Radioactivity: Alpha, Beta and Gamma
Unstable nuclei decay by emitting radiation. Three types, each with its own particle, charge, penetration and ionising power — and the properties are linked, which is what makes them worth understanding rather than memorising.
1. What radioactive decay is
Radioactive decay is the random and spontaneous breakdown of an unstable nucleus, emitting radiation.
Two words carry marks:
Random — you cannot predict which nucleus will decay next, or when. Spontaneous — the decay is not affected by anything outside the nucleus: not temperature, not pressure, not chemical state.
Ionising radiation means the emitted radiation can knock electrons off atoms, turning them into ions.
An ion is a charged atom — one that has lost or gained electrons. Losing an electron leaves a positive ion. This definition was a recorded difficulty, and the whole idea of “ionising power” depends on it.
2. The three types
| Alpha (α) | Beta (β) | Gamma (γ) | |
|---|---|---|---|
| What it is | a helium nucleus — 2 protons + 2 neutrons | a high-speed electron from the nucleus | a high-frequency electromagnetic wave |
| Symbol | ⁴₂He | ⁰₋₁e | ⁰₀γ |
| Charge | +2 | −1 | 0 |
| Mass | 4 | ~0 | 0 |
| Ionising power | very strong | moderate | weak |
| Penetration | paper / a few cm of air | ~3 mm aluminium | thick lead / concrete |
| Speed | slow (~10% of c) | fast | speed of light |
| Deflection in fields | deflected a little | deflected a lot, opposite way | not deflected |
A beta particle IS an electron — a high-speed one created in the nucleus when a neutron turns into a proton. It is not “like” an electron; it is one. What makes it a beta particle is where it comes from.
Gamma is a WAVE, not a particle. Calling gamma rays subatomic particles was a recorded error. Gamma is electromagnetic radiation, the same family as light and X-rays but much higher frequency.
The properties are connected
Learn the reason and you never have to memorise the table:
Alpha is big and highly charged, so it collides with and ionises atoms constantly — which means it loses energy quickly and therefore cannot penetrate far. Gamma has no charge and no mass, so it interacts weakly — it barely ionises, and therefore travels a long way through matter.
Ionising power and penetrating power are OPPOSITES. Strongly ionising = weakly penetrating. That single sentence recovers the whole table.
Deflection in electric and magnetic fields
- Alpha (+2): deflected towards the negative plate — only slightly, because it is heavy
- Beta (−1): deflected towards the positive plate — much more, because it is very light, and in the opposite direction to alpha
- Gamma (0): not deflected at all
3. Nuclide notation
A nuclide is written as ᴬ𝑍X, where: A = nucleon number (mass number) = protons + neutrons, on top Z = proton number (atomic number) = number of protons, on the bottom
Number of neutrons = A − Z. There is no separate symbol for it — you subtract. Confusion about how neutrons are represented in nuclide notation was a recorded error: they are not written, they are worked out.
| Particle | Charge | Mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | ~0 (1/1840) |
A neutron has ZERO charge, not −1. That was a specific recorded error — the neutral one is the neutron; the negative one is the electron.
Isotopes are atoms of the same element (same proton number Z) with different numbers of neutrons (different A).
4. Decay equations
The two rules: the totals on each side must balance.
Top numbers (A) must balance. Bottom numbers (Z) must balance.
Alpha decay
The nucleus loses 2 protons and 2 neutrons:
A decreases by 4, Z decreases by 2
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
Beta decay
A neutron turns into a proton, and an electron is emitted:
A stays the SAME, Z INCREASES by 1
¹³⁷₅₅Cs → ¹³⁷₅₆Ba + ⁰₋₁e
In beta decay the proton number goes UP, not down. Because the emitted electron has a charge of −1, the bottom line balances as 55 = 56 + (−1). Misunderstanding the change in atomic number during beta decay was a recorded error, and getting the direction wrong gives the wrong daughter element entirely.
The mass number does not change in beta decay — a neutron becomes a proton, so the total nucleon count is unchanged.
Gamma emission
The nucleus loses energy only:
A and Z both stay the same. Gamma is often emitted alongside alpha or beta decay, as the new nucleus settles.
To find an unknown particle, balance the numbers: whatever is missing from top and bottom tells you what it is.
Read the periodic table carefully for the daughter element. Confusing caesium with barium in a beta decay was a recorded error — check the new proton number against the table, and remember that beta decay moves you one place to the right.
5. Background radiation
Background radiation is the low-level ionising radiation always present around us.
Natural sources (the majority):
- Radon gas from rocks and soil — usually the largest single contributor
- Rocks and building materials
- Cosmic rays from space
- Food and drink
Artificial sources (a small fraction):
- Medical X-rays and treatments
- Nuclear power and weapons testing fallout
When measuring a source, subtract the background count rate first to get the true count rate from the source. Forgetting this is a standard exam trap.
6. Detecting radiation
- A Geiger–Müller (GM) tube connected to a counter measures the count rate in counts per second or per minute
- Photographic film darkens on exposure — the basis of film badges worn by workers
Identifying an unknown source experimentally: place absorbers between the source and detector.
- Stopped by paper → alpha
- Passes paper, stopped by a few mm of aluminium → beta
- Still detected through thick lead → gamma
Remember to subtract background at every stage.
7. Choosing a source for a job
Questions frequently ask why a particular type is used — and the answer always comes from the properties table.
Smoke detectors use ALPHA.
- Alpha ionises the air strongly, allowing a small current to flow; smoke absorbs the alpha, the current drops, and the alarm sounds
- It is safe because alpha cannot penetrate the casing or reach far into the room
Beta and gamma would not work in a smoke detector — this was a recorded misunderstanding. They ionise too weakly to produce the current, and they would escape the device, which is a hazard. The reason is both properties, not just one.
Thickness monitoring uses BETA — alpha would be stopped completely by the sheet and gamma would pass straight through regardless, so neither would respond to thickness changes.
Medical tracers and sterilisation use GAMMA — it penetrates the body or packaging to be detected or to kill bacteria.
Give reasons in terms of penetration AND ionising power. Tutors flagged that explanations should cover the properties of both the chosen radiation and the rejected ones.
8. Mistakes that cost marks
Saying a neutron has charge −1. It is 0.
Saying gamma is a particle. It is an electromagnetic wave.
Not knowing a beta particle is an electron from the nucleus.
Saying the proton number decreases in beta decay. It increases.
Changing the mass number in beta decay. It stays the same.
Getting alpha’s changes wrong — A drops by 4, Z by 2.
Unbalanced decay equations.
Confusing ionising power with penetrating power — they are opposites.
Forgetting to subtract background count rate.
Saying decay is affected by temperature or chemical state. It is spontaneous.
Giving only one property when justifying a choice of source.
Frequently asked questions
What are the three types of radiation? Alpha (helium nucleus), beta (high-speed electron) and gamma (electromagnetic wave).
What is a beta particle? An electron emitted from the nucleus when a neutron changes into a proton.
Which is most ionising? Alpha — and it is therefore the least penetrating.
Which is most penetrating? Gamma, stopped only by thick lead or concrete.
What stops each type? Alpha: paper. Beta: a few mm of aluminium. Gamma: thick lead.
What happens to A and Z in alpha decay? A decreases by 4, Z decreases by 2.
What happens in beta decay? A stays the same, Z increases by 1.
What does “random and spontaneous” mean? You cannot predict which nucleus decays when, and the rate is unaffected by external conditions.
What is background radiation? Low-level radiation always present, mostly from natural sources — especially radon gas.
Why is alpha used in smoke detectors? It ionises air strongly to create a current, and is safely absorbed by the casing.
Quick revision checklist
- I can define radioactive decay as random and spontaneous
- I know what ionisation means
- I know what alpha, beta and gamma each are
- I know their charges and masses
- I know a neutron is neutral
- I know the penetration of each and what stops it
- I know ionising and penetrating power are opposites, and why
- I know how each is deflected in a field
- I can read nuclide notation and find the neutron number
- I can write and balance alpha decay equations
- I can write and balance beta decay equations, with Z increasing
- I know gamma emission changes neither A nor Z
- I can name natural and artificial background sources
- I subtract background count rate
- I can identify a source using absorbers
- I can justify the choice of source using both properties
These notes cover alpha, beta and gamma radiation in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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