The Three Types of Nuclear EmissionCambridge IGCSE Physics: Revision notes

Nuclear emission is spontaneous, meaning it occurs without any external trigger or energy input. The nucleus emits radiation unpredictably and will eventually decay, but we cannot predict exactly when an individual nucleus will emit radiation.

Emission occurs randomly in direction, so particles and radiation are emitted in all possible directions from a nucleus with equal probability. This randomness means that if you observe many decaying nuclei, the emissions will be distributed equally in all directions around them.

These properties mean that:

• Nuclear decay cannot be stopped or speeded up by external factors (temperature, pressure, chemical state)
• We can only describe the probability of decay using the concept of half-life
• Individual emission events are unpredictable, but large samples show predictable statistical patterns

Alpha (α) particles are emitted nuclei of helium-4 atoms, consisting of 2 protons and 2 neutrons bound together.

Nature of alpha emission:

• Nucleus emits a helium-4 nucleus: $^A_Z X \rightarrow ^{A-4}_{Z-2} Y + ^4_2 He$
• The original nucleus loses 4 in mass number and 2 in atomic number
• Alpha particles have a positive charge (2+) due to their two protons

Key properties of alpha particles:

Why alpha particles ionise strongly:

• Large positive charge (+2e) creates strong electric field
• Interactions with electrons in materials cause many ionisation events
• Lower speed (compared to beta) means longer interaction time with atoms

Beta (β) particles are high-speed electrons emitted from the nucleus when a neutron decays.

Nature of beta emission:

• A neutron transforms into a proton: $n \rightarrow p + e^- + \bar{\nu}_e$
• The nucleus emits an electron (the beta particle) and an antineutrino
• The original nucleus gains 1 in atomic number but mass number stays the same
• Beta decay process: $^A_Z X \rightarrow ^A_{Z+1} Y + ^0_{-1}e + \bar{\nu}_e$

Key properties of beta particles:

Why beta particles ionise less strongly than alpha:

• Negative charge (-1e) has lower magnitude than alpha's +2e
• High speed (up to 95% c) means brief interaction with atoms, fewer ionisation events
• Lower kinetic energy transfer per interaction compared to slower, more highly charged alpha particles

Gamma (γ) radiation is high-energy electromagnetic radiation emitted from an excited nucleus.

Nature of gamma emission:

• The nucleus transitions from an excited state to a lower energy state by emitting a photon
• Often occurs after alpha or beta decay has left the nucleus in an excited state
• Represented as: $^A_Z X^* \rightarrow ^A_Z X + \gamma$
• Has no mass and no charge – it is pure energy in the form of electromagnetic waves

Key properties of gamma radiation:

Why gamma radiation ionises weakly:

• Zero charge means no direct electric field interaction with orbital electrons
• Ionisation occurs only through indirect processes (Compton scattering, pair production)
• High energy and speed mean brief, infrequent interactions with matter
• Despite high energy per photon, the number of ionisation events is small

The deflection of radiation depends critically on whether it has charge and mass.

Deflection in electric fields:

Deflection in magnetic fields:

Understanding the deflection differences:

For a charged particle in a magnetic field: $F = Bqv$ (magnetic force)

• Both alpha and beta experience a magnetic force
• Beta deflects more sharply because it has much less mass, so the same force produces greater acceleration: $a = F/m$
• Alpha, despite being faster in some cases, has ~7000 times more mass, resulting in minimal deflection
• Gamma cannot be deflected because it has zero charge and the magnetic force only acts on moving charges

Using deflection to identify radiation: In crossed electric and magnetic fields, or by observing deflection patterns, you can identify:

• No deflection in either field → gamma radiation
• Deflects one way in electric field, curves sharply in magnetic field → beta
• Deflects opposite way in electric field, minimal curve in magnetic field → alpha

Ionising ability is the power of radiation to remove electrons from atoms. It depends on two factors: kinetic energy (for particles) and electric charge.

Comparison of ionising abilities:

How charge affects ionising ability:

• Charge creates an electric field that reaches out and interacts with orbital electrons in surrounding atoms
• Alpha particles (+2e) create a stronger electric field than beta particles (-1e)
• More charge = stronger field = more frequent ionisations along the path
• A neutral particle (gamma) creates no electric field, so it cannot directly ionise through Coulomb interaction

How kinetic energy affects ionising ability:

• Kinetic energy determines interaction time: a slower particle spends longer near each atom, allowing more ionisations
• Alpha particles are slower (~5% c) than beta particles (~95% c), but this slower speed actually increases ionisation because they interact with more atoms
• Beta particles are faster (~95% c), so they pass through atoms more quickly, reducing ionisations per unit path length
• Despite high speed, beta particles ionise less than alpha because their lower charge dominates

Comparing directly:

1. Alpha vs Beta: Alpha ionises more strongly because its +2e charge creates a much stronger electric field than beta's -1e, even though beta is faster. The large charge difference outweighs the speed advantage.

2. Alpha vs Gamma: Alpha ionises directly; gamma only ionises indirectly (rare interactions). Alpha's charge gives it enormous advantage.

3. Beta vs Gamma: Beta ionises moderately because it is charged; gamma ionises very little because it is uncharged.