How radiation damages cells

1. Alpha, beta and gamma radiation are all ionising, which means each one carries enough energy to knock an electron out of an atom it passes.
2. The atom left behind has lost a negative charge, so it becomes a positive ion.
3. When this happens inside a living cell, the atom that is ionised is part of a molecule, so ionisation breaks or alters that molecule.
4. Cells are mostly water, so most ionisation happens in water molecules and produces highly reactive fragments that then attack proteins, cell membranes and DNA nearby.
5. A single ionisation is harmless because a cell can repair it, so the danger comes from the very large number of ionisations produced along the path of the radiation.
6. Alpha radiation produces the most ionisations per centimetre of its path, so it does the most damage in the small volume it reaches, while gamma radiation produces far fewer ionisations but spreads them right through the body.
7. From a source outside the body, beta and gamma radiation are the greater hazard, because alpha radiation is absorbed by the dead outer layer of skin and never reaches living tissue.

Tissue damage and mutation

1. There are two separate outcomes to learn: the cell is killed, or the cell survives with damaged DNA.
2. If enough molecules inside one cell are ionised, the cell can no longer function and it dies.
3. When large numbers of cells in the same place are killed, the organ or the skin stops working properly, and this is called tissue damage.
4. A very large exposure in a short time causes radiation sickness, with reddened skin like a burn, hair loss, vomiting and damage to the bone marrow that makes blood cells.
5. If instead the cell survives but its DNA has been altered, the instructions the cell copies when it divides are no longer the original ones, and this change is a mutation.
6. Most mutations have no noticeable effect, because they fall in a part of the DNA that the cell does not use or because the cell repairs them.
7. Some mutations affect the genes that control how often a cell divides, so the cell begins to divide uncontrollably.
8. The mass of cells that builds up is a tumour, and a tumour that spreads and invades other tissue is cancer.
9. A mutation in a sex cell can be passed on to a child, whereas a mutation in any other cell affects only the person exposed.
10. Tissues whose cells divide most often are the most sensitive, which is why bone marrow, the lining of the gut and a developing foetus are damaged by exposures that leave muscle or bone unharmed.

Why exposure changes risk

1. Radioactive decay is a random process, and so is the path each emitted particle takes through the body, so nobody can say which cell will be hit or which mutation will occur.
2. This is why exposure is described using risk, the chance that harm occurs, rather than as a certain outcome.
3. A greater exposure means more ionising particles pass through the body, which means more ionisations, which means a greater chance that a mutation happens in a gene that matters.
4. So the risk of cancer increases with exposure, but a person who is exposed will not necessarily develop cancer, and a person who is not exposed still can.
5. Everyone is exposed to background radiation all the time, so precautions are about keeping any extra exposure as small as is practical rather than reducing it to zero.

Precautions when using sources

1. Every precaution works by cutting the number of ionising particles that reach living tissue, and there are three ways of doing it.
2. Reduce the time spent near the source, because the total exposure is the rate at which radiation arrives multiplied by how long you stand there.
3. Increase the distance from the source, because the radiation spreads out in all directions, so the further away you stand the smaller the fraction of it that passes through you.
4. Use shielding between the source and the person, choosing a material that absorbs that type of radiation: a few millimetres of aluminium for beta, and thick lead or concrete for gamma.
5. In practice these three principles become a set of working rules that a technician or a teacher follows every time a source is used.
   1. Lift the source with long handling tongs rather than fingers, which puts the whole length of the tongs between the source and the hand.
   2. Point the open face of the source away from people and never look directly into it.
   3. Take the source out only for the reading itself and return it to its lead-lined, clearly labelled container immediately afterwards.
   4. Store that container in a locked cupboard and keep a record of when each source is signed out.
   5. Keep everyone else at the far side of the room while the source is out of its container.
   6. Never eat, drink or apply cosmetics in the area, and wash hands afterwards, so that no radioactive material can be swallowed.
6. Industrial sources that cannot be shielded by hand are handled by remote manipulators behind thick concrete walls, which is the same three principles applied on a larger scale.