Atoms and radioactivity, Year 10: radioactive decay and the types of radiation

The becquerel (Bq)

HintIt is named after the French scientist who discovered radioactivity in 1896.

The whyActivity is how quickly the unstable nuclei in a source are decaying. A source in which 500 nuclei decay every second has an activity of 500 of these units. School sources are a few tens of thousands; medical ones can be many millions.

Activity is the number of nuclei decaying each second in the source; count-rate is the number of decays a detector records each second

HintOne belongs to the radioactive material itself; the other depends on the instrument pointed at it.

The whyA detector such as a Geiger-Müller tube catches only the radiation that happens to travel into it, so the count-rate is always smaller than the activity. The two rise and fall together, though, which is why count-rate can be used to follow how the activity of a source changes.

To become more stable

HintThe clue is in the word used to describe the nucleus beforehand.

The whySome atomic nuclei are unstable. By emitting radiation such a nucleus changes into a different, steadier arrangement. The process is called radioactive decay, and nothing outside the nucleus triggers it.

random

HintThe same word describes the roll of a fair dice.

The whyEach unstable nucleus has a certain chance of decaying in any given second, but nobody can say which one will go. Only for a very large number of nuclei does a pattern appear, and that pattern is what half-life describes.

5

HintA rate 'per second' needs the time in seconds.

The whyCount-rate is how many decays a detector registers per second. Two minutes is 120 s, so the count-rate is 600 ÷ 120. Counting for a long time and dividing gives a steadier value than a single one-second reading, because decay is random.

An alpha particle

HintIt is named after the first letter of the Greek alphabet.

The whyBeing large and carrying a charge, it is strongly ionising: it knocks electrons off many atoms as it passes. For the same reason it does not get far, travelling only a few centimetres in air and being stopped by a sheet of paper or by skin.

A high-speed electron, ejected from the nucleus when a neutron turns into a proton

HintIt is a familiar particle, but it does not start out in the shells where you would expect to find it.

The whyThe nucleus contains no electrons to begin with. In beta decay a neutron changes into a proton, and the electron created in that change is thrown out at high speed. It is moderately ionising, travels about a metre in air and is stopped by a few millimetres of aluminium.

Alpha

HintThe most ionising kind is also the easiest to stop.

The whyIonising power and penetrating power run in opposite orders. The most ionising radiation gives up its energy quickly and is absorbed by a few centimetres of air; gamma is the least ionising and the most penetrating, with beta in between.

paper; aluminium; lead

HintThe absorbers get denser and tougher down the list: something you write on, a light metal used for foil and cans, then a very heavy metal.

The whyPenetrating power increases from alpha to beta to gamma. In air, alpha travels a few centimetres, beta about a metre, and gamma a very long way. These differences are what decide which kind of source suits a particular job.

Alpha would be stopped completely and gamma would pass through almost unchanged; only beta is partly absorbed

HintThe reading has to change when the foil gets a little thicker or thinner.

The whyIf the foil becomes thicker, less beta radiation reaches the detector and the count-rate falls; if it becomes thinner, the count-rate rises. Alpha would give a reading of zero whatever the thickness, and gamma would give nearly the same high reading whatever the thickness. Choosing the best source for a job is something AQA expects pupils to do.