Understanding Radiation: Becquerels and Sieverts
Following the recent problems at the Fukushima No.1 Nuclear Power Plant, the news has been teeming with talk of isotopes, radiation, contamination, and health risks. For many of us, all this talk of half lives and microSieverts can be a little confusing. In order to understand what we are being told in the news, we need to know a little about nuclear physics. The following series of articles will introduce a few fundamental concepts to help us better understand recent events (don’t worry, we’re going to keep this simple).
We now know about radiation, half lives, and radiation types. Next, let’s take a look at the units used to talk about radiation levels. The units we hear most often in the news at the moment are becquerels and sieverts.
Radiation can damage our bodies by breaking the chemical bonds in our cells. The amount of damage done depends on how much radiation we are exposed to. This in turn depends on how much radioactive material is present in our environment, our food and so on. So it is important to have some way to talk about amounts of radiation. This is what the unit becquerel is for.
A measurement in becquerels tells us something about how much radiation, and therefore how much radioactive material, is present in a given sample. It is a measure of radioactive decay, which is not the same as a simple count of radiation particles or photons, since some isotopes emit a combination of alpha, beta and gamma radiation as they decay (see Radiation Types).
One becquerel is defined as the decay of one atom of a radioisotope per second. So if a radiation detector (for example, a Geiger counter) detects the radiation of one decay coming from a sample in one second, then that sample would be said to contain one becquerel of radiation. A sample emitting radiation from one hundred decays per second would be said to have a radioactivity of 100 becquerels.
Radioactivity is often expressed in becquerels per unit of volume or weight, to express how much radioactive material is contained in a sample. But the unit of volume or weight is not fixed, so we may see becquerels per kilogram (Bq/kg), becquerels per litre (Bq/l), becquerels per cubic meter (Bq/m3), or becquerels per cubic centimetre (Bq/cm3). Of course we need to pay careful attention to this because 100 Bq/cm3 indicates 1000 times more radioactive material than 100Bq/l, which in turn indicates 1000 times more than 100Bq/m3.
So the becquerel is a useful measure of the amount of radiation emitted by a substance, but it tells us little about the effect of that radiation on our bodies. To understand that we first need to measure the amount of energy from the radiation absorbed by the body (called the absorbed dose). This is measured in joules per kilogram, or a special unit called grays.
But absorbed dose alone does indicate how much damage is done to the body, since different types of radiation cause different amounts of damage. So the absorbed dose is multiplied by a weighting factor (1 for beta and gamma radiation, and 20 for alpha radiation). The result is called equivalent dose and it is expressed in sieverts. This is essentially a measure of the amount of potential damage to the body from a given amount of radiation. Since one sievert is very large, we usually hear of the much smaller millisieverts (1/1000 sievert) or microsieverts (1/1000000 sievert).
These units can be used to express the total or accumulated equivalent dose, which is useful to talk about a one-time exposure, or a person’s lifetime dose. But often we want to talk about the dose rate, or the equivalent dose over a certain period of time. In this case the dose rate is expressed in micro(or milli)sieverts per hour (or per year).
We can use these different units depending on what we want to talk about. Exposure to radiation in a certain area is relatively constant, and your dose depends on how long you spend there, so it is useful to talk about this in terms of microsieverts per hour. But the dose from eating contaminated food depends on how contaminated the food is and how much of it you eat. So in this case it is more common to talk about becquerels per kilogram (or per litre, etc).