Radiation Protection Today - Spring 2022 - Page 36

Half Value Layer HVL ( mm )

Radionuclide Concrete Steel Lead
Caesium-137 48 16 6.5
Iridium-192 43 13 6
Cobalt-60 62 21 12
As can be seen , lead is a very effective material for shielding gamma radiation . A typical gamma-emitting sealed source used in industry is generally shielded to a safe level within an enclosure constructed of lead and lined with steel . 13-15cm thickness of lead will attenuate the typical activities of Cs-137 sealed sources used in industrial devices to safe levels . However , gamma radiation from Co-60 is more energetic , and requires 24-29cm of lead to attenuate the radiation to safe levels .
Because lead is expensive , concrete is preferred for shielding large areas like a calibration room / lab , where there is space to use a greater thickness of material . In X-ray tubes currently used in the UK , the X- rays are about half of the energy of gamma emissions from Cs-137 ( 662 keV ), so they require less shielding .
Sources of neutron radiation are the most complicated to shield as the radionuclides used typically also emit gamma radiation . The source then requires shielding of the gammas using materials like lead and steel , but also requires materials of low atomic number , like water or polythene , to attenuate neutrons . Boron-loaded polythene is the most effective material used nowadays to attenuate neutrons .
Corrigenda – Back to Basics Issue 2 ( fission vs fusion )
Shielded X-ray cabinet .
Thanks to Neil McColl for spotting these : p35 – Plutonium is also fissile . Thorium is fertile , which means it is not by itself fissile , but becomes fissile when exposed to a source of neutrons .
p35 – If used fuel is reprocessed , the fission products are concentrated , making the waste much more radioactive than the original fuel . This waste remains radioactive for a very long time .
p36 – The Watt is a unit of power , not energy .
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