Unit 5 Nuclear Decay

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  • Created by: megan
  • Created on: 21-03-13 16:01
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  • Nuclear Decay
    • Background Radiation
      • The natural environment exposes us the low levels or radiation from both natural and man made source.
      • Radiation levels are recorded in counts per second. unit - becquerel. Bq
      • On earth half the radiation is from naturally radioactive gases in the atmosphere, particluarly radon
        • radon is produced in the decay of uranium, and is present in certain rocks.
    • Types of Nuclear Radiation
      • Many nuclei are slightly unstable and there is a probability that they will decay
        • a nucleon may change from one type to another
        • The composition or energy state of the nucleus may change
      • When nuclear decay occurs the radiation particle emitted will leave the nucleus with kinetic energy
        • The particle released will ionise particles in its path, losing kinetic energy.
          • When all kinetic energy is transferred the radiation particle stops and is absorbed by the substance.
        • Many nuclei are slightly unstable and there is a probability that they will decay
          • a nucleon may change from one type to another
          • The composition or energy state of the nucleus may change
      • Alpha= 2 protons + 2 neutrons. large in size. +2 charge. Highly ionising
        • As it is ionises so quickly, it is easily absorbed by paper/skin
      • Beta = an electron. Emitted at high speed from the nucleus when a neutron decays into a proton. Smaller in size and charge it is less ionising
        • can penetrate much further than alpha, but blocked by aluminium
      • Gamma= High energy, high frequency electromagnetic radiation. photons with no max or charge so will rarely interact with particles= least ionising.
        • Gamma is never completely absorbed although their energy can be reduced by thick lead
    • Probability and Decay
      • For each second that it exists, there is a certain probability that the nucleus will decay. = Decay constant.
      • Each nucleus acts entirely independently
        • The likely hood that a particular nucleus will decay is not affected by factors outside the nucleus such as temp/pressure
      • A= -  ?N
        • A= activity (number decaying per second) N= the number of nuclei in the example.
      • N=N0 E^- ?t
    • Half life
      • The activity of a radio active sample decreases over time as the radioactive nuclei decays.
      • The rate at which the activity decreases depends on the particular isotope
        • A measure of this rate is the half life t 1/2
      • The time taken for half of the atoms of that nuclide within a sample to decay
      • ?=Ln2/ t(1/2)
    • Half life graphs
      • An experiment to determine half life of a substance will usually measure activity over time
      • The shape of a graph of activity against time shows exponential decay.
      • we can use the graph of activity against time to determine the half life of the substance by finding the time taken for the activity to halve.

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