Texas Industrial Radiography Gamma Practice Test

If the tenth-value layer of lead for Iridium 192 is 0.5", what will be the dose rate with 1.5" of lead shielding if the unshielded source dose rate is 100 mr/hr?

1. 0.1 mr/hr

2. 1.5 mr/hr

3. 15 mr/hr

4. 10 mr/hr

The correct answer is: 0.1 mr/hr

To determine the dose rate with 1.5 inches of lead shielding when the tenth-value layer (TVL) for lead with Iridium-192 is 0.5 inches and the unshielded dose rate is 100 mR/hr, it's essential to understand how the TVL works. The tenth-value layer indicates the thickness of a material needed to reduce the radiation dose by 90%. If the thickness of the lead shielding is 1.5 inches, we can evaluate how many TVLs that represents: 1.5 inches of lead shielding divided by the TVL of 0.5 inches results in 3 TVLs. Using the principle that each TVL reduces the dose rate by a factor of 10, we can calculate the dose rate after applying the shielding: 1. After the first TVL (0.5 inches), the dose rate would be 100 mR/hr * 0.1 = 10 mR/hr. 2. After the second TVL (1.0 inch), the dose rate would be 10 mR/hr * 0.1 = 1 mR/hr. 3. After the third TVL (1.5 inches), the dose rate would be 1 mR