Measuring radioactivity

Snack time! I’m going to have a banana. Do you want one too, Philip? - Noooo! Don’t eat that! Bananas are radioactive!

Philip has a point — bananas are a little radioactive, because they are rich in potassium. Some potassium atoms have unstable nuclei. This means that the nucleus “breaks down”, releasing energy or emitting tiny particles. This process is called radioactive decay and results in ionising radiation. Ionising radiation can be absorbed by our bodies and have quite harmful effects.

It can damage living cells, and lead to serious health issues. Does that mean bananas are dangerous to us?! Not at all! In fact, we are constantly exposed to some radiation — not just from bananas, but from nearly everything around us: cosmic rays, and radioactive elements in the soil, food, and even our own bodies! What matters is the amount and intensity of radiation we are exposed to.

So it is very important that we can measure it. Let’s say Philip stands close to some radioactive object. The atoms in this object decay and emit ionising radiation. If we can measure how many atoms decay over time, we can tell how much radiation is emitted. We can describe radioactivity with a unit named after the French physicist Antoine Henri Becquerel, who studied natural radioactivity.

One becquerel means that there is one atomic decay per second. The number of decays measured in becquerels tells us only how much radiation is emitted. But if Philip stands close enough, some of the radiation reaches him and can be absorbed by his body. To tell how much this radiation affects Philip, we need to take other factors into account too: the type of radiation, what parts of Philip’s body were exposed to radiation, and how much energy the body could have absorbed in total. Taking these factors into consideration, we can measure the effective radiation dose, to calculate the potential effects of radiation.

The international unit used in this case is sievert, named after the Swedish medical physicist Rolf Sievert. The higher the number of sieverts, the higher the potential health risk for Philip. The problem is that we can’t detect ionising radiation with our senses. Emitted particles are too small for us to see. We can’t touch, can’t feel or smell the radiation either.

So how do we know if we are exposed to ionising radiation? Or whether the level of radiation is safe or not? Well, we can detect and measure radioactivity with special devices! One such device that allows us to measure radioactivity is the Geiger-Müller counter. The device consists of a metal tube with a thin metal wire in the middle connected to electricity.

The tube is filled with a gas that does not conduct electricity. However, if the tube is hit by ionising radiation, this causes the gas inside to form ions, which do conduct electricity. Each time ions are formed, there is a little “spark” — an electric pulse. We can hear it as a “snap” or “click” in a connected loudspeaker. The more often radiation hits the Geiger-Müller tube, the more ions are created, so the more “clicks” we can hear.

This tube can also be connected to an automatic counter, to give a more precise result. - See, Philip, bananas are safe! - I guess I can have one then.