Section 1 of 9Two different ways of being exposed
People and objects can be exposed to radioactive sources in two quite different ways, and it is important not to confuse them.
- The radiation passes into or through the object, but nothing radioactive is left behind.
- When the source is removed or shielded, the exposure stops, and the irradiated object is not radioactive.
- The contaminating atoms keep on decaying and emitting radiation until they decay away, are washed off or are removed from the body.
Section 2 of 9Examples of irradiation
Having a dental X-ray, passing through an airport scanner, standing near a gamma source in the laboratory, or having a tumour treated with an external gamma beam are all irradiation.
Radiation from outside the body: gamma and, to a smaller extent, beta are the main hazards because they can penetrate skin. Alpha is stopped by skin and a few centimetres of air, so alpha irradiation from outside is mostly harmless.
Section 3 of 9Examples of contamination
Contamination occurs when radioactive dust, liquid or gas gets on skin, clothes or surfaces, or is swallowed, inhaled or enters through a wound. Examples are radioactive dust released in an accident, radon gas breathed in, or radioactive iodine from fallout settling on grass, then entering milk and then people.
Contamination is the more serious risk for alpha emitters.
- but if an alpha emitter is swallowed or inhaled, it sits against living tissue and its strong ionising power damages cells in a very small region over a long time.
- but a gamma emitter can leave the body as it is so penetrating,
- so a larger fraction of its energy escapes.
Section 4 of 9Controlling the risks
For irradiation the key principles are time, distance and shielding: use sources for as short a time as possible, keep as far away as possible (radiation intensity spreads out and weakens with distance), and put absorbing material such as lead or concrete between the source and people. Sources are held in lead-lined containers and handled with tongs.
For contamination the aim is to stop radioactive material getting on or into people: wear protective clothing, gloves and masks, work in sealed enclosures called glove boxes, do not eat or drink where sources are used, and wash hands.
Section 5 of 9Comparing them
| Irradiation | Contamination | |
|---|---|---|
| Source location | Outside the object | On or inside the object |
| Object becomes radioactive? | No | Yes, until contaminants decay or are removed |
| Exposure ends when | Source removed or shielded | Material removed or decayed |
| Main danger | Gamma, beta (penetrating) | Alpha emitters inside the body |
Linkthe half-life determines how long a contaminated area remains dangerous, and the type of radiation determines the type of protection needed.
Section 6 of 9Applying the ideas
When answering questions, first decide whether the radioactive material is outside or inside the object.
- and why a broken source is treated as an emergency: radioactive material may now be spread around.
Medical examples show both
- a patient given a radioactive drink for a scan is contaminated by design, for a short time, with a short half-life tracer that is excreted.
- A patient having an X-ray or an external gamma beam is irradiated.
- Both procedures are judged worthwhile because the medical benefit outweighs the small risk.
Section 7 of 9Worked examples
A patient swallows a capsule containing a radioactive tracer, while another patient is given a course of external gamma radiotherapy. Which patient is being contaminated, and which irradiated? Which will be radioactive after the treatment ends?
The first patient is contaminated, because radioactive atoms are inside the body. The second is irradiated by an external source.
Only the first patient remains radioactive until the tracer decays or is excreted; the second is not radioactive once the beam is switched off.
Explain why an alpha source held in the hand is less dangerous than the same source swallowed.
Outside the body, alpha particles travel only a few centimetres in air and are absorbed by the outer dead layer of skin, so they do not reach living cells.
If swallowed, the source is in direct contact with living tissue. All of the alpha energy is absorbed in a very small volume of cells, causing intense ionisation and cell damage, and exposure continues as long as the source remains in the body.
A technician has a gamma source for 30 minutes at a distance of 1 m. A rule says that doubling the time doubles the dose. How would the dose change if she worked for 15 minutes instead, and what other change would also reduce the dose?
Halving the time to 15 minutes halves the dose, since dose is proportional to exposure time.
Increasing the distance from the source, or placing lead shielding between her and the source, would also reduce the dose.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Irradiation | Exposure to radiation from an external source; the object does not become radioactive. |
| Contamination | Presence of unwanted radioactive material on or inside an object or person. |
| Shielding | Absorbing material, such as lead or concrete, placed between a source and people. |
| Dosimeter | Badge or device worn to record the total radiation dose received. |
| Dose | Measure of the radiation energy absorbed and its biological effect. |