6.1 Types of nuclear reactions

Nuclear reactions are the heart of atomic transformations. They come in various forms, from splitting heavy nuclei in fission to combining light ones in fusion. These processes release enormous energy and create new elements, powering stars and nuclear plants alike.

Understanding nuclear reactions is key to harnessing atomic power safely. We'll explore how different particles and energies trigger these reactions, their outcomes, and their applications in science and technology. This knowledge shapes our grasp of the universe's workings.

Nuclear Reactions Involving Nuclei Splitting

Fission and Spallation

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• Nuclear fission involves splitting heavy atomic nuclei into lighter nuclei
• Occurs spontaneously in some radioactive isotopes or induced by neutron bombardment
• Releases significant energy and additional neutrons (chain reaction)
• Fission of uranium-235 produces approximately 200 MeV of energy per reaction
• Spallation breaks nucleus into many smaller fragments through high-energy particle collisions
• Spallation typically requires particle accelerators to achieve necessary collision energies
• Produces neutron-rich isotopes useful for studying nuclear structure and properties

Radioactive Decay Processes

• Radioactive decay transforms unstable nuclei into more stable configurations
• Alpha decay emits helium nuclei (two protons and two neutrons)
• Beta decay involves electron or positron emission, converting neutrons to protons or vice versa
• Gamma decay releases high-energy photons to reduce nuclear excitation energy
• Electron capture occurs when an inner-shell electron combines with a proton, forming a neutron
• Spontaneous fission splits heavy nuclei without external stimulation (uranium-238)
• Half-life measures time for half of a radioactive sample to decay

Nuclear Reactions Involving Nuclei Combining

Fusion Processes

• Nuclear fusion combines lighter nuclei to form heavier elements
• Requires extremely high temperatures and pressures to overcome electrostatic repulsion
• Powers stars, including our Sun (hydrogen fusing into helium)
• Fusion of deuterium and tritium produces helium and releases a neutron
• Potential clean energy source, but technological challenges remain for controlled fusion
• Inertial confinement fusion uses lasers to compress and heat fusion fuel
• Magnetic confinement fusion employs strong magnetic fields to contain plasma (tokamak design)

Neutron Capture and Transmutation

• Neutron capture occurs when a nucleus absorbs a free neutron
• Can lead to the formation of heavier isotopes or induce fission in some elements
• Slow neutron capture (s-process) produces about half of elements heavier than iron in stars
• Rapid neutron capture (r-process) occurs in supernovae, creating very heavy elements
• Transmutation changes one element into another through nuclear reactions
• Artificial transmutation achieved through particle accelerators or nuclear reactors
• Natural transmutation happens in radioactive decay chains (uranium to lead)

Nuclear Reactions Induced by External Particles

Particle-Induced Nuclear Reactions

• Involve bombarding nuclei with high-energy particles to induce nuclear changes
• Proton-induced reactions can create new isotopes or elements (proton capture)
• Neutron-induced reactions include absorption, scattering, and fission
• Alpha particle bombardment can produce new elements (Rutherford's gold to mercury experiment)
• Deuteron-induced reactions often result in neutron emission
• Particle accelerators enable precise control of projectile energy and type
• Cross-section measurements determine reaction probabilities for different particles and energies

Photonuclear Reactions and Applications

• Photonuclear reactions occur when high-energy photons interact with nuclei
• Photodisintegration ejects nucleons from the nucleus (deuterium to proton and neutron)
• Photofission splits heavy nuclei using gamma rays instead of neutrons
• Nuclear resonance fluorescence identifies specific isotopes in materials
• Photoneutron sources produce neutrons for research and industrial applications
• Gamma-ray induced positron annihilation spectroscopy analyzes material properties
• Photonuclear reactions play a role in stellar nucleosynthesis and cosmic ray interactions