Understanding radiation units

Radiation units are confusing for three or four reasons.

  1. There are different units depending on whether you’re measuring how much radiation is being emitted or measuring how much is being received.
  2. There are different ways of quantifying the amount of radiation received depending on whether you’re doing physics or biology.
  3. For each of these measurements there are traditional units and SI units.

If you’re not familiar with scientific units, a fourth source of confusion is the prefixes for various powers of 10: milli-, micro-, etc.

The amount of radioactivity emitted by a source is measured in Becquerels or Curies. The SI unit the becquerel (Bq), one decay per second. The traditional unit Curie (Ci) is 3.7 × 1010 Bq and is about the radioactivity of a gram of radium.

The amount of radiation received by a source is measured in grays or rads. The SI unit Gray (Gy) corresponds to one joule of energy absorbed by one kilogram of matter. The traditional unit rad is 0.01 Gy.

The biological effect of radiation is measured in Sieverts or rems. Biologically effective dose is the amount of radiation received multiplied by the relative biological effectiveness (RBE) of the type of radiation source. For x-rays, the RBE is 1. For alpha rays, the RBE is 20. The SI unit of effective dose is the Sievert (Sv), which corresponds to one Gy of x-rays. A rem is 0.01 Sv.

Another unit of effect is the banana equivalent dose. A banana is 0.0001 mSv, or roughly the effective dose of radiation from eating a banana.

11 thoughts on “Understanding radiation units

  1. Not only that, but it is important to consider the type of radiation and the way it is received as well. Not only that, it obeys the inverse square law with distance. Not only that, but materials it passes though can effectively change the radiation. In some cases it is safer to keep a radiation source wrapped in paper than in lead foil.

    The RBE is an extraordinarily rough estimate of things which properly speaking shouldn ‘t even be lumped together. It exists because a rule of thumb is needed, but it has major problems.

    Think about this — if you are getting a certain dose from a distant source it can easily be more harmful than getting the same dose from a source you are holding in your hands. The reason is that for a distant source, the difference in the squared distance for different parts of your body is minimal. But for a proximate source the difference can be profound. Even though your hands are close to the source, they are much less harmed by radiation than your internal organs.

    Also, if you swallow a source you can be much worse off than even having it near you. Think about alpha emitters, and that guy who was assassinated with Polonium in his food or drink because his assassins knew that it is difficult to detect alpha particles and alpha particle radiation is not usually detected.

    Also, the term “radiation” by itself is not very specific. Ionizing radiation is what you want to watch out for.

    I have a Russian army surplus Geiger counter which just clicks, and has a green and red LED that alternate glowing based on the click rate as a rough indicator of danger. In a way it is unfortunate it does not display a number, but as you point out the units can be confusing and not necessarily easily related to the thing most people want to know, which is risk level. So clicks with a red LED when they get frequent is probably pretty good.

    A while back I found a table of “risk equilvalents” of ionizing radiation exposures, kind of like your banana equivalents. This listed different rates and exposure periods, and compared the risk to the risk of smoking cigarettes and driving on the freeway, etc. I though this was the best way of conveying the risk associated with an exposure I have ever seen. I wish more people reported something similar when talking about exposure to ionizing radiation.

    Still, “radiation” has that extra creepiness because unless it is extraorinarily high we can’t sense it, and the risks aren’t well understood, plus the risk can extend pretty far into the future from the exposure time.

    One of the creepiest things I’ve ever read is the list of human radiation experiments performed by legitimate investigators, along with descriptions including the type of person who was used.

  2. Alpha ray? Isn’t there a distinction between particles and rays? There are gamma rays and X-rays, but alpha and beta particles as well as neutron radiation (fast, slow, thermal), right? If I remember right, alpha radiation is basically a helium nucleus (2 neutrons and 2 protons).

  3. Yes, alpha and beta raidation are what we normally think of as particles and gamma is electromagnetic radiation (light wave) as are X-rays (the distinction is artificial and based on wavelength), neutrons are thought of as particles and move fast or slow.

    But you are forgetting your modern physics! Everything is both a particle and a wave (or ray), at the same time.

    I imagine ‘ray’ was the word used because ‘rays’ ‘radiate’, so ‘rays’ must be whatever is radiating in the phenomenon of radiation. If you don’t like it, ‘emission’ can be used for all forms of radiation.

    Fun beer and radiation trivia — the guy who invented the cloud chamber to detect alpha emissions got the idea while looking into his glass at bubbles being formed in beer.

  4. The reports says 752 microSieverts (first paragraph). But one banana = 0.0001 milliSieverts, which is 0.1 microSieverts. Dividing out, this looks like 7520 bananas/hour. I don’t know whether that level is harmful or not, but the banana equivalent metric becomes rather useless at this number.

  5. Egads. I can’t do arithmetic. The banana comparison is pointless when you do the calculation correctly.

  6. I’d say eating 7520 bananas an hour is clearly harmful! Probably 10 an hour is harmful over an extended period of time. 7520 per hour is over 125 per minute, or over two per second. No way that’s harmless!

    It’s the potassium in the bananas that make them radioactive, enough to light up the detectors at ports in the US. Which means the detectors are very good, not that bananas are very hot. At least they would catch something like the batch of radioactive steel that was imported from Mexico.

    I have heard from Geiger counter enthusiasts that salt substitue, which contains relatively large amounts of potassium chloride, will set off a Geieger counter.

    But back to risks: I have a single red-orange Fiestaware bowl, the kind that is famous because the glaze is colored by uranium. It is noticably much more active than the background. If you put the Russian counter next to it the red “danger” LED lights up. However, people have stored these in their houses and eaten food off of them for decades seemingly without adverse effect. That is not certain evidence that they are harmless, but they don’t seem to be especially harmful, even over the long term.

    Fun with bad math: try calculating the number of thermal calories it takes to warm up ice cream to body temperature. It is more than the calories contained in the ice cream! Of course, ice cream calories are really kilocalories. But it is fun to demonstrate that eating frozen ice cream will make you lose weight by leading to a net calorie loss.

  7. Can you recommend a source to better understand the radiation risk to medical personnel who assist at medical procedures such as thallium stress tests and nuclear medicine diagnostic imaging?
    Thank you

  8. Radioactivity 101 here–is a particle’s half-life the same if it is frozen as opposed to room temperature? Would it depend on the particle?

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