P6 - Radioactive materials

P6 - Radioactive materials

1. The Nuclear Atom

   • 1909 Rutherford Geiger Marsden alpha scattering experiment
     • Observations:
       • Most α particles passed straight through gold foil undeviated
       • Few particles deflected through small angles
       • Even fewer bounced straight back from foil
     • Conclusions
       • Atom mostly empty space
       • Mass + charge of atom concentrated in small are in centre (nucleus)
       • Nucleus is positive as positive α particles repelled
   • Strong nuclear force hold protons + neutrons in nucleus together. Balances repulsive electrostatic force between protons
   • Number of protons in nucleus determines the element
   • Isotopes
     • Atoms of an element with different numbers of neutrons. They have the same proton number, but different mass numbers.

2. Radioactive elements

   • Unstable elements that constantly emit ionising radiation to try to become more stable
   • Background radiation - low-level ionising radiation that is all around us
   • Radioactivity is random
   • Amount of radiation emitted depends only on amount of radiative element present. Behaviour of radioactive materials is not affected by physical/chemical processes

3. Ionising radiation

   • Types
     • As α, β + γ travel through air they ionise air molecules + lose energy
       • Alpha
         • Massive, easily knock off electrons
           • Lose energy quicker + travel few cm in air, stopped by paper
             • High ionisation, low penetration, charge 2+
       • Beta
         • Range in air of 1m, stopped by aluminium
           • Medium ionisation, medium penetration, charge 1-
       • Gamma
         • Not stopped by air, just spread out + become less intense, stopped by lead
           • Low ionisation, high penetration, charge neutral
   • Hazards
     • Damages living cells, depending on the + intensity
     • Ionising radiation collides with living cells + knocks electrons out of the atoms, leaving positive ions
     • High intensity radiation -> kill living cell, tissue damage, radiation sickness, cells become sterile
     • Lower intensity radiation can cause mutations -> cancer
     • Sievert (Sv)
       • One Sv of α, β or γ produced same biological effect, + is a measure of possible harm done to body
   • Uses
     • 1) Treating cancer - radiotherapy (gamma). Health tissue around tumour can be damaged, radiation must be focused on tumour
     • 2) Sterilising medical instruments - irradiated with gamma to kill bacteria (penetrate the packaging + kill microbes)
     • 3) Sterilising food - fresh food irradiated with gamma to kill micro-organisms. Makes shelf life of food longer

4. Half life

   • Time taken for half the nuclei in a sample to decay - it's specific to each radioactive element
   • Vary from fractions of a second to millions of years
   • Activity of a radioactive source (amount of radiation emitted) is measure of its rate of decay
   • Rate of decay faster at start when there are plenty of radioactive nuclei present, than at end when most nuclei have already decayed
   • Activity can never reach 0

5. Nuclear power generation

   • Nuclear fission produces radioactive waste
   • In a nuclear reactor...
     • Fuel rods contain pellets of uranium. Neutrons cause fuel to undergo fission. Energy released as kinetic energy of particles (heat)
     • Coolant (gas or liquid) circulated around reactor absorbs heat + transfers it to steam generator
     • Control rods (usually boron) absorb some neutrons, can be raised or lowered to control fission rate

6. Safety

   • Irradiation - exposure to radiation
   • Risk to health depends on level of radiation + length of exposure
   • Contamination - a surface or person is in contact with radioactive material
   • Radioactive waste can be contained or diluted to safe concentrations
   • High-level contamination (e.g. fallout from nuclear explosion) will need more intervention, e.g. administering iodine to affected people
   • People who work with radioactive sources regular have level of exposure monitored, e.g. with film badges
   • Lead shielding protects radiographers

7. Energy from the nucleus

   • Nuclear fusion - releases energy by 2 light nuclei (e.g. hydrogen) combining to create a larger nucleus
   • In nuclear fission, a neutron is fired at urnaim/plutonium nucleus to make it unstable
     • Nucleus breaks down into 2 smaller nuclei of similar size, + releases more neutrons
       • 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.
   • More + more neutrons will be released in each subsequent reaction, + chain reaction will get out of control unless number of neutrons is controlled
   • Energy released in fission + fusion calculated using E=mc^2
     • E - energy (joules); m - mass (kg); c - constant equal to speed of light in a vacuum

8. Harnessing fusion energy

   • 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
   • Isotopes of hydrogen are readily available + only small amounts needed
   • Despite huge potential, more energy consumed producing fusion reactions than is released