🪐Principles of Physics IV Unit 13 – Nuclear Reactions and Stability

Key Concepts and Terminology

• Atomic nucleus contains protons and neutrons held together by the strong nuclear force
• Isotopes are atoms of the same element with different numbers of neutrons
• Mass defect is the difference between the mass of an atomic nucleus and the sum of the masses of its constituent protons and neutrons
• Binding energy is the energy required to break apart a nucleus into its constituent protons and neutrons
  • Calculated using Einstein's equation: $E = mc^2$
• Radioactive decay is the spontaneous emission of particles or energy from an unstable atomic nucleus
• Half-life is the time required for half of a given quantity of a radioactive substance to decay

Types of Nuclear Reactions

• Alpha decay involves the emission of an alpha particle (two protons and two neutrons) from a nucleus
  • Reduces the atomic number by 2 and the mass number by 4
• Beta decay involves the emission of a beta particle (electron or positron) from a nucleus
  • Beta minus ($\beta^-$) decay converts a neutron into a proton, increasing the atomic number by 1
  • Beta plus ($\beta^+$) decay converts a proton into a neutron, decreasing the atomic number by 1
• Gamma decay involves the emission of high-energy photons (gamma rays) from a nucleus
  • Occurs when a nucleus transitions from a higher to a lower energy state
• Neutron capture occurs when a nucleus absorbs a neutron, increasing its mass number by 1
• Nuclear fission is the splitting of a heavy nucleus into lighter nuclei
• Nuclear fusion is the combining of light nuclei to form a heavier nucleus

Nuclear Stability and Decay

• Nuclear stability depends on the ratio of protons to neutrons in a nucleus
• Stable nuclei have a specific range of proton-to-neutron ratios
  • For light nuclei (low atomic number), the most stable ratio is approximately 1:1
  • For heavier nuclei, the most stable ratio has more neutrons than protons
• Nuclei outside the stable range are radioactive and undergo decay to reach a more stable configuration
• The nuclear shell model explains nuclear stability based on the arrangement of protons and neutrons in shells
  • Nuclei with completely filled shells (magic numbers) are more stable
• Radioactive decay follows an exponential relationship, with the decay rate proportional to the number of remaining unstable nuclei

Radioactivity and Half-Life

• Radioactivity is the emission of particles or energy from an unstable atomic nucleus
• The activity of a radioactive sample is the number of decays per unit time
  • Measured in becquerels (Bq), where 1 Bq = 1 decay per second
• The half-life ($t_{1/2}$) is the time required for half of a given quantity of a radioactive substance to decay
  • Calculated using the equation: $t_{1/2} = \frac{\ln(2)}{\lambda}$, where $\lambda$ is the decay constant
• The decay constant ($\lambda$) is the probability of a single atom decaying per unit time
• The remaining amount of a radioactive substance after a given time can be calculated using the equation: $N(t) = N_0e^{-\lambda t}$
  • $N(t)$ is the amount at time $t$, $N_0$ is the initial amount, and $t$ is the elapsed time

Nuclear Fission and Fusion

• Nuclear fission is the splitting of a heavy nucleus into lighter nuclei
  • Typically induced by the absorption of a neutron
  • Releases energy and additional neutrons, which can lead to a chain reaction
  • Used in nuclear power plants and atomic bombs
• Nuclear fusion is the combining of light nuclei to form a heavier nucleus
  • Requires extremely high temperatures and pressures to overcome the electrostatic repulsion between nuclei
  • Releases large amounts of energy
  • Occurs naturally in stars and is the goal of controlled fusion for energy production
• Both fission and fusion reactions convert mass into energy according to Einstein's equation: $E = mc^2$

Energy Release in Nuclear Reactions

• Nuclear reactions release energy due to the conversion of mass into energy
• The energy released is proportional to the mass defect, as described by Einstein's equation: $E = mc^2$
  • $E$ is the energy released, $m$ is the mass converted to energy, and $c$ is the speed of light
• The binding energy of a nucleus determines its stability and the energy released in nuclear reactions
  • Nuclei with higher binding energies per nucleon are more stable
• Fission reactions release energy when heavy nuclei split into lighter nuclei with higher binding energies per nucleon
• Fusion reactions release energy when light nuclei combine to form heavier nuclei with higher binding energies per nucleon
• The energy released in nuclear reactions is much greater than that released in chemical reactions

Applications and Real-World Examples

• Nuclear power plants use controlled fission reactions to generate electricity
  • Fission of uranium-235 or plutonium-239 heats water to produce steam, which drives turbines
• Radioisotopes are used in medical imaging and therapy
  • Technetium-99m is used in bone scans and other diagnostic procedures
  • Iodine-131 is used to treat thyroid disorders
• Carbon dating uses the radioactive decay of carbon-14 to determine the age of organic materials
• Smoke detectors use the ionizing radiation from americium-241 to detect smoke particles
• Nuclear fusion is the power source of stars, including the Sun
• Researchers are working on developing controlled fusion reactors for clean and abundant energy production

Safety and Environmental Considerations

• Radiation exposure can cause health effects, including an increased risk of cancer
• The International Commission on Radiological Protection (ICRP) sets limits on radiation exposure for workers and the public
• Nuclear waste, including spent fuel and contaminated materials, must be properly managed and disposed of
  • High-level waste requires long-term storage in deep geological repositories
• Nuclear accidents, such as Chernobyl and Fukushima, can have severe environmental and health consequences
• The proliferation of nuclear weapons is a global security concern
  • The Treaty on the Non-Proliferation of Nuclear Weapons (NPT) aims to prevent the spread of nuclear weapons
• Nuclear power plants have safety systems and protocols to prevent accidents and mitigate their consequences
  • Containment structures, emergency shutdown systems, and redundant cooling systems