Ionising power is how strongly radiation removes electrons from atoms. Penetration is how far it travels through a material before it is absorbed. The two are linked by one idea: every ionisation costs the radiation some energy, so the more strongly it ionises, the shorter its range.
This lesson pulls together the earlier ones in atomic and nuclear models. It relies on the comparison table and on the count-rate corrections from the previous lesson.
How does ionisation explain penetration?
Think of radiation as a moving particle with a fixed amount of energy to spend. When it ionises an atom, it gives up a little of that energy. When the energy runs out, it stops.
- Alpha ionises very many atoms in a short distance. Its energy is used up quickly, so it is stopped by paper or a few centimetres of air.
- Beta ionises less than alpha. It travels further, and a few millimetres of aluminium stops it.
- Gamma ionises rarely. Its energy is spent slowly, so it passes through most materials and is only reduced by thick lead or concrete.
So the ranking of ionising power (alpha, beta, gamma, from strongest to weakest) is the reverse of the ranking of penetration.
How do you identify a source from absorber readings?
Place different absorbers between the source and the detector, then compare the corrected count rates (measured minus background). A drop to background means “stopped”. A small drop means “partly reduced”.
Worked example
The background count rate is 25 counts/min. The table shows invented readings for three sources, P, Q and R, with different absorbers.
| Absorber | P | Q | R |
|---|---|---|---|
| None | 450 | 450 | 450 |
| Paper | 28 | 448 | 450 |
| 3 mm aluminium | 26 | 30 | 445 |
| 5 cm lead | 25 | 25 | 60 |
Step 1, P. Paper drops the reading from 450 to 28, which is almost background (25). So P is stopped by paper: alpha.
Step 2, Q. Paper changes the reading very little (450 to 448), but 3 mm aluminium drops it to 30, almost background. So Q passes paper and is stopped by aluminium: beta.
Step 3, R. Aluminium hardly changes the reading (445). Thick lead reduces it to 60, corrected 35 counts/min, which is much lower than the corrected 425 at the start, but not zero. So R is reduced by lead but not fully stopped: gamma.
Step 4, link to ionisation. P, which was stopped most easily, is the most strongly ionising. R, which passes the furthest, is the least.
The mistake to watch for
Mistaken answer: “Gamma is the most dangerous radiation because it is the most penetrating and therefore the most ionising.”
The student merged the two properties. Gamma is the least ionising of the three, because it is the most penetrating.
Hazard depends on both properties and on where the source is. Outside the body, penetrating radiation such as gamma can reach organs. Inside the body, alpha can be very damaging because it deposits all its energy in nearby tissue.
The correction is to write two separate statements: one about ionisation, one about penetration, then link them with “because each ionisation transfers energy”.
Check yourself
1. Why does alpha radiation have a short range in air?
Show answer
It is strongly ionising. It loses energy to many atoms along a short path, so its energy is used up within a few centimetres.
2. The background count is 20 counts/min. A source gives 380 counts/min with no absorber, 378 with paper, 22 with 3 mm aluminium and 20 with thick lead. What is the source?
Show answer
Paper does not reduce it, but aluminium reduces it to about background. So the source is emitting beta radiation. Alpha would have been stopped by paper, and gamma would not have been stopped by 3 mm aluminium.
3. Why might an alpha source be low risk on a desk but high risk if swallowed?
Show answer
On a desk, the alpha radiation is stopped by air, skin or clothing, so it does not reach sensitive tissue. If swallowed, the source is in direct contact with tissue, and the strongly ionising alpha particles deposit all their energy in a small region.
Where this leads next
You now have all five ideas in this topic. Use the mixed practice set to see whether you can apply them together. Go back to the topic overview to decide which lesson to revisit.
Some students find that they can state each fact but mix the two properties when a longer question is framed as a safety scenario. A teacher can help you practise this in online one-to-one Physics tuition.