Page 247 - Physics Coursebook 2015 (A level)

Chapter 16: Radioactivity
Gamma-radiation
Since γ-radiation is the least strongly ionising, it is the most penetrating. Lead can be used to absorb γ-rays. The intensity of the radiation decreases gradually as it passes through the lead. In principle, an infinite thickness of lead would be needed to absorb the radiation completely; in practice, a couple of centimetres of lead will reduce the intensity by half and 10 cm will reduce the intensity to a safe level in most situations.
The different penetrating properties of α-, β−- and γ-radiations can be summarised as follows:
When an electron (with a charge of magnitude 1.60 × 10–19 C) travels through a potential difference, energy is transferred. The energy change W is given by:
W = QV =1.60×10–19 ×1=1.60×10–19 J So we define the electronvolt as follows:
Therefore:
1 eV = 1.60 × 10–19 J
There is more about the electronvolt and its use in energy calculations in Chapter 30.
QUESTIONS
16 Explain why the most strongly ionising radiation (α-particles) is the least penetrating, while the least ionising (γ-rays) is the most penetrating.
17 A smoke detector (Figure 16.20) uses a source of α-radiation to detect the presence of smoke in the air. Find out how the smoke detector works and suggest why an α source is more suitable for this than a β− or γ source.
Figure 16.20 A smoke detector that uses the absorption of α-radiation as the principle of its operation.
■■ α-radiation is absorbed by a thin sheet of paper or a few centimetres of air.
■■ β−-radiation is absorbed by a few millimetres of metal.
■■ γ-radiation is never completely absorbed but a few centimetres of lead, or several metres of concrete, greatly reduces the intensity.
This is illustrated in Figure 16.19.
α
β−
γ
~1 mm
~2 mm
~2 cm
paper
aluminium
lead
Figure 16.19 A summary of the penetrating powers of α-, β−- and γ-radiations. The approximate thickness of the absorbing material is also shown.
The electronvolt (eV)
Alpha and beta particles move quickly; gamma photons travel at the speed of light. These types of radiation all have energy, but the energy of a single particle or photon is very small and far less than a joule. So we use another, much smaller unit of energy, the electronvolt, when considering the energy of individual particles or photons.
One electronvolt (1 eV) is the energy transferred when an electron travels through a potential difference of one volt.
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