CH1 1.2 Radioactivity


Types of radiation

Radioactivity is the spontaneous decay of unstable atoms with the emission of either alpha, beta or gamma radiation. 

  • Alpha (   ) rays are composed of a stream of fast moving helium nuclei (He 2+). They have limited penetrating power, so low they are unable to go through thin metal foil. Their mass is 4 units.
  • Beta (   ) rays are composed of a stream of fast moving electrons. They will penetrate a thin metal foil but are stopped by lead. Negligble mass.
  • Gamma (   ) rays are electromagnetic waves that have a very short wavelength, consequently leading them to have high energy. They are more penetrating than alpha or beta and can pass through lead. Negligble mass.
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The effects of a magnetic field

  • Alpha particles are positive, heavy and slow moving so are attracted slightly to the negative plate of an electric field
  • Beta particles are light and fast moving so are strongly attracted to the positive plate of an electric field
  • Gamma radiation is a short wavelength and is unaffected by an electric field 
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The consequences of alpha, beta and gamma

  • If an atom loses an alpha particle, the particle produced will contain two protons and two neutrons less than the original atom, the atomic number wil drop by  two and the mass number will drop by four.
  • If an atom loses a beta particle, then a change first occurs in the nucleus. A neutron changes into a proton plus and electron, and it is the electron that is lost. The resulting atom, contains one less neutron and one more proton, meaning the atomic number increases by one.
  • there are no definite changes in atomic structure during gamma emission 
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Half-life of radioactive isotopes

The half -life of a radioactive isoptipe is the time taken for half the unstable nuclei in the sample to decay. It is independant of the original mass, unaffected by changes in the temperature, and cannot be catalysed.

  • Radioactive decay is a first order process.
  • Half-lives are best described by graphing the radioactive count against time.
  • Radioactive counts are taken using a geiger counter.
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Artificial nuclear reactions

It is possible to change on element into another by bombarding with particles, protons, neutrons or even heavier atoms like carbon. The process produces unstable radioactive isotopes of elements such as lawrencium which does not naturally exist on earth.

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