Radiation Protection: Design of Protective Barriers

Study Notes

A medical physicist must be consulted in designing proper radiation shielding.
There are two basic types of barriers: Primary and Secondary barriers.
Primary radiation is the most intense and difficult to shield.
Any wall to which the useful beam can be directed is called a primary barrier.
Three of the four walls in a fixed x-ray lab are primary barriers, as well as the floor.
Lead bonded to sheet rock or wood paneling is often used as a primary barrier.
Concrete or brick may be used instead, with 4 inches of masonry being equal to 1/16 inch of lead.

The primary barrier is perpendicular to the central ray.
Lead is often used, with a thickness of at least 1/16 inch (1.6 mm) and a height of 7 feet.

Secondary barriers are designed to stop leakage and scatter radiation.
The primary beam should never be pointed at a secondary barrier.
The patient is the source of most scatter radiation produced.
Leakage radiation is emitted from the tube housing in all directions other than the primary beam.
Secondary barriers often have a lead glass window with a thickness of 1.5 mm of Pb.
The regulatory limit of leakage radiation is 100 mR/hr at 1m distance.

Workload is measured in mA per min or week (the amount of activity in an x-ray room).
Use factor is assigned to walls (1/4) and the floor (1), representing the amount of time the beam is directed at a wall or floor.
Occupancy factor (T) has limits of:
- 100 mR/week for controlled areas (occupied by radiation workers).
- 10 mR/week for uncontrolled areas (e.g. corridors, waiting rooms, restrooms).

Three qualities of the Primary Barrier:
- CR is perpendicular.
- Thickness of at least 1/16 inch of Pb (1.6 mm).
- Height of 7 feet.
Three qualities of the Secondary Barrier:
- CR is parallel.
- Thickness of at least 1/32 inch of Pb (0.8 mm).
- Height of 7 feet.