Voyage to the Antarctic: Tracking Radiation Across Sea & Sky
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Voyage to the Antarctic: Tracking Radiation Across Sea & Sky

By Dr. Zoomie

I will be traveling all of January so I thought it might be interesting to use this trip as the backdrop for a posting or two each week about what I’m seeing and how it fits in with ionizing radiation. In addition, I’ve got some radiation detectors with me so I can also write about what I’m measuring and what it might mean. What can be more fun?

My trip started on New Year’s Day when I boarded a flight from New York City to Buenos Aries Argentina, then on to Ushuaia Argentina the following day. After cooling my heels for a few days in Ushuaia I boarded the MV Sea Spirit to join a cruise to the Falkland and South Georgia Islands as well as to visit the Antarctic Peninsula. Today I’m 10 days into my trip and 5 days into the cruise.

As much as I’d like to write about penguins, albatrosses (albatrossi?), whales, and such…this is a radiation blog and unless the critters are irradiated they don’t really belong here (but – wow). So, instead, in this post I’d like to talk about the radiation measurements I’m making, to mention some things that surprised me (and why I think I’m seeing them), and other ionizing radiation stuff.

To start, I brought with me the bGeigieZen  (GPS and GM tube for dose rates and mapping), the RadiaCode (NaI for spectroscopy and a tiny little GM), a GQ GMC-320 (GM and mapping), and a few “professional-grade” instruments made by Kromek (dose rate, spectroscopy, and neutron count rate) and Mirion (dose rate). I used the bGeiegiZen, RadiaCode, and GQ on all the flights and (to date) much of the cruising with occasional reference to the other instruments for confirmation of odd readings or for specific measurements (e.g. only the Kromek will measure neutron radiation).  

One of the things I did was to capture a gamma spectrum at various locations on the trip here. Not surprisingly, the spectrum I collected on the ground looked a bit different from what I found at altitude or at sea. Specifically, it’s missing all of the gamma peaks from K-40 and the uranium and thorium decay series, which makes it sort of boring. But then I noticed a distinct peak at 511 keV (511 thousand electron volts of energy) – which is precisely the amount of energy that’s released when an electron or positron is turned into energy. Thinking about it, I speculated that this might be due to a higher number of high-energy cosmic rays present in the atmosphere at altitude, not having been stopped by the air. When these crash into atoms in the air they’ll knock lose protons or neutrons in the air and in the materials of one’s detectors and some of these collisions will create high-energy gammas (which then go on to create electrons and positrons as described in an earlier blog posting). So that made sense. But I also noticed an annihilation peak on the ground, which I’ve not seen in NYC or elsewhere and that was perplexing.

What I think might be happening is that, at the poles, Earth’s magnetic field curves inward and the charged particles that are spiraling around the magnetic lines of force find it easier to enter the Earth’s atmosphere and to penetrate to lower altitudes – this is why we have aurorae at the poles rather than the equator. Ushuaia is much closer to the South Pole than NYC is to the North (and, at present, we are at 60 degrees south latitude) so charged particles can reach closer to ground here than back home. This brings high-energy charged particles and the gammas they emit to within range of my instruments (I suspect), giving me the puzzling gamma peak characteristic of electron-positron destruction.

The other thing I noticed is that the gamma spectra I collected in the air and at sea lack so much of the structure I’m used to seeing – gamma peaks from uranium, radium, thorium, and all the other naturally occurring radionuclides; the only familiar natural gamma ray I’m seeing is potassium-40 (K-40) at 1460 keV. My guess on this one is that it’s from the potassium dissolved in seawater. Seawater has about ten times as much potassium as does the continental crust; even considering that the gamma radiation must pass through the bulk of the ship, there’s a LOT of water out there and a LOT of potassium – without doing the calculations I’m willing to bet that it’s enough to cause the K-40 peak I’m seeing.

So the gamma spectra at sea and at altitude are different than what I normally see sitting in my living room (or laboratory) in NYC, and I think we’ve figured out why; the other difference – lower-than-expected radiation dose rate – is much easier to explain. Most places I visit tend to have environmental radiation dose rates on the order of about 5-20 microR/hr (abbreviated µR/hr) while I’m reading no more than about 2 µR/hr and often less than 1 µR/hr at sea. The reason for this one is that, of the natural radiation we’re normally exposed t0 (about 300 mR annually), about 200 mR comes from radon inhalation, about 40 from radionuclides (e.g. K-40) in our bodies, about 30 from the rocks and soils, and about 30 from cosmic radiation. My radiation detectors aren’t sensitive to the alpha radiation emitted by radon (not that there’s much radon over the oceans), nor to the beta radiation that accounts for 80-90% of the dose from K-40. Not to mention that the seafloor is made of a rock (basalt) that’s nearly bereft of radioactivity, in addition to being well-shielded by the hundreds of feet of water beneath our keel. Thus, the only radiation my instruments are picking up is the cosmic radiation (again, about 30 mR/hr). This comes out to a dose rate of about 3 µR/hr, which is pretty close to what my instruments are showing me.

Having said that, I am a little puzzled by the fact that the neutron levels I’m measuring are lower than I’d expected while sitting in my office in NYC, as well as being lower than I’d expect if my hypothesis about the source of the annihilation gamma is accurate. Or, to put it another way, if there are enough charged particles penetrating deeply enough into the atmosphere to produce positrons, I’d expect them to produce neutrons as well – and they don’t seem to be doing so. My best guess on this one is that maybe the neutrons really are there, but the neutron detector I’m using might not be able to pick them up. The reason I’m thinking this is that neutrons have a huge range of energies, varying over several orders of magnitude from the eV and keV range into the MeV range (a factor of 1000 to 1 million), and no detector can measure that huge range of energies. So neutron detectors can be optimized for a particular energy range, and the energy range of neutrons emitted by nuclear weapons might not be the same as the energy range of neutrons created by cosmic ray interactions – the neutrons might simply be passing through the detector. I honestly don’t know, but it’s something I’ll look into when I get back home.

Finally, I know I haven’t been writing much lately, but hopefully it’ll pick up again when I’m back home in a few weeks and get back into the old rhythms of life.