Question

If the HVL of lead for I-131 is 3mm and a lead vial shield holding the iodine is 6 mm thick, what percentage of the original exposure rate will remain?

a. 75%
b. 50%
c. 25%
d. 12.5%

Answer 1

The original exposure rate of I-131 after passing through 6mm of lead (two Half Value Layers) is reduced to 25% of its initial value.

Step-by-step explanation:

The Half Value Layer (HVL) is the thickness of a material, in this case lead, required to reduce the radiation exposure rate by half. For Iodine-131 (I-131), the HVL is 3mm. When radiation passes through a layer of material, the exposure rate is halved for each HVL thickness. Therefore, passing through two such thicknesses, the exposure rate will be reduced by a factor of 4, or to 25% of the original exposure rate.

Since the lead vial shield is 6mm thick, which is equivalent to 2 HVLs (2 x 3mm), the percentage of the original exposure rate that will remain is:

1st HVL (3mm) reduces to 50%
2nd HVL (3mm) reduces to 50% of 50% = 25%

So, the correct answer is:

c. 25%