The maximum thickness for Ir-192 is closest to which value?

The maximum thickness for Ir-192 is closest to which value?

• A. 3.5 inches
• B. 2.5 inches
• C. 4.0 inches
• D. 5.0 inches

Answer: (A)

Shielding gamma radiation relies on exponential attenuation through a barrier. For Ir-192, the barrier effectiveness in lead is described by values like HVL (half-value layer) and TVL (tenth-value layer). A TVL is the thickness that reduces the dose by a factor of 10. The typical TVL for Ir-192 in lead is about 0.7 inches. To achieve a strong reduction in exposure suitable for work areas, designers commonly use several TVLs; using roughly five TVLs gives a substantial attenuation while keeping the barrier practical. Five TVLs × about 0.7 inches per TVL ≈ 3.5 inches of lead. That’s why the value closest to the required shielding thickness is 3.5 inches. Choices around 2.5 inches would provide less protection, while 4.0 or 5.0 inches would exceed the common design requirement.