Theme E — Nuclear and Quantum Physics
E.3.2 Background Radiation — IB Physics HL Revision Notes & Topical Questions
5 min read 9 parts Lesson 19 of 45 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
E.3.2 Background Radiation — IB PHYSICS SL notes: Identify the sources of background radiation and correct a measured count rate for it.Background radiation — the ionising radiation always present in the environment, whether or not a deliberate source is nearbyA Geiger–Müller tub…
- Why it matters
- Rocks and soil
- Cosmic rays
- Food and drink
- Nuclear medicine
- Nuclear accidents
Part 1 of 7What background radiation is
Background radiation
- Background radiation the ionising radiation always present in the environment, whether or not a deliberate source is nearby
- A Geiger–Müller tube on a bench with all sources removed still records counts.
Why it matters
- Why it matters it must be accounted for, otherwise a measured activity is too high
Sources
- Sources natural and artificial
Variation
- Variation the level varies with location, altitude and building materials
- Granite bedrock or poorly ventilated basements give much higher count rates than a coastal town with sandy soil.
Part 2 of 7Natural sources
Radon gas
- Radon gas it comes from uranium in rocks, soil and building materials such as stone and brick
- Uranium decays into radon, which is an alpha emitter and is dangerous if inhaled in large quantities.
- It is colourless, tasteless and odourless, so it is detected with a Geiger counter.
- It is often the largest single contribution, and in the UK by far the largest.
- It seeps into buildings and its level varies from place to place.
Rocks and soil
- Rocks and soil granite and some building materials contain uranium, thorium and potassium-40, giving gamma radiation
Cosmic rays
- Cosmic rays high-energy particles from the Sun, supernovae and other cosmic events, mainly protons and alpha particles
- Collisions with air molecules produce secondary radiation, including gamma rays.
- The dose rises with altitude, so air travellers receive more.
Carbon-14
- Carbon-14 all organic matter contains a tiny amount, and living things keep the amount almost constant by replacing their carbon
Food and drink
- Food and drink naturally occurring radioactive elements such as potassium-40 in bananas are present in tiny, harmless amounts
Part 3 of 7Artificial sources
Nuclear medicine
- X-rays, CT scans, radioactive tracers and radiation therapy
- Medical procedures are the largest part of the artificial share.
Fallout
- Fallout radioactive material thrown into the air after nuclear weapons explosions
- It is presently very low but would rise in areas of testing.
Nuclear accidents
- Nuclear accidents events such as Chernobyl give a large dose and can devastate areas for centuries, but are now extremely rare
Nuclear waste
- Nuclear waste it contributes little to background but is dangerous to those handling it
Industry and power
- Industry and power routine releases make a small contribution
Overall
- Overall for most people the artificial share of the dose is a minority, with waste and fallout under 1% in the UK
Part 4 of 7Correcting for background
Corrected count rate
- Corrected count rate measured count rate minus background count rate
- Both must be expressed per unit time before subtracting.
Measuring the background
- Measuring the background record counts with the source removed
- For example, 24 counts in 1 minute is 24 counts per minute, or 0.4 counts per second.
Measuring the source
- Measuring the source record the count rate with the source in place, then subtract the background
Improving accuracy
- Improving accuracy repeat readings and take averages, and count over a long time
Practical habits
- Keep other sources well away from the detector.
- Measure background at the start and end of the experiment.
- Use the same position and counting time for each reading.
Distance method
- Distance method when a source is moved far enough away, the count rate becomes constant and equals the background
Part 5 of 7Uncertainty in counts
Random uncertainty
- Random uncertainty the fractional uncertainty of N counts is about 1/√N
- 100 counts give about 10% and 10 000 counts give about 1%.
Subtraction
- Subtraction when two counts are subtracted, the uncertainties add
- A weak source near background gives poor precision and needs a long counting time.
Detection limit
- Detection limit if the source rate is smaller than the random fluctuations in the background, the source cannot be distinguished
Part 6 of 7Why it matters in experiments
Decay curves
- Decay curves the same background value is subtracted from every reading, and the corrected values are plotted
Without correction
- Without correction the curve levels off at a non-zero count rate instead of falling towards zero
- The half-life found from the graph is then wrong.
Safety
- Safety regulators compare the extra dose from a hospital procedure or power plant with natural variation in background
Part 7 of 7More from the specification
1Radon gas decay in environmental atoms
- Radon gas decay in environmental atoms spontaneous alpha emissions from atmospheric radon atoms provide a means of natural environmental background radiation.
2Sources of environmental background radiation
- Background radiation originates from cosmic rays, terrestrial radioactive rocks (radon gas), and artificial medical or industrial sources.
3Nuclear binding energy and mass defect
- The binding energy of a nucleus corresponds to the mass defect between constituent nucleons and the intact nucleus via E = delta m * c2.
4Corrected Geiger-Muller count rates
- Accurate measurement of radioactive source activity requires subtracting ambient background count rate from raw GM detector readings.
5Nuclear binding energy
- Nuclear binding energy energy required to completely separate an atomic nucleus into individual constituent protons and neutrons.
6Strong nuclear force
- Strong nuclear force attractive short-range force acting equally between nucleons to overcome electrostatic repulsion in stable nuclei.
7Radioactive isotopes decay
- Radioactive isotopes decay unstable isotopes in an environmental sample decay spontaneously over half-lives spanning days to millions of years.
8Decay equation
- An equation in nuclide notation with balanced nucleon and proton numbers.
9Neutron
- Neutral particle in the nucleus, relative mass 1.
10Isotope
- Atom of the same element with a different number of neutrons.
11Isotopes
- Nuclei with the same proton number but different neutron numbers.
12Electron
- Negatively charged particle in an atom.
13Binding energy
- The minimum energy required to separate the nucleons of a nucleus to infinity.
14Penetrating power
- How far radiation travels through a material.
15Nuclear notation
- Symbol showing mass number and atomic number.
16Beta-plus decay
- Proton changes to a neutron with emission of a positron and a neutrino.
Key terms
| Term | Meaning |
|---|---|
| Background radiation | Ionising radiation always present in the environment. |
| Radon | Radioactive gas from uranium decay, a major natural source. |
| Cosmic rays | High-energy particles from space producing secondary radiation. |
| Corrected count rate | Measured count rate minus background count rate. |
| Count rate | Number of detected counts per unit time. |
| Geiger–Müller tube | Detector that produces a pulse for each ionising particle entering it. |
Avoid these mistakes
1
Forgetting to subtract background, or subtracting counts taken over different times — convert to counts per unit time first.
2
Assuming background is constant everywhere — it changes with location, altitude and building materials.
3
Thinking background is purely man-made — most of it is natural.
4
Using a very short background measurement, which gives a large fractional uncertainty.
5
Saying background comes from the source itself — it is present when the source is removed.
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