P6 - Radioactive materials

The Nuclear Atom
1. 1909 Rutherford-Geiger-Marsden alpha scattering experiment
  1. Observations:
    1. most α particles passed straight through gold foil undeviated
    2. Few particles deflected through small angles
    3. Even fewer bounced straight back from foil
  2. Conclusions
    1. atom mostly empty space
    2. mass + charge of atom concentrated in small are in centre (nucleus)
    3. nucleus is positive as positive α particles repelled
2. Strong nuclear force hold protons + neutrons in nucleus together. Balances repulsive electrostatic force between protons
3. Number of protons in nucleus determines the element
4. Isotopes
  1. Atoms of an element with different numbers of neutrons. They have the same proton number, but different mass numbers.
Radioactive elements
1. unstable elements that constantly emit ionising radiation to try to become more stable
2. Background radiation - low-level ionising radiation that is all around us
3. Radioactivity is random
4. Amount of radiation emitted depends only on amount of radiative element present. Behaviour of radioactive materials is not affected by physical/chemical processes
Ionising radiation
1. Types
  1. As α, β + γ travel through air they ionise air molecules + lose energy
    1. alpha
      1. massive, easily knock off electrons
        lose energy quicker + travel less far (few cm in air)
    2. beta
      1. range in air of 1m
    3. gamma
      1. not stopped by air, just spread out + become less intense
2. Hazards
  1. Damages living cells (by interfering with structure of DNA, causing it behave incorrectly) depending on the + intensity
  2. Ionising radiation collides with living cells + knocks electrons out of the atoms, leaving positive ions
  3. High intensity radiation -> kill living cell, tissue damage, radiation sickness, cells become sterile
  4. Lower intensity radiation can cause mutations -> cancer
  5. Sievert (Sv)
    1. the unit of radiation absorbed equivalent dose
    2. one Sv of α, β or γ produced same biological effect, + is a measure of possible harm done to body
  6. Oxygen, hydrogen, nitrogen + carbon are highly susceptible to ionisation + are abundant in body
    1. Alpha particles don't pass through skin. Inside body alpha is highly damaging, but safe outside it
      1. Beta + gamma are more penetrating + pass through skin, so more dangerous outside body
3. Uses
  1. 1) Treating cancer - ionising radiation can kill cells, so can be used to kill cancerous cells in radiotherapy (usually gamma radiation). Some health tissue around tumour can be damaged, so radiation must be focused on tumour
  2. 2) Sterilising medical instruments - can irradiated with gamma radiation to kill bacteria (gamma can penetrate the packaging + kill microbes)
  3. 3) Sterilising food - fresh food irradiated with gamma radiation to micro-organisms. Makes shelf life of food longer
  4. 4) Detecting tumours - tumours can be detected using a radioactive tracer. A gamma emitter with half-life of few hours is injected; radiation is detected from outside to build up computer image of tumour.
    1. Usually beta or gamma emitters as they must be able to penetrate skin + tissue. Half-life needs to be few hours so it has time to reach affected parts of body in sufficient amounts, but not last so long that it damages body
Half life
1. time taken for half the nuclei in a sample to decay - it's specific to each radioactive element
2. Vary from fractions of a second to millions of years
3. activity of a radioactive source (amount of radiation emitted) is measure of its rate of decay
4. rate of decay faster at start when there are plenty of radioactive nuclei present, than at end when most nuclei have already decayed
5. activity can never reach 0
Nuclear power generation
1. 1/6 of UK's electric generated by nuclear power stations (use nuclear fission)
2. nuclear fission produces radioactive waste
3. In a nuclear reactor...
  1. fuel rods contain pellets of uranium. Neutrons cause fuel to undergo fission. Energy released as kinetic energy of particles (heat)
  2. coolant (gas or liquid) circulated around reactor absorbs hate + transfers it to steam generator
  3. control rods (usually boron) absorb some neutrons, can be raised or lowered to control fission rate
Safety
1. Irradiation - exposure to radiation
2. Risk to health depends on level of radiation + length of exposure
3. Contamination - a surface or person is in contact with radioactive material
4. Radioactive waste can be contained to prevent contamination, if radioactive waste cannot be contained it must be diluted to safe concentrations
5. High-level contamination (e.g. fallout from nuclear explosion) will need more intervention, e.g. administering iodine to affected people
6. People who work with radioactive sources (e.g. radiographers + nuclear power station workers) regular have level of exposure monitored
7. Film badges monitor radiation, level of exposure measured by how black the film has become
8. Lead shielding (in form of aprons/protective walls) protects radiographers
Energy from the nucleus
1. Nuclear fission - releases energy by heavy nucleus (e.g. uranium) splitting into 2 lighter nuclei
2. Nuclear fusion - releases energy by 2 light nuclei (e.g. hydrogen) combining to create a larger nucleus
3. Fusion + fission release more energy than chemical reactions because the energy that holds nuclei together (binding energy) is larger than energy that holds electrons in place
4. In nuclear fission, a neutron is fired at urnaim/plutonium nucleus to make it unstable
  1. Nucleus breaks down into 2 smaller nuclei of similar size, + releases more neutrons
    1. Neutrons released go on to initiate more fission reactions (chain reaction). Only one neutron from each fission needs to go on to initiate next fission.
5. Necessary to have critical mass of nuclear fuel for chain reaction to be viable
6. more + more neutrons will be released in each subsequent reaction, + chain reaction will get out of control unless number of neutrons is controlled
7. Energy released in fission + fusion calculated using E=mc
  1. E - energy (joules); m - mass (kg); c - constant equal to speed of light in a vacuum
Harnessing fusion energy
1. fusion releases much more energy per kg than fossil fuels. Its by-products aren't radioactive + it doesn't release carbon dioxide into the atmosphere
2. isotopes of hydrogen are readily available + only small amounts needed
3. despite huge potential, more energy consumed producing fusion reactions than is released

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P6 - Radioactive materials

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