P2bL11 Background Radiation

Key Words

Background Radiation - radiation coming naturally from the environment.

Cosmic ray - very high energy radiation coming from space

Geiger counter - device for detecting and counting radioactivity

Radon gas - a gaseous element that is radioactive.  Chemically it is inert, and is in the same chemical group (0) as helium, neon, argon, etc.

Grade E

Background radiation stems from a variety of sources, as shown in the pie-chart:

You will see that much of the radiation is entirely natural.  We even have radioactive carbon-14 in our bodies; the total amount of radiation is tiny and does us no harm.  High-flying pilots wear film badges, as they are exposed more to cosmic rays, so that if they are over-exposed, they are grounded for a time.

Grade C

The background radiation is different in different parts of the country.  This is due to the different types of rock.  The map of Europe shows where the background radiation is higher:

Most of Britain has a relatively low background count from rocks.  In some areas, like central France, the radiation from rocks is quite high.  This is is due to radon, a radioactive gas in the same chemical group as helium and neon.

To get a real-time measurement, we measure the radiation from a radioactive sample using a radiation detector called a Geiger-Müller tube.  This is connected to a rate-meter.

The radioactive decay is measured by the number of counts per second.  A computer can act as a rate-meter and store the results.  It will also plot a graph.

When we take readings it is important that we measure the background count.  There is radioactivity all around us; it's a natural part of the environment.  So we find out what the background count is, then we take that away from the count we get with the source.

People working with sources of radiation wear a film badge.  Each decay causes the deposit of a grain of silver, and it is possible measure the density of the deposits when the film is developed. 

Grade A

When we measure the decay of a radioactive material, we need to take a background count.  This involves leaving the Geiger counter running for a period of time, say 10 minutes (600 s).  Radioactive decay is random, so we could get no counts at all over a 10 s period, then 4 or 5 counts coming in at once.

Background count = total count ÷ time period

We need to subtract the background count from each reading.