# [ RadSafe ] What's a sievert? Definition, purpose etc

Thu Mar 17 22:44:41 CDT 2011

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1 Gy (100 rad) = 1 J/kg.
In other words only about absorbed energy.

Then add radiation quality (alpha, beta, gamma, energies of the particles etc) to take into account biological effects. Alpha is supposed to do more 20 times more damage than gamma radiation and so on. This weighting factor is usually named wR in the litterature.

This results in equivalent dose expressed as sievert, Sv. Thus 1 Gy of gamma radiation = 1 Sv. 1 Gy of alpha radiation = 20 Sv. The equivalent dose is for the specific organ or tissue in question.

In order to discuss the corresponding cancer risks tissue weighting factors (wT) are used like wT for breast cancer is 0.12 (for men+women divided by two as it is supposed to be used for an average population). Weighting for tissues/organs and summing up to 100 % results in effective dose (Sv) and this is what the numbers we may read/hear about.

As most dose in relation to acute situations probably comes from gamma radiation Gy and Sv are roughly the same thing.

Now, as the purpose of the sievert (Sv) mainly is to reflect cancer risks it is somewhat strange to see this used for acute exposures. The sievert is only defined for humans and ICRP states that it should only be used for decision making ("does practice A result in more dose than practice B"?) and for showing compliance with authority regulations.

A somewhat strange thing is that ICRP states that the sievert should not be used to calculate cancer risks - but the weighting factors etc have been derived from radiation exposed groups such as the atomic bomb survivors. From this a slope can be taken (about 5 % fatal cancers per Sv). This is the best estimate we have.

With the sievert being used by mass media in acute situations, I think however, it is best for simplified communication purposes not to confuse the media with one more unit (the Gy).

To my knowledge no person has survived an acute whole body exposure to more than 7 Gy - even with the best medical care etc. On the other hand most people with show no symptoms to an exposure to less than 1 Gy. (just call it Sv for the rescue observations we may see/hear about). In other words, if we read about 500 mSv/hour - just think "multiply by two" + add uncertainties and you may be killed (within about one month).

Two more references:
Chernobyl - total dose integrated over all time: 295 000 manSv.
Computed Tomography annually for the U.S. passed 600 000 manSv about 2-3 years ago.

Dose units are directly risk relevant and therefore much better to use than Bq or half lives which unfortunately often represent a confusing but favorite candy store for nuclear opponents et al..