⚛️Nuclear Physics Unit 7 – Interaction of Radiation with Matter

Key Concepts and Definitions

• Radiation refers to the emission and propagation of energy through space or a medium in the form of waves or particles
• Ionizing radiation has sufficient energy to ionize atoms or molecules by removing electrons from their orbitals
• Non-ionizing radiation lacks the energy to ionize matter but can still cause excitation of atoms or molecules
• Interaction of radiation with matter involves various processes such as absorption, scattering, and ionization
• Cross section is a measure of the probability of a specific interaction occurring between radiation and matter, expressed in units of area (barns)
  • Microscopic cross section ($\sigma$) refers to the probability of interaction for a single particle
  • Macroscopic cross section ($\Sigma$) takes into account the density of the material and is used for bulk interactions
• Mean free path is the average distance a particle travels between successive interactions, given by $\lambda = 1/\Sigma$
• Dosimetry is the measurement and calculation of the absorbed dose of ionizing radiation in matter, typically expressed in units of gray (Gy) or sievert (Sv)

Types of Radiation

• Alpha particles are helium nuclei (two protons and two neutrons) emitted during radioactive decay of heavy elements (uranium, radium)
  • Highly ionizing but short range due to their large mass and charge
  • Can be stopped by a sheet of paper or skin
• Beta particles are high-energy electrons or positrons emitted during radioactive decay or nuclear reactions
  • Can penetrate deeper than alpha particles but still have a relatively short range
  • Stopped by a few millimeters of aluminum or plastic
• Gamma rays are high-energy electromagnetic radiation emitted from excited atomic nuclei or during particle annihilation
  • Highly penetrating and can pass through significant amounts of matter
  • Require dense materials like lead or concrete for effective shielding
• X-rays are similar to gamma rays but originate from electron transitions in atoms rather than nuclear processes
• Neutron radiation occurs when free neutrons are emitted from nuclear reactions or radioactive decay
  • Can penetrate deeply into matter and cause secondary ionization through interactions with atomic nuclei
  • Moderated by materials with high hydrogen content (water, paraffin wax) to reduce their energy

Fundamental Interactions

• Electromagnetic interactions occur between charged particles and photons, governed by the electromagnetic force
  • Coulomb scattering involves the repulsion or attraction between charged particles
  • Photoelectric effect is the emission of electrons from matter due to the absorption of photons
  • Compton scattering is the inelastic scattering of photons by electrons, resulting in a decrease in photon energy
• Strong nuclear interactions are responsible for the binding of quarks within hadrons and the stability of atomic nuclei
  • Nuclear reactions such as fission and fusion are mediated by the strong force
  • Hadron-hadron interactions (proton-proton, neutron-neutron) are governed by the strong force
• Weak nuclear interactions are responsible for radioactive beta decay and neutrino interactions
  • Beta decay involves the conversion of a neutron into a proton (or vice versa) with the emission of an electron and antineutrino
  • Neutrino interactions have extremely small cross sections due to the weak nature of the interaction
• Gravitational interactions play a negligible role in the interaction of radiation with matter at the atomic and subatomic scales

Absorption and Attenuation

• Absorption is the process by which radiation energy is deposited in matter, resulting in the excitation or ionization of atoms
• Attenuation is the gradual loss of intensity of radiation as it passes through matter due to absorption and scattering processes
• Linear attenuation coefficient ($\mu$) is a measure of the fraction of radiation intensity lost per unit thickness of material
  • Depends on the type and energy of radiation and the properties of the absorbing material
  • Related to the macroscopic cross section by $\mu = \Sigma$
• Mass attenuation coefficient ($\mu/\rho$) is the linear attenuation coefficient divided by the density of the material, allowing for comparison between different materials
• Beer-Lambert law describes the exponential attenuation of radiation intensity as a function of material thickness: $I = I_0 e^{-\mu x}$
  • $I_0$ is the initial intensity, $I$ is the attenuated intensity, and $x$ is the material thickness
• Half-value layer (HVL) is the thickness of a material required to reduce the radiation intensity by half
  • Related to the linear attenuation coefficient by $HVL = \ln(2) / \mu$
• Tenth-value layer (TVL) is the thickness of a material required to reduce the radiation intensity to one-tenth of its initial value
  • Related to the linear attenuation coefficient by $TVL = \ln(10) / \mu$

Scattering Processes

• Scattering is the process by which radiation changes direction or energy upon interaction with matter
• Elastic scattering involves no change in the kinetic energy of the interacting particles (Rayleigh scattering)
• Inelastic scattering results in a change in the kinetic energy of the interacting particles (Compton scattering)
• Rayleigh scattering is the elastic scattering of photons by bound electrons, with no change in photon energy
  • Occurs predominantly for low-energy photons and high-Z materials
  • Responsible for the blue color of the sky due to the preferential scattering of shorter wavelengths
• Compton scattering is the inelastic scattering of photons by free or loosely bound electrons
  • Results in a decrease in photon energy and an increase in wavelength (Compton shift)
  • Energy and momentum are conserved in the interaction, with the electron receiving a portion of the photon's energy
• Thomson scattering is the elastic scattering of photons by free electrons at low energies
  • Classical limit of Compton scattering when the photon energy is much smaller than the electron rest energy
• Pair production is the creation of an electron-positron pair from a high-energy photon interacting with a nucleus
  • Requires a minimum photon energy of 1.022 MeV (twice the electron rest mass energy)
  • The photon disappears, and its energy is converted into the mass and kinetic energy of the electron-positron pair

Radiation Detection Methods

• Gas-filled detectors rely on the ionization of gas molecules by radiation, creating ion pairs that are collected by electrodes
  • Ionization chambers measure the total charge collected from ion pairs, proportional to the radiation energy
  • Proportional counters amplify the primary ionization through gas multiplication, providing energy information
  • Geiger-Müller counters operate at high voltages, producing a large output pulse for each detected event
• Scintillation detectors use materials that emit light when excited by ionizing radiation
  • Inorganic scintillators (NaI, CsI) have high density and atomic number, making them efficient for gamma-ray detection
  • Organic scintillators (plastic, liquid) are fast and have low afterglow, suitable for fast neutron and beta detection
  • Light output is converted into electrical signals using photomultiplier tubes or photodiodes
• Semiconductor detectors are solid-state devices that use the creation of electron-hole pairs in a semiconductor material (silicon, germanium) to detect radiation
  • Offer excellent energy resolution and fast response times
  • Require cooling to reduce thermal noise and maintain performance
• Neutron detectors rely on nuclear reactions to convert neutrons into charged particles that can be detected
  • Helium-3 proportional counters use the $^3$He(n,p)$^3$H reaction, producing protons and tritons
  • Boron trifluoride (BF3) counters use the $^{10}$B(n,α)$^7$Li reaction, producing alpha particles and lithium ions
  • Fission chambers use a thin layer of fissile material (uranium-235) to detect neutrons through induced fission reactions
• Dosimeters measure the absorbed dose of ionizing radiation in matter
  • Thermoluminescent dosimeters (TLDs) use materials that store radiation energy and release it as light when heated
  • Optically stimulated luminescence (OSL) dosimeters use materials that release stored energy when exposed to light
  • Film badges contain radiation-sensitive film that darkens upon exposure, providing a visual record of dose

Applications and Real-World Examples

• Medical imaging techniques use various types of radiation to create images of the human body for diagnostic purposes
  • X-ray radiography uses the attenuation of X-rays to create 2D projection images of bones and dense tissues
  • Computed tomography (CT) uses multiple X-ray projections to create detailed 3D images of internal structures
  • Positron emission tomography (PET) uses the annihilation of positrons emitted by radioactive tracers to map metabolic activity
  • Single-photon emission computed tomography (SPECT) uses gamma-emitting tracers to image the distribution of radioactivity in the body
• Radiation therapy employs ionizing radiation to treat cancer and other diseases
  • External beam radiotherapy uses high-energy photons (X-rays, gamma rays) or particles (electrons, protons) to deliver dose to tumors
  • Brachytherapy involves the placement of radioactive sources directly inside or near the tumor for localized treatment
  • Targeted radionuclide therapy uses radioactive molecules that selectively bind to cancer cells, delivering radiation from within
• Nuclear power plants generate electricity through the controlled fission of uranium or plutonium fuel
  • Radiation shielding and containment structures are designed to minimize the release of radioactive materials
  • Radiation monitoring systems ensure the safety of workers and the public
• Industrial radiography uses high-energy radiation (X-rays, gamma rays) to inspect materials for defects or irregularities
  • Widely used in the oil and gas, aerospace, and automotive industries for non-destructive testing
• Radiation is used in food irradiation to sterilize and preserve food products, extending their shelf life and reducing the risk of foodborne illnesses
• Radiocarbon dating is a technique that uses the radioactive decay of carbon-14 to determine the age of organic materials up to ~50,000 years old
• Radiation detection is crucial in nuclear security and non-proliferation efforts, preventing the illicit trafficking of radioactive materials

Advanced Topics and Current Research

• Particle therapy is an advanced form of radiation therapy that uses beams of charged particles (protons, carbon ions) to treat cancer
  • Offers improved dose localization and sparing of healthy tissues compared to conventional photon therapy
  • Active research areas include beam delivery techniques, treatment planning, and biological effects of high-LET radiation
• Radiation-induced bystander effect is a phenomenon where unirradiated cells exhibit biological responses similar to directly irradiated cells
  • Mediated by intercellular communication and signaling pathways
  • Has implications for understanding the non-targeted effects of radiation and the response of tissues to low doses
• Radiation hormesis is a controversial hypothesis suggesting that low doses of ionizing radiation may have beneficial effects on living organisms
  • Proposed mechanisms include stimulation of DNA repair and immune system response
  • Remains a topic of ongoing research and debate in the scientific community
• Radiation-induced genomic instability refers to the increased rate of genetic alterations in the progeny of irradiated cells
  • Can manifest as chromosomal aberrations, mutations, and altered gene expression
  • Contributes to the long-term effects of radiation exposure and the potential for carcinogenesis
• Space radiation is a significant concern for astronauts and spacecraft electronics
  • Consists of high-energy cosmic rays (protons, heavy ions) and trapped radiation belts around Earth
  • Research focuses on understanding the biological effects of space radiation and developing effective shielding materials
• Advanced detector technologies are being developed to improve the sensitivity, resolution, and efficiency of radiation detection
  • Examples include solid-state detectors (CZT, HPGe), noble liquid detectors (liquid xenon), and cryogenic detectors (transition edge sensors)
  • Applications range from fundamental physics research to nuclear security and medical imaging
• Computational modeling and simulation play an increasingly important role in studying the interaction of radiation with matter
  • Monte Carlo methods are used to simulate the transport and interactions of radiation in complex geometries
  • Molecular dynamics simulations provide insights into the nanoscale effects of radiation on materials
  • Machine learning techniques are being explored for data analysis, image reconstruction, and treatment planning optimization