The half-value layer for Co-60 gamma rays using lead is:

• A. 0.25 inches
• B. 0.75 inches
• C. 0.50 inches
• D. 1.0 inches

Answer: (C)

Half-value layer is the thickness of shielding that reduces the gamma-ray intensity by half for a specific energy and material. It’s found from the attenuation law I = I0 e^{-μt}, where μ is the linear attenuation coefficient. The HVL is t = ln(2)/μ.

For Co-60 gamma rays in lead, the HVL is about 0.5 inches (roughly 1.3 cm). This means a lead barrier of 0.50 inches cuts the gamma intensity in half. If you use less than that, the reduction is less than a factor of 2 (for example, 0.25 inch would leave about 0.707 of the original intensity). Using more than the HVL yields greater attenuation (e.g., 0.75 or 1 inch reduces more than half). The value 0.50 inches is the thickness that provides a 50% reduction, which is why it’s the correct HVL for this scenario.