☢️Radiochemistry Unit 4 – Radiation Detection and Measurement

Key Concepts and Terminology

• 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 but can cause excitation (radio waves, microwaves, visible light)
• Activity is the rate of decay of a radioactive substance, measured in becquerels ($Bq$) or curies ($Ci$)
  • $1 Bq = 1$ decay per second
  • $1 Ci = 3.7 \times 10^{10}$ decays per second
• Half-life 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 quantifies the energy deposited in a medium per unit mass, measured in grays ($Gy$) or rads
  • $1 Gy = 1$ joule per kilogram
  • $1 rad = 0.01 Gy$

Types of Radiation

• Alpha particles consist of two protons and two neutrons (helium nucleus), emitted from heavy nuclei during radioactive decay
  • Highly ionizing but short range, easily stopped by a sheet of paper or skin
• Beta particles are high-energy electrons or positrons emitted from nuclei during radioactive decay
  • Less ionizing than alpha particles but longer range, can penetrate skin and be stopped by a few millimeters of aluminum
• Gamma rays are high-energy electromagnetic radiation emitted from excited nuclei during radioactive decay or nuclear reactions
  • Highly penetrating, require dense materials like lead or concrete for shielding
• X-rays are similar to gamma rays but originate from electron transitions in atoms rather than nuclear processes
• Neutron radiation occurs when neutrons are ejected from nuclei during fission or fusion reactions
  • Can penetrate deeply into matter and cause activation of stable nuclei

Radiation Detection Principles

• Radiation detectors convert the energy deposited by radiation into measurable signals, such as electrical pulses or light
• Gas-filled detectors (ionization chambers, proportional counters, Geiger-Müller tubes) rely on the ionization of gas molecules by radiation
  • Applied voltage collects the generated charges, producing a measurable electrical signal
• Scintillation detectors use materials that emit light when excited by radiation (NaI, CsI, organic scintillators)
  • Light is converted to electrical signals by photomultiplier tubes or photodiodes
• Semiconductor detectors (silicon, germanium) operate based on the creation of electron-hole pairs in the semiconductor material by radiation
  • Applied voltage sweeps the charge carriers, generating an electrical signal proportional to the energy deposited
• Neutron detectors often rely on nuclear reactions that produce charged particles, which are then detected by conventional means
  • Examples include boron trifluoride ($BF_3$) and helium-3 ($^3He$) proportional counters

Common Detection Instruments

• Geiger counters are simple, portable instruments that detect ionizing radiation using a Geiger-Müller tube
  • Produce audible clicks and display count rate but provide no energy information
• Scintillation detectors, such as sodium iodide (NaI) detectors, are widely used for gamma-ray spectroscopy
  • Offer good efficiency and energy resolution, allowing identification of radioactive isotopes
• High-purity germanium (HPGe) detectors provide the highest energy resolution for gamma-ray spectroscopy
  • Require cooling to liquid nitrogen temperatures for optimal performance
• Liquid scintillation counters are used for detecting low-energy beta emitters, such as tritium and carbon-14
  • Sample is mixed with a scintillation cocktail, and the light output is measured
• Thermoluminescent dosimeters (TLDs) are passive devices that measure accumulated radiation dose
  • Heating the TLD causes it to emit light proportional to the absorbed dose

Measurement Techniques and Units

• Count rate is the number of radiation events detected per unit time, often expressed in counts per minute (cpm) or counts per second (cps)
• Activity concentration is the activity per unit volume or mass of a sample, typically expressed in $Bq/L$ or $Bq/kg$
• Efficiency calibration determines the relationship between the count rate and the activity of a source
  • Accounts for factors such as detector geometry, absorption, and backscattering
• Energy calibration establishes the relationship between the detector's response and the energy of the incident radiation
  • Allows for the identification of specific radioisotopes based on their characteristic gamma-ray energies
• Dose rate is the absorbed dose per unit time, commonly expressed in $Gy/h$ or $Sv/h$ (sieverts per hour)
• Dose equivalent (in sieverts) accounts for the biological effectiveness of different types of radiation
  • Calculated by multiplying the absorbed dose by a quality factor specific to the radiation type

Data Analysis and Interpretation

• Spectrum analysis involves identifying peaks in a gamma-ray spectrum and determining their energies and intensities
  • Allows for the identification and quantification of radioactive isotopes in a sample
• Background subtraction removes the contribution of natural background radiation from the measured spectrum
  • Improves the accuracy of quantitative analysis and lowers detection limits
• Decay correction adjusts the measured activity to account for the radioactive decay that occurred between the time of sample collection and the time of measurement
• Minimum detectable activity (MDA) is the smallest activity that can be reliably detected by a given measurement system
  • Depends on factors such as background level, counting time, and detector efficiency
• Uncertainty analysis quantifies the random and systematic errors associated with a measurement
  • Includes statistical counting uncertainties and uncertainties in calibration, sample preparation, and other factors

Safety Protocols and Shielding

• Time, distance, and shielding are the three primary methods for reducing radiation exposure
  • Minimizing time spent near a source, maximizing distance, and using appropriate shielding materials
• ALARA (As Low As Reasonably Achievable) principle guides radiation protection practices
  • Aims to keep exposures as low as possible, considering economic and societal factors
• Dosimetry monitoring, such as film badges or electronic dosimeters, tracks individual radiation exposures
  • Ensures compliance with regulatory limits and helps identify potential overexposures
• Proper handling and storage of radioactive materials, including the use of glove boxes and ventilation systems
  • Prevents the spread of contamination and minimizes inhalation or ingestion hazards
• Shielding materials are chosen based on the type and energy of the radiation
  • Lead is effective for gamma rays, while hydrogenous materials (e.g., water, plastic) are used for neutron shielding

Applications in Radiochemistry

• Radiotracers are used to study chemical and biological processes by labeling molecules with radioactive isotopes
  • Examples include carbon-14 for metabolic studies and technetium-99m for medical imaging
• Neutron activation analysis (NAA) is a sensitive technique for determining elemental composition
  • Sample is irradiated with neutrons, and the resulting radioactivity is measured to identify and quantify elements
• Radiometric dating uses the decay of radioactive isotopes to determine the age of materials
  • Carbon-14 dating is used for organic materials, while uranium-lead dating is used for rocks and minerals
• Nuclear medicine utilizes radioactive isotopes for diagnostic imaging and targeted therapy
  • Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are common imaging modalities
• Environmental monitoring assesses the presence and impact of radioactivity in the environment
  • Includes monitoring of air, water, soil, and biota for natural and anthropogenic radionuclides
• Nuclear forensics applies radiochemical techniques to investigate nuclear materials and events
  • Helps in identifying the origin and history of nuclear materials and in responding to nuclear incidents