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Thursday, March 24, 2011

How Radiation Is Measured

Posted by on Thu, Mar 24, 2011 at 6:05 AM

I suspect many of you, at this point, are pretty damn confused about Bq, Sieverts, Curies and the like. I know most reporters, struggling with the Fukushima story, have no clue at all what these scientific terms mean. Let's make you smart:

Measures of How much radioactivity: The Becquerel.

Radioactivity is when an unstable atom falls apart, releasing energy as it becomes a happier, more stable, atom. This process is called a 'decay'. We can detect a decay using a tool like a Geiger counter by looking for the energy that was released.

A Becquerel is a decay per second. So, when a news report says a liter of water in Tokyo has 210 Becquerels of radioactive iodine, we can translate that to a liter of water (about a quart) as having 210 radioactive Iodine atoms decaying each second.

How do they know it's radioactive Iodine that's creating the decays? By measuring the exact amount and kinds of energy being released in each decay. Every radioactive atom has a fairly unique pattern of energy release when it undergoes decay. This signature can be used to identify the radioactive atoms present.

The other implication of this is, not all decays are equal. Some atoms release far more energy during a decay. Both the kind and amounts of energy released per decay—as well as how many decays in a certain amount of time—all determine the effect of the radiation.

A Measure of the Effect of Radiation: Gray or Sievert

Now, we flip perspectives, from the radioactive atoms decaying and releasing energy to that of the recipients of all this energy.

What we often care about is the effect this energy might have on the recipient—such as the delicate thyroid gland of a young child...

...The first step is to figure how much energy an object is absorbing from a radioactive source. We measure energy in Joules. Next, we have to consider how much stuff this energy is being absorbed into. Putting the two together, we get Gray, or Joules (energy) per kilogram (amount of stuff).

Sieverts are a slight modification of Gray, reflecting an even deeper level of complexity about how living tissues deal with different kinds of radioactive decays. For the kinds of radiation people are being exposed to from Fukushima (gamma waves and beta decays), Gray and Sieverts are the same.

So, when someone says there is 1 sievert per hour of radioactivity around a reactor at Fukushima, what they're trying to tell you is per kilogram of person, there will be a joule of energy delivered by all the radioactive elements decaying nearby.

The energy delivered by radioactive elements to living tissue can cause problems in two ways: It can kill cells or it can damage the DNA in the cells more subtly, increasing the risk of cancer in the future, causing premature aging and perhaps other problems.

We understand far better how energy absorbed from radioactive decay can cause problems, and kill off parts of the body in the short-term. We have a much fuzzier sense of how lower doses of energy absorbed from radioactive decay increases the risks for subtler problems down the line.

I'm highly suspect—and you should be too—when people attempt make sense of the risks posed by small doses (measured in Gray or Sieverts) of radiation by comparing to things like chest x-rays, CT scans or airplane flights—or bananas eaten. We're just now beginning to appreciate that medical imaging isn't quite as safe as doctors or patients have long assumed. We're also beginning to realize that the duration and frequency of radiation exposures matter a lot for cancer risk—in ways scientists are just now beginning to tease out—important details completely ignored when you're measuring radiation exposures in Sieverts, and scaling linearly. In strange ways, quickly delivered, localized, higher doses of radiation are sometimes 'safer', killing off damaged cells rather than allowing them to live on, damaged, and cause cancers later. In short, living things are not all that much like an ananimate block of matter—and therefore radiation exposure shouldn't be measured for living things in the same way either.

Frankly, and honestly, it's next to impossible to accurately predict right now the ultimate health effects from the Fukushima disaster—despite all we've learned from other radioactive disasters.


Comments (19) RSS

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emma's bee 1
This is your best post by far on this issue, Jonathan. Thank you.
Posted by emma's bee on March 24, 2011 at 6:36 AM · Report this
mayor 2
As a child of the 60's and 70's, could you also provide conversion to REMs or mREMs? I understand those (and everybody older than 50 would appreciate it. You didn't fight the cold war like we did.
Posted by mayor on March 24, 2011 at 7:11 AM · Report this
Here's a good explanations of milisierverts:…

@mayor, here's an explanation of REMs…

Most important is that 100 REMs = 1 Sv
Posted by Lynx on March 24, 2011 at 7:26 AM · Report this
emma's bee 4
@2: 1 Gy=100 rad* or 100,000 mrad
1 Sv=100 rem** or 100,000 mrem

rad=Roentgen absorbed dose
rem=Roentgen equivalent man
Posted by emma's bee on March 24, 2011 at 7:28 AM · Report this
Can you explain how vegetables hit by radiation is a danger to humans? I understand how released energy hitting a human can change our cells but what is happening inside a radiated vegetable? Does radiation breed more radiation as the energy hits other atoms or is the mutation of the vegetable the actual danger?
Posted by cliche on March 24, 2011 at 7:32 AM · Report this
@5, the danger from contaminated food is not the mutation of the food itself. The danger is that radioactive atoms are incorporated into the food. For instance, radioactive atoms can be dispersed into grazing land for cows. The cows then eat that grass and incorporate the radiactive isotopes into their own bodies. We can end up accumulating them by drinking the milk from such cows.

The problem with radioactive atoms is that they emit radioactivity. If you have an isolated source of radioactivity, all you have to do is move away. However if you ingest radioactive materials, through contaminated food or drink, you carry those isotopes with you and they can irradiate your cells, which carries an increased risk of mutating them so that cell division becomes uncontrolled, causing cancer.
Posted by Lynx on March 24, 2011 at 7:37 AM · Report this
@6 - Thanks. Just so I got it; It sounds like the energy released from decay can be an actual radioactive atom and not just a electron particle? So the radioactive atom gets into the veggie and will itself decay and release even more radioactive atoms?
Posted by cliche on March 24, 2011 at 7:43 AM · Report this
Thank you Mr. Golob; a very informative post.

I take, however, a bit of objection to the supposition that the estimation of the risks provided by the relevant experts are somehow suspect, because of the complexity of living things and the way we deal with radiation exposure; while I don't disagree with the fundamental thesis that "living things are not all that much like an inanimate block of matter", my concern is that you're promoting a lack of confidence in existing standards based upon no evidence to the contrary, and in fact denigrating the attempt to make sense of some things which are unclear. At the very least, an attempt to make sense of things is a positive attempt; saying "I don't know why, but be afraid!" is purely negative.

As well, consider further that living things are complex in the face of recent studies about the effect of placebos, even when the placebo is revealed; there's a considerable amount of evidence that simply feeling that they are doing SOMETHING about their illnesses helps people, even when they know that that something is simply a placebo. In fact, the effect is quite strong. Considering that investigating the established risks in the face of concern likely counts as doing something about potential risks, the "placebo effect" may apply.

Degrading confidence in established scientific and medical standards increases the sense of powerlessness; if all the established placebo studies are to be trusted (and they are), denigrating the existing standards of risk without providing any alternate or reason can actually INCREASE the fundamental risk to the public from the (small) radiation exposure. That's not even considering the effect based upon the increase in stress / worry.

I'm not saying to not be skeptical; skepticism is valuable and useful. In fact, I'm being skeptical of your skepticism! But basis is important; you give no basis for your skepticism (other than an appeal to complexity). I'm skeptical of (the effects of) your skepticism based upon established scientific studies that indicate that a sense of powerlessness can be harmful to health; i.e., my skepticism is with basis.
Posted by jambalaya on March 24, 2011 at 7:52 AM · Report this
Vince 9
I understand the specifics a little better now. Thank you! My doctors want to x-ray me all the time but, unless it's an emergency, I only allow it, at most, once a year. All things cannot be measured. Like, when you just know in your mind that the "experts" don't know for sure, all the answers.
Posted by Vince on March 24, 2011 at 7:53 AM · Report this
@7, I'm not highly educated in such matters, since my field is more biochemistry, but that's not quite what I had in mind.

The energy released from decay can come in the form of particles, protons or neutrons, but the energetic emission is not of a different radioactive isotope.

What I mean is that radioactive contamination involves actual radioactive atoms getting into what's been contaminated. The inmediate danger near a radioactive reactor is the emission of energy yes, but the problem with contamination is that the source of that energy, the radoactive atoms themselves, escape into the environment. There, they get into the food chain and can cause cancer. Radioactive Iodine is a common byproduct in nuclear reactors. If it gets into the environment it can get into our food. Humans accumulate iodine in our thyroid gland. If our food has I-131, we will accumulate it and hence get ourselves a very small (but constant) source of radioactivity inside our own bodies. What actually harms us is the energy emitted when that isotope degrades into a more stable form.

I hope that made some sort of sense.
Posted by Lynx on March 24, 2011 at 7:59 AM · Report this
@10 - Yes, that makes sense. My confusion was that I thought the danger was always in moving energy of the decay itself. You have made it clear that is it the stationary byproduct of the decay that can also be dangerous. Thank you very much.
Posted by cliche on March 24, 2011 at 8:03 AM · Report this
Very good explanation. Radiation exposure is scary because it is unseen and unfelt. Because of it's association with nuclear weapons there is a great unease when talking about exposure.

What needs to be kept in mind is that we are already exposed to countless chemicals and other radiation in our lives on a daily basis. This is only one addition to the mix.

This issue became important because we can track it directly back to the source. But what about the presence of plastics in out bodies or the soup of radio waves in which we currently sit? I'm not trying to be alarmist just trying to place things in perspective.
Posted by kmq1 on March 24, 2011 at 8:14 AM · Report this
dirac 13
@8; "relevant experts are somehow suspect." In most cases, I'd agree with the sentiment expressed in your comment but particularly not so with this. Early on, we were offered (by people purporting rationality) radiation analogues in bananas or CT scans and told it was all super-fine even for people outside the immediate vicinity of Fukushima. I saw this offered so many times so matter-of-factly as if it was axiomatic (again offered by those claiming "rationality" with a practically fevered pitch). However, I am deeply skeptical of someone claiming that it's perfectly "safe" to have extra doses of radiation beyond background because someone else has arbitrarily deemed it a safe dose for them. Exposing yourself to bananas or CT scans or tranatlantic flights is somewhat volitional--a radiation leak is not.

Ionizing radiation from background likely plays a substantial role in aging. It can happen in one track and cause impact ionizations in DNA, which increases probability of misrepair. This is not cause for widespread alarm especially in the greater context of the earthquake and tsunami (and as kmq1 points out, environmental contamination is widespread) but underreporting it or dismissively downplaying it is also not acceptable.
Posted by dirac on March 24, 2011 at 8:22 AM · Report this
@13: I do, in fact, agree with you (and, for the most part, with Mr. Golob); I think that the risks can often be downplayed by officials. However, my point was that the process of investigating the risks and quantifying them can, through the placebo effect, be protective. Like I said, I do understand skepticism; recognize that there's a difference between dismissively downplaying something and self-reassurance (even if based upon suspect data).

To wit; it's reasonable to question whether government standards for radiation exposure are strict enough; but in my opinion it's a bit over the line to dismiss the efforts of individuals to put exposure in context (as in the case of the xkcd chart). It really comes down to the difference between "dismissive downplay" and "comforting reassurance". It's worth noting that there's a very similar differentiation between "reasonable skepticism" and "unfounded fearmongering". (Clarification: I'm not accusing anyone of "unfounded fearmongering", but pointing out that there's a spectrum of response in both the reaction to the radiation fear and in the reaction to the reaction, and that those spectra both have extremes.)
Posted by jambalaya on March 24, 2011 at 8:37 AM · Report this
Will in Seattle 15
So, does this mean I can keep using iodized salt on my fish and chips?
Posted by Will in Seattle on March 24, 2011 at 11:21 AM · Report this
Style note: When an SI unit is spelled out in English, it should always begin with a lower case letter (becquerel). (from the linked wikipedia article)
Posted by ff00ff on March 24, 2011 at 6:24 PM · Report this
"However, my point was that the process of investigating the risks and quantifying them can, through the placebo effect, be protective."

Just how is there a "placebo" effect in quantifying the risks of radiation exposure? Your body is affected by the radiation, whether you quantify the risk or not. Otherwise, the Japanese officials would be telling people "Hey, we've quantified the risk for you, so you can drink the milk/eat the vegetables even though there is indeed risk."
Posted by sarah68 on March 24, 2011 at 11:43 PM · Report this
tunanator 18
@16 Except when it's based on someone's name.
Posted by tunanator on March 25, 2011 at 12:50 AM · Report this
@18 Most of them are based on someone's name, and becquerels, ohms and kelvins are all written without a capital letter.
Posted by Insolitus on March 26, 2011 at 12:51 PM · Report this

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