📘 GCSE Physics · Notes & worksheet
Radioactivity and Nuclear Physics.
Notes
Atomic structure recap
Nuclear notation: ᴬ_Z X, where A is the mass number (protons + neutrons) and Z is the atomic number (protons). Isotopes of an element have the same Z but different A.
Ionising radiation
| Type | What it is | Ionising power | Penetration | Stopped by |
|---|---|---|---|---|
| Alpha (α) | helium nucleus (2 protons, 2 neutrons) | highest | lowest, a few cm of air | paper, skin |
| Beta (β⁻) | fast electron from the nucleus | medium | medium, about 1 m of air | a few mm of aluminium |
| Gamma (γ) | electromagnetic wave | lowest | highest | thick lead or concrete |
Also: neutron emission, and positron emission (β⁺).
Nuclear equations
Mass number and atomic number must balance on each side.
- Alpha: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
- Beta (β⁻): ¹⁴₆C → ¹⁴₇N + ⁰₋₁e (a neutron becomes a proton)
- Gamma: no change to A or Z.
Half-life
The time for the number of undecayed nuclei, or the activity (count rate), to halve. Decay is random but each isotope has a fixed half-life.
After n half-lives, the fraction remaining is (½)ⁿ. After 3 half-lives: 1/8 remaining and the ratio decayed : remaining is 7 : 1.
Correct count rates by subtracting background radiation.
Contamination and irradiation
- Irradiation: exposure to radiation from a source outside the body. Stops when the source is removed.
- Contamination: radioactive material gets onto or into the body, so exposure continues.
Alpha emitters are most dangerous inside the body; gamma emitters are most dangerous outside it.
Uses
Medical tracers (gamma emitters with a short half-life), radiotherapy, sterilising equipment, smoke detectors (alpha), thickness gauges (beta).
Nuclear fission and fusion
- Fission: a large nucleus (for example uranium-235) splits after absorbing a neutron, releasing energy and more neutrons that can start a chain reaction. In a reactor control rods absorb neutrons.
- Fusion: light nuclei join to make a heavier nucleus, releasing energy. It needs very high temperature and pressure, so is hard to sustain on Earth.
Models of the atom
Rutherford's alpha scattering experiment: most alpha particles went straight through (atom mostly empty space), a few deflected (small positive nucleus), very few bounced back (dense nucleus).
Common mistakes
- Saying half-life is the time for all nuclei to decay.
- Forgetting to subtract background radiation.
- Mixing up contamination and irradiation.
- Not balancing both mass number and atomic number.
Exam tips
- A half-life table helps: write counts down the page, halving at each step.
- For uses of isotopes, explain why the radiation type and half-life suit the job.
Worksheet
Prints the questions only, without answers. Choose “Save as PDF” in the print window to download.
Total: 28 marks.
- A radioactive isotope has a half-life of 6 hours. The initial count rate is 240 counts per minute. Calculate the count rate after 24 hours. (3)
- After 3 half-lives, what fraction of the original nuclei remains, and what is the ratio of decayed to undecayed nuclei? (2)
- A sample has an activity of 800 Bq. After 15 days it is 100 Bq. Calculate the half-life. (3)
- A Geiger counter reads 130 counts per minute near a source. The background count is 30 counts per minute. State the corrected count rate. (1)
- Complete the nuclear equations.
(a) ²³⁸₉₂U → ?Th + ⁴₂He (2)
(b) ¹⁴₆C → ¹⁴₇N + ? (2) - Describe the penetrating power and ionising power of alpha, beta and gamma radiation. (4)
- Explain why an alpha source is more dangerous inside the body than outside it, and why a gamma source is the opposite. (3)
- A doctor wants an isotope to trace blood flow through the body. Explain what properties the isotope should have. (3)
- Rutherford fired alpha particles at thin gold foil. Describe three observations and explain what each showed about the structure of the atom. (3)
- Describe how a chain reaction occurs in nuclear fission and how a reactor controls it. (2)
Show mark scheme and answers (tutor copy)
Try the worksheet first, then check your work here.
- 24 ÷ 6 = 4 half-lives (M1). 240 → 120 → 60 → 30 → 15 (M1). 15 counts per minute (A1).
- 1/8 remains (B1), and the ratio decayed : undecayed is 7 : 1. (B1)
- 800 → 400 → 200 → 100 is 3 half-lives (M1). 15 ÷ 3 (M1) = 5 days (A1).
- 130 − 30 = 100 counts per minute. (B1)
- (a) Th has mass number 234 and atomic number 90: ²³⁴₉₀Th. (B1, B1)
(b) ⁰₋₁e (a beta particle, an electron). (B1 for the particle, B1 for the correct notation or name) - Alpha: stopped by paper or skin, a few cm of air, strongly ionising (B1). Beta: stopped by a few mm of aluminium, about 1 m of air, weakly ionising compared with alpha (B1). Gamma: very penetrating, reduced by thick lead or concrete, weakly ionising (B1). Ionising power order: α > β > γ and penetration order: γ > β > α. (B1)
- Alpha is stopped by skin and a few cm of air, so outside the body it cannot reach living cells (B1). Inside the body all its energy is absorbed by cells over a very short distance, causing intense ionisation and damage (B1). Gamma passes through the body easily, so is dangerous outside (it reaches the cells), but it is less harmful when inside because most of it leaves. (B1)
- It should emit gamma radiation so it can pass out of the body and be detected (B1). It should have a short half-life (hours), so it does not remain radioactive in the patient for long (B1). It should be non-toxic and not emit alpha (or beta), which would cause damage in the body. (B1)
- Most alpha particles passed straight through: the atom is mostly empty space (B1). A few were deflected through large angles: there is a small, positive nucleus that repels alpha particles (B1). Very few bounced back: the nucleus is very dense and contains most of the mass. (B1)
- A neutron splits a uranium nucleus, releasing energy and more neutrons (B1) which go on to split further nuclei, causing a chain reaction. Control rods absorb neutrons to keep the reaction at a steady rate. (B1)