Radiation is measured in two kinds of units. One measures the number of radioactive decays; the other kind measures some aspect of its effect on the body. Both kinds of measures have definite limitations, and it can be very frustrating to try and derive one kind of measurement from another. One of the main problems is that there are several types of radioactive energy, and the effects of each kind on the human body are different. Also, there is a problem with different units having been used at different times in history.
Below are technical definitions; another page discusses levels of hazard associated with different levels of radiation.
Raw Radiation: Becquerels and Curies
The raw amount of radiation that’s present in a given situation is measured in becquerels or curies. The becquerel is the currently official unit, but we’re giving both here for two reasons. First of all, when we quote older documents, they often use curies. Second, the becquerel is a much smaller unit that is great for measuring things like medical x-rays, but often cumbersome for measuring the amount of radiation in an unintentional release. It is a nice, tidy measurement of one radioactive decay per second. The curie is 37,000,000,000 decays per second. So any measurement in curies can be converted to becquerels by multiplying by that number. It’s about the same ratio as the relationship between two inches and 100,000 miles. But there are times when you don’t really want to do the conversion, even though becquerels are the more officially approved unit nowadays. For example, the material released in the Chernobyl meltdown was radioactive to the tune of 51.4 million curies. That’s a really big number in becquerels.
Digging a little deeper, you can see a problem here already: both curies and becquerels measure only the number of decay events; they don’t hold any information about the type of radiation that was emitted. To access that information, you have to know what the radioactive material was. This is something of an artifact of instrumentation: the Geiger counter (the world’s most popular device for measuring radioactivity) only counts decays without conveying any information about what sort of decay it was. So to know how dangerous a radiation source is, you need more information.
Effects on Life: Grays/Sieverts, Rads/Rems and Roentgens
Grays and rads are units that measure how much of the radioactive energy that strikes your body is absorbed instead of just passing through. It is tissue-dependent: your bones absorb more gamma rays than the rest of you;, so when the radiation source includes gamma, they accrue more grays than your soft tissues. On the other hand, if the radiation is all alpha, none of the energy gets past your skin; so your bones don’t pick up any grays at all. Some sophisticated analyses of radiation exposure list different levels of grays for different tissues in the body. But most sources simply use a whole body measurement in grays, as if the radiation only had one level of effect on your whole body.
Grays are the modern, approved unit in this category and this time they are the bigger unit, with 100 rads equaling one gray. However, while these units measure the energy absorbed, they don’t allow for the extra-damaging effect of alpha particles (or neutrons, or big chunks of atomic nuclei, but we won’t go into that here).
The units that do attempt to capture the amount of actual ionizing damage are sieverts and rems. Sieverts correlate with grays and rems with rads. For beta and gamma radiation, 1 sievert = 1 gray (and 1 rem = 1 rad). But for alpha radiation, 1 gray = 20 sieverts and 1 rad = 20 rems.
Note that these biological-damage units only apply to mammalian flesh. For a measure of how much ionization takes place in an inanimate material (usually air), we use roentgens, which reflect an equivalent amount of ionization to rems.
Determining the level of biological damage
The above discussion is couched in units that are already defined in terms of biological tissue damage. But how does one arrive at tissue damage units from the cruder units of radiation that only measure the number of nuclear decays over time?
Different radionuclides emit electrons or gamma photons with different levels of energy, which means that they cause different amounts of tissue damage. These energy levels are known for each radionuclide, so if you are given a gross number of decays per second (in becquerels or curies) and the identity of the radioactive material – and your distance from the material – you can calculate the amount and rate of damage in sieverts, which carries reliable correlation to human health.
Here is a preliminary table of common radionuclides, showing some physical parameters and how much damage they cause. The effects have been keyed to megabecquerels (MBq) – one million radioactive decays per second – and grams of material. For each substance we have also given the volume of 1 gram relative to the volume of a penny. The health effects are given in milliSieverts per hour; as a reminder, 100 mSv per hour is the threshold at which deterministic health effects start to occur. Radium is included as a sort of benchmark – many people have a vague sense of how dangerous it is, as it has been responsible for the most human exposure under everyday conditions.
Isotope | half-life (years) | volume of 1g | MBq/ gram | at 1cm | at 5 feet | ||
|---|---|---|---|---|---|---|---|
| /penny | mSv/Mbq | mSv/hr/g | mSv/Mbq | mSv/hr/g | |||
| Cs-137 | 30 | 1/5 | 3,215,300 | 202 | 649,490,600 | 0.0007 | 2384 |
| Sr/Y-90 | 29 | 1/4 | 5,094,900 | 136 | 692,906,400 | 0.0034 | 2822 |
| U-238 | 4,500,000,000 | 1/10 | 0.01 | 1.26 | 0.016 | 0.0000539 | 0.0000005 |
| Pu-239 | 21,400 | 1/10 | 2294 | 4.5 | 10,323 | 0.000000002 | 0.0000044 |
| Ra-226 | 5.3 | 1/10 | 37,000 | 1.7 | 62,900 | 0.000075 | 2.763 |
So for cesium-137 and Sr/Y-90, one gram is enough to kill you outright in an hour at a distance of five feet, and an average of these two is about a thousand times as potent as radium.
Note also the tremendous fall-off in toxicity with distance. This means, conversely, that there’s a tremendous increase in virulence when radionuclides are taken internally, whether inhaled or swallowed. Note also that the above figures are only for beta and gamma radiation. But when taken internally, the heavily-damaging alpha radiation also needs to be taken into account.
Note: The dosage calculations were done with the beta and gamma calculators here. For Sr-90/Y-90, the assumption is that half of the radiation is coming from the strontium and half from the yttrium.