Texas Industrial Radiography Gamma Practice Test

After passing through 0.5" of lead, the dose rate from Iridium 192 drops to what fraction of its original value?

1. 1/10

2. 1/2

3. 1/5

4. 1/15

The correct answer is: 1/10

When considering the attenuation of radiation as it passes through a material like lead, the concepts of exponential decay and half-value layers (HVL) become relevant. The half-value layer is the thickness of a specified material that reduces the intensity of radiation to half its original value. Iridium-192 is a gamma-emitting isotope commonly used in industrial radiography. The attenuation characteristics of gamma radiation are well-documented, and lead is often utilized as a shielding material due to its high density and atomic number. For Iridium-192, the half-value layer in lead is approximately 1.2 cm (or 0.47 inches). Since 0.5 inches is slightly over half of the half-value layer, it will reduce the dose rate considerably but not down to half. Instead, after passing through 0.5 inches of lead, the radiation intensity falls to about 1/10th of its original value, taking into account the exponential decay characteristics of the radiation as it travels through the lead. This understanding aligns with the principle that as thickness increases, radiation intensity decreases significantly, often aligning closely with known values derived from specific shielding calculations. Therefore, after passing through 0.5 inches of lead, the fraction of the original dose