9. Nuclear Physics

• Changes in Atomic Models:

  • In 1803, Dalton believed everything was made up of indivisible particles
  • In 1897, atoms were thought to be positive bodies of matter with negative electrons (plum pudding model by Thompson)
  • In 1911, Rutherford discovered atoms were mostly empty space with electrons orbiting them
  • In 1913, it was discovered that electrons orbit in shells and emit EM radiation when moving between them
  • In 1919, the proton and neutron were discovered
  • Current belief: electrons exist in subshells and nucleons are made up of quarks (quantum mechanics)
• Plum Pudding Model:

  • Described the atom as a ball of positive charge with negatively charged electrons evenly distributed throughout it
• Rutherford Scattering:

  • Most alpha particles passed through with no deflection, suggesting the atom is mostly empty space
  • Some deflected by large angles, indicating a positively charged nucleus
  • A few deflected by more than 90 degrees, suggesting a very small but dense central nucleus
  • Disproved the Plum Pudding model
• Alpha Radiation:

  • Consists of two protons and two neutrons (helium nucleus)
  • Strongly ionising, slow moving, stopped by a few centimeters of air or paper
  • Positively charged, deflected in a magnetic field
  • Emitted a light spectrum identical to that of Helium
  • Used in smoke detectors
• Beta Radiation:

  • Beta-minus radiation consists of an electron, beta-plus radiation consists of a positron
  • Mildly ionising, fast moving, stopped by a few millimeters of aluminum
  • Negatively charged, deflected in a magnetic field
  • Existence proved by deflecting them in magnetic and electric fields
  • Used in thickness monitors
• Gamma Radiation:

  • Form of electromagnetic radiation, weakly ionising, travels at the speed of light
  • Stopped by a few several centimeters of lead or a few meters of concrete
  • Chargeless, unaffected by magnetic and electric fields
  • Used to sterilise medical equipment, kill cancerous cells, and as a medical tracer
• Safe Use of Radiation:

  • Procedures: never directly handle the source, use long armed tongs, display warning signage, minimize time of use, store in a lead box
• Background Radiation:

  • Originates from radon gas, rocks, cosmic radiation, nuclear weapon testing, and nuclear disasters
  • Present in very small quantities, corrected for in experiments
• Radioactive Decay:

  • Random process, activity measured in Becquerels, half-life, decay constant
• Distance of Closest Approach:

  • Method to calculate nuclear radius by converting kinetic energy to electric potential energy
• Electron Diffraction:

  • Method to determine nuclear radius using high-speed electrons that diffract through atoms
• Nuclear Fission:

  • Splitting of a large nucleus to produce two smaller nuclei, neutrons, and energy
  • Spontaneous fission is rare, commonly used fissile isotope is Uranium-235
• Nuclear Reactors:

  • Roles of constituents: control rods, moderator, fuel rods, coolant
• Nuclear Waste:

  • Procedures for safe disposal: cooling ponds, thick concrete containers, reinforced containers during transport, remote handling
• Nuclear Fusion:

  • Fusing of two smaller nuclei to form a single large nucleus and produce energy
• Mass Defect and Binding Energy:

  • Binding energy, mass defect, atomic mass unit, E=mc²
• Coulomb's Law applies here as energy:

  • have to assume that the particles will start stationary
  • have to assume the particles start as close to the nucleus as possible
• • Protons experience both electrostatic and gravitational forces
  • The strong nuclear force keeps the nucleus together

  A) typical nucleon separation

  
• Mass of a nucleus is found using a Mass Spectrometer

  • The nucleus passes through the slit into the mass spectrometer
  • Magnetic force acts upon the nucleus and causes circular motion
• Radius:

  $R=$$R_0A^*$$(1/3)$

  Where A = mass number and R0 = 1.05 fm or 1.4 fm (will be given in the question)
• Ionisation of gases is used by the Geiger tube to create a small charge and hence enable the detection of radiation

  • A charged particle moved within a magnetic field feels a force
• Radioactive sources:

  • Alpha AND gamma
  • Beta AND gamma
• Half life: the time that it would take for the mass/radioactive nuclei to decrease to half of the original value
• There is a 50% decay for each half life
• Decay constant = the probability of an individual nucleus decaying per second
• The bigger the half life, the smaller the decay constant is
• Proton/Neutron Ratio:

  • N = Z --> Stability line
  • Light isotopes up to Z = 20
  • Mass is an average of the abundance of the isotopes
  • Neutrons balance the repulsion of protons
• Beta+ Decay/Electron Capture:

  • Transforms a proton into a neutron
  • Diagonally up and left on the N-Z graph (north west)
• Beta- Decay:

  • Right and down on the N-Z graph (south east)
• Alpha Decay:

  • Diagonally to the bottom left (south west)
• • Isotopes decay, however the result may not be stable
  • Will undergo further decays to become stable
  • This is the Radioactive Series
  • Lead-208 is the last stable isotope in the periodic table
• In Industry, a number of properties are considered:

  • The half life of an isotope
  • The stability of the daughter isotope
  • Toxicity and the biochemical suitability of the parent/daughter nuclei
  • The type of the radiation that is emitted
• Radioactive Carbon Dating:

  • Upper atmosphere: cosmic rays can knock neutrons out of nuclei, which can interact with the nitrogen-14 atoms to create Carbon-14
  • Radioactive with a half-life of 5570 years
  • Is absorbed by some plants and enters the food chain
  • When something dies, the proportion of Carbon-14 reduces overtime
• There are different kinds of dating as the further back you go, the age of something becomes more uncertain
• Argon Dating:

  • Argon-40 can also be used
  • The half life is 1250 million years
  • This is formed via electron capture
• Radioactive Tracers:

  • Have a list of required properties that an isotope needs
  • Technetuim-99 has a half life of 6 hours and is a gamma emitter
  • It has an inert daugher isotope that has a very long half life and hence a low activity
  • Tracers are used to follow the path of a substance through a system
  • Has a half life stable enough for measurements to be made
• Remote Power Sources:

  • Gives out energy used to power sources via a Radioactive Thermal Generator
  • Needs to have a half life long enough to sustain the machine
  • Not too long of a half life that too much material is needed due to low activity

  P = AE
• Ionising Radiation:

  • Is anything that can cause electrons to be removed from atoms, creating ions
  • This process is harmful to living cells by either killing them or damaging DNA which causes mutations that can be passed to future cell generations and cause cancer
• These include:

  • X-Rays
  • Gamma
  • Alpha
  • Beta
  • Protons
  • Neutrons
• Monitoring Radiation:

  • Workers in the nuclear industry have to wear a film badge to monitor how much radiation they recieve
  • The badge contains a photographic film in a light proof case that will blacken if radiation strikes it
  • Also contains three different thicknesses of material, as well as different metals, that allow the user to know what radiation type has been absorbed