Nuclear Physics (AQA A-Level Physics): Revision Notes

Definitions

1. Activity

• Definition: The rate of radioactive decay of nuclei in a given sample, measured in Becquerels ($Bq$), where $1$ $Bq = 1$ decay per second.
• Explanation: Activity is proportional to the number of unstable nuclei present. Higher activity indicates a faster decay rate, commonly measured with a Geiger counter.

2. Alpha Decay

• Definition: The emission of an alpha particle ($2$ protons and $2$ neutrons) from an unstable nucleus to increase stability.
• Characteristics: Alpha particles are strongly ionising but have low penetration, being stopped by a few centimetres of air or paper.

3. Atomic Mass Unit (AMU)

• Definition: A unit for atomic masses; one AMU is defined as one-twelfth of the mass of a carbon$-12$ atom.
• Explanation: This unit simplifies atomic mass calculations and comparisons, where $1$ AMU $≈ 1.66 x 10⁻²⁷ kg.

4. Background Radiation

• Definition: Low levels of radiation present in the environment from natural sources (rocks, cosmic rays) and artificial sources (nuclear power, medical treatments).
• Explanation: Background radiation varies by location and contributes to the baseline radiation exposure.

5. Beta Decay

• Definition: The process where a beta particle is emitted from a nucleus when a proton turns into a neutron or vice versa.
• Types: Beta-minus decay (emits an electron) and beta-plus decay (emits a positron).
• Characteristics: Beta particles are weakly ionising and can be stopped by a few millimetres of aluminium.

6. Binding Energy

• Definition: The energy required to split a nucleus into its individual protons and neutrons.
• Explanation: Binding energy is related to the mass defect and reflects nuclear stability; higher binding energy per nucleon indicates a more stable nucleus.

7. Chain Reaction

• Definition: A self-sustaining sequence of nuclear fission reactions where neutrons from each fission event cause further fissions.
• Application: Chain reactions are controlled in nuclear reactors to maintain a steady energy output.

8. Closest Approach

• Definition: A technique for estimating nuclear radius by observing how close an alpha particle can approach a nucleus before being repelled.
• Explanation: The point at which kinetic energy converts to electric potential energy indicates the effective radius of the nucleus.

9. Contamination

• Definition: The presence of radioactive material on or in another object, making it radioactive.
• Explanation: Contaminated objects must be handled carefully to prevent radiation exposure.

10. Control Rods

• Definition: Rods made of neutron-absorbing materials (like boron or cadmium) used in nuclear reactors to regulate the fission rate.
• Function: By adjusting their position, control rods manage the number of neutrons available for fission, ensuring a controlled reaction.

11. Coolant

• Definition: A fluid that removes heat from a reactor's core, which is then used to produce energy.

12. Critical Mass

• Definition: The minimum amount of fissile material needed to sustain a nuclear chain reaction.
• Explanation: Below this mass, neutrons escape without triggering further fission events.

13. Electron Capture

• Definition: A process in which an inner electron is drawn into the nucleus, causing a proton to convert into a neutron and emitting an electron neutrino.

14. Fission

• Definition: The splitting of a heavy nucleus into two smaller nuclei, releasing neutrons and energy.
• Explanation: Fission releases significant energy and is used in nuclear reactors for power generation.

15. Fusion

• Definition: The joining of two light nuclei to form a heavier nucleus, releasing energy.

16. Gamma Decay

• Definition: The emission of gamma rays from an excited nucleus to release excess energy.
• Characteristics: Gamma rays are weakly ionising but highly penetrating, requiring thick lead to stop them.

17. Half-Life

• Definition: The average time it takes for half of the radioactive nuclei in a sample to decay.

18. Inverse Square Law

• Definition: A principle stating that the intensity of gamma radiation decreases in proportion to the square of the distance from the source.
• Formula: $I \propto \frac{1}{d^2}$
• Explanation: Doubling the distance from a source reduces the radiation intensity to a quarter.

19. Irradiation

• Definition: Exposure to radiation without making the exposed object radioactive.

20. Mass Defect

• Definition: The difference in mass between a nucleus and the sum of the masses of its individual nucleons.
• Explanation: This "missing mass" is converted into binding energy according to $E = mc^2$.

21. Moderator

• Definition: A material (e.g., graphite or water) used in reactors to slow down neutrons so they can be absorbed by fissile nuclei, sustaining fission.
• Explanation: Fast neutrons are less effective at sustaining fission, so moderators are essential in reactors.

22. Radioactive Dating

• Definition: The use of radioactive isotopes with known half-lives to determine the age of objects.

23. Radioactive Waste

• Definition: The by-products of nuclear fission that are radioactive and require careful handling and storage.
• Explanation: Radioactive waste can remain hazardous for thousands of years, necessitating long-term storage solutions.

24. Random Nature of Radioactive Decay

• Definition: Decay is unpredictable; it's impossible to know exactly when a nucleus will decay or which one will decay next.
• Explanation: Decay probabilities are described statistically, but individual decay events are random.

25. Rutherford Scattering

• Definition: An experiment in which alpha particles were fired at thin gold foil to study atomic structure.
• Discovery: Rutherford's experiment demonstrated that atoms have a small, dense, positively charged nucleus, surrounded by mostly empty space.