☢️Radiochemistry Unit 5 – 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 remove electrons from atoms or molecules, creating ions (alpha, beta, gamma, X-rays)
• Non-ionizing radiation lacks the energy to ionize atoms or molecules but can cause excitation (radio waves, microwaves, visible light)
• Activity ($A$) is the rate of decay of a radioactive substance, measured in becquerels ($Bq$) or curies ($Ci$)
  • $1 Bq = 1 \text{ decay per second}$
  • $1 Ci = 3.7 \times 10^{10} \text{ decays per second}$
• Half-life ($t_{1/2}$) is the time required for half of a given quantity of a radioactive substance to decay
• Exposure is a measure of the ionization produced in air by X-rays or gamma radiation, expressed in roentgens ($R$)
• Absorbed dose is the energy absorbed per unit mass of material, measured in grays ($Gy$) or rads
  • $1 Gy = 1 \text{ J/kg}$
  • $1 \text{ rad} = 0.01 \text{ Gy}$

Types of Radiation

• Alpha radiation consists of heavy, positively charged particles (helium nuclei) emitted from the nucleus of an atom
  • Low penetrating power but high ionizing power
  • Can be stopped by a sheet of paper or skin
• Beta radiation involves the emission of electrons ($\beta^-$) or positrons ($\beta^+$) from the nucleus during radioactive decay
  • Moderate penetrating power and ionizing power
  • Can be stopped by a few millimeters of aluminum or plastic
• Gamma radiation is high-energy electromagnetic radiation emitted from the nucleus during radioactive decay
  • High penetrating power but low ionizing power
  • Requires dense materials like lead or concrete for shielding
• X-rays are similar to gamma rays but originate from the electron shell of an atom rather than the nucleus
  • Produced by the deceleration of charged particles or electronic transitions in atoms
• Neutron radiation occurs when neutrons are ejected from the nucleus during nuclear reactions or spontaneous fission
  • Can penetrate deeply into matter and cause activation of stable nuclei
  • Moderated by materials rich in light elements (hydrogen, carbon)

Radiation-Matter Interactions

• Photoelectric effect occurs when a photon transfers all its energy to an electron, ejecting it from the atom
  • Dominant for low-energy photons and high-Z materials
• Compton scattering involves the inelastic scattering of a photon by a nearly free electron, resulting in a lower-energy photon and a scattered electron
  • Prevalent for intermediate-energy photons and low-Z materials
• Pair production is the creation of an electron-positron pair from a high-energy photon in the presence of a nucleus
  • Requires photon energy greater than $2m_ec^2$ (1.022 MeV)
• Rayleigh scattering is the elastic scattering of photons by bound electrons, with no energy loss
  • Significant for low-energy photons and high-Z materials
• Charged particles (alpha, beta) primarily interact through Coulomb forces, causing ionization and excitation of atoms and molecules along their path
• Neutrons interact through elastic and inelastic scattering, capture reactions, and nuclear reactions with atomic nuclei

Absorption and Attenuation

• Attenuation is the reduction in intensity of radiation as it passes through matter due to absorption and scattering
• Linear attenuation coefficient ($\mu$) describes the fraction of photons removed from a beam per unit thickness of material
  • Depends on photon energy and material properties (density, atomic number)
• Mass attenuation coefficient ($\mu/\rho$) is the linear attenuation coefficient divided by the material density
  • More fundamental property, independent of physical state
• Intensity of a photon beam after passing through a thickness $x$ of material is given by $I = I_0e^{-\mu x}$
  • $I_0$ is the initial intensity
• Half-value layer (HVL) is the thickness of a material required to reduce the intensity of a beam by half
  • Related to the linear attenuation coefficient by $HVL = \ln(2) / \mu$
• Charged particle range is the average distance a particle travels before losing all its energy
  • Depends on particle type, energy, and material properties (density, atomic number)
• Bragg peak is the sharp maximum in energy deposition near the end of a charged particle's range
  • Exploited in radiation therapy to deliver high doses to tumors while sparing healthy tissue

Radiation Effects on Materials

• Ionization and excitation can lead to the formation of free radicals, which are highly reactive and can initiate chemical reactions
• Radiolysis is the dissociation of molecules due to the absorption of radiation energy
  • Can result in the formation of new chemical species and changes in material properties
• Radiation-induced polymerization involves the initiation and propagation of polymer chains by free radicals generated through radiation interactions
  • Used in the production of plastics, adhesives, and coatings
• Radiation damage occurs when the accumulation of defects and changes in material structure alters the physical and mechanical properties
  • Embrittlement, swelling, and degradation of materials exposed to high radiation doses
• Radioluminescence is the emission of light from materials following the absorption of radiation energy
  • Used in scintillation detectors and dosimetry systems
• Radiation-induced conductivity is the temporary increase in electrical conductivity of insulators due to the generation of charge carriers by radiation
  • Exploited in radiation-hardened electronics and sensors

Detection and Measurement Techniques

• Gas-filled detectors (ionization chambers, proportional counters, Geiger-Müller tubes) rely on the ionization of gas molecules by radiation
  • Collect and measure the resulting electrical charge or current
• Scintillation detectors use materials that emit light when exposed to radiation (NaI, BGO, plastic scintillators)
  • Light is converted to an electrical signal by photomultiplier tubes or photodiodes
• Semiconductor detectors (HPGe, Si(Li), CdTe) are based on the creation of electron-hole pairs in a semiconductor material by radiation
  • Provide excellent energy resolution for gamma and X-ray spectroscopy
• Thermoluminescent dosimeters (TLDs) store radiation energy in crystal defects and release it as light when heated
  • Used for personal and environmental dosimetry
• Film badges contain radiation-sensitive photographic emulsions that darken upon exposure
  • Provide a permanent record of radiation exposure
• Neutron detectors often rely on nuclear reactions that produce charged particles (BF3, 3He, fission chambers)
  • Measure the resulting ionization or scintillation light
• Spectrometry techniques (alpha, beta, gamma) measure the energy distribution of radiation to identify radioactive sources and quantify activity

Applications in Radiochemistry

• Radiotracers are radioactive isotopes used to track chemical and biological processes
  • Used in nuclear medicine for diagnostic imaging (PET, SPECT) and therapy
• Neutron activation analysis (NAA) involves the irradiation of samples with neutrons to induce radioactivity
  • Measures the resulting gamma spectra to determine elemental composition
• Radiometric dating techniques use the decay of radioactive isotopes to determine the age of materials
  • Carbon-14 dating for organic materials, uranium-lead dating for rocks and minerals
• Radiation synthesis and modification of materials, such as the production of polymers, nanomaterials, and pharmaceuticals
  • Utilizes the unique chemical effects of radiation
• Radiation sterilization of medical devices, food, and other products
  • Inactivates microorganisms without leaving harmful residues
• Nuclear forensics applies radiochemical techniques to investigate nuclear incidents, illicit trafficking, and environmental contamination
  • Analyzes the isotopic composition and physical characteristics of nuclear materials
• Radionuclide generators produce short-lived radioactive isotopes for medical and industrial use
  • Molybdenum-99/technetium-99m generator for nuclear medicine imaging

Safety and Shielding

• Time, distance, and shielding are the primary methods for reducing radiation exposure
  • Minimize time spent near sources, maximize distance, and use appropriate shielding materials
• ALARA principle (As Low As Reasonably Achievable) guides radiation protection practices
  • Optimize procedures and equipment to minimize doses while maintaining operational efficiency
• Biological effects of radiation depend on the absorbed dose, dose rate, and radiation type
  • Deterministic effects (skin erythema, cataracts) have a threshold dose and severity increases with dose
  • Stochastic effects (cancer, genetic mutations) have no threshold and probability increases with dose
• Radiation shielding materials attenuate or absorb radiation to reduce exposure
  • Lead, concrete, and steel for gamma and X-rays; hydrogenous materials (water, plastic) for neutrons
• Contamination control involves the containment and removal of radioactive materials from surfaces and environments
  • Protective clothing, decontamination procedures, and proper waste management
• Dosimetry and monitoring programs ensure that radiation exposures are accurately measured and controlled
  • Personal dosimeters (film badges, TLDs), area monitors, and bioassay measurements
• Regulatory agencies (NRC, EPA, IAEA) establish and enforce standards for radiation protection, licensing, and transportation of radioactive materials
  • Ensure the safe and secure use of radiation sources and compliance with regulations