Alpha, beta and gamma radiation differ in what they are, how strongly they ionise, and what stops them. Ionising power and penetrating power move in opposite directions.
This lesson is part of SPM Physics nuclear physics. Next, see how the radiations appear in balancing simple nuclear equations.
How do the three compare?
| Property | Alpha | Beta | Gamma |
|---|---|---|---|
| Nature | Helium nucleus | Fast electron | Electromagnetic wave |
| Charge | +2 | −1 | None |
| Ionising power | Strongest | Medium | Weakest |
| Stopped by | Paper, a few cm of air | A few mm of aluminium | Thick lead or concrete (weakened) |
Worked example: reading absorption data
An original experiment places absorbers between a source and a detector. The background count is 20 counts per minute.
| Absorber | Measured count (per min) | Net count (per min) |
|---|---|---|
| None | 400 | 380 |
| Paper | 240 | 220 |
| 3 mm aluminium | 60 | 40 |
| 10 cm lead | 22 | 2 |
Step 1: subtract the background from every reading to get the net count.
Step 2: paper lowers the net count from 380 to 220, so alpha radiation was present (160 counts removed).
Step 3: aluminium lowers it from 220 to 40, so beta radiation was present (180 counts removed).
Step 4: lead lowers it from 40 to 2, so gamma radiation was present (38 counts removed), and almost nothing remains.
The source emits all three types.
The mistake that costs marks
The common slip is to mix up ionising power and penetrating power. Alpha is the most ionising but the least penetrating, and gamma is the reverse.
| Statement | Wrong | Right |
|---|---|---|
| Most penetrating radiation | Alpha | Gamma |
| Most ionising radiation | Gamma | Alpha |
A second slip is to forget the background count. Without subtracting it, every conclusion about how much radiation an absorber removes is off by the background.
Why does alpha ionise so strongly?
It is large and carries a charge of +2, so it interacts strongly with the atoms it passes. It transfers its energy in a short distance, which is why paper stops it.
Check yourself
A source gives a net count of 300 per minute. With paper, the net count is still 298. With 3 mm aluminium it is 20. Which radiations are present, and which is absent?
Answer
Paper removes only 2 counts, so alpha is absent (or negligible).
Aluminium removes 298 − 20 = 278 counts, so beta is present and is the dominant radiation.
The remaining 20 counts pass through aluminium, so gamma is present, and lead would be needed to reduce it.
What to study next
Continue with balancing simple nuclear equations or interpreting half-life graphs. Test yourself with the nuclear physics practice set.
The units and significant-figure checker supports answer presentation. If you want a teacher to go through your reasoning on new data, see online one-to-one Physics tuition.