P5.2.4P5. Nuclear Physics
Half-life — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 11 parts Lesson 7 of 8 1 quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
P5.2.4 Half-life — COMBINED SCIENCE notes: Define half-life, find it from data and graphs, and explain how it and the type of radiation decide the use of an isotope.Half-life = the time taken for half the nuclei of an isotope in any sample to decayIt is also the time for the acti…
- Constant value
- Decay curve
- Two deciding factors
- Smoke detector
- Thickness gauge
- Sterilising equipment
Part 1 of 9Half-life
Half-life
Half-life the time taken for half the nuclei of an isotope in any sample to decay
- It is also the time for the activity or count rate of the sample to fall to half.
- Falling from 100% to 50% takes the same time as falling from 50% to 25%.
Constant value
- Constant value the half-life is constant for a particular isotope
- Half-lives range from a fraction of a second to billions of years.
- Different isotopes have different half-lives.
Part 2 of 9Fraction remaining
| Number of half-lives | Proportion remaining |
|---|---|
| 0 | 1 |
| 1 | 1/2 |
| 2 | 1/4 |
| 3 | 1/8 |
| 4 | 1/16 |
- Decay curve
- each time the activity halves, another half-life has passed
- All isotopes give the same shape of curve and only the time scale changes.
Part 3 of 9Finding half-life from a graph
1
Read the original activity or count rate where the line crosses the y-axis.
2
Halve this value and find it on the y-axis.
3
Go across to the best-fit curve and then straight down to the x-axis.
4
The time read on the x-axis is the half-life.
5
Draw the lines with a ruler to read the most accurate value.
Part 4 of 9Removing background radiation
1
Measure the background count with no source present.
2
Carry out the experiment with the source.
3
Subtract the background count from each reading to get the corrected count.
4
Use the corrected counts to find the half-life, because background radiation does not decay.
Part 5 of 9Choosing an isotope
- Two deciding factors
- the type of radiation and the half-life of the isotope
- The radiation must have the right penetration and ionisation for the job.
- The half-life must be long enough for the job and short enough to limit harm.
Part 6 of 9Alpha and beta uses
Smoke detector
- Smoke detector an alpha source ionises the air in the detector and produces a current
- Smoke absorbs the alpha particles, so the current stops and the alarm is triggered.
- Alpha is used because it is strongly ionising and absorbed by smoke, whereas beta and gamma would pass through.
- A long half-life means the source does not need replacing often.
Thickness gauge
- Thickness gauge beta radiation is partly absorbed by thin materials such as paper, card and aluminium foil
- A detector on the other side measures the particles that pass through.
- If the material gets thicker, fewer particles are detected and the machine adjusts to keep the thickness constant.
- Very thick materials use gamma radiation.
- A half-life of 10-20 years keeps the count rate steady from day to day.
Part 7 of 9Gamma uses
Sterilising equipment
- Sterilising equipment gamma is the most penetrating radiation, so all sides of the instruments are irradiated without removing the packaging
Irradiating food
- Irradiating food gamma kills microorganisms so the food lasts longer and food-borne infection is less likely
Radiotherapy
- Radiotherapy beams of gamma rays are aimed at a cancerous tumour
- Cancer cells are more susceptible to radiation than healthy cells.
- The beams are moved around to reduce harm to healthy tissue.
Source half-life
- Source half-life about 5 years for sterilisation and radiotherapy, so the source is not replaced often
Part 8 of 9Medical tracers and dating
Tracer
Tracer a radioactive isotope used to track the movement of substances such as blood around the body
- A PET scan detects the emissions to locate a tumour.
- A half-life of several hours gives time for the scan and lets the radiation leave the body quickly.
Radioactive dating
- Radioactive dating carbon-14 is used for once-living materials
Part 9 of 9More from the specification
More from the specification
Atomic structure
- Atoms contain protons, neutrons and electrons;
- protons carry positive charge, electrons negative, and neutrons are neutral.
Unstable nuclei
- Nuclei with too many neutrons or protons are unstable and decay emitting radiation.
Deflection
- Alpha particles are deflected slightly toward the negative plate and beta toward the positive in electric fields;
- gamma is not deflected.
Half-life
- Time for half the unstable nuclei in a sample to decay, or for the activity to halve.
Background radiation
- Low level radiation from rocks, cosmic rays and sources; the amount must be subtracted from measurements.
Atomic structure
- Atoms contain protons, neutrons and electrons;
- protons carry positive charge, electrons negative, and neutrons are neutral.
Unstable nuclei
- Nuclei with too many neutrons or protons are unstable and decay emitting radiation.
Deflection
- Alpha particles are deflected slightly toward the negative plate and beta toward the positive in electric fields;
- gamma is not deflected.
Half-life
- Time for half the unstable nuclei in a sample to decay, or for the activity to halve.
Background radiation
- Low level radiation from rocks, cosmic rays and sources; the amount must be subtracted from measurements.
Unstable nucleus
- A nucleus that decays by emitting radiation to become more stable.
Electric field
- A region where a charge experiences a force.
Magnetic field
- Area around a magnet where its effects can be detected.
Kinetic energy
- Energy stored in a moving object.
Neutron
- Neutral particle in the nucleus.
Proton
- Positive particle in the nucleus.
Charge
- A quantity measured in coulombs that is moved round a circuit.
Electron
- Negative particle that moves around the nucleus.
Deflected
- Having its direction changed.
Element
- A substance made of only one kind of atom.
Penetrating power
- How far radiation travels through a material before it is stopped.
Key terms
| Term | Meaning |
|---|---|
| Half-life | The time taken for half the nuclei of an isotope in a sample to decay. |
| Activity | The rate at which the nuclei in a sample decay. |
| Corrected count | The measured count with the background count subtracted. |
| Radiotherapy | The treatment of cancer using beams of gamma rays. |
| Tracer | A radioactive isotope used to track the movement of substances in the body. |
| Smoke detector | A device in which alpha particles ionise air and smoke stops the current. |
| Radioactive dating | Using the decay of carbon-14 to find the age of once-living material. |
Avoid these mistakes
1
Thinking half-life is half the time taken for the sample to disappear — the sample never reaches zero and the half-life is the time to halve
2
Forgetting to subtract background radiation — use the corrected count when finding the half-life
3
Thinking a longer half-life means more dangerous radiation — it means slower decay, and the sample stays radioactive for longer
4
Thinking half-life changes with temperature or sample size — it is constant for an isotope
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