Unstable nuclei
Some nuclei are unstable. To become more stable they give out radiation: this is radioactive decay. It is random: there is no way to say which nucleus will decay next, or when.
- Activity is the rate at which nuclei in a source decay, measured in becquerels: 1 Bq is one decay per second.
- A Geiger-Muller tube and counter measure the count rate: the number of decays it detects each second or minute.
- Radiation is always around us from rocks, the air and space: the background count. Subtract it before using a reading.
- A tube records 1500 counts in 5 minutes; background is 0.4 counts per second
- 5 minutes = 300 s, so count rate = 1500 ÷ 300 = 5 counts per second
- corrected count rate = 5 − 0.4 = 4.6 counts per second
Four kinds of nuclear radiation
| What it is | Charge | Ionising | Range in air | Stopped by | |
|---|---|---|---|---|---|
| alpha (α) | 2 protons + 2 neutrons (a helium nucleus) | +2 | strongly | a few cm | paper or skin |
| beta (β) | a fast electron from the nucleus | −1 | moderately | about 1 m | a few mm of aluminium |
| gamma (γ) | an electromagnetic wave | 0 | weakly | very far | reduced by thick lead or concrete |
| neutron (n) | a neutron | 0 | (indirectly) | far | thick water or concrete |
Ionising means knocking electrons off atoms. Each time radiation ionises an atom it loses some energy. Alpha ionises so strongly that it loses all its energy within a few centimetres of air; gamma ionises so weakly that it goes a long way.
Do not mix up the two ideas. Alpha is the most ionising and the least penetrating; gamma is the opposite.
Which radiation is it?
Put absorbers between the source and the detector, one at a time, and compare each reading with the one before:
- no absorber 520, paper 270, aluminium 22, background 20 (counts per minute)
- paper takes off 250: alpha is present
- aluminium takes off 248: beta is present
- behind aluminium it is the same as background: no gamma
Watch decay happen at random
In the radioactive decay simulation, set the half-life slider to 10 s and the starting nuclei to 100, then press Start. Watch the grid: the nuclei change colour in no pattern at all. Reset and run it again: different nuclei go first each time. That is what random means.
Name the radiation
Use what each one is, its charge, or what stops it.