🫴Physical Science Unit 14 – Nuclear Physics and Radioactivity

Atomic Structure Basics

• Atoms consist of a dense, positively charged nucleus surrounded by a cloud of negatively charged electrons
• The nucleus contains protons (positively charged) and neutrons (electrically neutral), while electrons orbit the nucleus in shells
• Atomic number represents the number of protons in an atom's nucleus and determines the element's identity
• Mass number is the sum of the number of protons and neutrons in an atom's nucleus
• Isotopes are atoms of the same element with different numbers of neutrons, resulting in varying mass numbers
  • For example, carbon-12 and carbon-14 are isotopes of carbon with 6 and 8 neutrons, respectively
• Electrons occupy discrete energy levels or shells around the nucleus, with each shell having a specific electron capacity
• The distribution of electrons in these shells determines an atom's chemical properties and bonding behavior

Nuclear Physics Fundamentals

• Nuclear physics focuses on the study of the atomic nucleus, its structure, and the interactions between nucleons (protons and neutrons)
• Strong nuclear force is the fundamental force that binds protons and neutrons together within the nucleus, overcoming the electrostatic repulsion between positively charged protons
• Binding energy is the energy required to break apart a nucleus into its constituent protons and neutrons
  • Binding energy per nucleon varies with the mass number, with the most stable nuclei having the highest binding energy per nucleon (iron-56)
• Nuclear stability depends on the ratio of protons to neutrons in the nucleus
  • Stable nuclei typically have a 1:1 ratio of protons to neutrons for lighter elements and a slightly higher neutron-to-proton ratio for heavier elements
• Radioactivity is the spontaneous emission of radiation from an unstable atomic nucleus
• Nuclear reactions involve changes in the composition or energy of atomic nuclei, such as fusion (combining light nuclei) and fission (splitting heavy nuclei)

Types of Radioactivity

• Alpha decay involves the emission of an alpha particle (two protons and two neutrons) from the nucleus
  • Alpha particles have a positive charge and are relatively heavy, limiting their penetration power
• Beta decay occurs when a neutron transforms into a proton, emitting an electron (beta minus particle) and an antineutrino, or a proton transforms into a neutron, emitting a positron (beta plus particle) and a neutrino
  • Beta particles have a negative or positive charge and are more penetrating than alpha particles
• Gamma decay involves the emission of high-energy photons (gamma rays) from an excited nucleus as it transitions to a lower energy state
  • Gamma rays have no charge or mass and are highly penetrating
• Neutron emission can occur in some heavy, unstable nuclei, releasing a neutron from the nucleus
• Electron capture is a process where a proton captures an inner shell electron, converting into a neutron and emitting a neutrino
  • This process is sometimes accompanied by the emission of characteristic X-rays

Radioactive Decay Processes

• Radioactive decay is a random process where an unstable nucleus releases energy in the form of radiation to achieve a more stable configuration
• The decay process follows an exponential relationship, with the rate of decay proportional to the number of unstable nuclei present
• Decay constant (λ) is the probability of a single atom decaying per unit time and is unique to each radioactive isotope
• Activity (A) represents the number of decays per unit time and is measured in becquerels (Bq) or curies (Ci)
  • Activity decreases exponentially over time as the number of unstable nuclei decreases
• Decay equations describe the relationship between the initial number of unstable nuclei (N₀), the number remaining after time t (N), and the decay constant (λ):
  • $N = N₀e^{-λt}$
  • $A = A₀e^{-λt}$, where A₀ is the initial activity
• Radioactive decay series involve a sequence of decays from a parent isotope through various daughter isotopes until a stable isotope is reached
  • Examples include the uranium-238 and thorium-232 decay series

Half-Life and Decay Rates

• Half-life (t₁/₂) is the time required for half of a given quantity of a radioactive isotope to decay
  • Half-life is a characteristic property of each radioactive isotope and is independent of the initial amount present
• The relationship between half-life and decay constant is given by: $t_{1/2} = \frac{\ln(2)}{\lambda} \approx \frac{0.693}{\lambda}$
• After one half-life, 50% of the original radioactive material remains; after two half-lives, 25% remains; after three half-lives, 12.5% remains, and so on
• The number of half-lives elapsed can be calculated using the equation: $n = \frac{t}{t_{1/2}}$, where n is the number of half-lives, t is the elapsed time, and t₁/₂ is the half-life
• Decay rates can be used to determine the age of objects through radiometric dating techniques
  • For example, carbon-14 dating is used to date organic materials up to approximately 50,000 years old

Nuclear Reactions and Fission

• Nuclear reactions involve changes in the composition or energy of atomic nuclei
• Fusion reactions combine light nuclei to form heavier nuclei, releasing energy in the process
  • Fusion is the primary energy source in stars and is being researched for potential use in fusion power reactors
• Fission reactions involve the splitting of heavy nuclei into lighter fragments, releasing energy and neutrons
  • Fission can be induced by bombarding heavy nuclei (such as uranium-235) with neutrons
• Chain reactions occur when the neutrons released from one fission event trigger additional fission events in nearby nuclei
  • Controlled chain reactions are used in nuclear power plants to generate electricity
  • Uncontrolled chain reactions can result in a rapid release of energy, as in nuclear weapons
• Critical mass is the minimum amount of fissionable material required to sustain a chain reaction
• Nuclear reactors use moderators (such as water or graphite) to slow down neutrons and control the rate of fission, while control rods (made of neutron-absorbing materials) are used to regulate the reactor's power output

Radiation Detection and Measurement

• Radiation detectors are devices used to measure and quantify ionizing radiation
• Geiger-Müller counters detect ionizing radiation by measuring the electrical pulses created when radiation interacts with a gas-filled tube
  • GM counters are sensitive to alpha, beta, and gamma radiation but do not distinguish between them
• Scintillation detectors use materials that emit light when exposed to ionizing radiation, which is then converted into an electrical signal by a photomultiplier tube
  • Scintillation detectors are efficient at detecting gamma rays and can provide information about the energy of the radiation
• Semiconductor detectors (such as silicon or germanium) operate by measuring the electrical charges created when radiation interacts with the semiconductor material
  • Semiconductor detectors offer excellent energy resolution and are used for precise measurements of radiation energy
• Radiation dosimetry involves measuring the amount of radiation absorbed by an object or person
  • Absorbed dose is measured in grays (Gy), which represents the energy absorbed per unit mass (1 Gy = 1 J/kg)
  • Equivalent dose takes into account the biological effects of different types of radiation and is measured in sieverts (Sv)
• Radiation shielding uses materials (such as lead, concrete, or water) to reduce the intensity of radiation passing through them, protecting people and equipment from excessive exposure

Applications and Implications

• Nuclear power plants generate electricity by harnessing the energy released from controlled nuclear fission reactions
  • Nuclear power provides a low-carbon alternative to fossil fuels but raises concerns about safety and waste management
• Radioisotopes are used in various medical applications, such as diagnostic imaging (e.g., positron emission tomography or PET scans) and cancer treatment (radiation therapy)
• Radioactive tracers are used in industrial and scientific applications to study the flow of materials, monitor wear and tear, and detect leaks or flaws
• Carbon-14 dating is a radiometric dating method used to determine the age of organic materials up to approximately 50,000 years old
  • Other radiometric dating methods (such as uranium-lead dating) are used to date rocks and minerals on geological timescales
• Radiation exposure can have detrimental effects on living organisms, causing DNA damage, cell death, and an increased risk of cancer
  • The severity of the effects depends on the type and amount of radiation, as well as the duration of exposure
• Nuclear weapons rely on uncontrolled fission or fusion reactions to release immense amounts of energy in a short period, causing devastating blast, heat, and radiation effects
• The study of nuclear physics has led to advancements in our understanding of the fundamental structure of matter and the forces that govern the universe