24.2Nuclear and Particle Physics
Nuclear Decay and Reactions — American System Physics Revision Notes & Topical Questions
10 min read 7 parts Lesson 4 of 5 1 quiz
Lesson summary
Describe the nature and properties of alpha, beta and gamma radiation.Alpha decayAZX → A−4Z−2Y + 42He.Beta-minus decayAZX → AZ+1Y + 0−1e (+ antineutrino); a neutron becomes a proton.Gamma emissionAZX* → AZX + γ (no change in A or Z).In any nuclear equation, nucleon numbers balanc…
- Activity = number of decays ÷ time
- Typical fission
- Decay is random
- Fusion powers the Sun
- Radium-226 (Z = 88) decays by alpha emission to radon (Rn). Write the equation
- Radioactive decay
Section 1 of 7Nuclear Decay and Reactions
Essential knowledge
Decay is random
Decay is random it is impossible to predict when a particular nucleus will decay, but for a large sample the fraction decaying per second is constant. This is why the count rate fluctuates and why many readings must be averaged.
Alpha particles are heavy and doubly charged, so they ionise many atoms per mm and lose energy quickly: range a few cm in air. Beta particles are lighter and ionise less, travelling further. Gamma rays rarely interact, so they penetrate far.
In electric and magnetic fields, alpha and beta are deflected in opposite directions (opposite charges); beta is deflected more because it has much smaller mass. Gamma is undeflected.
In beta-minus decay a neutron in the nucleus turns into a proton and an electron; the electron is ejected. So Z increases by 1 and A stays the same; the element changes. In beta-plus decay (positron emission) a proton becomes a neutron and Z decreases by 1.
Half-life is constant for a given isotope, from fractions of a second to billions of years. After 1 half-life ½ remains, after 2 ¼, after 3 ⅛. Carbon-14 (half-life 5730 years) is used to date once-living material.
Uses
Uses alpha in smoke detectors (ionises air, short range); beta for thickness control of paper or aluminium foil; gamma for sterilising equipment and food, and for killing cancer cells; gamma tracers with short half-lives in medicine.
Hazards
Hazards ionising radiation damages or kills cells and can cause mutations and cancer. Alpha is most dangerous inside the body; beta and gamma are most dangerous outside it. Precautions: keep sources in lead-lined boxes, handle with tongs, keep distance, minimise exposure time, wear dosimeter badges.
Fission
Fission a slow neutron is absorbed by U-235, which becomes unstable and splits into two daughter nuclei plus 2–3 fast neutrons, releasing kinetic energy of the products. The neutrons can trigger further fissions (chain reaction).
In a reactor, a moderator (water or graphite) slows neutrons so they are more likely to cause fission; control rods (boron or cadmium) absorb neutrons to keep exactly one further fission per fission; a coolant carries thermal energy to a heat exchanger to make steam that drives turbines.
Fusion powers the Sun
Fusion powers the Sun hydrogen nuclei fuse to helium. Very high temperatures give nuclei enough kinetic energy to overcome electrostatic repulsion so the strong force can act. Fusion releases more energy per kg than fission, but sustaining it on Earth is technically very difficult.
Section 2 of 7Process & sequence
1
Measure the background count rate with no source present (e.g. counts over 5 minutes) and find the average per minute.
2
Place the source a fixed distance from a Geiger–Müller tube connected to a counter.
3
Record the count rate at regular time intervals.
4
Subtract the background count rate from every reading to get the corrected count rate.
5
Plot corrected count rate against time and draw a smooth curve of best fit.
6
Read off the time for the count rate to halve from any starting value; repeat for several starting points and average to find the half-life.
Controlled chain reaction in a reactorwrite these steps in this order; each step is a marking point.
1
A slow neutron is absorbed by a U-235 nucleus.
2
The nucleus splits into two daughter nuclei plus 2–3 fast neutrons and energy.
3
The moderator slows the new neutrons.
4
Control rods absorb excess neutrons so only one causes another fission.
5
Thermal energy is carried by the coolant to make steam for turbines.
Section 3 of 7Compare
| Property | Alpha α | Beta β− | Gamma γ |
|---|---|---|---|
| Nature | 42He nucleus | Fast electron 0−1e | EM wave |
| Charge | +2 | −1 | 0 |
| Ionising ability | Strong | Moderate | Weak |
| Penetration / stopped by | Paper, skin, few cm air | Few mm aluminium | Thick lead, concrete (reduced, not fully stopped) |
| Deflection in E or B field | Yes, small (heavy) | Yes, large, opposite direction | None |
| Effect on nucleus | A − 4, Z − 2 | A unchanged, Z + 1 | No change in A or Z |
| Feature | Fission | Fusion |
|---|---|---|
| Process | Heavy nucleus splits | Light nuclei join |
| Example | U-235 + n | 2H + 3H → 4He + n |
| Conditions | Slow (thermal) neutrons, critical mass | Very high temperature (about 107–108 K) and pressure |
| Where used | Nuclear power stations | Stars; experimental reactors |
| Waste | Long-lived radioactive products | Little long-lived waste |
Section 4 of 7Worked examples
1Radium-226 (Z
88) decays by alpha emission to radon (Rn). Write the equation.
- 22688Ra → 22286Rn + 42He. Check: 226 = 222 + 4; 88 = 86 + 2. Radon-222 with Z = 86.
2200 Bq
A sample has an activity of 3200 Bq. Its half-life is 6.0 hours. What is its activity after 24 hours?
- Number of half-lives n = 24/6.0 = 4. Activity = 3200 × (½)4 = 3200/16 = 200 Bq.
311 460 years
A wooden artefact has 25% of the carbon-14 of living wood. The half-life of C-14 is 5730 years. How old is it?
- 25% = ¼ = (½)2, so 2 half-lives have passed. Age = 2 × 5730 = 11 460 years (about 1.1 × 104 years).
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Radioactive decay · Alpha particle (α) · Beta-minus particle (β−) · Gamma ray (γ) · Ionising radiation · Half-life (T½) · Activity (A) · Background radiation · Nuclear fission · Chain reaction · Nuclear fusion · Critical mass
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary, never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.
Words to avoid
1
half-life is half the time it takes to decay completelyhalf-life is the time for half of the unstable nuclei (or the activity) to decay.
2
gamma rays are completely stopped by leadgamma is greatly reduced (absorbed) by thick lead or concrete.
3
beta particles come from the electron shellsa beta particle is an electron emitted from the nucleus when a neutron changes into a proton.
4
heating a sample speeds up decaydecay is random and spontaneous, unaffected by temperature or chemical conditions.
5
control rods slow down neutronsthe moderator slows neutrons; control rods absorb neutrons.
6
fission is joining nucleifission is splitting a heavy nucleus; fusion is joining light nuclei.
Instant feedbackCheck yourself
Q1. Which radiation is stopped by a sheet of paper?
- A. Gamma
- B. Beta
- C. Alpha
- D. X-rays
Show answer
C. Alpha is highly ionising and has a very short range.
Q2. When a nucleus emits a beta-minus particle, its proton number
1
A. increases by 1
2
B. decreases by 1
3
C. decreases by 2
4
D. is unchanged
Show answer
A. A neutron becomes a proton.
Q3. A source has half-life 2 days. What fraction remains after 6 days?
- A. 1/6
- B. 1/3
- C. 1/4
- D. 1/8
Show answer
D. 6/2 = 3 half-lives: (½)3 = 1/8.
Q4. 23490Th → 23491Pa + X. What is X?
1
A. An alpha particle
2
B. A beta particle
3
C. A gamma ray
4
D. A neutron
Show answer
B. A unchanged, Z up by 1: beta-minus 0−1e.
Q5. In a nuclear reactor, what is the function of the control rods?
1
A. To absorb neutrons
2
B. To slow neutrons
3
C. To carry heat away
4
D. To shield gamma
Show answer
A. Boron/cadmium rods absorb neutrons to control the rate.
Q6. Why does fusion need very high temperatures?
1
A. To melt the fuel
2
B. To create neutrons
3
C. To give nuclei enough energy to overcome electrostatic repulsion
4
D. To stop gamma rays
Show answer
C. Only then can nuclei get close enough for the strong force to act.
Marked practiceQuizzes for this lesson
Last updated: