How is radioactivity measured - in quantity?

We measure the quantity of radioactivity in several different units, but they are all related to a single basic characteristic of radioactive materials - the rate at which they "decay." This question calls for some discussion of some of the terms used to describe radioactivity. So, this will take a few minutes.

First, remember that everything is made of atoms. Most atoms are "stable," that is, they won't change into another type of atom all by themselves. Radioactive atoms are said to be "unstable" because they can and do change into other types of atoms spontaneously. When a radioactive atom goes through such a transition, it always changes to a more stable atom. We call this change a "decay" or "disintegration," although the atom doesn't really disintegrate. The "decay" of radioactive material into stable material is also accompanied by the emission of ionizing radiation. This is why the resulting atom is more stable - the radiation is the release of "excess" energy contained in the unstable atom. The radiation emitted makes it possible to locate and quantify radioactive materials - even in very small amounts.

Here's a quiz: The earth is billions of years old. If all radioactive things eventually become stable, why is there any of it around? The answer is at the end.

Now, we've really defined the quantity of radioactivity in this sample. We said there were 1,000 atoms when we started. We could just say it that way, but as I mentioned in the very first sentence, we normally use the "decay rate" to express how much is there. So, what is the decay rate? Well, if I know I have 1,000 of them, and the half-life is 12 years, I could say the rate is 500 decays per 12 years. BUT, that's really the decay rate averaged over the first 12 years. After 12 years the new average decay rate is 250 decays per 12 years. So, these are "average activity" units, but you can see that the activity of the material is decreasing constantly. More common units to express activity are "decays per minute," or "decays per second." To get the actual initial decay rate in our sample, there is a simple formula that tells us the activity is about 0.0000018 decays per second.

There are two "special" units of radioactivity. One is called the Becquerel and is equal to 1 decay per second. The other special unit is called the Curie. One curie is equal to 37,000,000,000 decays per second. So we can say our activity is 0.0000018 becquerels. See if you can convert that to curies.

Now, we can't tell how many radioactive atoms are in something by looking at it, so we must measure the radiation emission to figure it out. Remember the electron that got kicked out of the atom during the decay? That's called a beta particle, and it is the radiation emitted each time an H-3 atom decays. This beta radiation can be detected with specialized instruments. It is by measuring this radiation that we normally determine the quantity of radioactivity present.

Let's imagine we put our water sample into a device that could detect each beta particle emitted from the H-3. How long will it be for us to detect it? Well, with the small amount we have, we'd get tired of waiting. We would have to analyze (or "count") our sample for many months to measure the decay rate. It turns out that there are tens of thousands of H-3 atoms in every drop of rain, so if I take a sample of a teaspoon or so, I'll have enough to measure the activity.