Welcome to Atomic and Nuclear Physics!
Have you ever wondered what makes the Sun shine, how a smoke alarm detects a fire, or what is really happening deep inside the center of an atom? In this chapter for CCEA GCSE Double Award Science Unit P1, we will explore the tiny world of the atomic nucleus and the enormous energy it can release.
Don't worry if physics sometimes feels intimidating! We will break everything down into clear, bite-sized steps with memory tricks and examiner tips so you can feel completely confident for your exam.
---1. Atomic Structure and Nuclear Notation
Inside the Atom
Everything in the universe is made of atoms. An atom consists of two main parts:
1. A tiny, dense, positively charged nucleus at the very center containing protons and neutrons.
2. Orbiting electrons arranged in energy levels (shells) around the outside of the nucleus.
The Subatomic Particles
You need to know the relative charge and relative mass of each particle:
• Proton: Relative Charge = \(+1\), Relative Mass = \(1\)
• Neutron: Relative Charge = \(0\) (neutral), Relative Mass = \(1\)
• Electron: Relative Charge = \(-1\), Relative Mass = \(\frac{1}{1840}\) (very small / negligible)
Nuclide Notation
In physics, we write an element symbol with two numbers: \(^{A}_{Z}\text{X}\)
• Mass Number (\(A\)): Also called the nucleon number. This is the total number of protons + neutrons in the nucleus.
• Atomic Number (\(Z\)): Also called the proton number. This is the number of protons in the nucleus (it tells you which element it is).
• Number of Neutrons: Calculated easily by subtracting the bottom number from the top number: \(\text{Neutrons} = A - Z\).
Example: For Carbon-14 written as \(^{14}_{6}\text{C}\):
• Protons = \(6\)
• Electrons = \(6\) (in a neutral atom, protons = electrons)
• Neutrons = \(14 - 6 = 8\)
What is an Isotope?
Isotopes are atoms of the same element with the same number of protons (same atomic number) but a different number of neutrons (different mass number).
Memory Trick: Remember the letter P in Isotopes = same Proton number!
Key Takeaway: The nucleus contains protons (\(+1\)) and neutrons (\(0\)), while electrons (\(-1\)) orbit outside. \(A\) is the total mass (protons + neutrons), and \(Z\) is the atomic number (protons).
---2. Radioactivity and the Three Types of Radiation
What is Radioactive Decay?
Some atomic nuclei have an unstable balance of protons and neutrons. To become more stable, these nuclei give out ionizing radiation. Radioactive decay is completely random and spontaneous:
• Random: We cannot predict exactly which nucleus will decay next, or precisely when it will decay.
• Spontaneous: The decay happens naturally on its own and is unaffected by external factors like temperature or pressure.
The Three Types of Nuclear Radiation
1. Alpha Particles (\(\alpha\) or \(^{4}_{2}\text{He}\)):
• Nature: A helium nucleus made of \(2\text{ protons} + 2\text{ neutrons}\).
• Charge: \(+2\)
• Ionizing Power: Strongly ionizing (because of its large mass and \(+2\) charge, it easily knocks electrons off other atoms).
• Penetrating Power: Low. It travels only \(3\text{--}5\text{ cm}\) in air and is completely stopped by a thin sheet of paper.
2. Beta Particles (\(\beta\) or \(^{0}_{-1}\text{e}\)):
• Nature: A fast-moving, high-speed electron ejected from the nucleus.
• Charge: \(-1\)
• Ionizing Power: Moderately ionizing.
• Penetrating Power: Moderate. It travels several meters in air and is stopped by a few millimeters (typically \(3\text{--}5\text{ mm}\)) of aluminium.
3. Gamma Rays (\(\gamma\)):
• Nature: A high-energy, short-wavelength electromagnetic wave.
• Charge: \(0\) (neutral, uncharged). Mass = \(0\).
• Ionizing Power: Weakly ionizing.
• Penetrating Power: Very high. It passes easily through air and materials. Its intensity is reduced (never completely stopped) by several centimeters of lead or thick concrete.
Key Takeaway: Alpha is a heavy \(+2\) helium nucleus (stopped by paper); Beta is a fast \(-1\) electron (stopped by aluminium); Gamma is an uncharged electromagnetic wave (reduced by lead or concrete).
---3. Nuclear Decay Equations
Nuclear equations show what happens when a nucleus decays. The golden rule for balancing nuclear equations is simple:
• Total Mass Number (top numbers) on the left must equal the total on the right.
• Total Atomic Number (bottom numbers) on the left must equal the total on the right.
Alpha Decay Equation
When a nucleus emits an alpha particle (\(^{4}_{2}\alpha\)), it loses \(2\) protons and \(2\) neutrons:
• The mass number (\(A\)) decreases by \(4\).
• The atomic number (\(Z\)) decreases by \(2\).
\(^{A}_{Z}\text{X} \rightarrow ^{A-4}_{Z-2}\text{Y} + ^{4}_{2}\alpha\)
Example: Decay of Radium-226 into Radon:
\(^{226}_{88}\text{Ra} \rightarrow ^{222}_{86}\text{Rn} + ^{4}_{2}\alpha\)
Check: Top: \(226 = 222 + 4\). Bottom: \(88 = 86 + 2\). Balanced!
Beta Decay Equation
Crucial Examiner Fact: How can a nucleus emit an electron when there are no electrons in the nucleus? Inside an unstable nucleus, a neutron turns into a proton and an electron. The proton stays inside the nucleus, and the high-speed electron is fired out as a beta particle (\(^{0}_{-1}\text{e}\)).
• The mass number (\(A\)) remains unchanged.
• The atomic number (\(Z\)) increases by 1 (because a new proton has formed!).
\(^{A}_{Z}\text{X} \rightarrow ^{A}_{Z+1}\text{Y} + ^{0}_{-1}\text{e}\)
Example: Decay of Carbon-14 into Nitrogen:
\(^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + ^{0}_{-1}\text{e}\)
Check: Top: \(14 = 14 + 0\). Bottom: \(6 = 7 + (-1)\). Balanced!
Gamma Emission
Gamma emission simply releases excess energy from the nucleus. There is no change to the mass number (\(A\)) and no change to the atomic number (\(Z\)).
Common Pitfall Alert:
In beta decay, students often mistakenly decrease the atomic number. Remember: because the beta particle has a charge of \(-1\), the new atomic number becomes \(Z - (-1) = Z + 1\)!
Key Takeaway: In alpha decay: top drops by \(4\), bottom drops by \(2\). In beta decay: top stays the same, bottom goes up by \(1\). In gamma: nothing changes.
---4. Half-Life, Activity, and Background Radiation
Activity and the Becquerel
Activity is the rate at which unstable nuclei decay in a sample. It is measured in Becquerels (\(\text{Bq}\)).
\(1\text{ Bq} = 1\text{ decay per second}\).
What is Half-Life (\(t_{1/2}\))?
You can define half-life in either of two ways for your exam:
1. The time taken for half the unstable nuclei in a radioactive sample to decay.
2. The time taken for the activity (or count rate) of a radioactive isotope to fall to half of its initial value.
Step-by-Step Half-Life Calculations
Worked Example: A radioactive isotope has an initial activity of \(800\text{ Bq}\) and a half-life of \(10\text{ minutes}\). What is its activity after \(30\text{ minutes}\)?
• Step 1: Find how many half-lives have passed: \(\text{Number of half-lives} = \frac{30\text{ minutes}}{10\text{ minutes}} = 3\text{ half-lives}\).
• Step 2: Halve the initial activity \(3\) times:
Start: \(800\text{ Bq}\)
After 1 half-life (\(10\text{ min}\)): \(800 \div 2 = 400\text{ Bq}\)
After 2 half-lives (\(20\text{ min}\)): \(400 \div 2 = 200\text{ Bq}\)
After 3 half-lives (\(30\text{ min}\)): \(200 \div 2 = 100\text{ Bq}\)
Answer: The activity after \(30\text{ minutes}\) is \(100\text{ Bq}\).
Background Radiation
Background radiation is the low-level ionizing radiation that is constantly present in our environment all around us.
• Natural Sources: Radon gas (released from rocks and soil), cosmic rays from space, rocks and building materials, food and drink (such as potassium-40 in bananas).
• Man-Made Sources: Medical applications (X-rays, radiotherapy), fallout from historical nuclear weapons testing, radioactive waste/discharges from nuclear power plants.
Corrected Count Rate
When measuring a source in the lab, the detector records both the source and background radiation. To find the true activity of the source alone:
\(\text{Corrected Count Rate} = \text{Total Measured Count Rate} - \text{Background Count Rate}\)
Key Takeaway: Half-life is the time it takes for activity (or nuclei remaining) to halve. Always subtract background radiation to find the true count rate of a sample!
---5. Uses, Dangers, and Safety of Radioactivity
Everyday and Medical Uses
• Smoke Detectors (Alpha Emitter, e.g., Americium-241): Alpha particles ionize air particles inside a gap, creating a small electric current. If smoke enters, it absorbs the alpha particles, stopping the current and triggering the alarm.
• Thickness Control in Paper/Metal Foil (Beta Emitter): Beta radiation passes through paper on a production line to a detector. If the paper becomes too thick, fewer beta particles get through, and the computer commands the rollers to press closer together.
• Medical Tracers and Imaging (Gamma Emitter, e.g., Technetium-99m): A gamma source with a short half-life is injected or swallowed. Doctors use external detectors to trace organ function. Gamma is used because it easily penetrates out of the body to the detector, and a short half-life ensures the patient does not remain radioactive for long.
• Sterilisation (Gamma Rays): Gamma rays penetrate plastic packaging to kill bacteria on medical instruments and food without damaging the equipment or making the food radioactive.
Dangers of Ionizing Radiation
Ionizing radiation can knock electrons off atoms inside living human cells. This can lead to:
• DNA mutations causing cancer.
• Cell and tissue damage or cell death.
Irradiation vs. Contamination (Common Exam Confusion!)
• Irradiation: An object is exposed to radiation from an external source. The object does not become radioactive (e.g., getting a dental X-ray or sterilising medical tools).
• Contamination: Unwanted radioactive material is physically present on the skin, clothes, or inside the body. The object continues to emit radiation until the material is removed.
Safety Precautions
To reduce radiation dose:
• Keep your distance from sources (use long-handled tongs).
• Minimize exposure time.
• Use protective shielding (such as lead-lined aprons, lead boxes, or thick barriers).
Key Takeaway: Alpha is ideal for smoke alarms; Beta controls paper thickness; Gamma is used for medical tracers and sterilisation. Irradiation exposes an object to rays; contamination means radioactive dust is actually on or inside it.
---6. Nuclear Fission and Nuclear Fusion
Nuclear Fission (Splitting Nuclei)
Nuclear Fission is the splitting of a large, unstable nucleus (such as Uranium-235 or Plutonium-239) into two smaller, lighter daughter nuclei, with the release of two or three neutrons and large amounts of energy.
The Chain Reaction:
1. A slow-moving neutron is absorbed by a Uranium-235 nucleus, making it unstable.
2. The nucleus splits into two smaller daughter nuclei and releases \(2\) or \(3\) fast neutrons plus energy.
3. These emitted neutrons can then be absorbed by other Uranium-235 nuclei, triggering further fission reactions. This continuous cycle is called a chain reaction.
Inside a Nuclear Reactor:
• Moderator (Graphite or Water): Slows down fast-moving neutrons so that they can be easily captured by Uranium-235 nuclei.
• Control Rods (Boron or Cadmium): Absorb excess neutrons to regulate the rate of the chain reaction or shut it down safely.
Nuclear Fusion (Joining Nuclei)
Nuclear Fusion is the process where two small, light nuclei (such as isotopes of hydrogen: deuterium and tritium) collide at high speed and fuse together to form a larger, heavier nucleus (such as Helium), releasing huge amounts of energy.
• Where it happens: Fusion is the natural energy process that powers the Sun and all other stars.
• Conditions Required: Fusion requires extremely high temperatures and extremely high pressures/densities.
• Why? Protons in both nuclei are positively charged. High temperatures give the nuclei enough kinetic energy to move fast enough to overcome the strong electrostatic repulsion between the positive charges so they can collide and fuse.
Quick Comparison Memory Trick:
• Fission = Fissure \(\rightarrow\) Splitting a large nucleus apart.
• Fusion = Fuse \(\rightarrow\) Joining small nuclei together.
Key Takeaway: Fission splits heavy nuclei (Uranium-235) in nuclear power plants using a moderator (slows neutrons) and control rods (absorbs neutrons). Fusion fuses light hydrogen nuclei in stars, needing extreme temperature and pressure to overcome positive electrostatic repulsion.
---Chapter Quick Review Checklist
Before your exam, make sure you can:
• State the mass and charge of protons (\(1, +1\)), neutrons (\(1, 0\)), and electrons (\(\frac{1}{1840}, -1\)).
• Calculate protons, neutrons, and electrons from \(^{A}_{Z}\text{X}\) notation.
• Compare Alpha, Beta, and Gamma in terms of nature, charge, ionizing power, and penetrating power.
• Balance alpha (\(^{4}_{2}\alpha\)) and beta (\(^{0}_{-1}\text{e}\)) decay equations.
• Define half-life and solve activity halving problems.
• Explain the difference between irradiation and contamination.
• Explain how moderators and control rods function in a fission reactor.
• Explain why fusion requires very high temperatures and pressures (to overcome electrostatic repulsion).