Fear and Loathing on the Learning Curve: Observations on Life, Tech and Web Design from a Slightly Misanthropic Mind

A Word on Radiation Units

Having been fol­low­ing quite a bit of the cov­er­age of the tra­gic events in Japan over the last week, I noticed how many dif­fer­ent units of meas­ure­ment are being thrown around by vari­ous media out­lets. Regardless of your thoughts on the sever­ity of the nuc­lear situ­ation, it’s not easy to aggreg­ate facts when some pub­lic­a­tions are talk­ing about microsiev­erts, some about bequer­els, and more. My phys­ics know­ledge has been well neg­lected since leav­ing school, but I thought I’d do a bit of research and attempt to sum­mar­ise these vari­ous units and how they relate to one another.

Note that I’m not a phys­i­cist, nor am I qual­i­fied in any­thing more rel­ev­ant than the abil­ity to sum­mar­ise pages from Wikipedia. Please treat the con­tent of this post accordingly.

Most of these units are meas­ure­ments of ion­iz­ing radi­ation — that is, meas­ure­ments of radio­act­ive particles or elec­tro­mag­netic waves that have enough energy to cause ion­iz­ing reac­tions in other particles.

The becquerel is a meas­ure of the radio­activ­ity of a mater­ial. An amount of mater­ial that decayed at the rate of one nuc­leus per second would be said to have 1 Bq of radio­activ­ity. The meas­ure­ment is dir­ectly related to the quant­ity of the mater­ial in ques­tion as a lar­ger quant­ity of a radio­act­ive sub­stance will pro­duce more nuc­lei in decay. A related unit is the curie, which is essen­tially the same type of meas­ure­ment using a dif­fer­ent numeric basis.

The siev­ert is a meas­ure­ment of the bio­lo­gical effect of a radi­ation dose. Its defin­i­tion incor­por­ates sev­eral char­ac­ter­ist­ics of the radi­ation expos­ure — being weighted by the parts of the body that were exposed, for example, and the type of radi­ation involved. The defin­i­tion is built upon the gray unit (below).

The gray is a more straight­for­ward meas­ure­ment of phys­ical radi­ation absorp­tion, with one gray being defined as the absorp­tion of one joule of energy by one kilo­gram of mat­ter — con­tras­ted with the siev­ert (above) where not all mat­ter is con­sidered equal. For some types of radi­ation, the gray and the siev­ert are con­sidered equi­val­ent. There is also the older unit of rad, which meas­ures the same thing, but with one rad being defined as the absorp­tion of 0.01 joule of energy per kilo­gram of matter.

Older mater­ial on nuc­lear top­ics (such as the Wikipedia page on the Chernobyl dis­aster, which I ima­gine has seen some­thing of an uptick in traffic recently) often refers to the rönt­gen, which seems to be a less favoured meas­ure­ment of radi­ation expos­ure. Much like the rela­tion­ship between grays and siev­erts, the rönt­gen has a coun­ter­part known as “rönt­gen equi­val­ent in man” or rem, which is an adjust­ment to take account of the bio­lo­gical prop­er­ties of humans. It has been largely sur­passed by the sievert.

Lastly there is the banana equi­val­ent dose (BED) which attempts to place radi­ation expos­ure meas­ure­ments in con­text by com­par­ing them to the amount of expos­ure one might receive from eat­ing a banana (meas­ured in becquer­els, see above). Bananas have a high radio­act­ive potassium con­tent com­pared to other foods (though still very minor), and the BED attempts to coun­ter­act the prob­lem of the lack of con­text of the vari­ous radio­act­ive units (“what does that num­ber really mean?”) by com­par­ing them to the banana, which is uni­ver­sally recog­nised to be harmless.

It’s also worth being aware of back­ground radi­ation — we are con­stantly exposed to a tiny level of radi­ation from nat­ural sources in the earth, in the atmo­sphere and in space.

Hopefully this sheds a bit of light on what all these dif­fer­ent terms mean. Once again, I just sum­mar­ised this from Wikipedia, and am not a phys­i­cist. If you spot any errors or want to add any­thing help­ful, please feel free to leave a comment.