ASNT Industrial Radiography Radiation Safety Practice Test

What type of radiation shielding reduces exposure to one-half at a thickness of 0.2 inches for Ir-192?

1. Iron

2. Concrete

3. Lead

4. Copper

The correct answer is: Lead

The appropriate choice is lead, as it is well known for its effectiveness in radiation shielding, especially when it comes to gamma rays, such as those emitted by Iridium-192 (Ir-192). The concept of half-value thickness is key here; it represents the thickness of a material needed to reduce the intensity of radiation by half. For Ir-192, a specific thickness of lead—0.2 inches—can reduce gamma radiation exposure to one-half of the initial level. Lead's high atomic number and density make it particularly effective in attenuating gamma radiation. This property allows it to effectively absorb and scatter the gamma rays produced by radioactive isotopes like Ir-192. The other materials listed do not provide the same level of attenuation at the same thickness. While iron, concrete, and copper do offer some degree of shielding, they require greater thicknesses to achieve the same level of reduction in radiation exposure as lead.