Radioactivity Units of Measurement

Recently, the ongoing nuclear disaster that is Fukushima Daiichi Nuclear Power Station was given the ultimate designation on the International Nuclear and Radiological Event Scale (INES), a 7.  Many media outlets covered the rationale behind the reclassification, including an article from the New York Times. The IAEA also has a pamphlet online that explains the various designations.

Fukushima I Nuclear Power Plant 1975 (Made based on National Land Image Information, Color Aerial Photographs, Ministry of Land, Infrastructure, Transport and Tourism)

What’s common to both the NYT article and the IAEA pamphlet is the use of terminology—in the form of units—that attempts to quantify some aspect of measuring radioactivity. Most of the articles that discuss the upgrading (the use of the word upgrade here is unfortunate, as if the Japanese people have somehow had their lot improved) of the Fukushima Daiichi disaster to a level 7 also mention that at least some part of the rationale for the higher designation is the amount of radioactive materials that have been released into the environment. Most articles cite a report from the Nuclear Safety Commission that puts the total amount of released radioactive materials at 630,000 terabecquerels (TBq). (By comparison, the Chernobyl disaster is estimated to have had a total release of 5.2 million TBq.) The INES pamphlet also makes use of radiation measurement units. For example, in a level 2 incident, members of the public would have to receive dosages of greater that 10 millisieverts (mSv).

Articles about the Fukushima Daiichi accident have contained a bewildering array of terms that are used in measuring radiation: curies, rads, rems, becquerels, and sieverts are among those that we recall reading over the last month. A couple of others used for measuring radiation, but that we haven’t seen mentioned, are grays and roentgens.

One of the reasons for the multiplicity of the radiation measurement units is the dilemma that we have faced in the United States for far too long now, the use of U.S. common units versus SI units (SI stands for the International System). The other reason for the wide array of units is vastly more interesting: there exist a range of types of radiation measurements that one might want to take. For the purposes of this post, we’re going to address four types of radiation measurements and their respective units: 1) Activity, 2) Exposure, 3) Absorbed dose, and 4) Equivalent dose.

Marie and Pierre Curie with daughter Irene

Measuring activity is effectively asking the question, “How much?” In other words, one type of measurement assesses how much radioactivity has been released into the environment. As was mentioned earlier, in the case of Fukushima Daiichi, the answer to that question is 630,000 TBq. The becquerel is an SI unit that that corresponds to a single radioactive decay (or disintegration) event per second. The U.S. unit that describes activity is the curie (named for Pierre and Marie Curie, about whom we’ve written before), and it’s tied to the radioactive decay processes of a sample of radium (an element the Curies discovered). So, one curie is equal to the amount of radioactive decay that take place in radium in one second. In mathematical terms, that’s 3.7 x 10 to the 10th decays (37,000,000,000, if you’re keeping score at home). The conversion relation for curies and becquerels is an relatively straightforward: 1 curie = 3.7e+10 becquerels. The important think to keep in mind is that both curies and becquerels are measurements of activity.

X-ray of Anna Bertha Röntgen's Hand

The next radiation measurement worth exploring is exposure. Although you might naturally assume that exposure would imply a human or some other tangible target, that wouldn’t be exactly right. (In fact, that’s closer to dosage, which we discuss below.) Instead, radiation exposure measures the ionization of a mass of dry air, whether or not any person is exposed to it. In fact, using the term exposure has an even more constrained meaning in that it is only applicable to energy deposited in the air by gamma and x-rays (not, say, for beta decay). (See more about radiation terms HERE.) The U.S. unit for measure exposure is the roentgen, and the SI equivalent is coulombs/kilogram. But there are better measures than exposure for describing the biological dangers that one might face by coming into contact with radioactivity.

Dosages come in several flavors; two of the most common are absorbed dose and equivalent dose. Each of these dosage measures attempts to quantify the biological impact of coming into contact with radioactive materials. The first measure—absorbed dose—addresses the effects of the energy that is deposited—or absorbed—as ionizing radiation interacts with the body. The U.S. unit for this measurement is the rad, which is an acronym for radiation absorbed dose. The SI unit for absorbed dosage, and one that we haven’t yet seen in the press, is the gray (gy). One gray is equivalent to 100 rads.

wikicommons, no conditions
Alpha Decay

The final dose measurement of particular interest to us right now is the radiation measure that we have encountered most frequently in the media: equivalent dose. The equivalent dose differs from the absorbed dose in that it accounts for differing types of radioactivity. Different types of radiation—alpha-, beta-, gamma-, x-rays—interact with matter in distinctive and unequal ways. The equivalent dose introduces a quality factor to address the discrete biological damage regimes of each type of radioactivity. The U.S. unit for equivalent dose is the rem. Again, this is an acronym; it stands for roentgen equivalent man—or roentgen equivalent in man, meaning mammal. To come up with a measurement in rems, it is necessary to multiply rads by the quality factor for the specific radiation type. The SI unit of equivalent dose is the sievert (sv). It can be exchanged with rems in the same way that rads and grays are converted: a single sievert is equivalent to 100 rems. In other words, equivalent dose—as expressed in rads or sieverts—is really important because it’s a way to weight absorbed dosage in a way that better reflects what radioactivity means to the human beings who come in contact with it.

(For a big conversion chart of these and other measurements for radiation, click HERE.)

The INES grades radiological events as either incidents (levels 1-3) or accidents (levels 4-7).  No number on this scale is exactly equivalent to any of the four ways of measuring radioactivity, and other factors, in addition to radioactivity levels, are considered when assessing the extent of a radiological event. We’d like to suggest another term for the worst nuclear accidents: cataclysm. Thankfully, there have been only two level-7 accidents; Chernobyl and Fukushima Daiichi clearly meet the intended definition of cataclysm, a word whose etymology comes from the Greek kataklysmós, meaning flood deluge. In the case of these two events, it hasn’t been torrents of water that have changed the earth’s surface, rendering the area uninhabitable. Instead, showers of radioactive isotopes—such as the cesium, iodine, and strontium we discussed last week—have turned the areas around Chernobyl and Fukushima Daiichi into a wasteland.


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