Why do thermal neutrons have a higher probability of being captured by fissile nuclei than fast neutrons?

Thermal neutrons move more slowly, so they spend more time in the vicinity of the nucleus, increasing the chance of capture.

Define a chain reaction in the context of nuclear fission.

A chain reaction occurs when neutrons released from one fission event go on to cause further fission events in other nuclei.

What condition must be met for a fission chain reaction to be self-sustaining?

At least one neutron from each fission event must cause another fission event.

What is meant by critical mass?

The minimum mass of fissile material required to sustain a chain reaction.

Why can a chain reaction not be sustained if the mass of fissile material is below the critical mass?

Too many neutrons escape from the surface without causing fission, so there are insufficient neutrons to maintain the reaction.

AQA A-Level Physics 7408

Measurements and Their Errors

Topic overview Use of SI units and their prefixes Limitation of physical measurements Estimation of physical quantities

Particles and Radiation

Topic overview Constituents of the atom Stable and unstable nuclei Particles, antiparticles and photons Particle interactions Classification of particles Quarks and antiquarks Applications of conservation laws The photoelectric effect Collisions of electrons with atoms Energy levels and photon emission Wave-particle duality

Waves

Topic overview Progressive waves Longitudinal and transverse waves Principle of superposition of waves and formation of stationary waves Interference Diffraction Refraction at a plane surface

Mechanics and Materials

Topic overview Scalars and vectors Moments Motion along a straight line Projectile motion Newton's laws of motion Momentum Work, energy and power Conservation of energy Bulk properties of solids The Young modulus

Electricity

Topic overview Basics of electricity Current–voltage characteristics Resistivity Circuits Potential divider Electromotive force and internal resistance

Further Mechanics and Thermal Physics

Topic overview Circular motion Simple harmonic motion (SHM)Simple harmonic systems Forced vibrations and resonance Thermal energy transfer Ideal gases Molecular kinetic theory model

Fields and Their Consequences

Topic overview Fields Newton's law Gravitational field strength Gravitational potential Orbits of planets and satellites Coulomb's law Electric field strength Electric potential Capacitance Parallel plate capacitor Energy stored by a capacitor Capacitor charge and discharge Magnetic flux density Moving charges in a magnetic field Magnetic flux and flux linkage Electromagnetic induction Alternating currents Transformers

Nuclear Physics

Topic overview Rutherford scattering α, β and γ radiation Radioactive decay Nuclear instability Nuclear radius Mass and energy Induced fission Safety aspects

AstrophysicsOptional topic

Topic overview Astronomical telescope consisting of two converging lenses Reflecting telescopes Single dish radio telescopes, I-R, U-V and X-ray telescopes Advantages of large diameter telescopes Classification by luminosity Absolute magnitude, M Classification by temperature, black-body radiation Principles of the use of stellar spectral classes The Hertzsprung-Russell (HR) diagram Supernovae, neutron stars and black holes Doppler effect Hubble's law Quasars Detection of exoplanets

Medical PhysicsOptional topic

Engineering PhysicsOptional topic

Topic overview Concept of moment of inertia Rotational kinetic energy Rotational motion Torque and angular acceleration Angular momentum Work and power First law of thermodynamics Non-flow processes The p–V diagram Engine cycles Second Law and engines Reversed heat engines

Turning Points in PhysicsOptional topic

Topic overview Cathode rays Thermionic emission of electrons Specific charge of the electron Principle of Millikan's determination of the electronic charge, e Newton's corpuscular theory of light Significance of Young's double slits experiment Electromagnetic waves The discovery of photoelectricity Wave–particle duality Electron microscopes The Michelson-Morley experiment Einstein's theory of special relativity Time dilation Length contraction Relativistic mass and energy

ElectronicsOptional topic

Topic overview MOSFET (metal-oxide semiconducting field-effect transistor)Zener diode Photodiode Hall effect sensor Difference between analogue and digital signals LC resonance filters The ideal operational amplifier Inverting amplifier configuration Non-inverting amplifier configuration Summing amplifier configuration Real operational amplifiers Combinational logic Sequential logic Astables Principles of communication systems Transmission media Time-division multiplexing Amplitude (AM) and frequency modulation (FM) techniques

Nuclear Physics

Induced fission

4 things to know
Papers 2 and 3

Overview

Chain reactions, thermal neutrons, and the roles of the moderator and the control rods.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is a chain reaction, what makes it self-sustaining, and what are thermal neutrons?

Process

What does a moderator do, and why do nuclei of similar mass to neutrons work best?

Process

What do control rods do, and which materials serve as moderators?

Key term

Give three examples of coolant materials used in nuclear reactors

Questions from this subtopic

Each one with the model answer underneath.

1
What are thermal neutrons?
Model answer
Low-energy neutrons that have been slowed down to approximately the same kinetic energy as the surrounding atoms.
2
Why do thermal neutrons have a higher probability of being captured by fissile nuclei than fast neutrons?
Model answer
Thermal neutrons move more slowly, so they spend more time in the vicinity of the nucleus, increasing the chance of capture.
3
Define a chain reaction in the context of nuclear fission.
Model answer
A chain reaction occurs when neutrons released from one fission event go on to cause further fission events in other nuclei.
4
What condition must be met for a fission chain reaction to be self-sustaining?
Model answer
At least one neutron from each fission event must cause another fission event.
5
What is meant by critical mass?
Model answer
The minimum mass of fissile material required to sustain a chain reaction.
6
Why can a chain reaction not be sustained if the mass of fissile material is below the critical mass?
Model answer
Too many neutrons escape from the surface without causing fission, so there are insufficient neutrons to maintain the reaction.

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Measurements and Their Errors

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Mechanics and Materials

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