SP6 radioactivity

• jj thompson discovered the plum pudding model- atoms where spheres of positive charge with negative electrons stuck in them.
• rutherford fired a beam of alpha particles at thin gold foil.

  most particles went straight through, but some deflected back.

  rutherford realised this meant most of the mass was concentrated in the nucleus, and the nucleus must have a positive charge as it repelled the alpha particles.
• bohr tweaked rutherford's idea by suggesting electrons were in fixed orbits at set distances from the nucleus.
• if an electron absorbs EM radiation, it can be moved up to a higher energy level, then fall back to its original level, so it will emit the same amount of energy absorbed.
• an alpha particle is a helium nucleus.

  it doesn't penetrate very far and can be absorbed by paper, but they are highly ionising.
• a beta minus particle is a fast moving electron released by the nucleus with the charge -1.

  a beta plus particle is a fast moving positron with the charge +1.

  both are moderately ionising, and beta minus is stopped by aluminium.

  positrons have smaller range as when they hit an electron the two destroy each other and produce gamma rays- this is annihilation.
• gamma rays penetrate far into materials and travel a long distance through air, but they are weakly ionising as they do not collide with atoms as much but when they do, they cause damage.

  gamma rays are absorbed by lead.
• beta minus decay increases charge of the nucleus, while beta plus decreases it.

  neutron emission decreases the mass of the nucleus.
• activity is measured in becquerels, with a geiger-muller tube which clicks when exposed to radiation.

  it can be attached to a counter, which displays the number of clicks per second/count rate

  you can also use photographic film, which becomes darker the more radiation its exposed to.
• the half life is the average time taken for the number of radioactive nuclei of an isotope to halve.

  short half lives quickly become safe, while longer half lives release small amounts of radiation for a long time.
• irradiation is exposure to radiation- it does not make something radioactive, but effects can be reduced with lead lined boxes and photographic film badges which monitor exposure.
• contamination is radioactive particles getting onto objects.

  to reduce effects, gloves and tongs should be used, and protective suits.
• smoke detectors contain a weak source of alpha radiation, which causes ionisation so a current of charged particles flow.

  smoke absorbs the particles- current falls and the alarm sounds.
• food can be irradiated with a high dose of gamma rays which kills all microbes.

  medical equipment can be sterilised using gamma rays.

  the radioactive source must have a reasonably long half life, and a very strong emitter of gamma rays.
• medical tracers are injected into the body and progress is followed using an external detector.

  all isotopes must be beta or gamma, as they can pass out of the body and are weakly ionising, and they should have a short half life.

  gamma tracers can be used to detect leaks in underground pipes.

  beta radiation is used in thickness control- direct radiation through the product and put a detector on the other side, when amount of radiation changes the control unit adjusts the rollers to get the correct thickness.
• how does PET scanning work?

  1. inject patient with positron emitting isotope with a short half life
  2. positrons meet electrons in the body and annihilate, emitting gamma rays in opposite directions.
  3. gamma rays are detected, location of tumour can be found with triangulation.
  4. important to make isotopes close to hospital due to short half life.
• nuclear fission is the splitting up of an atomic nuclei, and it can be used to release energy from uranium/plutonium atoms in chain reactions.

  • a slow moving neutron is fired at a large, unstable nucleus, and is absorbed, making the atom more unstable and causing it to split.
  • two radioactive daughter nuclei are formed, as well as energy.
  • each time a uranium atom splits up, it spits out two or more neutrons, which hit other nuclei, causing them to split also.
  • this is a chain reaction.
• how are chain reactions controlled?

  1. slow moving chain reactions are captured by the fuel rods.
  2. slow moving neutrons are called thermal neutrons.
  3. fuel rods are placed in a moderator to slow down fast moving neutrons.
  4. control rods, often made of boron, absorb excess neutrons, limiting rate of fission- they are raised and lowered.
  5. if a chain reaction is left unchecked, it can cause a runaway reaction which could lead to a meltdown.

  
• in nuclear fusion, two light nuclei collide at a high speed and fuse to create a larger nucleus.

  heavier nucleus does not have as much mass- some of the mass of the lighter nuclei is converted to energy and released.

  nuclear fusion is the energy source of stars.
• nuclear fusion requires extremely high pressure and temperature as the nuclei need to get close to fuse, and electrostatic repulsion must be overcome.

  no materials can withstand these temperatures, so reactors are hard and expensive to build.
• how do nuclear power stations work?

  • energy released by fission is transferred to thermal energy store of the moderator which is transferred to the thermal energy store of the coolant, then to the store of the cold water passing through the boiler. this causes water to boil, and energy is transferred to kinetic energy store of the steam.
  • this energy is transferred to kinetic store of the turbine and then to the generator, which transfers energy away electrically.