What the Hell is Radiation Measured In?

If you're anything like me, you've been reading about the incident a the Fukushima power plant, then eventually read up a little on Three Mile Island and Chernobyl. I've noticed that whenever they talk about radiation it's always measured in some other way.

From this article:

The National Institute of Radiological Sciences said that 3.9 million becquerels per square centimeter of radiation had been detected in the water that the three workers stepped in — 10,000 times the level normally seen in coolant water at the plant.

The injured workers’ dosimeters suggested exposure to 170 millisieverts of radiation.

They measure it one way in one paragraph, then in the next freaking paragraph they measure it differently!

Well, here are all the ways radiation is measured:

Curie (Ci) / Becquerels (Bq)

Curies/Becquerels are measures of actual radioactive radiation. Curies (obviously named for Marie Curie) are the conventional unit, whereas Becquerels (named for another radiation pioneer, Henri Becquerel) are the SI unit. You know, SI, like kilometers instead of miles.

They are defined as:
1 Ci = 3.7×10¹⁰ decays per second = 37 GBq

Obviously, one Becquerel is one radiatoactive decay per second. So, every time an atom emits one tiny chunk of radiation, it is 1 Bq.

Technically, the "becquerels per square centimeter" the article used isn't really a measure of radiation, but it does provide a measurable reference for radiation.

rad / Gray (Gy)
Rads/Grays are measures of absorbed ionizing radiation. Ionizing radiation is radiation that can detach electrons from atoms or molecules. One Gray (the SI unit) is defined as:

1 Gy = 1 Joule of energy absorbed / 1 kilogram of absorbing medium -- usually human tissue
1 rad = .01 Gy

It was named for Louis Gray, who was a British physicist.

Anyway, if you're exposed to some for of radiation (X-rays, for example) that can knock your electrons loose, the amount of energy you absorb is measured in Grays.

rem / Sievert (Sv)
Okay, now it gets fun. Sieverts (named for Rolf Sievert, a Swedish physicist who apparently exposed all sorts of things to radiation) are an attempt to take the absorbed dose (in Gy) and relate it to the biological effects of the radiation.

1 Sv = (1 Gy)W
1 rem = (1 rad)W

W here is a weighting factor that depends on the type of radiation and the type of tissue being measured. Gamma rays and X-rays are going to have different effects on the same tissue.

As a point of comparison that the article didn't feel the need to provide, 170 mSv (the radiation dose the workers were exposed to) is about half of one entire year's worth of natural radiation (300 mSv). One dental X-ray is up to .15 mSv. One mammogram is .7 mSv. I, for one, wouldn't volunteer for 243 simultaneous mammograms.

Ultra Bonus Extra Radiation Measurement:
Roentgen (R)
This one... hmmm... I'm not even going to try to figure this one out. Why don't you read about it for yourself?