Atomic Structure and Nuclear Forces

Structure of the Atom

• Nucleons are protons and neutrons, which make up the nucleus
• Isotopes are atoms of the same element (same number of protons) but different numbers of neutrons, leading to different masses
• Isotopes of the same element have identical chemical properties
• Isotopes have different physical properties (e.g., mass and stability)
• Electrostatic force is the force between protons due to their positive charge
• Strong nuclear force holds the nucleus together
• Larger atoms (atomic numbers greater than 82) require more neutrons than protons to maintain stability due to increasing electrostatic repulsion between protons

Fundamental Forces

• Strong Nuclear Force acts between quarks and gluons
• The strong nuclear force acts between protons and neutrons, and is responsible for nuclear binding
• The range of the strong nuclear force is ~3-4 femtometers
• The strong nuclear force is strongly attractive at short distances
• The strong nuclear force becomes repulsive at extremely short ranges to prevent the collapse of nuclei
• Electromagnetic Force acts between electrically charged particles (e.g., protons and electrons)
• The electromagnetic force is responsible for the repulsive force between positively charged protons in the nucleus
• The weak nuclear force affects quarks and leptons and is responsible for particle decay
• Beta-minus decay involves a neutron transforming into a proton, emitting an electron and an antineutrino
• Beta-plus decay involves a proton transforms into a neutron, emitting a positron and a neutrino
• Gravity affects all particles with mass, but is negligible at atomic and subatomic scales

Standard Model of Particle Physics

• Quarks are fundamental particles that make up hadrons
• Quarks come in six "flavors": up, down, charm, strange, top, and bottom
• Protons are composed of two up quarks and one down quark (uud)
• Neutrons are composed of one up quark and two down quarks (udd)
• Leptons include electrons, muons, tau particles, and their corresponding neutrinos
• Gauge Bosons are the force carriers for the fundamental forces
• Photons carry the electromagnetic force
• Gluons carry the strong nuclear force
• W and Z bosons carry the weak nuclear force
• The Higgs Boson gives other particles mass through the Higgs field
• Hadrons include baryons and mesons
• Baryons are composed of three quarks (e.g., protons and neutrons)
• Mesons are composed of a quark and an antiquark

Forces in Stable Nuclei

• In stable nuclei, the strong nuclear force and the electrostatic force are balanced
• The strong nuclear force is attractive at short ranges and counteracts the repulsive electrostatic force between protons
• As the size of the nucleus increases, additional neutrons are needed to maintain stability

Radioisotopes and Decay

• Spontaneous Transmutation: A radioisotope nucleus undergoes decay by emitting particles, which changes the identity of the element or isotope
• Artificial Transmutation: Changing the number of protons in a nucleus, often by bombarding it with particles like neutrons
• Alpha decay involves emission of an alpha particle and reduces the atomic number by 2 and the mass number by 4
• Beta-minus (β-) decay involves a neutron transforming into a proton, emitting an electron and an antineutrino
• Beta-plus (β+) decay involves a proton transforming into a neutron, emitting a positron and a neutrino
• Gamma radiation involves released high-energy photons without changing the number of protons or neutrons in the nucleus

Key Concepts in Radiation

• Alpha Particles: Highly ionizing but have low penetration power, and can be stopped by a sheet of paper
• Penetration of Alpha Particles: ~3 cm in air, stopped by paper
• Beta Particles: High-speed electrons and positrons emitted during beta decay
• Penetration of Beta Particles: Can penetrate a few millimeters of aluminium foil
• Gamma Rays: High-energy electromagnetic radiation
• Penetration of Gamma Rays: Can pass through thick lead or several meters of concrete

Decay Equations and Energy Considerations

• Alpha Decay: mass number decreases by 4, and the atomic number decreases by 2
• Beta Decay: A neutron decays into a proton, emitting an electron and an antineutrino
• Rest Energy: The energy equivalent of a particle's mass is given by Einstein's equation: E=mc^2
• m is the rest mass and c is the speed of light
• Half-life is the time it takes for half of the nuclei in a sample of a radioactive substance to decay

Background Radiation

• Average Background Radiation in Australia is ~1.5-2.0 mSv annually
• Average Background Radiation Worldwide is ~2.4 mSv
• Food & Drink account for ~10% of radiation exposure
• Cosmic Rays account for ~20% of radiation exposure
• Rocks, Soil, & Building Materials account for ~20% of radiation exposure
• Medical Radiation (X-rays, CT scans) accounts for ~15% of radiation exposure
• Radon Gas accounts for ~35% of radiation exposure
• Air Travel: ~0.5%
• Nuclear Industry: ~0.5%

Nuclear Energy

• Fission occurs when large nuclei (e.g., Uranium-235) split into smaller nuclei, releasing energy
• Fusion occurs when small nuclei (e.g., hydrogen isotopes) fuse to form a larger nucleus, releasing energy
• Binding energy: The energy required to break a nucleus into its constituent nucleons
• The binding energy per nucleon indicates the stability of the nucleus; a higher binding energy per nucleon means greater stability
• Fuel Rods contain enriched uranium or plutonium
• Moderator is a material (e.g., water, graphite) that slows down neutrons to increase the chance of fission
• Control Rods are made of materials like boron or cadmium, these absorb neutrons to control the rate of the fission chain reaction
• Coolant is typically water, heavy water, or liquid sodium, used to remove heat from the reactor core
• Shielding such as lead or concrete protects workers and the environment from harmful radiation

Induced Fission

• To achieve a controlled nuclear reaction, the probability of fission events must be high enough, and neutrons must be moderated to sustain the chain reaction
• Challenges in induced fission includes ensuring adequate neutron flux, maintaining neutron control, and safety