Sodium coolant with a specific activity of 0.05 Ci/cm³ passes through a long pipe 8 inches in diameter. By treating the pipe as a line source and ignoring attenuation in the sodium itself, estimate the thickness of a cylindrical concrete shield, wrapped around the pipe that reduces the dose rate to 2.5 mrem/hr. 

[Note: The activity of sodium is due to ²⁴Na, and 2 γ-rays of 2.75 MeV and 1.37 MeV are emitted by the activation reaction.]