3.1 Types of nuclear reactions

Nuclear reactions are the cornerstone of nuclear physics, involving changes in atomic nuclei through various processes. These reactions are crucial for energy production, medical treatments, and scientific research, and can be categorized into fusion, fission, radioactive decay, and particle interactions with nuclei.

Understanding these reactions is essential for harnessing nuclear power, developing medical treatments, and advancing our knowledge of the universe. From the fusion reactions powering stars to the fission reactions in nuclear power plants, each type of nuclear reaction has unique characteristics and applications.

Types of nuclear reactions

• Nuclear reactions form the foundation of nuclear physics, involving changes in atomic nuclei through various processes
• Understanding different types of nuclear reactions is crucial for applications in energy production, medical treatments, and scientific research
• Nuclear reactions can be broadly categorized into fusion, fission, radioactive decay, and particle interactions with nuclei

Fusion vs fission reactions

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• Fusion combines light nuclei to form heavier elements, releasing energy in the process
• Fission splits heavy nuclei into lighter elements, also releasing energy
• Fusion requires extremely high temperatures and pressures, while fission can occur spontaneously or be induced
• Energy released per nucleon is generally higher in fusion reactions compared to fission

Radioactive decay processes

• Spontaneous emission of particles or energy from unstable nuclei
• Common types include alpha, beta, and gamma decay
• Each decay process has characteristic properties and decay rates
• Radioactive decay is governed by the law of radioactive decay, describing the exponential decrease in the number of radioactive nuclei over time

Particle interactions with nuclei

• Involves collisions between subatomic particles and atomic nuclei
• Can lead to various outcomes, including scattering, absorption, or emission of particles
• Neutron interactions are particularly important in nuclear reactors and nuclear weapons
• Proton and heavy ion interactions are used in particle accelerators for research and medical applications

Fusion reactions

• Fusion reactions power stars and have potential for clean energy production on Earth
• Require overcoming the electrostatic repulsion between positively charged nuclei
• Research focuses on achieving sustained fusion reactions for energy generation

Thermonuclear fusion basics

• Occurs at extremely high temperatures, typically millions of degrees Celsius
• Requires sufficient energy to overcome the Coulomb barrier between nuclei
• Most promising fusion reactions involve isotopes of hydrogen (deuterium and tritium)
• Fusion cross-section increases with temperature, peaking at specific energies for different reactions

Fusion in stars

• Powers the sun and other stars through a series of fusion reactions
• Main sequence stars primarily fuse hydrogen into helium through the proton-proton chain or CNO cycle
• More massive stars can fuse heavier elements up to iron in their cores
• Stellar nucleosynthesis produces elements heavier than hydrogen and helium in the universe

Controlled fusion for energy

• Aims to harness fusion reactions for sustainable energy production on Earth
• Two main approaches: magnetic confinement fusion (tokamaks) and inertial confinement fusion
• Challenges include achieving sufficient plasma confinement time, temperature, and density (Lawson criterion)
• Ongoing research projects include ITER and National Ignition Facility

Fission reactions

• Fission reactions are the basis for current nuclear power generation and nuclear weapons
• Involve the splitting of heavy nuclei into lighter fragments, neutrons, and energy
• Can be induced by neutron absorption or occur spontaneously in some heavy elements

Induced fission mechanism

• Typically initiated by neutron absorption in fissile nuclei (uranium-235, plutonium-239)
• Absorbed neutron causes the nucleus to become unstable and split into two or more fragments
• Releases additional neutrons, enabling chain reactions
• Energy released primarily as kinetic energy of fission fragments and neutrons

Spontaneous fission

• Occurs naturally in some heavy elements without external stimulation
• Probability increases with atomic number and is significant for elements beyond uranium
• Important in the design of nuclear weapons and in the production of neutron sources
• Contributes to the background radiation from natural radioactive materials

Chain reactions

• Self-sustaining series of fission reactions
• Each fission event produces neutrons that can induce further fissions
• Controlled in nuclear reactors using moderators and control rods
• Uncontrolled chain reactions form the basis of nuclear weapons
• Criticality describes the balance between neutron production and loss in a chain reaction

Alpha decay

• Type of radioactive decay involving the emission of an alpha particle (helium-4 nucleus)
• Typically occurs in heavy nuclei with atomic numbers greater than 82
• Results in the transmutation of the parent nucleus to an element with atomic number decreased by 2

Alpha particle emission process

• Quantum tunneling phenomenon allows alpha particles to escape the nuclear potential well
• Probability of tunneling depends on the energy of the alpha particle and the height of the potential barrier
• Geiger-Nuttall law relates the decay constant to the energy of the emitted alpha particle
• Alpha decay often leaves the daughter nucleus in an excited state, leading to subsequent gamma emission

Alpha decay energy spectrum

• Typically consists of discrete energy peaks corresponding to transitions to specific energy levels in the daughter nucleus
• Fine structure in alpha spectra provides information about nuclear energy levels
• Total decay energy (Q-value) is shared between the alpha particle and the recoiling daughter nucleus
• Alpha particle energies typically range from 4 to 9 MeV for naturally occurring alpha emitters

Applications of alpha decay

• Used in smoke detectors (americium-241 source)
• Alpha particle therapy in targeted cancer treatments
• Radioisotope thermoelectric generators for space probes (plutonium-238)
• Alpha spectroscopy for material analysis and nuclear forensics

Beta decay

• Weak interaction process involving the transformation of a neutron into a proton or vice versa
• Three main types: beta-minus decay, beta-plus decay, and electron capture
• Results in the emission of an electron or positron and an antineutrino or neutrino

Beta-minus decay

• Neutron in the nucleus transforms into a proton, emitting an electron and an antineutrino
• Increases the atomic number of the element by one
• Common in neutron-rich nuclei
• Continuous energy spectrum of emitted electrons due to three-body decay process

Beta-plus decay

• Proton in the nucleus transforms into a neutron, emitting a positron and a neutrino
• Decreases the atomic number of the element by one
• Occurs in proton-rich nuclei
• Requires sufficient energy to create the positron-electron pair (1.022 MeV)

Electron capture

• Inner shell electron is captured by a proton in the nucleus, forming a neutron and emitting a neutrino
• Competes with beta-plus decay in proton-rich nuclei
• Probability increases for heavier elements due to closer proximity of inner electrons to the nucleus
• Results in characteristic X-ray emission as outer electrons fill the vacancy left by the captured electron

Gamma emission

• Electromagnetic radiation emitted by excited nuclei transitioning to lower energy states
• Often follows other nuclear decay processes or nuclear reactions
• Does not change the atomic number or mass number of the nucleus

Nuclear excited states

• Result from various nuclear processes (radioactive decay, nuclear reactions)
• Characterized by specific energy levels and angular momentum quantum numbers
• Metastable states (isomers) have relatively long lifetimes before gamma emission
• Nuclear shell model and collective models describe the structure of nuclear excited states

Gamma ray characteristics

• High-energy photons typically in the range of 0.1 to 10 MeV
• Highly penetrating radiation compared to alpha and beta particles
• Energy corresponds to the difference between initial and final nuclear energy states
• Angular distribution of emitted gamma rays provides information about nuclear spin states

Internal conversion

• Competing process to gamma emission
• Excitation energy transferred directly to an atomic electron, ejecting it from the atom
• Probability increases with atomic number and decreases with transition energy
• Results in characteristic X-ray emission as outer electrons fill the vacancy left by the ejected electron

Neutron reactions

• Interactions between neutrons and atomic nuclei
• Crucial in nuclear reactors, nuclear weapons, and neutron activation analysis
• Neutron cross-sections vary widely with neutron energy and target nucleus

Neutron capture

• Absorption of a neutron by a target nucleus, forming a heavier isotope
• Often followed by gamma emission (radiative capture)
• Important in the production of radioisotopes and in stellar nucleosynthesis
• Resonance capture occurs at specific neutron energies, leading to sharp peaks in cross-section plots

Neutron-induced fission

• Neutron absorption triggers the fission of heavy nuclei (uranium-235, plutonium-239)
• Releases additional neutrons, enabling chain reactions in nuclear reactors and weapons
• Cross-section for fission varies with neutron energy, with thermal neutrons often preferred
• Fission products typically include two medium-mass fragments and 2-3 neutrons

Neutron scattering

• Elastic scattering preserves the total kinetic energy of the neutron and target nucleus
• Inelastic scattering leaves the target nucleus in an excited state
• Used in neutron diffraction techniques to study material structures
• Important in neutron moderation in nuclear reactors, slowing down fast neutrons

Proton-induced reactions

• Interactions between accelerated protons and target nuclei
• Used in particle accelerators for research and medical applications
• Cross-sections generally increase with proton energy due to reduced Coulomb barrier effects

Proton capture

• Absorption of a proton by a target nucleus, forming a heavier element
• Important in stellar nucleosynthesis, particularly in the proton-proton chain and CNO cycle
• Used in the production of specific radioisotopes for medical and research purposes
• Often followed by gamma emission or particle evaporation from the compound nucleus

Proton-induced spallation

• High-energy protons cause the emission of multiple particles from the target nucleus
• Produces a wide range of isotopes, including neutron-rich species
• Used in the production of exotic isotopes for research
• Important in cosmic ray interactions and the design of accelerator-driven systems

Proton therapy applications

• Use of accelerated protons for cancer treatment
• Protons deposit most of their energy at a specific depth (Bragg peak)
• Allows for more precise targeting of tumors compared to conventional radiotherapy
• Reduces damage to surrounding healthy tissue, particularly beneficial for pediatric cancers

Heavy ion reactions

• Collisions between nuclei heavier than helium
• Studied using particle accelerators to explore nuclear structure and dynamics
• Can lead to the formation of superheavy elements and exotic nuclear species

Nuclear collisions

• Classified by impact parameter into central, peripheral, and grazing collisions
• Energy dissipation and angular momentum transfer depend on collision geometry
• Coulomb excitation can occur without direct nuclear contact
• Time-dependent Hartree-Fock calculations used to model collision dynamics

Fusion-evaporation reactions

• Complete fusion of projectile and target nuclei to form a compound nucleus
• Compound nucleus de-excites by evaporating particles (neutrons, protons, alpha particles)
• Used in the synthesis of new elements and studies of nuclear structure
• Cross-sections limited by competition with other reaction channels at high energies

Fragmentation reactions

• Projectile or target nucleus breaks into multiple fragments
• Important mechanism for producing exotic nuclei far from stability
• Studied using in-flight separation techniques at fragmentation facilities
• Momentum distributions of fragments provide information about nuclear structure

Nuclear transmutation

• Process of changing one element or isotope into another
• Occurs naturally through radioactive decay and cosmic ray interactions
• Artificially induced for various scientific and practical applications

Artificial transmutation methods

• Particle-induced reactions (neutron capture, proton bombardment)
• Photonuclear reactions using high-energy gamma rays
• Spallation reactions using high-energy protons or heavy ions
• Nuclear fission and fusion processes

Natural transmutation processes

• Radioactive decay chains (uranium and thorium series)
• Cosmic ray interactions in the atmosphere (production of carbon-14)
• Stellar nucleosynthesis in stars and supernovae
• Spontaneous fission of very heavy elements

Applications in nuclear medicine

• Production of radioisotopes for diagnostic imaging (technetium-99m, fluorine-18)
• Therapeutic radioisotopes for cancer treatment (iodine-131, lutetium-177)
• Neutron activation analysis for elemental composition studies
• Radiotracer techniques in biological and environmental research