How can I shield an area of radiation?
Protection from radiation (best by distance) can consist of ribbons or ropes and warning flags to demarcate the area where radiographs are made, or concrete bunkers with doors which automatically switch off the X-ray equipment as soon as they are opened. Both methods have the same objective: i.e. to prevent unauthorised people entering the area of radiation. An area of radiation can be defined as an area in which the radiation level exceeds the per- mitted value of 10 μSv/h.
There are three ways to achieve a reduction in intensity:
- by erecting a demarcation barrier at an appropriate distance,
- by erecting an absorbing barrier,
- by a combination of methods 1 and 2.
Distance
Since radiation is subjected to the inverse square law, its intensity is reduced with the incre- ase in distance to the square.
Absorbing barrier and distance Whenever radiation penetrates a material, the absorption process reduces its intensity. By placing a high-density material such as lead around the source of radiation, the quantity of transmitted radiation will decrease. To determine the material thickness required for a certain reduction in radiation, a factor known as the half-value thickness (HVT) is used. The table shows the HVT-values for lead for various types of gamma sources
Example
To reduce 2.56 mSv/h, measured at 1 meter distance, to 10 μSv/h the required distance according the inverse square law is 2560/10 = 16 metres. To achieve the same by placing a shield, the number of HVTs is calculated as follows:
- Required intensity reduction is 2560 / 10 = 256 x
- Number of HVTs is, log 256 / log 2 = 8
The example above demonstrates that an intensity of 2.56 mSv/h can be reduced to 10 μSv/h by increasing the distance to 16 metres, or place shielding material of 8 HVTs as close as possible to the source. If either of these methods cannot be used on its own, a combination of the two could be considered.