☢️Radiochemistry Unit 1 – Radiochemistry: Intro to Radioactivity

Key Concepts and Definitions

• Radioactivity the spontaneous emission of radiation from unstable atomic nuclei
• Radioactive decay the process by which an unstable atomic nucleus loses energy by emitting radiation
• Half-life the time required for a quantity of a radioactive substance to decay to half of its initial value
• Ionizing radiation radiation with sufficient energy to ionize atoms or molecules (alpha, beta, and gamma radiation)
• Radioisotopes isotopes of an element that are radioactive and undergo radioactive decay
• Specific activity the rate of decay of a radioactive substance per unit mass, measured in becquerels per gram (Bq/g) or curies per gram (Ci/g)
• Becquerel (Bq) the SI unit of radioactivity, defined as one nuclear decay per second
  • Curie (Ci) a non-SI unit of radioactivity, defined as 3.7 × 10^10 decays per second (the activity of 1 gram of radium-226)

Historical Background

• Henri Becquerel discovered radioactivity in 1896 while studying phosphorescent materials (uranium salts)
• Marie Curie coined the term "radioactivity" and discovered the radioactive elements polonium and radium
• Ernest Rutherford classified radiation into alpha, beta, and gamma types based on their penetrating power and ability to be deflected by magnetic fields
• Frederick Soddy and Kasimir Fajans independently discovered the concept of isotopes and the radioactive displacement law
• James Chadwick discovered the neutron in 1932, completing the modern understanding of atomic structure
• Enrico Fermi and his team achieved the first artificial radioactivity by bombarding elements with neutrons in 1934
• The Manhattan Project during World War II led to the development of nuclear weapons and the first controlled nuclear chain reaction

Types of Radioactivity

• Alpha decay emission of an alpha particle (two protons and two neutrons, equivalent to a helium-4 nucleus) from an atomic nucleus
  • Alpha particles have a positive charge and are highly ionizing but have low penetrating power
• Beta decay emission of a beta particle (an electron or positron) from an atomic nucleus
  • Beta minus ($\beta^-$) decay involves the emission of an electron and an antineutrino, resulting from the conversion of a neutron into a proton
  • Beta plus ($\beta^+$) decay involves the emission of a positron and a neutrino, resulting from the conversion of a proton into a neutron
• Gamma decay emission of a gamma ray (high-energy photon) from an atomic nucleus
  • Gamma rays have no charge, are highly penetrating, and often accompany alpha or beta decay
• Electron capture a proton captures an inner shell electron, converting into a neutron and emitting a neutrino
• Spontaneous fission the spontaneous splitting of a heavy atomic nucleus into two smaller fragments, releasing neutrons and energy

Radioactive Decay Processes

• Radioactive decay is a random process governed by probability and follows first-order kinetics
• The decay rate is proportional to the number of radioactive nuclei present and is characterized by the decay constant ($\lambda$)
• The half-life ($t_{1/2}$) is related to the decay constant by $t_{1/2} = \ln(2) / \lambda$
• The activity of a radioactive sample decreases exponentially over time according to the equation $A(t) = A_0 e^{-\lambda t}$, where $A_0$ is the initial activity
• Secular equilibrium occurs when the half-life of the parent isotope is much longer than that of the daughter isotope, resulting in equal activities of both isotopes
• Transient equilibrium occurs when the half-life of the parent isotope is longer than that of the daughter isotope, but not by a large margin
• No equilibrium occurs when the half-life of the parent isotope is shorter than that of the daughter isotope

Measuring Radioactivity

• Geiger-Müller counters detect ionizing radiation by the ionization of a gas in a sealed tube, producing an electrical pulse
  • GM counters are sensitive but cannot distinguish between different types of radiation or their energies
• Scintillation counters detect ionizing radiation by the production of light in a scintillator material, which is then converted to an electrical signal by a photomultiplier tube
  • Scintillation counters can distinguish between different types of radiation and their energies based on the intensity and duration of the light pulses
• Semiconductor detectors (germanium or silicon) detect ionizing radiation by the creation of electron-hole pairs in a semiconductor material, producing an electrical signal
  • Semiconductor detectors offer high energy resolution and are used for gamma-ray spectroscopy
• Liquid scintillation counting is used for measuring low-energy beta emitters (such as tritium or carbon-14) in liquid samples
• Autoradiography uses photographic film or phosphor screens to visualize the spatial distribution of radioactivity in a sample

Applications in Chemistry and Beyond

• Radiometric dating techniques (carbon-14, uranium-lead, potassium-argon) are used to determine the age of rocks, fossils, and archaeological artifacts
• Radiotracers are used to study chemical reactions, metabolic pathways, and environmental processes by tracking the movement of labeled compounds
• Positron emission tomography (PET) uses radiotracers to produce 3D images of metabolic processes in the body for medical diagnosis and research
• Radiation therapy uses targeted high-energy radiation (gamma rays or X-rays) to destroy cancer cells while minimizing damage to healthy tissue
• Food irradiation uses gamma radiation to sterilize and preserve food by killing bacteria and other pathogens
• Radioisotope thermoelectric generators (RTGs) use the heat generated by radioactive decay to produce electricity for spacecraft and remote installations
• Smoke detectors use a small amount of americium-241 to ionize the air in a chamber, allowing for the detection of smoke particles

Safety and Handling Procedures

• The three principles of radiation protection are time, distance, and shielding
  • Minimize the time spent in proximity to a radioactive source
  • Maximize the distance from a radioactive source, as radiation intensity decreases with the square of the distance (inverse square law)
  • Use appropriate shielding materials (lead, concrete, water) to reduce exposure to radiation
• Always wear personal protective equipment (PPE) when handling radioactive materials, including lab coats, gloves, and dosimeters
• Use fume hoods or glove boxes when working with volatile or dispersible radioactive materials to prevent inhalation or contamination
• Properly label and store radioactive materials in designated areas, with appropriate shielding and security measures
• Monitor work areas and personnel regularly for contamination using Geiger counters or other suitable detectors
• Dispose of radioactive waste according to institutional, state, and federal regulations, using appropriate containers and labeling
• Maintain accurate records of radioactive material inventory, use, and disposal for regulatory compliance and safety audits

Current Research and Future Directions

• Targeted alpha therapy (TAT) uses alpha-emitting radioisotopes conjugated to targeting molecules (such as antibodies) for precise cancer treatment with minimal side effects
• Theranostics combines diagnostic imaging and targeted radiotherapy, using the same molecular target for both purposes (e.g., gallium-68 for imaging and lutetium-177 for therapy)
• Nanoparticle-based radiopharmaceuticals offer improved biodistribution, pharmacokinetics, and targeting compared to traditional small-molecule radiotracers
• Radioimmunotherapy (RIT) uses radiolabeled antibodies to deliver radiation directly to cancer cells expressing specific antigens, minimizing damage to healthy tissue
• In situ beta radiation therapy uses implantable devices containing beta-emitting radioisotopes (such as yttrium-90) for localized treatment of solid tumors
• Accelerator-driven subcritical reactors (ADSRs) are being developed as a safer and more efficient alternative to traditional nuclear reactors, using a particle accelerator to drive the fission process
• Radiation-induced chemical reactions are being explored for the synthesis of novel materials, such as polymers and nanostructures, under mild conditions