Radiation confusion

By Marcus Wilson 17/03/2011 8

I don’t have any clearer picture than any of you on what is happening in the Japanese nuclear power stations at the moment. I’ve only got the official statements to go on, the same as anyone else.  But one thing I can talk about a bit is the nature of radioactive risk and to try to untangle the plethora of different measures of radiation and their units.

Radiation can cause damage to cells in your body, and this in turn can cause nasty effects such as cancer or genetic issues with any children you have in the future. The biology of this I’m not so sure about, but in terms of the physics, each radioactive particle carries energy and that energy can cause damage when it hits you. It’s rather like a cricket ball carrying kinetic energy that will damage any car that gets in the way. Now, you can’t escape radiation. We are continually bombarded by cosmic rays (and, if you live for example in a granite region – radiation from rocks as well)  – and so lots of these minature cricket balls are hitting you every second. Each one of these has the potential to cause damage to a cell that leads to something nasty. 

This means that the more that hit you, the greater the risk. The risk is cumulative. A dose-equivalent (I’ll mention what that means in a moment) of 1 milliSievert for 10 days gives the same level of risk as 10 milliSieverts for one day.  It also means there is no such thing as a ‘safe’ level of radiation. Any radiation at all gives a chance of an adverse effect on you.  It’s like playing a backwards lottery.  Imagine a zillion tickets, of which a zillion minus one are winning tickets. Just one is a losing ticket. Each time you are exposed to a radioactive particle you enter the lottery, staking your life.  Chances are that you’ll win, and live, – but, play the lottery enough times, and your chances of at some point buying that losing ticket become greater overall.

So what about those units?  An easy measure of radioactivity is ‘Activity’. This is a measure of the number of radioactive decays (when an atom changes its status, emitting an alpha, beta or gamma particle – or sometimes more than one) every second.  It’s measurable with a Geiger counter.  The Systeme Internationale units are the becquerel (after Henri Becquerel, the discoverer of radioactivity), but often the old curie unit is used.

Activity doesn’t correlate well with risk to you. A further measure, that is relatively easy to do in a lab, is exposure (measured in roentgens). This is a measure of the amount of ionization of air that a radioactive material causes – ionization being when a molecule has electrons stripped from it by the particle.

Then we bring in Dose.   The dose is a measure of the energy absorbed (e.g. by you). This is measured in grays. It’s a physics-based measure, not a biological-based measure, so still isn’t the best measure of your risk factor. A better measure is the dose equivalent. The unit is the sievert, and it is a reasonable measure of the chance of causing cell damage that leads to cancer etc.

Remember, it’s a cumulative risk, so you want to add up the total dose equivalent you have had over your life. To give you some idea, a typical individual gets about 2 milliSieverts per year. I was hearing on the radio today that they are talking about 200 mSv per hour at places within the Fukushima complex – that’s basically a lifetime’s worth in an hour. You would not want to be there for a particularly long time. But, even so, if you were not exposed for too long a period of time, it would still not be a terribly large risk.

Radiation safety decisions really come down to ‘risk’.  I’ll close by saying that we are pretty bad at estimating risk generally speaking, and when presented with a figure we might scream in fright, but be prepared to take much greater risks everyday, for example when driving to work or crossing the road.  (E.g. do a quick estimate on your chance of dying in a car crash this year – given that about 400 New Zealanders die a year out of a population of 4 million – the calculation is an easy one.  Would you be prepared to enter a backwards lottery with those chances?  Most people do.)

8 Responses to “Radiation confusion”

  • significantly less confused, especially about grays. thank you. even so, still wish i never had to look up what millisieverts were. sharing.

    • Glad the explanation helped! You might notice that as we get further down the progression Becquerel -> Roentgen -> Gray -> Sievert it gets more and more vague. The exposure (Roentgens) is fairly easy to measure (measure the ionization caused by the radiation), but the dose (grays) is more difficult because the amount of energy absorbed by tissue depends a lot on the shape of the tissue and its composition. So the calculation of dose is a little vague. Dose equivalent is more vague still, because the damage to tissue is hard to quantify – it depends on type of radiation (alpha, beta, gamma – and should add X-ray in there as a lower-energy form of gamma) as well as the energy absorbed.

      I could go on to add there is another measure, the EFFECTIVE dose equivalent, which is also confusingly measured in sieverts. This is used when different parts of the body are exposed, because different bits of you have different susceptibilities to being damaged. It’s kind of a ‘whole-body’ average.

      If you want to be really scared, one can estimate your chance of getting cancer due to a return flight from Auckland to London. (You get a much higher cosmic ray exposure at high altitudes). I get my students to do this as an assignment question sometimes. It’s quite scary when you see the answer, BUT then you think about your chance of getting cancer due to other reasons at some point in your life, which is pretty high, and it falls a bit more into perspective. Living is a risky business. Chances are you’ll die of something one day…

  • Marcus said…
    This is a measure of the number of radioactive decays (when an atom changes its status, emitting an alpha, beta or gamma particle – or sometimes more than one) every second.

    This is what disturbed Einstein about the theory of Quantum Mechanics. In QM , there is no causation to the decay process at all, it is probabilistic. What causes the decay? Its chance, says QM. But Einstein hit back at the proponents at the time saying, that God doesn’t play dice with the universe. Bohr replied that Einstein don’t tell God what to do. QM mathematics is indisputable in its prediction capability, so one just have to take it without questioning its validity (ie, its philosophical foundations & formulations), since it is very successful because to date, it has never been refuted.

    I take Feynman’s approach to QM. Just shut-up and calculate.

  • A point to add here, is that not all radiations are harmful to the human body. There are about 65 billion solar neutrino particles that pass through every square centimeter perpendicular to the direction of the sun on any object here on earth (from Wikipedia). This is because of their charge neutrality.

    • Actually, not just because they are neutral. Neutrons are neutral, but they can do damage. The neutron can interact with protons and other nucleons through the strong nuclear force, but the mysterious neutrino only interacts through the weak nuclear force, meaning for most of the time it does nothing at all.

  • The neutron can interact with protons and other nucleons

    That’s true, but this interaction is still elusive to physicists to figure out how to capture it instrumentally (electronics), because it is indistinguishable from noise because it is very very weak . The first person/group/corporation to successfully do so, ie, capturing/detecting neutrinos instrumentally at above noise-level can make billions in neutrino-based telecommunication system. This means that international telecommunication will not require transmission lines/systems as fibre-optic or satellites. Just beam neutrinos from a transmitter thru the earth’s core from one side (which then emerges out on) to the other side of the earth to a receiver there. An excellent alternative wireless system.

    The idea is not new because someone in the late 1970s / early 1980s had file a patent claim on such telecommunication system based on neutrino.

    “Neutrino communication arrangement”

    The US military are into neutrino-based communication R&Ds as well.

    “Neutrinos for Submarine Communications”

  • So, at some level, the effect of radiation is specific to each person who is exposed?

    We fly quite a bit – especially the super long ones to the US and to Europe. I guess it is a little scary, come to think of it. But, the other option is to go nowhere, and that’s not the life we want.

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