The three types of nuclear emission Revision Notes

Nuclear radiation is emitted spontaneously from unstable atomic nuclei. 'Spontaneous' means that the emission occurs without any external trigger or influence; it is an inherent property of the unstable nucleus. This contrasts with induced processes, where an external factor causes emission.

The instability arises because the nucleus has an imbalance of protons and neutrons or excess energy. To reach a more stable state, the nucleus emits radiation in the form of alpha particles, beta particles, or gamma rays. This emission is a random process governed by quantum mechanics and nuclear forces.

The rate of emission for a large number of nuclei is statistically predictable (characterised by half-life), but the exact time at which a single nucleus emits radiation cannot be predicted.

• Spontaneous emission means no external cause triggers the radiation.
• Unstable nuclei emit radiation to become more stable.
• Emission occurs randomly in time for individual nuclei but statistically predictable for many nuclei.

The direction in which nuclear radiation is emitted is random. This means that the emitted particle or photon can travel in any direction with equal probability.

This randomness arises because the nucleus has no preferred orientation in space and the emission process is governed by quantum probabilities. There is no directional bias or pattern in the emission.

As a result, detectors placed around a radioactive source will detect radiation coming from all directions, and the intensity detected depends on the solid angle subtended by the detector relative to the source.

• Emission direction is equally probable in all directions.
• No preferred emission direction due to isotropic nature of nuclei.
• Detection depends on detector position and geometry.

Nuclear decay is a quantum mechanical process where the nucleus transitions from a higher energy state to a lower energy state by emitting radiation.

The process is probabilistic: each nucleus has a certain probability per unit time to decay, described by the decay constant (λ).

The decay constant relates to the half-life (T₁/₂) by the equation T₁/₂ = ln(2)/λ. This statistical nature means that while the average behaviour of many nuclei is predictable, individual decay events are random in time and direction.

• Decay is a quantum probabilistic process.
• Decay constant (λ) defines probability of decay per second.
• Half-life T₁/₂ = ln(2)/λ describes average decay time.
• Individual decay events are unpredictable in time and direction.

Because emission direction is random, radiation detectors must be positioned appropriately to measure radiation intensity accurately.

The random emission means that only a fraction of emitted radiation is detected, depending on the detector's size and distance from the source.

This randomness also affects shielding design, as radiation can penetrate in any direction, requiring comprehensive coverage to protect against exposure.

• Detector placement affects measured radiation intensity.
• Only a fraction of emitted radiation is detected due to random direction.
• Shielding must cover all directions to be effective.

Nuclear emissions are particles or waves emitted spontaneously from an unstable atomic nucleus during radioactive decay. The three main types are alpha (α) particles, beta (β) particles, and gamma (γ) rays, each differing in composition and properties.

Alpha particles consist of two protons and two neutrons bound together, identical to a helium-4 nucleus. They carry a positive charge of +2e (where e = 1.6 × 10⁻¹⁹ C).

Beta particles are high-speed electrons emitted from the nucleus when a neutron transforms into a proton (beta-minus decay). They carry a negative charge of -1e and have much smaller mass than alpha particles (mass approximately 1/1836 of a proton). Beta-plus particles (positrons) also exist but are not required here. Gamma rays are electromagnetic waves (photons) with no mass and no electric charge, emitted when the nucleus transitions from a higher to a lower energy state after alpha or beta decay.

• Alpha (α): helium nucleus, +2 charge, mass ≈ 4 u (atomic mass units).
• Beta (β): electron, -1 charge, negligible mass compared to nucleons.
• Gamma (γ): electromagnetic radiation, zero charge, zero rest mass.

Ionisation occurs when radiation removes electrons from atoms or molecules, creating ions. The ionising power depends on the charge, mass, and speed of the radiation.

Alpha particles have the highest ionising power because of their large mass and +2 charge. They interact strongly with matter, causing many ionisations over a short distance.

Beta particles have moderate ionising power. Their smaller mass and single negative charge cause fewer ionisations per unit path length than alpha particles, but more than gamma rays due to their charged nature and ability to interact with electrons in matter directly.

• Alpha: very high ionising effect, causing dense ionisation tracks.
• Beta: moderate ionising effect, less dense ionisation than alpha.
• Gamma: low ionising effect, interacts mainly via secondary processes like Compton scattering and photoelectric effect.

Penetration refers to how far radiation can travel through materials before being stopped. It depends on the radiation's mass, charge, and energy.

Alpha particles have very low penetrating ability. They can be stopped by a few centimetres of air or a sheet of paper because their large mass and charge cause rapid energy loss through collisions.

Beta particles penetrate further than alpha particles but are stopped by a few millimetres of aluminium or plastic. Their smaller mass and charge allow deeper penetration but still limited by interactions with matter's electrons and nuclei.

• Alpha: stopped by paper or skin; range in air a few centimetres.
• Beta: stopped by a few millimetres of aluminium; range in air up to a metre.
• Gamma: highly penetrating; requires several centimetres of lead or metres of concrete to reduce intensity significantly.

The following table summarises the key differences between alpha, beta, and gamma emissions.

Because alpha particles are highly ionising but poorly penetrating, they are dangerous if alpha-emitting materials are ingested or inhaled, causing severe internal damage. External alpha radiation is generally harmless as it cannot penetrate skin.

Beta particles can penetrate skin and cause radiation burns; protection requires shielding with plastic, glass, or aluminium.

Gamma rays are the most penetrating and require dense shielding such as thick lead or concrete. They pose external and internal hazards and require strict safety protocols in handling sources.

• Use paper or clothing to block alpha radiation.
• Use aluminium or plastic shields for beta radiation.
• Use lead or concrete shielding for gamma radiation.

Alpha particles (α) consist of two protons and two neutrons, making them helium nuclei. They carry a positive charge of +2e (where e = 1.6 × 10⁻¹⁹ C) and have relatively large mass (approximately 6.64 × 10⁻²⁷ kg).

Beta particles (β) are high-speed electrons emitted from the nucleus during beta decay. They carry a negative charge of -1e and have a much smaller mass (9.11 × 10⁻³¹ kg) compared to alpha particles.

Gamma rays (γ) are electromagnetic waves with no mass and no electric charge. They are photons with very high frequency and energy.

• Alpha particles: +2e charge, heavy mass (helium nucleus).
• Beta particles: -1e charge, very light (electron).
• Gamma rays: no charge, no mass (electromagnetic radiation).

An electric field exerts a force on charged particles given by F = qE, where q is the particle's charge and E is the electric field strength (in newtons per coulomb, N/C).

Alpha particles, being positively charged, are deflected towards the negative plate (cathode) in an electric field.

Beta particles, negatively charged, are deflected towards the positive plate (anode). Their deflection is greater than alpha particles due to their smaller mass and higher acceleration under the same force (F = ma). The direction of deflection is opposite to that of alpha particles because of opposite charge sign.

• Force on charged particle: F = qE.
• Alpha particles deflect towards negative plate.
• Beta particles deflect towards positive plate.
• Gamma rays are not deflected (no charge).

A charged particle moving with velocity v perpendicular to a magnetic field B experiences a magnetic force F = qvB, perpendicular to both velocity and magnetic field (Lorentz force).

The force causes the particle to move in a curved path. The radius r of curvature is given by r = mv/(qB), where m is mass.

Alpha particles curve in one direction due to their positive charge; beta particles curve in the opposite direction due to their negative charge. Gamma rays, having no charge, continue straight without deflection.

• Magnetic force: F = qvB (perpendicular to velocity and field).
• Radius of curvature: r = mv/(qB).
• Alpha and beta particles curve in opposite directions.
• Gamma rays are undeflected.

Beta particles experience greater deflection than alpha particles in both electric and magnetic fields because of their smaller mass and higher acceleration for the same force.

Alpha particles, despite having a larger charge (+2e), have much greater mass, resulting in less curvature in magnetic fields and less deflection in electric fields compared to beta particles.

Gamma rays are unaffected by electric or magnetic fields due to zero charge.

• Beta particles deflect more than alpha particles.
• Alpha particles have larger charge but much larger mass.
• Gamma rays show no deflection.

To draw the deflection of radiation in an electric or magnetic field:

1. Draw two parallel plates labelled positive (+) and negative (-) for electric fields, or indicate magnetic field direction with crosses (×) for into the page or dots (•) for out of the page.

2. Draw straight paths for gamma rays passing through the field without deflection, indicating no charge and no force acting on them.

• Alpha particles curve towards negative plate or in one direction in magnetic field.
• Beta particles curve towards positive plate or opposite direction in magnetic field.
• Gamma rays travel straight through fields.

Alpha particles (α) consist of two protons and two neutrons, identical to a helium nucleus. They have a relatively large mass (~6.64 × 10⁻²⁷ kg) and carry a +2 electric charge.

Beta particles (β) are high-speed electrons (β⁻) or positrons (β⁺) emitted from the nucleus. They have a much smaller mass (~9.11 × 10⁻³¹ kg) and carry a single electric charge (−1 for electrons, +1 for positrons).

Gamma rays (γ) are electromagnetic waves with no rest mass and no electric charge. They have very high frequency and energy but are uncharged photons.

• Alpha particles: heavy, +2 charge, mass ~6.64 × 10⁻²⁷ kg
• Beta particles: light, ±1 charge, mass ~9.11 × 10⁻³¹ kg
• Gamma rays: no mass, no charge, electromagnetic radiation

Ionisation occurs when radiation interacts with atoms, knocking out electrons and creating ions.

Charged particles (alpha and beta) ionise atoms primarily through Coulomb interactions with orbital electrons, causing ionisation along their paths.

Alpha particles, with a +2 charge, exert a stronger electric field and cause more ionisation per unit length than beta particles, which have a single charge (±1). Gamma rays, being uncharged, interact less directly and cause ionisation mainly via secondary processes such as the photoelectric effect, Compton scattering, or pair production, resulting in lower ionising power.

• Ionisation caused by electric charge via Coulomb forces
• Alpha particles have stronger ionising effect due to +2 charge
• Beta particles have moderate ionising effect with ±1 charge
• Gamma rays have low ionising effect due to no charge

Kinetic energy influences how far radiation can penetrate and how much ionisation it causes.

Alpha particles have relatively high kinetic energy but lose it quickly due to their large mass and charge, resulting in high ionisation but very short range (a few centimetres in air).

Beta particles have lower mass and moderate kinetic energy, allowing them to penetrate further (up to a few metres in air) with moderate ionisation along their path, losing energy more gradually than alpha particles due to lower charge and mass. Gamma rays have very high energy photons that penetrate deeply through materials, with low ionisation per unit distance because they interact less frequently.

• Alpha: high kinetic energy, short range, high ionisation
• Beta: moderate kinetic energy, longer range, moderate ionisation
• Gamma: very high energy, very long range, low ionisation

The penetration ability of radiation is inversely related to its ionising power.

Alpha particles can be stopped by a sheet of paper or the outer dead layer of skin due to their strong interactions and short range.

Beta particles penetrate further and require a few millimetres of plastic, glass, or aluminium to be stopped effectively, as their lower charge and mass allow deeper penetration before losing energy completely. Gamma rays penetrate deeply and require dense materials such as several centimetres of lead or concrete for effective shielding.

• Alpha stopped by paper or skin
• Beta stopped by few mm of plastic or aluminium
• Gamma requires dense shielding like lead or concrete

Alpha particles have the highest ionising power because their +2 charge creates strong electric fields that strip electrons efficiently, despite their short range due to rapid energy loss.

Beta particles have moderate ionising power due to their single charge and smaller mass, allowing deeper penetration but less ionisation per unit length than alpha particles.

Gamma rays have the lowest ionising power because they are uncharged photons that interact less frequently, but their high energy allows them to penetrate deeply into materials.

• Ionising power: alpha > beta > gamma
• Penetration: gamma > beta > alpha
• Charge and kinetic energy determine ionisation and penetration