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Which one of the following ionising radiation types can be stopped after travelling through 3 cm of air, 0.2 mm of water, or a piece of paper or skin?
  • A
    X-ray radiation
  • B
    Alpha radiation
  • C
    Beta radiation
  • D
    Gamma radiation

Refer to figure.

Alpha particles are composite particles consisting of two protons and two neutrons tightly bound together. Alpha particles are relatively slow and heavy compared with other forms of nuclear radiation.

Alpha radiation, because they are highly ionising, are unable to penetrate very far through matter and are brought to rest by a few centimetres of air or less than a tenth of a millimetre of biological tissue. 

  • It is therefore the least penetrating. It can be stopped (or absorbed) by a sheet of paper or a human hand.

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