Radioactive Half-Life Graphs, Contamination vs Irradiation, Fission & Fusion
Unstable atomic nuclei decay at random into more stable nuclei by emitting alpha (α), beta (β), or gamma (γ) ionising radiation; although individual decays cannot be predicted, the time taken for half of a large number of radioactive nuclei to decay (the half-life) is constant for any given isotope. Safely handling radioactive sources requires distinguishing between irradiation (exposing an object to radiation from an external source) and contamination (the unwanted presence of radioactive atoms on or inside an object). In Triple Science GCSE Physics, nuclear energy is explored through controlled nuclear fission chain reactions in reactors and nuclear fusion inside stars.
- Properties of Alpha (α), Beta (β), and Gamma (γ)
- 1. Alpha (α): 2 protons + 2 neutrons (⁴₂He); +2 charge; strongly ionising; weakly penetrating (range ~5 cm in air, stopped by paper or skin). 2. Beta (β): high-speed electron (⁰₋₁e) ejected when a neutron turns into a proton; -1 charge; moderately ionising; moderately penetrating (range ~1 m in air, stopped by ~5 mm of aluminium). 3. Gamma (γ): high-frequency electromagnetic wave; 0 charge, 0 mass; weakly ionising; highly penetrating (infinite range in air, reduced by thick lead or several metres of concrete).
- Nuclear Decay Equations
- Both total mass number (top) and total atomic charge number (bottom) must balance across the arrow. Alpha decay example: ²³⁸₉₂U -> ²³⁴₉₀Th + ⁴₂He. Beta decay example: ¹⁴₆C -> ¹⁴₇N + ⁰₋₁e. Gamma emission (⁰₀γ) causes no change to the mass number or atomic number of the nucleus. Neutron emission (¹₀n) decreases mass number by 1 and leaves atomic number unchanged.
- Activity (Becquerels) & Half-Life
- Radioactive decay is random. Activity is the rate at which a source of unstable nuclei decays, measured in becquerels (Bq, where 1 Bq = 1 decay per second). Count rate is the number of decays recorded per second/minute by a Geiger-Muller (GM) tube. Half-life is the time taken for the number of radioactive nuclei of the isotope in a sample to halve (or the time taken for the count rate / activity to fall to half its initial level).
Radioactive Half-Life Graphs, Contamination vs Irradiation, Fission & Fusion: Exponential Half-Life Curve & Radiation Shielding
Adjust the number of elapsed half-lives to calculate undecayed nuclei N = N₀ × (½)ⁿ and compare alpha, beta, and gamma absorption.
Key Concepts & Exam Definitions (6)
- Properties of Alpha (α), Beta (β), and Gamma (γ)
- 1. Alpha (α): 2 protons + 2 neutrons (⁴₂He); +2 charge; strongly ionising; weakly penetrating (range ~5 cm in air, stopped by paper or skin). 2. Beta (β): high-speed electron (⁰₋₁e) ejected when a neutron turns into a proton; -1 charge; moderately ionising; moderately penetrating (range ~1 m in air, stopped by ~5 mm of aluminium). 3. Gamma (γ): high-frequency electromagnetic wave; 0 charge, 0 mass; weakly ionising; highly penetrating (infinite range in air, reduced by thick lead or several metres of concrete).
- Nuclear Decay Equations
- Both total mass number (top) and total atomic charge number (bottom) must balance across the arrow. Alpha decay example: ²³⁸₉₂U -> ²³⁴₉₀Th + ⁴₂He. Beta decay example: ¹⁴₆C -> ¹⁴₇N + ⁰₋₁e. Gamma emission (⁰₀γ) causes no change to the mass number or atomic number of the nucleus. Neutron emission (¹₀n) decreases mass number by 1 and leaves atomic number unchanged.
- Activity (Becquerels) & Half-Life
- Radioactive decay is random. Activity is the rate at which a source of unstable nuclei decays, measured in becquerels (Bq, where 1 Bq = 1 decay per second). Count rate is the number of decays recorded per second/minute by a Geiger-Muller (GM) tube. Half-life is the time taken for the number of radioactive nuclei of the isotope in a sample to halve (or the time taken for the count rate / activity to fall to half its initial level).
- Irradiation vs Contamination & Medical Tracers (Triple)
- Irradiation exposes an object to external radiation without making it radioactive. Contamination deposits radioactive atoms onto/into an object. In medicine (Triple), gamma-emitting tracers (e.g., Technetium-99m) are injected or swallowed to image internal organs: they must emit gamma (so radiation passes out of the body to a gamma camera and is weakly ionising) and have a short half-life (~6 hours, so activity falls rapidly to safe background levels after the scan).
- Nuclear Fission & Chain Reactions (Triple Science)
- Nuclear fission is the splitting of a large, unstable nucleus (such as Uranium-235 or Plutonium-239). Spontaneous fission is rare; usually, the unstable nucleus must first absorb a slow-moving neutron. The nucleus splits into two smaller daughter nuclei roughly equal in size, plus two or three neutrons, plus gamma radiation and a huge amount of kinetic energy. The emitted neutrons can be absorbed by more U-235 nuclei, triggering a chain reaction (controlled in a nuclear reactor using boron control rods, or uncontrolled in a nuclear weapon).
- Nuclear Fusion (Triple Science)
- Nuclear fusion is the joining of two light nuclei (e.g., two isotopes of hydrogen: deuterium ²₁H and tritium ³₁H) to form a heavier nucleus (helium, ⁴₂He, plus a neutron). Some of the mass of the nuclei is converted directly into energy (by E = mc²) and radiated away. Fusion is the process that powers all stars, requiring extremely high temperatures (millions of °C) and pressures so the positively charged nuclei have enough kinetic energy to overcome electrostatic repulsion.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
Give the two-part definition of 'half-life'.
A radioactive sample has an initial activity of 1,600 Bq and a half-life of 8 days. Calculate its activity after 32 days, and state the net decline as a ratio.
How do you correct a GM tube count-rate measurement for background radiation?
Name three sources of natural background radiation and two man-made sources.
Why is an alpha-emitting isotope like Americium-241 with a half-life of 432 years used in domestic smoke alarms?
Why does nuclear fusion require extremely high temperatures and pressures (Triple Science)?
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Polonium-210 (²¹⁰₈₄Po) decays by emitting an alpha particle to form an isotope of lead (Pb), whereas Iodine-131 (¹³¹₅₃I) decays by emitting a beta particle to form an isotope of xenon (Xe). Write balanced nuclear decay equations for both decays, and explain why Iodine-131 (half-life = 8 days) can be used as a medical tracer to check thyroid function whereas Polonium-210 (half-life = 138 days) would be fatal if injected. [6 marks]
Click “Check Mark Scheme” to compare your answer, 1. Alpha decay of Polonium-210: ²¹⁰₈₄Po -> ²⁰⁶₈₂Pb + ⁴₂He [1 mark]. 2. Beta decay of Iodine-131: ¹³¹₅₃I -> ¹³¹₅₄Xe + ⁰₋₁e [1 mark]. 3. Polonium-210 emits alpha radiation, which has very low penetrating power and would be completely stopped by body tissues, so no radiation could ever escape the body to be detected by an external camera [1 mark]. 4. Also, alpha radiation is extremely strongly ionising; inside the body, it transfers all its energy to nearby cells, causing severe DNA mutations, cell death, and radiation sickness, compounded by its long 138-day half-life [1 mark]. 5. Iodine-131 emits beta (and gamma) radiation, which is much less ionising than alpha and can penetrate out through the neck tissue to be detected externally [1 mark]. 6. Its shorter half-life of 8 days means the activity decays relatively quickly after the medical scan is completed, limiting the total radiation dose to the patient [1 mark].
- Q2
A hospital physicist measures the count rate of a radioactive isotope using a GM tube. The background count rate in the laboratory is 30 counts per minute (cpm). At time t = 0 hours, the total measured count rate with the source present is 990 cpm. At t = 18 hours, the total measured count rate is 150 cpm. Calculate the half-life of the radioactive isotope. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Subtract the background count rate (30 cpm) from both readings to find the corrected count rates from the source alone [1 mark]. 2. Corrected initial count rate at t = 0 h: 990 - 30 = 960 cpm; Corrected count rate at t = 18 h: 150 - 30 = 120 cpm [1 mark]. 3. Determine the number of half-lives for the corrected count rate to fall from 960 cpm to 120 cpm: 960 -> 480 (1) -> 240 (2) -> 120 (3 half-lives) [1 mark]. 4. Since 3 half-lives take 18 hours, 1 half-life = 18 / 3 = 6 hours [1 mark].
- Q3
Describe the process of nuclear fission of a Uranium-235 nucleus in a nuclear power station, and explain how the control rods and moderator keep the chain reaction proceeding at a steady, safe rate. (Triple Science) [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. A large, unstable Uranium-235 nucleus absorbs a slow-moving (thermal) neutron [1 mark]. 2. The nucleus splits into two smaller daughter nuclei of roughly equal size, releasing two or three fast-moving neutrons, gamma radiation, and a large amount of kinetic energy [1 mark]. 3. The emitted neutrons can go on to be absorbed by other Uranium-235 nuclei, causing them to split and creating a chain reaction [1 mark]. 4. Moderator (graphite or water): Slows down the fast-moving neutrons released by fission so they can be successfully absorbed by other U-235 nuclei [1 mark]. 5. Control rods (made of boron): Absorb excess neutrons; they are raised or lowered into the reactor core so that, on average, exactly one neutron from each fission goes on to trigger one further fission, keeping power output constant and preventing an explosion [1 mark].
- Q4
Strawberries sold in supermarkets are sometimes exposed to gamma rays from a sealed Cobalt-60 source inside a lead-lined chamber to kill bacteria and mould spores. Explain whether the strawberries are irradiated or contaminated, and explain why the strawberries are completely safe for customers to eat afterwards. [3 marks]
Click “Check Mark Scheme” to compare your answer, 1. The strawberries are irradiated (exposed to gamma radiation from an external sealed source), not contaminated [1 mark]. 2. The high-energy gamma rays pass straight through the plastic packaging and the strawberries, destroying the DNA of bacteria and mould spores so the fruit stays fresh longer [1 mark]. 3. Because no radioactive Cobalt-60 atoms ever touch or enter the strawberries (no contamination), the strawberries do not become radioactive themselves and emit no radiation after leaving the chamber [1 mark].
Common Exam Questions (4)
How do mass number and atomic number change in Alpha (α) vs Beta (β) decay equations?
1. Alpha decay emits a helium nucleus (⁴₂He): the mass number (top number) decreases by 4, and the atomic number (bottom proton number) decreases by 2. 2. Beta decay occurs when a neutron in the nucleus turns into a proton and emits a fast-moving electron (⁰₋₁e): the mass number stays the same, and the atomic number increases by 1.
What is the difference between irradiation and contamination?
Irradiation is exposing an object to radiation from an external source (e.g., having a hospital X-ray or sterilising surgical instruments with gamma rays); an irradiated object does not become radioactive itself, and the risk stops as soon as the source is removed. Contamination is when unwanted radioactive atoms get onto or into an object (e.g., inhaling radon gas or spilling a source on skin); the contaminated object IS now radioactive and continues to emit radiation until decontaminated or until the isotope decays.
Why is an alpha source the most dangerous inside the body (contamination), but the least dangerous outside the body (irradiation)?
Alpha particles are the most strongly ionising radiation (large mass and +2 charge), but have very low penetrating power (stopped by a few cm of air or dead skin cells). Outside the body, alpha particles cannot penetrate the skin to reach living tissue. Inside the body (if inhaled or swallowed), alpha particles cannot escape and transfer all their ionising energy to a tiny localised area of living cells, causing severe DNA damage and cancer.
What are the roles of the moderator, control rods, and coolant in a nuclear fission reactor? (Triple Science)
1. Moderator (graphite or water): slows down fast-moving neutrons released by fission so they can be absorbed efficiently by other Uranium-235 nuclei. 2. Control rods (boron or cadmium): lowered into or raised out of the reactor core to absorb excess neutrons, ensuring exactly one neutron per fission goes on to cause another fission (steady, controlled chain reaction). 3. Coolant (water or CO₂ gas): absorbs the thermal energy released by fission and transfers it to a boiler to make steam for turbines.
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