Sievert, Gray, Rem, and Rad - Why are there so many different ways to measure radiation exposure?

Japan's unfolding nuclear disaster has introduced Americans to the confusing practice of measuring radiation exposure. According to some stories, the water nearby to the No. 2 Fukushima reactor has a radioactivity level of 1,000 millisieverts per hour. But other articles describe radiation levels in terms of millirem per year. And a few sources have referred to exposure in terms of millirad or nanogray per hour. Why don't all radiation experts just use the same unit?

Because some people are afraid to switch to the metric system. As with distance, weight, and temperature, doses of radiation can be expressed in either SI units (sieverts) or U.S. customary units (rem). U.S. scientists and engineers in most fields had switched to metric units by 1964, when the National Bureau of Standards (now the National Institute of Standards and Technology) officially adopted the international system. But nuclear physicists never made the full switcheroo. That's because a wholesale change in measurement could lead to mistakes, at least during the transition—and even a small mistake can be very dangerous when it comes to radiation exposure. (There is an historical argument for being cautious: In 1999, NASA lost contact with the Mars Climate Orbiter because of a mix-up between metric and customary units [PDF].) On the basis of this concern, the U.S. Nuclear Regulatory Commission still requires plants to report radiation releases in rem, while the rest of the world uses sieverts. For the record, one rem is equivalent to one-hundredth of a sievert.

Sieverts and rem are just two of the many units you might see associated with radiation levels. Scientists use different terms to describe radiation depending on where it is and what it's doing at the time of measurement. For example, when radiation is first emanating from its source, physicists refer to the rate...

FOund on another website

becquerels

The International system of units uses the becquerel.

One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second.

The Bq is used rather than the second for the unit of activity measure to avoid dangerous mistakes: a measurement in becquerels is proportional to activity, and thus a more dangerous source of radiation gives a higher reading.

A measurement in seconds is inversely proportional. This 'activity' varies over time, half-life and all that.

millisievert defined (ya, real helpful)
A unit of ionizing radiation absorbed dose equivalent in the International System of Units, obtained as a product of the absorbed dose measure in grays and a dimensionless factor, stipulated by the International Commission on Radiological Protection, and indicating the biological effectiveness of the radiation.

grays defined
Unit of absorbed radiation dose of ionizing radiation (for example, X-rays), and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter (usually human tissue).

Huh?

(money quote) For X-rays and gamma rays, these are the same units as the sievert (Sv). To avoid any risk of confusion between the absorbed dose (by matter) and the equivalent dose (by biological tissues), one must use the corresponding special units, namely:

the gray instead of the joule per kilogram for absorbed dose and

the sievert instead of the joule per kilogram for the dose equivalent (ie radiation exposure in living things, like you). The word "gray" is both the singular and plural spelling.

In (US) English please!

The unit gray measures absorbed radiation which is absorbed into any material.
The unit sievert specifically measures absorbed radiation which is absorbed by a person.

OK, now just WTF does this mean?

The rem and millirem (abbreviated mrem), are in wider use among the general public, many industries, and government here in the US.

Sieverts, as measured in the United States in "rem"
• 1 rem = 0.01 Sv = 10 mSv
• 1 mrem = 0.00001 Sv = 0.01 mSv = 10 μSv
• 1 Sv = 100 rem = 100,000 mrem (or millirem)
• 1 mSv = 100 mrem = 0.1 rem
• 1 μSv = 0.1 mrem

I have to admit, I had never heard of a becquerel. Thought I would pass this along, in the US we don't normally see SI or metric measurements in the news.

Now, go drink your milk.

Sieverts are weighted according to the type of radiation, the energy range of the radiation, and the target area within the human body. They’re equivalent to Grays only for the issue of X-rays and gamma rays.

Here’s CIPM’s recent re-definition of Sievert:

http://www.bipm.org/en/CIPM/db/2002/2/

And since I’m guessing that won’t help much, here’s a powerpoint bit of slideware to help you:

http://lowdose.energy.gov/ppt/Powerpoint_Dose.ppt

Pay attention to the slides about “weighting factors.”

One of the biggest problems engineers and physicists have when talking with laymen about very technical issues like this is that laymen get careless about units and dimensions. Here, for example, people in the press are tossing around “sieverts” and “becquerels” as tho they’re both a ‘measure of radiation.’

No, they’re not. Becquerel is a unit of radioactive decay, regardless of whether or not any human is going to be in the area to be exposed.

Sievert is a unit to express relative levels of exposure risk to humans and possibly other mammalian life as an “equivalent dose.” If there is a source of radiation where there is and won’t be any human in the vicinity, (eg, deep space, etc), it is pretty pointless to use sieverts as a unit to describe the radioactivity, since a dose has to be applied to a subject. Since what nuke people are trying to get at with a sievert is a dosage level, it is pointless to talk about sieverts without talking about a unit of time; eg, “sieverts per hour” or whatever unit of time you wish to discuss. As the slideware I referenced points out, a bottle of aspirin consumed in 50 seconds ie probably going to be fatal. A bottle consumed over 50 years? Not a whole bunch of health risks there, and possibly it might have even been prescribed. This is why your doctor doesn’t say merely “consume this bottle of pills until they’re all gone,” he tells you “consume this bottle of pills, X pills in T hours, until the prescription is consumed.”

The other thing that hampers rational discussion is that the US population still hasn’t converted to the metric or SI unit system. In engineering school, we were actually given a choice on the first day of physics class. The professor gave us a very simple particle physics question, gave us all the formulas and methods to solve the problem, then he told us that we had to pick either the metric system or the US/imperial system for our units. We couldn’t switch, and he wrote down which system everyone picked.

The guys who picked the US/imperial system invariably failed the exercise, because in the US system, pounds are used for weight, not mass, and weight and mass are not the same thing, and not even the same quantity. Mass in the US/imperial system is dimensioned as “slugs,” not “pounds.”

After that, everyone saw the error of our ways and we jumped on board with SI units immediately. Suddenly, everything gets so easy, so quickly, it isn’t even funny.

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