Penetration and ionisation

1. Alpha particles, beta particles and gamma rays can pass through different thicknesses of material before they are stopped, and this is their penetrating power.
2. As radiation passes through a material, it ionises atoms by knocking electrons out of them, and it loses some of its energy each time it does so.
3. Radiation is absorbed once it has transferred all of its energy to the material.
4. Radiation that is strongly ionising loses its energy quickly, so it is stopped after only a short distance and has a low penetrating power.
5. Radiation that is weakly ionising loses its energy slowly, so it travels a long way and has a high penetrating power.
6. Penetrating power and ionising power therefore follow opposite orders for the three types of radiation.

Alpha particles

1. Alpha particles are the least penetrating of the three types of radiation.
2. They travel only about $5cm$ in air before they are stopped.
3. They are stopped by a sheet of paper or by the outer layer of dead skin.
4. Alpha particles are the most strongly ionising, because they are large and carry a charge of $+2$, so they collide with and ionise many atoms along a short path.

Beta particles

1. Beta particles are more penetrating than alpha particles but less penetrating than gamma rays.
2. They travel up to about $1m$ in air.
3. They pass through paper but are stopped by a sheet of aluminium a few millimetres thick, typically about $3mm$ to $5mm$.
4. Beta particles are moderately ionising, because they have a charge of $-1$ and a very small mass.

Gamma rays

1. Gamma rays are the most penetrating of the three types of radiation.
2. They can travel hundreds of metres through air and pass easily through paper and aluminium.
3. Gamma rays are never completely stopped, but their intensity is greatly reduced by several centimetres of lead or by a thick wall of concrete.
4. Gamma rays are only weakly ionising, because they have no charge and no mass.

Identifying the radiation

1. The type of radiation from a source can be identified by finding which materials reduce the count rate from a detector.
2. If a sheet of paper makes the corrected count rate fall sharply, the source emits alpha particles.
3. If paper has little effect but a few millimetres of aluminium makes the count rate fall sharply, the source emits beta particles.
4. If a significant count rate remains after the aluminium and is only reduced by thick lead, the source emits gamma rays.
5. Many sources emit more than one type, so a count rate that drops at each stage shows a mixture of radiations.
6. The same differences explain how radiation is chosen for a task, such as using beta particles to check the thickness of paper in a factory, because changes in the paper thickness noticeably change how many beta particles get through.