⚛️Intro to Applied Nuclear Physics Unit 4 – Neutron Physics: Principles and Applications

Neutron Basics

• Neutrons are subatomic particles with no electric charge and a mass slightly greater than protons
• Consist of two down quarks and one up quark bound together by the strong nuclear force
• Classified as hadrons, particles composed of quarks, and baryons, a subtype of hadrons
• Have a mean lifetime of approximately 879 seconds (14.6 minutes) when free and outside the nucleus
  • Undergo beta decay, transforming into a proton, an electron, and an electron antineutrino
• Play a crucial role in nuclear stability by allowing heavier elements to exist without excessive Coulomb repulsion
• Discovered by James Chadwick in 1932 through experiments with beryllium radiation
• Neutron wavelength can be calculated using the de Broglie equation: $\lambda = \frac{h}{mv}$, where $h$ is Planck's constant, $m$ is the neutron mass, and $v$ is the neutron velocity

Neutron Sources and Production

• Neutrons can be produced through various nuclear reactions, such as fission, fusion, and spallation
• Fission reactions in nuclear reactors are a common source of neutrons
  • Fissile isotopes like uranium-235 and plutonium-239 undergo induced fission when bombarded with neutrons
  • Each fission event releases an average of 2-3 neutrons, sustaining the chain reaction
• Fusion reactions, such as those in the sun and stars, produce neutrons through the combination of light nuclei
  • Deuterium-tritium fusion is a promising source for controlled fusion reactors: $^2_1\text{H} + ^3_1\text{H} \rightarrow ^4_2\text{He} + ^1_0\text{n} + 17.6 \text{ MeV}$
• Spallation neutron sources use high-energy proton beams to bombard heavy metal targets (tungsten, tantalum)
  • Proton-nucleus collisions cause the emission of neutrons from the target material
• Radioisotope neutron sources, such as americium-beryllium (AmBe) and californium-252, undergo spontaneous fission or $(\alpha, n)$ reactions
• Neutron generators use deuterium-deuterium or deuterium-tritium fusion reactions to produce neutrons on-demand

Neutron Interactions with Matter

• Neutrons interact with matter through various mechanisms, primarily scattering and absorption
• Elastic scattering occurs when a neutron collides with a nucleus and conserves kinetic energy
  • Used for neutron moderation to reduce neutron energy
  • Important for neutron thermalization in nuclear reactors
• Inelastic scattering involves the transfer of kinetic energy from the neutron to the target nucleus
  • Excites the nucleus to a higher energy state, which then de-excites by emitting gamma rays
• Radiative capture (n, $\gamma$) reactions occur when a neutron is absorbed by a nucleus, forming a heavier isotope and releasing gamma radiation
  • Essential for neutron absorption and control in nuclear reactors
• Charged particle reactions, such as (n, p) and (n, $\alpha$), result in the emission of protons or alpha particles
• Neutron-induced fission (n, f) reactions are crucial for energy production in nuclear reactors and the creation of radioisotopes
• Neutron activation analysis utilizes neutron capture reactions to identify and quantify elements in a sample

Neutron Cross-Sections

• Neutron cross-sections quantify the probability of a specific interaction between a neutron and a target nucleus
• Measured in units of barns (b), where 1 barn = $10^{-24}$ cm$^2$
• Microscopic cross-section ($\sigma$) represents the effective target area of a single nucleus for a given reaction
  • Depends on the incident neutron energy and the target isotope
• Macroscopic cross-section ($\Sigma$) accounts for the number density of target nuclei in a material
  • Calculated as $\Sigma = N \sigma$, where $N$ is the number density of the target isotope
• Total cross-section ($\sigma_t$) is the sum of individual reaction cross-sections: $\sigma_t = \sigma_s + \sigma_a + \sigma_f + ...$
  • $\sigma_s$: scattering cross-section
  • $\sigma_a$: absorption cross-section
  • $\sigma_f$: fission cross-section
• Neutron cross-section data is essential for reactor design, shielding calculations, and activation analysis
• Evaluated Nuclear Data File (ENDF) and Joint Evaluated Fission and Fusion (JEFF) libraries compile cross-section data for various isotopes and reactions

Neutron Moderation and Absorption

• Neutron moderation is the process of reducing the energy of fast neutrons to thermal energies (~0.025 eV at room temperature)
• Moderators are materials with low atomic mass and high scattering cross-sections, such as water, heavy water, and graphite
  • Efficient energy transfer through elastic collisions with light nuclei
• Neutron absorption is the capture of neutrons by nuclei, removing them from the system
• Absorbers are materials with high neutron absorption cross-sections, such as boron, cadmium, and gadolinium
  • Used for reactor control, shielding, and neutron detection
• Neutron moderators and absorbers play crucial roles in nuclear reactor design and operation
  • Moderators ensure efficient fission chain reactions by thermalizing neutrons
  • Control rods containing absorbers regulate reactor power and provide shutdown capabilities
• Neutron balance in a reactor is maintained by carefully selecting moderator and absorber materials and geometries
• Neutron temperature and energy distribution in a moderated system can be described by the Maxwell-Boltzmann distribution

Neutron Detection Methods

• Neutron detection is essential for monitoring neutron fluxes, radiation safety, and experimental measurements
• Gas-filled detectors, such as boron trifluoride (BF3) and helium-3 (3He) proportional counters, rely on neutron-induced charged particle reactions
  • $^{10}$B(n, $\alpha$)$^7$Li and $^3$He(n, p)$^3$H reactions produce ionization in the gas, which is collected and amplified
• Scintillation detectors use materials that emit light when interacting with neutrons or secondary charged particles
  • Lithium-6 enriched lithium iodide (6LiI) and lithium glass scintillators are common choices
  • Light output is converted to electrical signals using photomultiplier tubes or photodiodes
• Activation foils and detectors measure neutron fluence by inducing radioactivity in materials through neutron capture reactions
  • Gold, indium, and dysprosium foils are frequently used, with subsequent gamma spectroscopy to determine activation levels
• Fission chambers contain fissile materials (U-235, Pu-239) that undergo neutron-induced fission, producing measurable charged fission fragments
• Bonner sphere spectrometers consist of a thermal neutron detector surrounded by polyethylene spheres of varying sizes
  • Enables neutron energy spectrum measurements by unfolding the detector responses
• Neutron imaging techniques, such as neutron radiography and tomography, utilize the penetrating nature of neutrons to visualize internal structures and compositions

Applications in Nuclear Reactors

• Neutron physics is the foundation of nuclear reactor design, operation, and safety
• Fission reactors rely on the chain reaction of neutron-induced fission in fuel materials (U-235, Pu-239)
  • Neutron moderation, absorption, and leakage must be carefully balanced to maintain criticality
• Reactor core design involves optimizing fuel arrangement, moderator and coolant selection, and control rod placement
  • Light water reactors (LWRs) use ordinary water as both moderator and coolant
  • Heavy water reactors (HWRs) employ deuterium oxide (D2O) as moderator, allowing the use of natural uranium fuel
  • High-temperature gas-cooled reactors (HTGRs) use graphite moderator and helium coolant for increased efficiency and safety
• Neutron flux distribution and energy spectrum in the reactor core affect power production, fuel burnup, and material activation
  • Multigroup neutron diffusion theory and Monte Carlo simulations are used to model reactor neutronics
• Reactor control systems, including control rods and soluble neutron absorbers, regulate neutron population and reactor power
• Neutron instrumentation, such as ex-core and in-core detectors, monitor neutron flux levels for safety and operational purposes
• Neutron activation of structural materials and coolant contributes to the radiological characteristics of a reactor
  • Influences reactor maintenance, decommissioning, and waste management strategies

Other Practical Applications

• Neutron activation analysis (NAA) is a sensitive and non-destructive technique for elemental analysis
  • Samples are irradiated with neutrons, inducing radioactivity proportional to the concentration of target elements
  • Gamma spectroscopy of the activated sample reveals the presence and quantity of specific isotopes
  • Applications include environmental monitoring, archaeological studies, and materials science
• Neutron scattering techniques probe the structure and dynamics of materials at the atomic and molecular level
  • Elastic scattering (diffraction) provides information on crystal structures and phase transitions
  • Inelastic scattering (spectroscopy) reveals vibrational, rotational, and magnetic excitations
  • Small-angle neutron scattering (SANS) investigates nanoscale structures and macromolecular assemblies
• Neutron radiography and tomography utilize the attenuation of neutrons by materials to create images of internal structures
  • Complementary to X-ray imaging, as neutrons interact differently with matter
  • Applications in non-destructive testing, cultural heritage studies, and fuel cell research
• Boron neutron capture therapy (BNCT) is an experimental cancer treatment modality
  • Boron-10 compounds are selectively accumulated in tumor cells and irradiated with neutrons
  • Neutron capture reactions produce high-LET alpha particles and lithium-7 nuclei, causing localized cell damage
• Neutron sources and beamlines at research reactors and spallation facilities enable a wide range of scientific and industrial applications
  • Materials characterization, radioisotope production, and radiation effects studies