A primer on how radiation exposure is actually measured so that you can judge for yourself whether the figures coming from Fukushima are worrisome or not.
March 26, 2011 - 12:00 am
In the last years of the 19th century, Henri Becquerel, along with Marie and Pierre Curie, discovered that some materials spontaneously emitted mysterious rays, like X-rays, that could penetrate matter and expose photographic plates. This property was eventually labeled “radioactivity” — a property that caused certain atoms to spontaneously break down and emit energy.
This was, frankly, a major shock: there had been a whole theory behind chemistry built up around unbreakable, indivisible things called “atoms” — the very name means “indivisible” or “uncuttable.”
But science recovered, and now radioactivity is something we’re used to, at least until something like the Chernobyl, Fukushima, or Three Mile Island accidents makes people think about it again.
Since we’re not faced with thinking about radioactivity in daily life, the units and methods of measuring radioactivity aren’t part of daily life either, not like weight and temperature are. As a result, many people get confused about them. The worst confusion, in fact, seems to be among people who are reporting about radioactivity and radiation in the media.
We would like to be able to get something as clear as a weather report, telling us how hot it is.The problem is, we’re used to the idea of temperature, we have some intuitions about it. We know that 104°F is a hot day or a high fever. But what about radiation exposure?
So let’s look at radiation in some detail and see if there’s something similar.
Becquerel and Roentgen and Curie, oh my!
Start off with Becquerel’s and the Curies’ discovery. Becquerel found out that a particular material known to glow in ultraviolet light, the uranium compound potassium uranyl sulfate, would expose a carefully wrapped photographic plate even through light-proof wrappings. Within a few years, Becquerel, the Curies, Ernest Rutherford, and others proved this radioactivity was being produced by a process that not only emitted energy but transmuted one “immutable” atom into a different kind of atom. These bits of energy came off in discrete packets, called “quanta,” and any particular transmutation produced the same amount of energy in the same form every time.
In fact, the radiation was normally in one of four forms, which we now describe as:
- “Alpha” rays, fast moving particles that are the nuclei of helium atoms stripped of their electrons
- “Beta” rays, electrons with no nucleus attached,
- “Gamma” rays, a form of very high energy X-ray,
- and free, fast-moving neutrons, one of the particles that make up the nucleus.
There are other forms, like free fast-moving protons, and more exotic particles, but they aren’t really important for this; the main four are enough to explain what’s happening at Fukushima.
Take a deep breath, we’re going to be underwater for a little while here.
How much radioactivity is it?
Of course, the first question we want to ask is “how much radioactivity is there?” and frankly, the news readers usually go astray right away at this most basic question. We measure the amount of radioactivity simply by measuring how often an atom decays and transmutes to another kind of atom, freeing some energy. The international unit used to answer the question “how radioactive is it?” is the Becquerel, named for guess who, and represents by definition one radioactive decay per second.
Another unit, named the Curie after Marie, or Pierre, or after both of them (there’s a fun little story of scientific politics that goes with that), is defined to be the number of radioactive decays from one gram of radium in one second, a really big number: 3.7×1010 decays per second.
The outcome of the scientific-political struggle was that a Becquerel is an inconveniently small unit, and the Curie is inconveniently large, so you’ll usually see numbers in tens of thousands or millions of Becquerel, or of milli- or microCurie — 0.001 Curie or 0.000001 Curie.
But how much radiation is it?
The thing is, for questions like health effects, we don’t really care about how many decays there are per second — we’re concerned with how much energy it transmitted to something else. If you’re having baseballs thrown at you, you don’t care how many are thrown near as much as you care how many hit you, and how hard.
When an X-ray hits an atom, it can ionize the atom; it knocks an electron off, giving it an electrical charge. So, to measure the amount of damage being done, we need a measure of absorbed energy. The first unit defined for this was defined as liberating a certain amount of charge in one cubic centimeter of dry air; this unit is called a Roentgen (or Röntgen, the more traditional way of spelling the name) and named for the discoverer of X-rays, Wilhelm Röntgen.
There’s a similar international unit called the Gray, which is defined by the amount of energy absorbed instead of the ionization produced. But since “standard air” absorbs energy as a known constant rate, we can compare Roentgen and Gray directly; it turns out that 1 Gray (Gy) is about 115 Roentgen (R). There is also an exactly comparable unit, of Roentgen absorbed dose (rad). By definition, there are exactly 100 rads to 1 Gray.
Why is there air?
Of course, people aren’t air, and there’s a complication for this whole measurement that different kinds of radiation — alpha, beta, gamma, or neutrons — transfer energy in different ways, with more or less efficiency. A fast moving neutron or alpha particle transfers a lot more energy when it hits than a gamma ray or an electron does. So there are defined units of “biological equivalent dose” that tell us the effect of a dose on a person. This is defined by using a “quality factor” or “weighting factor” for different kinds of radiation. Here’s a table of common weighting factors:
|Type and energy range||Weighting factor|
|electrons, positrons, muons, or photons (gamma, X-ray)||1|
|neutrons <10 keV||5|
|neutrons 10–100 keV||10|
|neutrons 100 keV – 2 MeV||20|
|neutrons 2 MeV – 20 MeV||10|
|neutrons >20 MeV||5|
|protons other than recoil protons and energy >2 MeV||2|
|alpha particles, fission fragments, nonrelativistic heavy nuclei||20|
(keV and MeV are thousands and millions of electron volts, respectively; an electron volt is a measure of a particle’s energy.)
Multiply the absorbed energy in Gray times the weighting factor and you get the human equivalent dose, measured in Sievert.
For our purposes, all the radiation types we’re interested in talking about in connection with the Fukushima accident are in the first row. But the other rows are interesting because you can see how the weighting factor changes for different kinds of radiation; in particular, high energy neutrons don’t do as much damage as the middle range. Basically, a high energy neutron finds it “hard to hit the target” — it’s likely to zip right through without transferring energy at all.
At least talking about Fukushima, all we’re concerned with are beta and gamma radiation, so we can just use a weighting factor of 1. One Gray, times the weighting factor of 1, gives a human equivalent dose measured in Sievert, so 1 Gy of gamma rays is 1 Sv of dose.