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Basics - Poster 6
Time , Distance and Shielding
One of the key principles of radiological protection is to keep doses As Low As Reasonably Practicable ( ALARP ). This is achieved by various combinations and variations of 3 basic Parameters : Time of exposure Distance from the source of radiation Shielding to reduce the radiation
Dose is the quantity that matters in reducing risk , but the quantity -1 that is most readily measurable is the dose rate , often in μSv h or -1 mSv h .
Dose = Dose rate x time
Minimising the time of exposure minimises the dose . This is important where things are being manipulated within radiation fields such as in industrial radiography , laboratory work and interventional radiology .
The key is to plan the work , have everything you need available and , where necessary , practice without the presence of a radiation source .
The Inverse Square Law states that the radiation intensity is inversely proportional to the distance squared from the point source .
Inverse Square Law
This is shown schematically on the right Dose rate is reduced by the square of the distance If you double the distance the dose rate reduces by 4
The different types of radiation have different penetration capabilities .
The amount of radiation transmitted reduces with increasing
thickness .
The higher the energy of the radiation the greater the penetration ( see below ).
Half Value Layer ( HVL ) is the thickness of material to reduce the intensity of the transmitted radiation to half of the incident radiation .
Half Value Layer HVL ( mm )
Radionuclide Concrete Steel Lead
Caesium-137 48 16 6.5
Iridium-192 43 13 6
Stopped by a few cm of air
Will penetrate paper , plastic and other thin materials
Will penetrate metals and thick objects
Density
Maximum Beta Range ( mm )
Material g cm - 3
1.1 MeV 2.3 MeV
Air 0.0012 3800 8800
Water ( soft tissue )
1.0 4.6 11
Plastic ( acrylic ) 1.2 4.0 9.6
Glass ( Pyrex ) 2.2 2.2 5.6
Aluminium 2.7 2.0 4.2
Cobalt-60 62 21 12
Copper 8.9 0.5 1.0
Images provided by courtesy of Public Health England