and analysis of the radiation dose experienced by airline passengers and crews has been done. This is a Fourmilab “basement science” experiment (well, not in the basement, but in a shrieking aluminium death tube) you can do yourself for amusement. If you do this on a crowded flight, your seatmate may inquire what're you're up to. “Measuring the cosmic radiation dose we're receiving on this flight.” This can either lead to a long and interesting conversation about atmospheric absorption of cosmic rays, background radiation, and radiation hormesis or, more likely, your having an empty seat next to you for the remainder of the flight. Think of it as win-win. There were only seven passengers on this flight (I don't go to places that are too crowded—nobody goes there), so this didn't come up during this experiment.

Return Flight

A couple of weeks later, the return flight was on an Embraer E190 regional turbofan airliner. The altitude of the flight was never announced en route, but this aircraft has a service ceiling of 12,000 m and usually cruises around 10,000 m, substantially higher than the turboprop I took on the outbound flight. I expected to see a higher radiation level on this flight, and I did.

Radiation monitor: 5.07 μSv/h

Did I ever! Most of the readings I obtained during cruise were around 3.8 μSv/h, more than thirty times typical sea level background radiation. (I'd show you one of these readings, but there was substantial turbulence on the flight and all of my attempts to photograph the reading are blurred.) During the cruise, I got several substantially higher values such as the 5.07 μSv/h shown above—more than forty times sea level.

Why was there such variation in background radiation during the cruise? I have no idea. If I had to guess, it would be that at the higher altitude there is more exposure to air showers, which might account for the greater variance than observed at sea level or lower altitude in flight. Or, maybe the gremlin on the wing was wearing a radioactive bracelet.