Physics - P2.6 - Radioactivity

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P2.6.1 - Observing Nuclear Radiation

  • Nuclei of radioactive substances is unstable - become stable in radioactive decay: emit radiation and turn into other elements
  • Three types of radiation emitted: alpha, beta and gamma radiation
  • Can't predict when unstable nucleus will decay - random process, unaffected by external conditions
  • Background radiation - around us all the time - from radioactive substances in environment, from space, from devices eg. X-ray tubes
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P2.6.2 - The Discovery of the Nucleus

  • Plum pudding model - scientists thought atoms were spheres of positive charge with electrons stcuk into them
  • Rutherford, Geiger and Marsden devised alpha particle scattering experiment - fired alpha particles at thin gold foil:
    • most of the alpha particles passed through: most of the atom is empty space
    • some alpha particles deflected through small angles: nucleus has positive charge
    • a few alpha particles rebounded at large angles: nucleus has large mass and very large positive charge
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P2.6.3 - Nuclear Reactions

  • Alpha decay: 2 protons and neutrons lost, emitted as alpha particle
  • Beta decay - neutron turns in proton and electron, electron is emitted
  • No of protons = no of electrons - lose or gain either: becomes charged: ion - atoms of same element with different no of nuetrons: isotope
  • Alpha particle - two protons and neutrons, relative mass = 4, charge = +2
  • Nucleus emits alpha particle: atomic number goes down by 2 and mass number by 4 - radium emits alpha particle and becomes radon
  • Beta particle - high speed electron from nucleus, mass = 0, charge = -1
  • Nucleus emits beta particle: proton stays: atomic number goes up by one and mass number is unchanged, electron instantly emitted - carbon-14 emits beta particle and becomes nitrogen
  • Nucleus emits gamma radiation - no change in numbers: gamma ray is electromagnetic wave released from nucleus - no charge or mass
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P2.6.4 - More about Alpha, Beta and Gamma Radiatio

  • Nuclear radiation moves through material: collides with materials atoms - knocks electrons off them creating ions: ionisation - ionisation in living cells damages of kills cell
  • Alpha particles - relatively large: lots of collisions with atoms: strongly ionising, don't penetrate far into material, stopped by paper, skin or few centimeters of air, positive charge, deflected by electric/magnetic fields
  • Beta particles - smaller and faster: less ionising and penetrate further - stopped by few meters of air or thin aluminium - negative charge and deflected by electric/magnetic fields, opposite direction to alpha particle
  • Gamma rays - electromagnetic waves: travel long way through material before colliding - weakly ionising, very penetrating - several cm of lead or meters of concrete needed to absorb most of the radiation - not deflected by electric/magnetic fields
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P2.6.5 - Half-Life

  • Measure radioactivity of sample of radioactive material by measuring count rate from it
  • Radioactivity of sample decreases over time - how quickly count rate falls to 0 depends on isotope - some take minutes, others millions of years
  • Half-life used to measure how quickly radioactivity decreases
  • Half-life: time taken for count rate of original isotope to half
  • Or defined by time taken for number of unstable nuclei in sample to half
  • Half-life is the same for any particular isotope
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P2.6.6 - Radioactivity at Work

  • Alpha sources - used in smoke alarms - not dangerous: poorly penetrating - needs half-life of several years
  • Beta sources - used for thickness monitering in manufature of paper or metal foil - alpha particles would be stopped and gamma rays would pass through it - needs half-life of several years: decrease in count rate is due to changes in thickness
  • Gamma and beta sources - used as tracers in medicine - injected or swallowed by patient and progress around body monitered by dectector outside - needs half-life of few hours: patient isn't exposed to too much radiation
  • Radioactive dating - find out age of ancient material - carbon dating used for wood and other organic material - uranium dating used for igneous rock
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