Section 8 - Nuclear physics

• Dalton thought that everything was made of indivisible particles
• Thompson made the plum pudding model - atoms were positive bodies of matter with negative electrons
• Rutherford discovered that atoms were mostly empty space with electrons orbiting them
• Bohr discovered that electrons orbit in shells and emit EM radiation when they move between them
• during the rutherford scattering experiment, alpha particles were fired at a thin sheet of gold foil
• during the rutherford scattering experiment:

  • most alpha particles passed through with no deflection, suggesting that the atom is mostly empty space and that the nucleus is very small
  • some were deflected by large angles, suggesting that there is a positively charged nucleus to repel the alpha particles
  • a few were reflected backwards, suggesting that the central nucleus was very small but very dense as it changed the direction of the very fast moving alpha particles
• an alpha particle consists of 2 protons and 2 neutrons.

  alpha radiation is:

  • strongly ionising
  • slow moving
  • stopped by a few cm of air or paper
  • positive charge
  • deflected in a magnetic field
• alpha radiation is used in smoke detectors. the particles ionise the air particles between two metal plates, allowing a current to flow between them. when smoke enters the detector, the alpha particles are restricted from ionising the air, resulting in the current flow decreasing. this current reduction triggers the alarm.
• beta radiation consists of an electron and beta plus radiation consists of a positron.

  it is:

  • mildly ionising
  • fast moving
  • stopped by a few mm of aluminium
  • positive or negative charge
  • deflected in a magnetic field
• beta radiation is used in thickness monitors. the particles are emitted on one side of the material, and detected on the other. if the material is too thick, the number of beta particles detected will be too low, and will trigger the machine to reduce the material thickness.
• gamma radiation is a form of electromagnetic radiation. it is very high frequency and:

  • is weakly ionising
  • travels at the speed of light
  • stopped by several cm of lead or a few m of concrete
  • no charge
  • unaffected by magnetic and electric fields
• gamma radiation is used to sterilise medical equipment and to kill cancerous cells, as well as being used as a medical tracer in diagnosis
• safe use of radiation:

  • never touch the source
  • use long armed tongs to increase your distance from the source
  • display signs to warn others that radioactive sources are in use
  • keep the time that the source is being used to a minimum
  • store in a lead box when not in use
• sources of background radiation:

  • radon gas
  • rocks
  • cosmic radiation
  • nuclear weapon testing
  • nuclear disasters
  • medical equipment
• background radiation is only present in very small quantities and so isn't harmful, but it must be corrected for when carrying out experiments - you should always take a background count reading and subtract this from any further measurements you take
• radioactive decay is a random process. which nucleus will decay, and when it will happen is unpredictable and determined only by chance.
• activity = the number of nuclei that decay per second, measured in becquerels (Bq)
• half life = time it takes for the number of radioactive nuclei to halve, for a given isotope
• decay constant = the probability of a decay occuring in a unit time
• one method of calculating an approximation for nuclear radius is to calculate the distance of closest approach
• distance of closest approach:

  • when you fire an alpha particle at a gold nucleus, as it approaches, kinetic energy is converted to electric potential energy. at the point where it turns around, the potential energy is equal to the initial kinetic energy, and the distance the particle is from the nucleus is the distance of closest approach. using the equation for electrical potential, you can calculate the distance
• using distance of closest approach to calculate an approximation for nuclear radius will always be an overestimate for the nuclear radius since the particle will never reach the nucleus
• another method of determining nuclear radius is by using electron diffraction
• using electron diffraction to determine nuclear radius:

  • high speed electrons are fired at a thin sheet of metal
  • as they pass through the atoms they diffract
  • a diffraction pattern consisting of concentric circles is formed on a screen behind the material
  • the nuclear radius can be calculated using measurements from the pattern
• nuclear fission is the splitting of a large nucleus to produce two smaller nuclei, two or three neutrons, and energy
• nuclear fission:

  • spontaneous fission is rare
  • if a nucleus absorbs a 'thermal neutron', it can induce fission
• induced fission:

  • a thermal neutron is absorbed by a nucleus
  • a thermal neutron is a slow moving neutron that can induce fission
• if nuclear fission is uncontrolled, chain reactions occur, which is when the neutrons produced in a fission reaction go on to induce further fission reactions
• in nuclear fission, critical mass is the minimum amount of a 'fissile substance' (a substance that can undergo nuclear fission) needed to maintain a steady flow of fission chain reactions
• in nuclear reactors, control rods absorb neutrons to prevent them going on to induce further fission. the lower the rods are inserted, the more neutrons that are absorbed and so the fewer the number of fission reactions that occur
• in nuclear reactors, the moderator is responsible for slowing down the neutrons released in fission reactions so that they are at the speed of a thermal neutron and can go on to induce further fission
• in nuclear reactors, fuel rods consist of a fissile material. each rod contains less that the critical mass so that the reactions don't become uncontrolled
• in nuclear reactors, the coolant carries away the thermal energy produced by fission reactions to generate steam and turn generators
• high level nuclear waste should be cooled in cooling ponds to reduce the temperatures to safe levels
• high level waste (such as used fuel rods, irradiated reactor components), should be stored in thick concrete containers underground for hundreds of years.

  when being transported, it should be stored in reinforced containers in case of accidents, and should be processed as close to the plant as possible
• low level nuclear waste contains only short lived radioactivity (such as contaminated clothing, cleaning supplies), should be stored close to the surface as it will not take very long to stop being radioactive
• nuclear fusion is the fusing of two smaller nuclei, to form a single large nucleus and produce large quantities of energy.
• nuclear fusion produces much larger quantities of energy per unit of fuel than nuclear fission. however it needs very high temperatures and pressures, so currently it is a challenging and unsustainable form of energy production
• binding energy = energy required to split up the nucleus into its individual nucleons. the greater the binding energy per nucleon, the more stable the nucleus.
• mass defect is the difference between the mass of the nucleus and its individual constituents