I’m reading an interesting-yet-frustrating book about Chernobyl called “Wormwood Forest: A Natural History of Chernobyl” by Mary Mycio. It is interesting simply because I didn’t know much about what happened at Chernobyl immediately after the disaster and in the two decades since.
The frustrating thing is that the narrative style is disjointed and rambling. Fascinating tidbits are sprinkled amongst sort of boring day-to-day info about her travels there. Also frustrating is her random use of units.
The units aren’t really her fault — just as with temperature and length measurements, there are “common” and “system international” (si, or metric) units for radioactivity, too. But to further confuse the issue, radioactivity is divided into four different categories : Radioactivity, Absorbed Dose, Dose Equivalent, and Exposure.
Radioactivity | Absorbed Dose | Dose Equivalent | Exposure | |
Common Units | curie (Ci) | rad | rem | roentgen (R) |
SI Units | becquerel (Bq) | gray (Gy) | sievert (Sv) | coulomb/kilogram (C/kg) |
The first unit, Radiation, is simply a measure of how many atoms spontaneously disintegrate in a unit of time. As you would expect from SI units, the becquerel is exceptionally straightforward at one disintegration per second. The curie is 37 billion becquerels. Why? That is how much radiation is in one gram of the element radium.
The rad and grey are both a measure of how much radiation is absorbed by the tissue of a person. Again, SI rules the day with a simple definition of a grey as one Joule of energy absorbed per kilogram. The older rad unit is 0.01 grey. If you’re good on SI prefixes, you’d realize that a rad is a centigrey (not to be confused with the old name for the Celcius temperature unit, centigrade).
Next up : Dose Equivalent. These units attempt to assess how much damage the radiation would do to living tissue based on the type of radiation. The effectiveness of radiation in damaging tissue is known as its relative biological effectiveness (r.b.e.). Alpha particles have a higher ability to damage tissue than X-Rays or gamma rays, so it may have an r.b.e. of up to 20 whereas the r.b.e for x-ray and gamma radiation is 1. The seivert is the SI unit for dose equivalent, and it equal to the greys times the r.b.e. The rem is the common unit for dose equivalent, and it is equal to the rads times the r.b.e. So 1 seivert is 100 rems.
So while this is confusing, imaging how frustrating it is to understand Mary Mycio’s book when she bandies about becquerels and curies in the same paragraph. It is like saying, “It was so hot in the desert today — it got up to 40 degrees C, but it was a mild 70 degrees F here in Minneapolis.” Not so intuitive.