P6

P6

1. Plum pudding model

   • In 1897 they thought the atom was just a blob of charged things
   • Scientists did an experiment where they fired alpha particles at a thin gold leaf and observed it under a microscope
   • They expected the particles to go straight through
   • They found the particles went off in different directions
   • This is because some of the + a particles hit the nucleus of a gold atoms and bounced off
   • They adjusted their theory to a nucleus with protons and neutrons and electrons spinning around it

2. Radioactive elements

   • Different atoms have various isotopes which have the same number of protons but just different amounts of neutrons
   • Isotopes have the same chemical properties as they still have the same number of protons
   • You call an isotope by its atomic mass like Carbon-12 and Carbon-14
   • Some isotopes are stable some are unstable and will decay
   • Inside the nucleus there is a strong nuclear force that holds the protons and neutrons together in the atom. It has to balance the electrostatic repulsion between the protons
   • Some combinations of protons and neutrons are very stable
   • Some simply dont work and the atom will split into particles that will just fly away

3. Radiation types

   • Alpha
     • A nucleus may give out an alpha particle which is 2 protons and 2 neutrons
     • The atomic number of the atom goes down by 2 and the mass goes down by 4
     • This is the nucleus of helium 4
     • Range in air is very short
     • Stopped by paper clothes and skin
     • Very ionising
   • Beta
     • It is an electron that comes from the nucleus
     • A neutron forms into a proton and an electron
     • The proton number of the atom goes up by 1 and the mass number stays the same
     • Range is several cm in air
     • Stopped by a few mm of aluminium
     • Medium ionisation
   • Gamma
     • Very short wavelength electromagnetic radiation
     • It is just a way for nucleuses to get rid of energy
     • Atomic number and mass stay the same
     • Long range in air
     • Stopped by cm's of lead
     • Low ionisation
   • Radioactivity is a fundamental property of an unstable nucleus
   • Any physical or chemical changes made to the material have no effect on its radioactivity

4. Background radiation/Health effects

   • A geiger counter will click every time it is detecting radiation
   • Most people live without getting harmed by background radiation. We have evolved to not be affected by it
   • Ionising radiation can ionise atoms and molecules turning them into ions
   • Ions are more chemically reactive than atoms and these reactions can kill or damage cells leading to cancer
   • The activity of a radioactive source is the number of nuclei that are decaying (by emmiting radiation) per second
   • Activity is measured in becquerels (bq)
   • An activity of 20 bq means 20 nuclei decaying per second
   • Radiation dose is a measure of how much harm is caused to a human body by exposure to ionising radiation
   • Radiation dose is measured in sieverts (sv) or millisieverts
   • Irradiation- exposed to a radioactive source
     • Gamma is the worst
   • Contamination- Having the radiation in your body by eating drinking or inhaling it
     • Much worse as it is continuous and it gets close enough to your body for alpha and beta particles to reach
     • Alpha is worse as it is in range to harm you

5. Half life

   • It is the time taken for half of the nuclei in a radioactive sample to decay, it is different in all elements
   • Can vary from a few seconds to millions of years
   • The activity can never reach 0

6. Uses of ionising radiaton

   • 1. Treating cancer. Can be used to kill cancerous cells
   • 2. Sterilising medical instruments. Kills bacteria
   • 3. Sterilising food. To kill microorganisms in the food
   • 4. Detecting tumours. Using a radioactive tracer

7. Energy from the nucleus

   • Nuclear fission
     • Releases energy by a larger nucleus splitting into two smaller nuclei and giving out neutrons
     • A neutron is fired at uranium or plutonium nucleus to make it unstable so it breaks down to 2 smaller nuclei and 1-3 neutrons
     • The neutrons released by a previous reaction can set off another one
     • In nuclear fission there is a very small loss of mass
     • The mass lost is converted to energy with equation energy = mass x speed of light 2
     • The only way to protect a serious chain reaction from going on is by having control rods in the reactor to absorb some neutrons
     • The amount of energy released can be controlled by lowering or raising the rods out of the water
     • Electricity is created as the coolant absorbs the heat of the reaction and it turns to steam to turn a turbine
   • Nuclear fusion
     • Nuclei stick together because
       • Strong nuclear force holding them together
       • Right number of neutrons reduce the force of repulsion
     • It is joining to small nuclei (isotopes of hydrogen) together to release energy
     • The mass of the particles after the reaction is smaller so we can use formula e = mc2 to find the energy
     • Sun does fusion all the time to create energy
     • Needs very high temperatures and pressures to happen

8. Nuclear waste

   • Fission creates radioactive waste
   • Low level waste (LLW)
     • Contaminated clothing, paper. It is burnt, put in containers and then put in a landfill
   • Intermediate Level Waste (ILW)
     • Chemical sludges, reactor parts, sealed in steel drums and put in concrete trenches
   • High Level Waste
     • Spent fuel rods , very radioactive. Mixed with molten glass and contained in steel drums and needs to be cooled

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