Natural radioactivity can make things fun for radiation safety professionals like me – it gives us a chance to tell the public that radiation isn’t quite as harmful as they might think, for example, and it lets us confirm that our instruments are working properly. But it does carry with it some drawbacks. For example, we can’t clean up a site properly (especially one contaminated with naturally occurring radioactivity) unless we know what was there to begin with. And if natural radiation is everywhere then those who are frightened of radiation always have something to be worried about. Not only that, but if you’re responsible for interdiction – trying to find subtle indications of a radiological or nuclear weapon – then background radiation can complicate your job considerably.
First, let me talk a bit about background radiation. There is always radiation in our environment and your instruments should always have a reading above zero. This comes from natural radioactivity in our own bodies, from cosmic radiation, and from radioactivity in rocks and soils (and things made from them). If you’re responsible for a fixed location (a city, for example) then your readings from cosmic radiation and from the radioactivity in peoples’ bodies won’t be changing much. But in a city, the local architecture can have a significant impact on your radiation readings and if you don’t account for this then you can end up missing something, or responding to an elevated reading that doesn’t amount to anything. Here are some examples of things I’ve seen, ranging from fairly straightforward to a bit of a head-scratcher.
- As one example, I was doing a radiation survey once and realized that I was seeing radiation levels higher than what I expected. As I approached a building my detector sounded an alarm, which had me concerned. I knew that granite could cause elevated readings (gray, pink, and red granites often contain elevated levels of natural radioactivity), but I didn’t see any granite buildings in the area. I was finally able to determine that the elevated radiation readings were coming from a brick building in front of me and approached it. I surveyed the building, assuming that radiation was shining through the bricks from something on the inside, but I noticed that the levels were remarkably uniform – everyplace I checked had almost identical readings. This had me stumped because if somebody was hiding radioactivity in a particular room I should see a “hot’ spot. I finally realized that the radiation was coming from the bricks – more precisely, from naturally radioactive potassium in the clays from which the bricks were made.
- Another incident happened to me near a natural gas processing plant. In this case, we not only got elevated radiation readings, but one of our RIIDs (a RIID is a RadioIsotope IDentifier) identified the presence of highly enriched uranium. For a number of reasons we were confident that there was none of that in the area, but we still had to explain the reading. In this case, it was important to remember that oil and natural gas deposits are associated with elevated levels of uranium. Not only that, but one of the decay products of uranium is radium-226 – anyplace you find uranium, you’re also going to find the radium (where it comes from is the decay of U-238, which decays through over a dozen steps before turning into stable lead). Radium-226 turns out to emit gamma radiation with an energy that’s almost identical to that of U-235, the isotope from which reactor fuel and nuclear weapons are made. The gamma radiation emitted by U-235 is so close to that of Ra-226 that the most common type of RIID can’t “see” the difference. So in this case, it was natural radium from the nearby natural gas facility that was causing the elevated radiation dose rates and that was fooling our RIID. In this case, we confirmed this hypothesis by identifying other natural radionuclides from the uranium decay series. We could also have used a very precise type of RIID (called a high-purity germanium detector), which is sensitive enough to distinguish between Ra-226 and U-235, but didn’t need to in this case.
- The other case I wanted to mention came on
a cold winter’s day when we were driving around performing radiation surveys. At one point our instrument indicated that it had detected cobalt-60, a nuclide used for industrial radiography, cancer therapy, and research, but one that’s also a concern for possible malicious use. This was obviously a concern so we tried to track it down. As we drove around, though, the readings just didn’t make any sense – it seemed as though we’d driven into an area where Co-60 was everywhere; including some areas where we’d previously not detected anything. I have to admit I was puzzled – it just didn’t act like we were seeing a single source of Co-60; they were more consistent with somebody already having spread contamination around a large area. But if that was the case, then how could we explain Co-60 “detections” in areas previously unaffected? What finally settled the issue for me was taking a look at the instrument output – what we call the spectrum. Every single gamma-emitting radionuclide gives off gamma radiation with a very specific pattern of gamma ray energies – a fingerprint of sorts. In the case of Co-6o, this fingerprint is two gammas with energies of 1170 and 1330 thousand electron volts (or keV). It turns out that radioactive potassium-40 (K-40) has a single gamma ray with an energy of 1460 keV. So the K-40 gamma is close in energy to one of the Co-60 gammas – but not close enough to mis-identify this way. The other factor is that the type of detectors we were using (crystals of sodium iodide, or NaI) are sensitive to the effects of temperature – the gamma energy readings you see can change as these detectors heat up or cool down. I realized that since the detectors were inside the vehicle they had been warming up as we drove around and this temperature change was making the K-40 gamma look like the higher-energy Co-60 gamma. What I saw when I looked at the energy spectrum was a single gamma peak instead of the double Co-60 peak; this is what clued me in to the instrument problem. To verify this, we drove out of the area and simple rebooted the instrument – when it started up and re-calibrated itself the “Co-60” vanished, to be replaced by the real K-40 that had been there all along. On the other hand, had I seen the double peak I would have known we had a problem.
There’s a lot more I can say on this topic, but not without boring you! So to sum up, you should remember that you are always surrounded by natural radiation and this is going to give you readings – sometimes it will give you alarms. Being aware of this is important – you don’t want to over-respond to a granite wall, but you don’t want to miss something important either. What’s important is to be able to trouble-shoot and confirm your readings.