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

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

• A. 0.50 inches
• B. 1.0 inches
• C. 2.0 inches
• D. 1.50 inches

Answer: (D)

The idea tested is how much shielding is needed to cut a gamma ray beam to one-tenth of its original intensity. For gamma rays in a material, the intensity decreases exponentially with thickness: I = I0 e^{-μx}, where μ is the linear attenuation coefficient. The tenth-value layer is the thickness that makes I equal to I0/10, which means TVL = ln(10)/μ. The ratio of TVL to the half-value layer (HVL, the thickness to cut I by half) is about ln(10)/ln(2) ≈ 3.32. For cobalt-60 gamma rays in lead, the HVL is around 0.5 inches, so the TVL is about 0.5 × 3.32 ≈ 1.66 inches. Practical data round this to about 1.5 inches. So the thickness that yields a tenfold reduction is roughly 1.5 inches of lead.

The idea tested is how much shielding is needed to cut a gamma ray beam to one-tenth of its original intensity. For gamma rays in a material, the intensity decreases exponentially with thickness: I = I0 e^{-μx}, where μ is the linear attenuation coefficient. The tenth-value layer is the thickness that makes I equal to I0/10, which means TVL = ln(10)/μ. The ratio of TVL to the half-value layer (HVL, the thickness to cut I by half) is about ln(10)/ln(2) ≈ 3.32. For cobalt-60 gamma rays in lead, the HVL is around 0.5 inches, so the TVL is about 0.5 × 3.32 ≈ 1.66 inches. Practical data round this to about 1.5 inches. So the thickness that yields a tenfold reduction is roughly 1.5 inches of lead.