4.2.1a Radioactive decay, activity and count-rate

Unstable nuclei give out radiation to become more stable

1. Some atomic nuclei are unstable.
2. An unstable nucleus releases energy by giving out nuclear radiation, which makes it more stable.
3. You cannot predict when any single nucleus will decay, so it is a completely random process.
4. This giving out of radiation from unstable nuclei is called radioactive decay.

Activity measures how quickly a source decays

1. Activity is the number of nuclei that decay each second in a source.
2. It is measured in becquerel (Bq).
3. A source with an activity of $50Bq$ has $50$ nuclei decaying every second.

Count-rate is what the detector actually records

1. Activity describes the source, while count-rate describes what a detector picks up.
2. A Geiger–Müller (GM) tube records a count each time it detects radiation from the source.
3. The count-rate is the number of counts divided by the time in seconds.
4. The count-rate is lower than the activity, because a detector only catches the radiation that reaches it, not every decay.

4.2.1b Types of nuclear radiation

Unstable nuclei can emit four types of nuclear radiation

1. When an unstable nucleus decays it can give out one of four types of nuclear radiation.
2. These are alpha particles, beta particles, gamma rays and neutrons.

Alpha: two protons and two neutrons, the most ionising

1. An alpha particle ($\alpha$) is two protons and two neutrons joined together, which is the same as a helium nucleus.

1. It is relatively large and heavy and carries a $2+$ charge, so it is the most strongly ionising.
2. Because it interacts so strongly it has the shortest range: only a few centimetres in air, and it is stopped by a sheet of paper or by skin.

Beta: a fast electron made as a neutron becomes a proton

1. A beta particle ($\beta$) is a high-speed electron thrown out of the nucleus.
2. It is produced when a neutron in the nucleus turns into a proton.
3. It is much lighter and less ionising than an alpha particle, so it travels further: about a few metres in air, and it is stopped by a few millimetres of aluminium.

Gamma: high-energy EM radiation, the most penetrating

1. A gamma ray ($\gamma$) is electromagnetic radiation released from the nucleus.
2. It has no mass and no charge, so it is the least ionising.
3. It is the most penetrating: it has a long range in air and needs thick lead or several metres of concrete to reduce it.

Neutrons can also be emitted from the nucleus

1. Some unstable nuclei give out a neutron ($n$).
2. A neutron has no charge, which lets it travel a long way and pass into other nuclei.

Ionising power and penetration run in opposite orders