Absorption of radiation

Side view of a hand, a block of aluminium and a block of lead. Alpha radiation is stopped by the hand, beta radiation passes through the hand but is stopped by aluminium, and gamma radiation passes through the hand and aluminium but is stopped by lead.

The energy of the three radiations is absorbed by the material through which the radiation passes. The amount of energy which is absorbed depends on the type of radiation and the type of the absorbing material.

  • The range of the alpha radiation in an absorbing material is less than that of beta or gamma. The alpha radiation transfers more energy to an absorber than beta or gamma radiation. Alpha radiation is absorbed by the thickness of the skin or by a few centimetres of air.
  • Beta radiation is more penetrating than alpha radiation. It can pass through the skin, but it is absorbed by a few centimetres of body tissue or a few millimetres of aluminium.
  • Gamma radiation is the most penetrating of the three radiations. It can easily penetrate body tissue. It requires a few centimetres of lead or about 1 metre of concrete to absorb it.

Alpha, beta and gamma radiation (pt 1/2)

Alpha, beta and gamma radiation (pt 2/2)

The question below will test your knowledge of the absorption of alpha, beta and gamma radiation.

Question

The diagram shows the readings obtained when a Geiger Muller tube is placed near a piece of radioactive rock. The readings obtained when the rock is wrapped in a sheet of paper and in a thick layer of aluminium foil are also shown.

Diagram showing readings gained while Geiger Muller tube is placed near rocks.  Radioactive rock has very high reading. Rock wrapped in paper has high reading. Rock wrapped in aluminium foil has low reading.

The radiation emitted by the radioactive rock is:

  • A alpha only
  • B beta only
  • C gamma only
  • D alpha and beta only
  • E alpha, beta and gamma.

You should have noticed that some of the radiation is absorbed by the sheet of paper. This suggests that some of the radiation from the rock is alpha radiation.

Then you should have noticed that when the rock is wrapped in a thick layer of aluminium foil the count rate of the radiation detected is reduced. This suggests that some of the radiation from the rock is beta radiation.

You should also have noticed that even when the rock is wrapped in the thick layer of aluminium foil some radiation is still being detected. Since beta radiation is absorbed by a few mm of aluminium, this suggests that gamma radiation is also coming from the rock.

Well done if you selected E as your answer. If you selected D you made a good attempt, but you did not use fully all of the information that was given to you in the diagram. If you selected A, B or C then you should go back and revise this topic again.