Alpha, beta, gamma: ionisation, penetration, range in air, and deflection in electric & magnetic fields
β’οΈ Describe the nature and composition of alpha (Ξ±), beta (Ξ²) and gamma (Ξ³) radiation
π¬ Compare the ionising power of the three types of radiation
𧱠Explain the penetrating power and range in air of each radiation type
β‘ Predict how Ξ±, Ξ² and Ξ³ are deflected in electric and magnetic fields
π‘οΈ Link penetration and ionisation to appropriate shielding materials
π Use data to identify an unknown radiation type from its behaviour
π΄ Alpha Radiation (Ξ±)
An alpha particle consists of 2 protons and 2 neutrons β identical to a helium-4 nucleus. It carries a charge of +2.
Ξ± particle = β΄βHe | charge = +2e | mass β 4 u
Because it is relatively large and carries a double positive charge, the alpha particle interacts strongly with the electrons of atoms it passes through, stripping them away and creating ion pairs. This makes alpha radiation the most ionising of the three types.
High ionising power means alpha particles lose energy very quickly. Their range in air is only about 3β7 cm and they are stopped by a thin sheet of paper or a few centimetres of air.
Despite being the most ionising, alpha is the least penetrating. Dead skin cells are usually sufficient to stop alpha particles from outside the body. However, if an alpha source is ingested or inhaled, it can cause serious damage to internal tissue because of that same high ionising power.
Deflection in fields: Because alpha carries a positive charge, it deflects in the direction of the electric force on positive charges β towards the negative plate in an electric field. In a magnetic field, it follows the right-hand rule (or left-hand rule for force on positive charge moving in a field direction). The relatively large mass and charge give it a moderate radius of curvature compared to beta.
Property
Value / Description
Composition
2 protons + 2 neutrons (He-4 nucleus)
Charge
+2
Mass (u)
β 4
Range in air
3β7 cm
Stopped by
Paper / a few cm of air
Ionising power
Highest
π΅ Beta Radiation (Ξ²)
A beta particle is a fast-moving electron emitted from the nucleus when a neutron decays into a proton. It carries a charge of β1.
Ξ² particle = β°ββe | charge = β1e | mass β 1/1836 u (very small)
Beta particles are much lighter and faster than alpha particles. Their smaller size and single charge mean they interact less frequently with matter, giving them intermediate ionising power β more ionising than gamma, but far less than alpha. They create roughly 100 times fewer ion pairs per cm in air compared to alpha particles.
Beta particles have a range of up to about a few metres in air and are stopped by a few millimetres of aluminium (or similar metal sheet).
Beta particles travel at a significant fraction of the speed of light, which makes their paths easily curved in fields. Because they carry a negative charge, they deflect in the opposite direction to alpha particles in both electric and magnetic fields. In an electric field, beta deflects towards the positive plate.
In a magnetic field, beta particles curve with a smaller radius than alpha particles (due to much smaller mass), and in the opposite direction to alpha (due to opposite charge).
Property
Value / Description
Composition
Fast electron from nucleus
Charge
β1
Mass (u)
β 0.00055
Range in air
Up to ~1β2 m
Stopped by
Few mm of aluminium
Ionising power
Medium
π’ Gamma Radiation (Ξ³)
Gamma radiation is electromagnetic radiation (a photon) emitted from an unstable nucleus after alpha or beta decay. It has no mass and no charge.
Ξ³ = electromagnetic wave | charge = 0 | mass = 0 | speed = c (3 Γ 10βΈ m/s)
Because gamma photons carry no charge and no mass, they interact very weakly with matter. This makes gamma the least ionising type of radiation but also the most penetrating. Gamma rays can pass through many centimetres of lead or several metres of concrete before being significantly attenuated.
Gamma radiation is never fully stopped β it is only attenuated (reduced in intensity). Thick lead or thick concrete are used as shielding.
Gamma radiation has an effectively unlimited range in air β it will travel many hundreds of metres with only gradual weakening by the inverse square law. In medical and industrial applications, this penetrating power makes gamma rays extremely useful (e.g. cancer radiotherapy, sterilisation of equipment, industrial thickness gauging).
Deflection in fields: Because gamma carries no charge, it is not deflected by electric or magnetic fields. This is a key distinguishing feature in examinations.
Property
Value / Description
Composition
Electromagnetic photon
Charge
0
Mass
0
Range in air
Effectively unlimited
Stopped by
Many cm of lead / metres of concrete
Ionising power
Lowest
β‘ Deflection in Electric and Magnetic Fields
The behaviour of radiation in fields depends entirely on charge. Gamma, with zero charge, is unaffected. Alpha (+2) and beta (β1) are deflected in opposite directions.
In an electric field between parallel plates: alpha deflects towards the negative plate; beta deflects towards the positive plate; gamma travels straight through.
In a magnetic field (e.g. into the page), using Fleming's Left-Hand Rule for the force on a moving charge:
Alpha (+, moving right) β force downwards
Beta (β, same velocity direction) β force upwards (opposite to alpha)
Gamma (no charge) β undeviated
The radius of curvature in a magnetic field depends on momentum. Alpha particles (heavy, slower) have a larger radius than beta particles (light, fast), even though alpha has greater charge. Beta curves more sharply and in the opposite direction.
Radius of curvature: r = mv Γ· (qB)
where m = mass, v = speed, q = charge, B = magnetic flux density
This equation (Higher context) shows that for similar energies, the lighter beta particle has a much smaller r, giving it a tighter curve despite having half the charge of alpha.
π Summary Comparison Table
Property
Alpha (Ξ±)
Beta (Ξ²)
Gamma (Ξ³)
Nature
He-4 nucleus
Fast electron
EM radiation
Charge
+2
β1
0
Mass (u)
4
~0
0
Ionising power
Highest β β β
Medium β β
Lowest β
Penetrating power
Lowest β
Medium β β
Highest β β β
Range in air
3β7 cm
~1β2 m
Unlimited
Stopped by
Paper
Al (few mm)
Thick Pb/concrete
Deflected by fields?
Yes (+)
Yes (β)
No
Speed
~5% c
~90% c
c
π Example 1: Identifying radiation type from penetration data
A student places different materials between a radioactive source and a Geiger counter. The count rate drops significantly when a sheet of paper is placed between them. What type of radiation is the source emitting? Explain your answer.
1Recall the shielding materials for each radiation type: alpha is stopped by paper; beta is stopped by a few mm of aluminium; gamma requires thick lead or concrete.
2The count rate drops significantly with only a sheet of paper β this means the radiation is absorbed by paper.
3Beta and gamma both pass through paper, so they would not cause a significant drop with paper alone.
4Only alpha radiation is stopped by paper, as it has the lowest penetrating power.
β The source emits alpha (Ξ±) radiation. Alpha particles have the lowest penetrating power and are stopped by a thin sheet of paper due to their large mass and +2 charge causing frequent ionisation and rapid energy loss.
π Example 2: Deflection in an electric field
Three radiation sources β one alpha, one beta, one gamma β are placed in front of a pair of parallel plates. The upper plate is positive and the lower plate is negative. Describe the path of each type of radiation between the plates.
1Recall: charged particles are deflected by electric fields. Gamma has no charge, so it is not deflected.
2Alpha carries a +2 charge. Positive charges are attracted to the negative plate (lower plate). So alpha deflects downward (towards the negative/lower plate).
3Beta carries a β1 charge. Negative charges are attracted to the positive plate (upper plate). So beta deflects upward (towards the positive/upper plate).
4Gamma carries no charge. It is not affected by electric fields and travels in a straight line between the plates.
β Alpha β curves towards the negative (lower) plate. Beta β curves towards the positive (upper) plate. Gamma β travels in a straight line, undeflected.
π Example 3: Comparing ionising power
Explain why alpha radiation is the most ionising type but has the shortest range in air, while gamma radiation is the least ionising but has the greatest range in air.
1Alpha particles are large (mass β 4 u) and carry a charge of +2. As they travel through air, their strong electric field rips electrons from nearby atoms, creating many ion pairs. They interact strongly and frequently with air molecules.
2Because alpha particles lose energy so rapidly through these frequent ionising collisions, they run out of energy quickly β hence a short range of only 3β7 cm in air.
3Gamma photons carry no charge and have no mass. They rarely interact with atoms as they pass through air and when they do, it is through indirect processes (photoelectric effect, Compton scattering). This makes them weakly ionising.
4Because gamma loses energy so infrequently, it can travel very large distances β effectively unlimited range in air β before its intensity is significantly reduced.
β High ionising power = rapid energy loss = short range. Low ionising power = slow energy loss = long range. There is an inverse relationship between ionising power and penetrating power / range in air.
π Example 4: Deflection in a magnetic field (Higher)
A beam of alpha particles and a beam of beta particles both travel horizontally to the right into a region where a magnetic field points vertically upward (out of the page). Using the motor effect, determine the direction of the force on each type of radiation.
1Use Fleming's Left-Hand Rule for positive charges: point the First finger in the direction of the field (out of page), point the seCond finger in the direction of conventional current (= direction of motion for positive charge = to the right). The thuMb gives the force direction.
2For alpha (+2, moving right, B out of page): First finger = out of page, second finger = right β thumb points downward. Alpha experiences a downward force.
3For beta (β1, moving right): the conventional current direction is opposite to electron motion, so point the second finger to the left (opposite to motion). First finger = out of page, second finger = left β thumb points upward. Beta experiences an upward force.
4Note: beta is much lighter than alpha, so it curves with a much smaller radius, producing a tighter arc despite its smaller charge magnitude.
β Alpha β deflected downward. Beta β deflected upward (opposite direction). Beta curves more sharply due to its much smaller mass. Gamma (if present) would be undeflected.
Question 1: Which type of nuclear radiation consists of 2 protons and 2 neutrons?
Question 2: A radiation source is tested with different materials. The radiation passes through paper but is stopped by 5 mm of aluminium. What type of radiation is it?
Question 3: Which radiation type is NOT deflected by an electric field?
Question 4: Place the three radiation types in order of increasing penetrating power (least penetrating first).
Question 5: In an electric field, alpha particles deflect towards the negative plate. In which direction do beta particles deflect?
Challenge 1: A scientist detects radiation from an unknown source. The radiation is deflected towards the positive plate in an electric field and has a range of about 4 cm in air. Identify the radiation type and explain three pieces of evidence that support your answer. [6 marks]
β Alpha radiation.
Evidence 1: Deflected towards the positive plate β the radiation carries a positive charge. Gamma has no charge; beta is negatively charged. Alpha (+2) is the only type that deflects towards the negative plate β wait, towards the positive plate would indicate negative charge (beta). Re-read: deflected towards the positive plate = negative charge = beta... but range in air is only 4 cm which is typical of alpha.
β οΈ Contradiction in the question β exam skill point: Alpha deflects towards the negative plate (positive charge). If deflection is towards the positive plate, that means negative charge = beta. But 4 cm range in air = alpha. In a real exam, look for consistency. If deflected towards the negative plate AND range β 4 cm β definitely alpha. Three pieces of evidence for alpha: (1) +2 charge β deflects to negative plate; (2) range in air 3β7 cm; (3) stopped by paper / not penetrating.
Challenge 2: Explain why alpha radiation, despite being the most ionising, is considered less dangerous than gamma radiation as an external source, but more dangerous as an internal source. [4 marks]
β External source: Alpha particles are stopped by the outer layer of dead skin cells (or by just a few cm of air). They cannot penetrate into living tissue from outside the body. Gamma, being highly penetrating, passes through skin and body tissues, irradiating internal organs. Therefore gamma is more dangerous externally.
Internal source: If an alpha source is ingested or inhaled, it deposits all its energy directly in living tissue because it cannot escape β its short range means all ionising events occur in nearby cells. The high ionising power causes severe localised cell damage, potentially causing cancer or cell death. Gamma radiation, being weakly ionising, would deposit much less energy per unit path length inside the body, causing less localised damage.
Challenge 3 (Higher): A magnetic field directed into the page causes an alpha particle beam to curve downward. On the same diagram, a beta particle beam enters from the same point travelling in the same direction. Describe and explain the path of the beta beam, including how its radius of curvature compares to the alpha beam. [5 marks]
β Direction: Beta carries a charge of β1, which is opposite in sign to alpha (+2). Since alpha curves downward, beta curves upward (opposite direction) because the magnetic force on an opposite charge moving in the same direction is reversed.
Radius of curvature: r = mv Γ· (qB). Beta particles have a much smaller mass (β 1/7344 of alpha mass for same energy) but travel at much higher speed (~90% c vs ~5% c for alpha). For similar kinetic energies, beta has much greater speed but far smaller mass. The product mv (momentum) for beta is smaller than for alpha at comparable energies, but beta's charge q is half that of alpha. Overall, beta curves with a smaller radius than alpha β it follows a tighter arc.
Summary: Beta curves in the opposite direction to alpha (upward) with a smaller radius of curvature, producing a tighter, more sharply curved path.
Challenge 4 (Extended Writing): A radiation detector is used to investigate an unknown source. Describe an experimental procedure to determine whether the source emits alpha, beta, gamma, or a mixture of radiation types. Include the materials needed and how you would interpret the results. [6 marks]
β Method:
1. Use a Geiger-MΓΌller (GM) tube and counter. Record background count rate with no source present.
2. Place the source a fixed distance (e.g. 5 cm) from the GM tube. Record the count rate with no absorber.
3. Place a thin sheet of paper between source and detector. Record count rate.
4. Place a few mm of aluminium sheet between source and detector. Record count rate.
5. Place several cm of lead between source and detector. Record count rate.
Interpretation:
β’ If count rate drops significantly with paper β alpha component present.
β’ If count rate (after removing paper) drops with aluminium β beta component present.
β’ If count rate (after removing aluminium) is still above background β gamma component present (gamma is only attenuated by lead, not eliminated).
β’ Subtract background count rate from all readings to get corrected count rates.
Safety: Keep source in lead-lined container when not in use. Use tongs to handle source. Minimise time near source. Keep distance from source as large as possible.