Units of radiation

A couple of weeks ago, we posted an item about the Christchurch earthquake, but now of course the world has been shocked by an even bigger earthquake and accompanying tsunami in Japan. For a good explanation of what happened geologically in Japan, see this excellent guest blog at Scientific American.

Currently though, we’re all waiting to see what happens at the damaged Fukushima nuclear plant, and whether a third catastrophe of a complete meltdown can be averted. As I type this, the level of radiation at the plant is 400 millisieverts per hour; while in Tokyo, 250 km away, 0.809 microsieverts were measured in an hour. But exactly what is a sievert?

Measuring radiation in a way that accurately expresses the level of danger is actually rather complicated. A dazzling array of units have been developed, including curies, röntgens, rads, rems, even a banana equivalent dose.

But these days, the preferred SI unit is the sievert, which tries to capture the biological effect of the radiation. It’s based on the gray, which measures the physical dose of radiation. One gray is equal to one joule of radiation absorbed by one kilogram of matter. To get a sievert (Sv), the gray is then multiplied by a factor Q, also known as the Relative Biological Effectiveness, which depends on the type of radiation.

According to the Australian Radiation Protection and Nuclear Safety Agency, the average amount of natural background radiation we’re exposed to is 1.5 millisieverts (thousandths of a sievert) per year. An additional 1 millisievert (mSv) – the recommended maximum extra dose per year – increases your risk of cancer by a factor of 1 in 17,000, which is roughly equivalent to smoking 100 cigarettes.

So the levels of 400 mSv per hour at the Fukushima plant are significant indeed, and the people there working to control the meltdown are putting themselves at great risk. We can only hope for success soon, so that they and the people in the surrounding areas can be safe once more.


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