5.2Nuclear Physics
Radioactivity — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
25 min read 56 parts Lesson 7 of 17 Unit quiz
Lesson summary
5.2 Radioactivity (Part 1) — IGCSE PHYSICS notes: Name sources of background radiation, measure count rate with a Geiger-Müller tube and correct readings for background.Definition — background radiation is the radiation that exists around us all the time.Natural sources — radioac…
- Man-made sources
- Rocks and buildings
- Food and drink
- Cosmic rays
- Nuclear waste
- Nuclear fallout
Part 1 of 7What background radiation is
Definition
- Definition background radiation is the radiation that exists around us all the time.
Natural sources
- Natural sources radioactive elements that have always existed on Earth and in space.
Man-made sources
- Man-made sources human activities add to the radiation we receive.
Variation
- Variation the count rate of background radiation varies significantly from place to place.
Part 2 of 7Natural sources
Rocks and buildings
- Rocks and buildings uranium and thorium occur naturally in rocks, and building materials such as stone and brick are slightly radioactive.
- Uranium decays into radon gas.
Radon gas
- Radon gas an alpha emitter given off by some rocks;
- it is colourless, tasteless and odourless and is dangerous only if large amounts are inhaled.
Food and drink
- Food and drink radioactive elements reach food and water from rocks and soil, such as potassium-40 in bananas, in minuscule amounts.
Cosmic rays
- Cosmic rays the Sun and supernovae send out high-speed protons;
- when they hit air molecules they produce gamma radiation.
- Exposure is greater at high altitude.
Carbon-14
- Carbon-14 all organic matter contains a tiny amount;
- living things replace carbon continually, so its level stays almost constant.
Part 3 of 7Man-made sources
Medical
- X-rays, CT scans, radioactive tracers and radiation therapy.
Nuclear waste
- Nuclear waste adds little to background levels but is dangerous to the people handling it.
Nuclear fallout
- Nuclear fallout residue thrown into the air after a nuclear explosion, such as at Hiroshima;
- higher near weapon test sites.
Nuclear accidents
- Nuclear accidents such as Chernobyl, which can devastate areas for centuries but are now extremely rare.
Part 4 of 7Detecting radiation and count rate
Count rate
- Count rate the number of decays detected per second or per minute.
Geiger-Müller (GM) tube
- Geiger-Müller (GM) tube each time it absorbs radiation it sends an electrical pulse to a counter, which clicks and shows the count rate.
- A higher click frequency means more radiation is entering the tube.
Distance
- Distance the count rate falls as the tube moves further from the source, because the radiation spreads out.
Other detectors
- Other detectors photographic film (badges), ionisation chambers, scintillation counters and spark counters.
Part 5 of 7Correcting for background radiation (Extended)
1
Measure the count rate with no source present to find the background count.
2
Measure the count rate with the source in place.
3
Subtract the background from the source reading to get the corrected count rate.
4
In a distance experiment, the count rate that stops changing at large distances is the background value.
Part 6 of 7Why count rate falls with distance
Spreading out
- Spreading out radiation travels in all directions from the source, so the same number of particles spreads over a larger area as distance increases.
Absorption in air
- Absorption in air alpha and beta particles are absorbed by the air within a short distance.
Background plateau
- Background plateau once the count rate stops falling, only background radiation is being detected.
Part 7 of 7More from the specification
More from the specification
Radiation deflection in transverse magnetic fields
- Radiation deflection in transverse magnetic fields applying uniform magnetic fields causes opposite curvature of alpha and beta trajectories, leaving gamma rays undeflected.
Radioactive Decay of Unstable Nuclei
- Unstable radioactive isotopes randomly emit ionising radiation to transition to a more stable nuclear state.
- (alpha, beta, or gamma)
Alpha Particles and Nuclear Composition
- An alpha particle consists of two protons and two neutrons carrying a +2 charge with high ionising power and low penetrating ability.
- (a helium-4 nucleus)
Radioactive Half-Life Definition
- The half-life (t_½) of a radioactive sample is the time taken for half the unstable nuclei in the sample to decay,
- or for count rate to halve.
Nuclear Penetrating Power Comparison
- Alpha particles are stopped by paper, beta particles penetrate paper but are absorbed by a few millimetres of aluminium,
- and gamma rays require dense lead shielding.
Unstable nucleus
- A nucleus with an imbalance of protons and neutrons that may decay.
Electric field
- Region of space in which a charge experiences a force.
Magnetic field
- Area around a magnet where its effects can be detected.
Decay equation
- An equation in nuclide notation with balanced nucleon and proton numbers.
Kinetic energy
- K = ½mv2, energy of motion.
Neutron
- Neutral particle in the nucleus, relative mass 1.
Half-life
- Time for half the nuclei of an isotope to decay.
Charge
- Quantity of electricity in coulombs, Q = It.What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
Isotope
- Atom of the same element with a different number of neutrons.
Penetrating power
- How far radiation travels through a material.
Ionising power
- How strongly radiation ionises atoms.
Key terms
| Term | Meaning |
|---|---|
| Background radiation | Radiation present all around us from natural and man-made sources. |
| Count rate | The number of radiation counts detected per second or per minute. |
| Geiger-Müller tube | A detector that sends an electrical pulse to a counter each time it absorbs radiation. |
| Corrected count rate | The measured count rate minus the background count rate. |
| Radon | A radioactive gas that decays by emitting alpha particles. |
| Cosmic rays | High-energy particles from the Sun and space. |
Avoid these mistakes
1
Thinking background radiation is all man-made — most of it is natural.
2
Calling the GM tube "a radiation detector" in an exam answer.
3
Forgetting to convert hours to seconds for counts per second.
4
Forgetting to subtract background from the measured count rate.
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