In the weeks since the Japanese earthquake, tsunami, and nuclear crisis, there have been hundreds of numbers dealing with the radiation levels tossed about by the media. Now that matters have somewhat quieted I thought it might be useful to discuss the various units of measure and what they mean.
First of all, there are two types of units which, unfortunately, seem to be constantly misunderstood. On the one hand we have units which measure radiation as the actual process of decaying atoms. The classic unit of measure is the curie (Ci). One curie is defined as 3.7x1010 decays per second. This has been replaced over time with the SI unit of the becquerel (Bq). One becquerel is equal to “the activity of a quantity of radioactive material in which one nucleus decays per second”. The curie was based on the activity of 1 gram of radium-226, but this is a fairly useful benchmark. Close enough in fact that 1 curie equals 3.7x1010 becquerels.
Another useful unit is cpm or counts per minute, which is defined as “the number of atoms in a given quantity of radioactive material that are detected to have decayed in one minute”. The key word here is ‘detected’, which allows for some inaccuracies. The similar unit of dpm, disintegrations per minute, is the actual (not just reported) number of atoms to decay in one minute. One becquerel = one disintegration per second, so 60 dpm.
Now, each of these units describes radiation emitted by or observed in a radioactive material. The other class of units, more familiar to the average individual, measure absorbed radiation and radiation exposure. The most well-known is probably the rad, which was first proposed in 1918 and is a measure of absorbed radiation. It was standardized in the older CGS (centimeter-gram-second) system of measure in 1953 to be defined as “the dose causing 100 ergs of energy to be absorbed by one gram of matter”. 1 rad = 0.01 J/kg. (A little background: the ‘erg’ is another CGS unit and is “the amount of work done by a force of one dyne exerted for a distance of one centimeter”. 1 erg = 100 nanoJoule. And a ‘dyne’ (also CGS) is exactly 10 microNewtons. The Newton of course is 1kg m/s2.)
Before the rad was standardized, the Roentgen (R) was introduced as a measure of radiation exposure. Named after the German physicist Wilhelm Röntgen, one R is defined as “the amount of radiation required to liberate positive and negative charges of one electrostatic unit of charge in 1 cm3 of dry air at STP”. 1R = 1 rad. And 1 rad * Q = 1 rem, which is the unit of “roentgen equivalent mammal”. The concept of a “radiation dose equivalent” involves taking an absorbed dose and applying some weighting representing the tissue which is doing the absorbing. In most cases, 1 rad equals 1 rem, but because of the lethality of the dosages it is often expressed in millirems. For example, the average person is exposed to 45 millirems of radiation from cosmic rays each year.
Now we enter the present. The SI unit of absorbed radiation is the gray (Gy) and it is defined as “the absorbption of one joule of ionizing radiation by one kilogram of matter”. That is, one gray is one joule of energy absorbed by one kilogram of matter. One gray = 100 rad and, put another way, 1 Gy = 1 J/kg. Though formally defined in 1975, in the United States the rad remains the dominant unit of measure. So why haven’t we seen Gy in the news? The gray measures the absorption of radiation by any matter, and generally we are more interested in the absorption by human beings. To that end there is the sievert, which you are more likely to have seen in the media recently (along with the becquerel).
The sievet (Sv) is the SI unit of dose equivalent radiation. The sievert is to the gray what the rem is to the rad. A weighing factor is applied based on the type of tissue absorbing the radiation. 1 Sv = 1 Gy * the weighing factor (W, or less often Q). Bone usually has a factor 0.01, while the lungs are 0.12. Since the US is more likely to use rems (because we stubbornly and stupidly refuse to standardize any of our systems of measure with the rest of the developed world), 0.01 Sv = 1 rem. Eating a banana delivers a dose of 0.0001 mSv or 0.00001 rem. (Actually, there is also the “banana equivalent dose” or BED, which attempts to express radiation exposure in terms of the amount received from consuming one banana. As it turns out one of my favorite nuts, the Brazil nut, can often exceed 400 Bq/kg per serving, which is 4 times more radioactivity than is found in a banana. This unit is not widely used.) For a really great visualization of radiation doses in sieverts, visit XKCD.
In Japan recently, seawater being tested registered around 40-60 becquerels of radioactivity, and earlier reports had some tap water samples at around 200 Bq. Some produce in the region has measured at 2000 Bq/kg for the common iodine-131 and cesium-137 isotopes. Several articles even report 54,000 Bq of iodine-131 in spinach and other leafy vegetables. As engineers and civillians continue to be exposed to radiation we will undoubtedly see a continued flurry of numbers in the news. And as this incident is constantly being compared to Chernobyl and Three Mile Island, it’s especially important to keep in mind the various units of measure. The fact is, we know that radiation is bad, but we often have no idea how much of it is being absorbed by human beings. The more radioactive elements like Plutonium don’t travel very far, but the lighter ones like cesium and iodine do get carried in the wind to far off places.
We will all be experiencing some temporary increase in our radiation exposure, but it will likely be minimal for most people. However, I strongly believe this should serve as a warning for nuclear power in general. Consider how much of this strife was caused not by the active reactors but by the waste fuel in the storage pools. Even when buried miles underground this stuff stays radioactive for hundreds if not thousands of years! And not just the spent fuel rods, so often talked about. The entire containment vessel is radioactive at the end of a reactor’s life. Entire sections of these facilities must be treated as radioactive waste. Now, I’m not saying that all nuclear power is bad. In fact, there are quite a few alternative reactor designs which not only promise to be safer, but consume more of their fuel and thus produce less waste. Nuclear IS an option, but it must be restrained. These older reactor designs are not the way forward, and as we’ve seen in the last few weeks, they probably shouldn’t even be allowed to remain operational.
The Japanese Red Cross and other relief organizations have been doing tremendous fundraising work to help the victims of the disaster. Many companies are offering deals where half or all of the profits from a sale will be donated. Tumblr has already raised $50,000 through small donations. Please consider contributing or simply showing your love and support.
And don’t forget, tonight from 8:30-9:30 is Earth Hour 2011! While I don’t particularly promote this nonsense of turning off all lights for an hour, I do believe we should all be fighting for the reduction of light pollution in our cities. Be vocal, be active, make change.