Nuclear Physics

• Rutherford's alpha scattering experiment involved firing alpha particles to a thin gold sheet of foil. The results were

  • Most went straight through- the atom was mostly empty space
  • Few deflected at angles- the nucleus is positively charged
  • Very few deflected straight back- the nucleus is very dense and contains most of the atoms' mass
• Radioactive waste- there are 3 levels

  • High level waste
  • Intermediate level waste
  • Low level waste
• Radiation is where an unstable nucleus emits energy in the form of EM waves or subatomic particles in order to become more stable
• Rutherford scattering demonstrated the existence of a nucleus
• Before the alpha scattering experiment, scientists believed in Thomson’s plum pudding model
• Thomson's plum pudding model described the atom as a sphere of positive charge with small areas of negative charge evenly distributed throughout
• Rutherford scattering led to the production of a new model for the atom known as the nuclear model
• Rutherford's apparatus included an alpha source and gold foil in an evacuated chamber covered in a fluorescent coating
• Types of radiation:

  • Alpha radiation: range in air 2-10 cm, highly ionising, deflected by electric and magnetic fields, absorbed by paper
  • Beta radiation: range around 1 m, weakly ionising, deflected by electric and magnetic fields, absorbed by aluminium foil
  • Gamma radiation: infinite range, very weakly ionising, not deflected by electric and magnetic fields, absorbed by several metres of concrete or several inches of lead
• Identifying radiation types experiment:

  1. Find background count with no source present

  2. Measure count rate with source close to GM tube

  3. Place paper between source and GM tube, if count rate decreases significantly, source emits alpha radiation

  4. Repeat with aluminium foil and lead to identify beta and gamma radiation
• Uses of gamma radiation in medicine:

  • As a detector
  • To sterilise surgical equipment
  • In radiation therapy to kill cancerous cells
• Radioactive decay:

  • Random process
  • Decay constant (λ) is the probability of a nucleus decaying per unit time
  • Exponential decay formula: N = N0e^(-λt)
  • Half-life (T1/2) is the time taken for the number of nuclei to halve
• Nuclear instability reasons for decay:

  1. Too many neutrons - decays through beta-minus emission

  2. Too many protons - decays through beta-plus emission or electron capture

  3. Too many nucleons - decays through alpha emission

  4. Too much energy - decays through gamma emission
• Nuclear radius estimation:

  • Estimate by calculating the distance of closest approach of a charged particle to the nucleus
• Electric potential energy is equal to the initial kinetic energy of a particle at its distance of closest approach
• The equation for electric potential is V = 1 / (4πε₀) * r * Q
• Electron diffraction is a method for calculating nuclear radius accurately
• The De Broglie wavelength of electrons is used to estimate nuclear radius
• The relationship between nuclear radius and nucleon number can be found by plotting a log graph
• The binding energy of a nucleus is the energy required to separate it into its constituents
• Nuclear fission is the splitting of a large nucleus into two daughter nuclei
• Nuclear fusion is the joining of two smaller nuclei to form a larger nucleus
• The critical mass is the minimum mass of fuel required to maintain a steady chain reaction
• A nuclear reactor includes a moderator, control rods, and a coolant
• Enriched uranium is used as fuel in nuclear reactors
• Control rods are used to absorb neutrons in a reactor to control chain reactions
• Spent fuel rods are considered high-level nuclear waste
• Radioactive waste must be stored in geologically stable locations for thousands of years
• Nuclear power stations produce no polluting gases but generate radioactive waste
• Benefits of using nuclear power

  • Nuclear releases less CO2 resulting in cleaner air
  • Nuclear releases less CO2 so doesn’t contribute heavily to global warming
  • Small amount of fuel can produce a large amount of energy
  • Nuclear power can be produced continuously
  • Some nuclear power stations can quickly adjust their output
  • Producing medical isotopes can help the medical field
• Risks of nuclear power

  • safety concerns - nuclear meltdowns+ radiation causing severe long lasting effects
  • nuclear waste- needs to be treated and disposed of properly
  • The fuel used has a very long half life so need to be stored in geologically stable locations
• Examples of nuclear waste

  • High Level = unstable nuclei from fission of U-235 & spent fuel rods
  • Medium level = low level radioactive material and containers
  • Low level = Lab equipment and protective clothing
• How to dispose of high level waste

  1. Store it under water for a decade or so
  2. Uranium and plutonium can be recycled
  3. The rest is stored underground in steel containers
• How to dispose of medium level waste

  • Encased in concrete and stored in specialist buildings
  • Need reinforced concrete walls
• How to dispose of low level waste

  • Incinerate salad metal drums
  • Bury in large trenches
• The likely initial mode is decay is beta- minus decay as there is usually a high N/Z ratio so a neutron changes into a proton