5.2Nuclear Physics

Radioactivity — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

25 min read 56 parts Lesson 7 of 17 Unit quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision

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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

Section 1 of 56What 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.

Section 2 of 56Natural 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.

Section 3 of 56Man-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.

Section 4 of 56Detecting 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.

Section 5 of 56Correcting for background radiation (Extended)

Measure the count rate with no source present to find the background count.

Measure the count rate with the source in place.

Subtract the background from the source reading to get the corrected count rate.

In a distance experiment, the count rate that stops changing at large distances is the background value.

Section 6 of 56Why 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.

Section 7 of 56More 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.

Section 8 of 56Random and spontaneous decay

Spontaneous

Spontaneous the emission of radiation from a nucleus happens by itself, in a random direction.

Random

Random every nucleus has an equal chance of decaying, and it is impossible to know which nucleus will decay next or when.

Unaffected by conditions

Unaffected by conditions the rate of decay does not depend on the surroundings.

Only probability

Only probability we can estimate only the chance of a nucleus decaying in a given time.

Section 9 of 56Unstable nuclei

Cause

Cause an imbalance of forces inside the nucleus, usually because it has too many protons or neutrons, or is very large.

Example

Example carbon-14 is unstable because it has two extra neutrons compared with stable carbon-12.

Decay

Decay an unstable nucleus emits a high-energy particle or wave, which carries away energy and makes the nucleus more stable.

Section 10 of 56Nature of the three radiations

Alpha (α)

Alpha (α) a helium nucleus of 2 protons and 2 neutrons, charge +2.

Beta (β−)

Beta (β−) a fast-moving electron made when a neutron changes to a proton and an electron in the nucleus, charge −1.

Gamma (γ)

Gamma (γ) an electromagnetic wave with the highest energy of the electromagnetic spectrum and no charge.

Section 11 of 56Comparing alpha, beta and gamma

Radiation	Range in air	Penetration	Ionising effect
Alpha	A few cm	Low; stopped by a thin sheet of paper	High
Beta	A few tens of cm	Moderate; stopped by a few mm of aluminium or Perspex	Moderate
Gamma	Infinite	High; only reduced by several cm of lead	Low

Going down the table, range and penetration increase while ionisation decreases.

Section 12 of 56Identifying a source from absorber tests

1Passes through paper

Passes through paper not alpha.

2Stopped by a few mm of aluminium

Stopped by a few mm of aluminium beta.

3Still detected through aluminium

Still detected through aluminium gamma; thick lead is needed to reduce it.

4Thickness matters

Thickness matters beta passes through aluminium that is thinner than a few millimetres.

Section 13 of 56Ionisation (Extended)

Ionisation

Ionisation an atom gains or loses electrons to become charged;
- radiation mostly knocks electrons out so the atom becomes positive.

Dependence

Dependence the ionising effect depends on the radiation’s kinetic energy and charge.

Section 14 of 56Exam practice phrases

Describing alpha

"a helium nucleus of two protons and two neutrons".

Describing beta

"a fast-moving electron emitted from the nucleus".

Describing gamma

"electromagnetic radiation of very short wavelength".

Section 15 of 56More from the specification

More from the specification

Lorentz deflection of ionizing particles in magnetic fields

Lorentz deflection of ionizing particles in magnetic fields trajectories of ionizing emissions curve predictably through magnetic fields, providing a means of charge determination.

Properties of Radioactive Emissions

Alpha particles are helium nuclei with high ionising power and low penetration;
- beta particles are fast-moving electrons;
- gamma rays are uncharged, highly penetrating electromagnetic radiation.

Absorbing Materials for Radiation

Alpha radiation is absorbed by a sheet of paper or skin, beta radiation is stopped by a few millimetres of aluminium,
and gamma rays are attenuated by thick lead or dense concrete.

Background Radiation and Detection

A Geiger-Müller (GM) tube connected to a counter measures radiation in counts per minute;
- background radiation from rocks, cosmic rays,
and radon must be subtracted from sample measurements.

Radioactive Half-Life

The half-life of a radioactive isotope is the time taken for half the unstable parent nuclei in a sample to decay or for the corrected count rate of the source to halve.

Background radiation

Radiation that is always present in the environment.

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.

Half-life

Time for half the nuclei of an isotope to decay.

Source

Reservoir at T_H providing heat Q_H

Isotope

Atom of the same element with a different number of neutrons.

Section 16 of 56What controls ionising power

Charge

Charge the greater the charge, the more ionising: alpha (+2) is the most ionising, beta (−1) is moderate and gamma (0) is least.

Kinetic energy

Kinetic energy kinetic energy is ½mv², so a heavier or faster particle is more ionising.

Alpha

Alpha has the greatest mass, so is the most ionising.

Beta

Beta very light but fast, so it has a lot of kinetic energy and is moderately ionising.

Gamma

Gamma has no mass or charge, so it is weakly ionising.

Section 17 of 56Deflection in electric fields (Extended)

Deflection in electric fields (Extended)

Charged particles only

Charged particles only a particle is deflected in an electric field if it has charge;
- gamma rays have none and travel straight on.

Between charged plates

Between charged plates alpha particles are deflected towards the negative plate and beta particles towards the positive plate.

Amount of deflection

Amount of deflection beta particles are lighter, so they are deflected more than alpha particles.

Section 18 of 56Deflection in magnetic fields (Extended)

Condition

Condition a charged particle is deflected only if it is moving across the field.

Opposite directions

Opposite directions alpha and beta particles curve in opposite directions because their charges are opposite.

Gamma

Gamma no charge, so it is not deflected.

Section 19 of 56Why nuclei are unstable

Neutron–proton balance

Neutron–proton balance stable light nuclei have about as many neutrons as protons;
- heavy nuclei need more neutrons to offset the repulsion between protons.

Hydrogen isotopes

Hydrogen isotopes hydrogen-1 is stable, hydrogen-2 (deuterium) is stable and tritium is unstable.
- (hydrogen-3, 2 neutrons to 1 proton)

Heavy nuclei

Heavy nuclei in a very large nucleus the forces holding it together are weaker;
- uranium-238 decays by a chain of alpha and beta emissions.

Section 20 of 56Parent and daughter nuclei

New element

New element alpha or beta decay changes the proton number, so the daughter nucleus is a different element.

Example

Example plutonium-239 decays by alpha emission to uranium-235, losing 2 protons and 2 neutrons.

Section 21 of 56Effect of each decay on the nucleus (Extended)

Decay	Particle emitted	Nucleon number A	Proton number Z
Alpha	2 protons and 2 neutrons	Decreases by 4	Decreases by 2
Beta	Electron (from a neutron changing to a proton)	No change	Increases by 1
Gamma	Electromagnetic wave	No change	No change

Alpha and beta decay both make the nucleus more stable by reducing the number of excess neutrons;
- gamma emission only reduces the nucleus’s energy.

Section 22 of 56Explaining deflection

Why deflected

Why deflected an electric field pushes on a charged particle;
- a magnetic field pushes on a moving charged particle at right angles to its motion.

Direction rule

Direction rule use Fleming’s left-hand rule for magnetic fields, remembering that the current direction for beta particles is opposite to their motion.

Gamma

Gamma has no charge, so it passes through both fields without bending.

Section 23 of 56Link between decay and stability

Too many neutrons

Too many neutrons beta decay turns a neutron into a proton and moves the nucleus towards stability.

Too heavy

Too heavy alpha decay removes two protons and two neutrons and reduces the size of the nucleus.

Excess energy

Excess energy gamma emission takes away energy after an alpha or beta decay.

Section 24 of 56More from the specification

More from the specification

Radiation ionization measurement

Radiation ionization measurement counting pulses with Geiger-Müller tubes behind varied shielding thickness provides an empirical means to identify radiation types.

Radioactive Half-Life

The half-life of a radioactive isotope is the time taken for half the unstable parent nuclei in a sample to decay,
or for source count rate to fall by half.

Background Radiation Correction

Ambient background radiation from cosmic rays and rocks must be recorded using a radiation detector and subtracted from measured counts to obtain true source activity.

Penetration of Radiation Through Materials

Alpha particles are stopped by thin paper, beta particles penetrate paper but are absorbed by aluminium,
and dense lead is required to attenuate gamma rays.

Spontaneous Nuclear Decay

Radioactive decay is a random atomic process unaffected by external chemical or physical conditions,
meaning individual nuclear decay events cannot be predicted.

Background radiation

Radiation that is always present in the environment.

Penetrating power

How far radiation travels through a material.

Decay equation

An equation in nuclide notation with balanced nucleon and proton numbers.

Half-life

Time for half the nuclei of an isotope to decay.

Source

Reservoir at T_H providing heat Q_H

Detector

An instrument that records the particles produced in collisions.

Section 25 of 56Decay equations (Extended)

Format

Format nuclide notation shows the parent nucleus before the arrow and the products after it.

Balancing rule

Balancing rule the sums of the nucleon numbers and of the proton numbers must be the same on both sides.

Alpha decay

^A_ZX → ^A−4_Z−2Y + ⁴₂He.

Beta decay

^A_ZX → ^A_Z+1Y + ⁰₋₁e.

Gamma decay

^A_ZX → ^A_ZX + γ, so no change to the nucleus apart from its energy.

Section 26 of 56Method for decay equations

Find the nucleon number of the parent: protons plus neutrons.

For alpha, subtract 4 from the nucleon number and 2 from the proton number.

For beta, keep the nucleon number and add 1 to the proton number.

Check that both totals balance across the arrow.

Section 27 of 56Half-life

Definition

Definition the half-life of an isotope is the time taken for half the nuclei of that isotope in a sample to decay.

Activity

Activity the activity of a sample also halves in one half-life, whether it falls from 100% to 50% or from 50% to 25%.

Constant

Constant each isotope has its own fixed half-life, ranging from fractions of a second to billions of years.

Section 28 of 56Fraction remaining

Number of half-lives	Fraction remaining
0	1
1	1/2
2	1/4
3	1/8
4	1/16

Section 29 of 56Using half-life in calculations

Using half-life in calculations

Find the fraction remaining: final amount ÷ initial amount.

Match it to a whole number of half-lives (1/2 → 1, 1/4 → 2, 1/8 → 3).

Multiply the number of half-lives by the half-life to get the time.

Section 30 of 56Finding half-life from a graph (Extended)

Subtract the background count from every reading to get the corrected count rate.

Read the starting value C₀ where the curve meets the vertical axis.

Halve this value and draw a line across to the curve, then down to the time axis.

This time is the half-life; repeat for another halving as a check.

Section 31 of 56Half-life and the activity of a sample

Activity

Activity falls by half in each equal time interval equal to the half-life.

Shape of curve

Shape of curve a decay curve falls quickly at first then more slowly, and never reaches exactly zero.

Short half-life

Short half-life a source decays quickly and is very active but soon becomes safe.

Long half-life

Long half-life a source decays slowly, stays active for a long time and is a long-term hazard.

Section 32 of 56Percentage remaining

Half-lives passed	Percentage remaining
0	100%
1	50%
2	25%
3	12.5%
4	6.25%

Section 33 of 56More from the specification

More from the specification

Deflection of alpha and beta rays in magnetic fields

Deflection of alpha and beta rays in magnetic fields charged nuclear radiation particles curve across magnetic fields, providing a means of identification.

Alpha, beta, and gamma radiation

Alpha particles are helium nuclei with high ionizing power and low penetration;
- beta particles are fast electrons;
- gamma rays are uncharged electromagnetic photons.

Radioactive decay law and half-life

Unstable atomic nuclei decay randomly;
- half-life is the time required for half the radioactive nuclei in a sample to decay.

Ionization and radiation shielding

Alpha radiation is stopped by paper or skin, beta by a few millimetres of aluminium,
and gamma radiation is attenuated by thick lead or concrete.

Ionising power of radiations

Ionising power of radiations alpha particles have high ionising power and double positive charge, easily stopped by paper or thin materials.

Beta particle properties

Beta particle properties fast-moving high-energy electrons emitted from unstable nuclei when a neutron transforms into a proton.

Radiation detector types

Radiation detector types photographic film and Geiger-Muller tubes detect ionisation caused by emitted radioactive particles.

Penetrating power

How far radiation travels through a material.

Ionising power

How strongly radiation ionises atoms.

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.

Kinetic energy

K = ½mv², energy of motion.

Radiation

Transfer by electromagnetic waves, requiring no medium

Charge

Quantity of electricity in coulombs, Q = It.
What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)

Detector

An instrument that records the particles produced in collisions.

Electron

Negatively charged particle in an atom.

Section 34 of 56Half-life from a graph

Subtract the background radiation from the readings.

Read the initial activity on the vertical axis and halve it.

Draw a ruled line across to the curve and down to the time axis.

The time read off is the half-life.

Half-life from a graph

Example: the activity of a technetium sample halves after 6 hours, so its half-life is 6 hours.

Drawing the lines with a ruler gives the most accurate reading.

Section 35 of 56Half-life from numbers of atoms

Method

Method count how many times the number of un-decayed atoms has halved, then divide the time by that number.

Example

2 000 000 atoms fall to 500 000 in one year: 2 000 000 → 1 000 000 → 500 000 is two half-lives, so one half-life is 6 months.

Section 36 of 56Choosing a radioisotope

Choosing a radioisotope

Radiation type

Radiation type the type of radiation determines how penetrating and ionising it is, which decides what job it can do.

Half-life

Half-life a long half-life means the source does not need replacing often; a short one suits medical tracers.

Section 37 of 56Smoke alarms (alpha)

Smoke alarms (alpha)

An alpha source ionises the air in the detector and creates a small current.

Smoke absorbs the alpha particles, so the current falls.

A microchip detects the drop and sounds the alarm.

Smoke alarms (alpha)

Alpha is used because it is the strongest ioniser and is absorbed by smoke; beta or gamma would pass straight through the smoke.

A long half-life isotope is used, so the detector rarely needs replacing.

Section 38 of 56Sterilising equipment and food (gamma)

Sterilising instruments

Sterilising instruments gamma is the most penetrating, so it reaches all sides of the instruments, even through the packaging.
The source has a half-life of about 5 years so it lasts a long time.

Irradiating food

Irradiating food gamma rays kill microorganisms, so food lasts longer and the risk of food-borne infection falls.
Irradiated food carries the Radura symbol, and different countries permit different foods.

Section 39 of 56Monitoring thickness

Principle

Principle material passes between a source and a detector; thicker material absorbs more radiation,
so a lower count is detected, and the machine adjusts to keep the thickness constant.

Beta

Beta used for thin materials such as paper, card and aluminium foil, because it is partly absorbed.

Gamma

Gamma used for very thick materials because it penetrates more.

Half-life

Half-life about 10 to 20 years, so the count rate stays steady from day to day.

Section 40 of 56Medical uses of gamma and tracers

Radiotherapy

Radiotherapy gamma beams are aimed at a tumour from different directions to kill cancer cells while limiting harm to healthy tissue.
The source has a half-life of about five years.

Tracers

Tracers a radioactive isotope followed round the body, such as in a PET scan, to find a tumour.
It has a half-life of several hours, long enough to scan and then leave the body quickly.

Section 41 of 56Choosing the radiation for a job

Job	Radiation	Reason
Smoke detector	Alpha	Strongly ionises air; absorbed by smoke
Paper thickness gauge	Beta	Partly absorbed by thin sheets
Steel thickness gauge or sterilisation	Gamma	Penetrates thick materials and packaging
Medical tracer	Gamma with a short half-life	Detected outside the body and soon decays

Section 42 of 56Evaluating the choice of isotope

Evaluating the choice of isotope

Half-life too short

Half-life too short the source would need replacing too often or the readings would keep changing.

Half-life too long for a tracer

Half-life too long for a tracer the patient would stay radioactive for a long time.

Type of radiation

Type of radiation must match the penetration needed for the job.

Section 43 of 56More from the specification

More from the specification

Mass spectrometry sorting using magnetic fields

Mass spectrometry sorting using magnetic fields accelerating radioactive isotopes through magnetic fields deflects ions along circular paths to determine isotopic abundance.

Alpha Particle Nature and Nuclear Emission

An alpha particle is an emitted helium nucleus containing two protons and two neutrons;
- it carries a relative mass of 4 and an electric charge of +2e, emitted by heavy unstable nuclei to increase stability.

Nuclear Composition and Atomic Notation

The atomic nucleus consists of positively charged protons and uncharged neutrons;
- in nuclide notation AZX, Z represents atomic (proton) number and A represents nucleon (mass) number.

Beta and Gamma Radioactive Decay

In beta-minus decay, a neutron within an unstable nucleus transforms into a proton while emitting a fast-moving electron (beta particle) and antineutrino;
- gamma emission releases excess nuclear energy as electromagnetic radiation.

Nuclear Fission and Nuclear Power

In a nuclear power reactor, heavy uranium-235 nuclei absorb thermal neutrons and split into lighter daughter nuclei, releasing additional neutrons and large quantities of kinetic and thermal energy.

Alpha particle nuclear composition

Alpha particle nuclear composition alpha radiation consists of helium-4 nuclei composed of two bound protons and two neutrons carrying positive charge.

Unstable nuclei and radioactive decay

Unstable nuclei and radioactive decay unstable parent nuclei undergo spontaneous nuclear decay, emitting ionising radiation to achieve greater structural stability.

Nuclear power generation

Nuclear power generation controlled nuclear fission reactions within reactor cores release massive thermal power from splitting uranium atomic nuclei.

Penetrating power

How far radiation travels through a material.

Ionising power

How strongly radiation ionises atoms.

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.

Nucleus

Small, dense, positively charged centre of the atom.

Neutron

Neutral particle in the nucleus, relative mass 1.

Proton

Positive particle in the nucleus, relative charge +1 and relative mass 1.

Charge

Quantity of electricity in coulombs, Q = It.
What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)

Electron

Negatively charged particle in an atom.

Section 44 of 56How radiation harms living things

Ionisation of DNA

Ionisation of DNA if the atoms in DNA are ionised, the DNA strand is damaged.

Possible outcomes

Possible outcomes the cell may die, or the DNA may be mutated when it is copied.

Cancer

Cancer if a mutated cell keeps replicating, a tumour may develop.

Cell repair

Cell repair sometimes the cell repairs the DNA, but incorrect repair causes mutations.

Section 45 of 56Effects of high doses

Skin burns

Skin burns similar to severe sunburn.

Lower immunity

Lower immunity fewer white blood cells, so a person is more open to infection.

Dose

Dose the amount of radiation received is the dose, measured in sieverts (Sv);
- one sievert is a very large dose and causes acute radiation poisoning.

Section 46 of 56Safe handling of sources

Storage

Storage keep sources in lead-lined boxes, away from people.

Time

Time handle them for as short a time as possible and return them to their boxes immediately.

Distance

Distance keep everyone as far as possible and use tongs so the source is held at arm’s length.

Warning signs

Warning signs mark areas with the radioactivity warning sign.

Records

Records note the dates and times of use, and wash hands afterwards.

Section 47 of 56Three key precautions (Extended)

Precaution	Why it works	Examples
Reduce exposure time	A shorter time gives a smaller dose	Return sources to their boxes at once
Increase distance	Radiation spreads out, so the dose falls greatly	Tongs; power plants in remote areas; waste buried far from towns
Use shielding	Barriers absorb the radiation	Lead, water or concrete; lead aprons; X-ray operators behind a lead barrier

Section 48 of 56Transport and disposal

Transport

Transport used nuclear fuel travels in casks that withstand fire, cold and immersion in water.

Long half-life waste

Long half-life waste it decays slowly and stays radioactive and a contamination risk for a very long time.

Disposal

Disposal long half-life waste is buried deep underground so it cannot escape into the environment.

Section 49 of 56Weighing risk and benefit

Benefits

Benefits radiation is used to diagnose and treat disease, sterilise equipment and preserve food.

Risks

Risks high doses damage cells, so exposure must be controlled and sources used only when necessary.

Different people

Different people radiographers and workers in nuclear plants wear badges with photographic film to monitor their exposure.

Section 50 of 56Alpha, beta and gamma as hazards

Alpha

Alpha most dangerous inside the body because it is so strongly ionising, but easily stopped outside by skin or paper.

Beta

Beta can damage skin and tissue just under the surface.

Gamma

Gamma passes through the body, so shielding and distance are needed.

Section 51 of 56Protection of different groups of people

Group	Hazard	Precaution
Student using a source	Radiation dose from handling	Tongs, distance, short time, lead-lined box
Radiologist	Repeated exposure to X-rays	Lead apron and barrier, dose badge
Nuclear plant worker	Exposure to radioactive materials	Shielding, protective clothing, monitoring
General public	Contamination from waste	Secure storage and burial of long half-life waste

Section 52 of 56Principles to apply to any question

Limit the time spent near the source.

Keep as far as possible from it.

Place a suitable absorber between the source and people.

Keep sources locked away and record their use.

Section 53 of 56Examples of radiation doses

Background

Background an average person receives a few millisieverts each year.

Medical scan

Medical scan a chest X-ray gives a small dose, while higher doses are used only when the benefit outweighs the risk.

High dose

High dose a dose of several sieverts received in a short time is life-threatening.

Section 54 of 56More from the specification

More from the specification

Beta ray deflection in magnetic fields

Beta ray deflection in magnetic fields applying transverse magnetic fields deflects high-speed beta electrons, providing an analytical means of radiation separation.

Characteristics of nuclear radiation

Alpha particles are helium nuclei with high ionizing power and short range (~5 cm in air);
- beta particles are high-speed electrons;
- gamma rays are uncharged electromagnetic photons.

Random nature of radioactive decay

Nuclear decay is a spontaneous, random quantum process where the probability of decay per unit time is constant for a given isotope.

Half-life measurement and background correction

Radioactive half-life is the time taken for half the radioactive nuclei to decay;
- measured counts must be corrected by subtracting background radiation.

Alpha particle composition

Alpha particle composition comprises two protons and two neutrons tightly bound, carrying a net positive charge of +2.

Geiger-Muller counter tube

Geiger-Muller counter tube detects ionising radiation emitted by unstable radioisotopes, registering pulses as a recorded count rate.

Radioactive decay in samples

Radioactive decay in samples unstable parent nuclei in a radioactive sample decay randomly into daughter isotopes over time.

Penetrating power

How far radiation travels through a material.

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.

Proton

Positive particle in the nucleus, relative charge +1 and relative mass 1.

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.

Detector

An instrument that records the particles produced in collisions.

Electron

Negatively charged particle in an atom.

Section 55 of 56Key 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.

Term	Meaning
Radioactive decay	Spontaneous random change of an unstable nucleus with emission of radiation.
Alpha particle	A helium nucleus emitted by an unstable nucleus.
Beta particle	A fast electron emitted when a neutron changes into a proton.
Gamma ray	High-energy electromagnetic radiation emitted from a nucleus.
Unstable nucleus	A nucleus with an imbalance of protons and neutrons that may decay.
Ionisation	The process of an atom gaining or losing electrons.

Term	Meaning
Ionising power	How strongly radiation removes electrons from atoms.
Parent nucleus	The original nucleus before decay.
Daughter nucleus	The new nucleus formed after decay.
Kinetic energy	The energy of motion, ½mv².
Decay chain	A series of decays from one nucleus to a stable one.
Excess neutrons	More neutrons than needed for stability.

Term	Meaning
Half-life	The time for half the nuclei of an isotope in a sample to decay.
Activity	The rate at which nuclei in a sample decay.
Decay equation	An equation in nuclide notation with balanced nucleon and proton numbers.
Corrected count	Count rate with the background count subtracted.
Alpha particle symbol	⁴₂He.
Beta particle symbol	⁰₋₁e.

Term	Meaning
Radiotherapy	Treating cancer with beams of radiation.
Tracer	A radioactive isotope used to follow substances around the body.
Sterilisation	Killing microorganisms, for example with gamma rays.
Thickness gauge	A device using radiation to monitor and control the thickness of a material.
Radura	The symbol on irradiated food.
PET scan	A scan that detects emissions from a tracer.

Term	Meaning
Dose	The amount of radiation received by a person, measured in sieverts.
Sievert (Sv)	The unit of radiation dose.
Mutation	A change in DNA that may be passed on when a cell copies itself.
Shielding	A barrier that absorbs radiation, such as lead or concrete.
Cask	A special container used to transport used nuclear fuel.
Contamination	Unwanted radioactive material on or in objects or people.

Section 56 of 56Exam technique

Thinking background radiation is all man-made — most of it is natural.

Calling the GM tube "a radiation detector" in an exam answer.

Forgetting to convert hours to seconds for counts per second.

Forgetting to subtract background from the measured count rate.

Saying decay can be predicted for one nucleus — it is random.

Saying gamma is stopped by paper — it is the most penetrating.

Mixing up ionising power and penetrating power — they are opposite in order.

Thinking beta particles are electrons from the orbit — they come from the nucleus.

Thinking beta is more ionising than alpha — alpha is more ionising.

Saying alpha and beta curve the same way in a field — they curve in opposite directions.

Saying beta decay changes the nucleon number — it stays the same.

Saying gamma decay forms a new element — it does not.

Not balancing both numbers in a decay equation.

Using the half-life as the total decay time — the sample never fully disappears.

Forgetting to subtract background before finding half-life.

Using +1 for the nucleon number in beta decay.

Saying gamma is used in smoke alarms — alpha is used.

Using a short half-life source in a smoke alarm or thickness gauge — a long half-life is needed.

Forgetting thin aluminium lets beta through — only a few mm of aluminium stops it.

Reading the half-life from the wrong axis.

Thinking a short half-life means a source is the most dangerous for the longest time — long half-life waste stays dangerous longer.

Handling sources with bare hands — always use tongs.

Believing shielding by paper is enough — dense materials such as lead are needed.

Confusing dose (sieverts) with count rate.

Last updated: 15 October 2026