Final answer:
Secondary radiation barriers require thick lead shielding to block gamma rays effectively. To absorb all but one in 1000 gamma rays, approximately 10 layers of 0.170 mm thick lead are needed.
Step-by-step explanation:
Secondary radiation barriers typically require thick lead shielding to effectively block gamma rays. The thickness of the shielding determines its effectiveness in providing protection. In this case, we need to find the thickness of lead that will absorb all but one in 1000 gamma rays.
Since half of the gamma rays are absorbed by a 0.170-mm-thick lead shielding, we can use this information to find the thickness of lead needed. We know that the second layer of lead is also of equal thickness and absorbs half of the remaining gamma rays.
To find the thickness of lead needed to absorb all but one in 1000 gamma rays, we need to find the number of layers of lead required. We can set up the equation: 0.170 mm * (1/2)^n = 1/1000, where 'n' is the number of layers of lead.
Solving for 'n', we find that approximately 10 layers of lead are needed. Since each layer is equal in thickness to 0.170 mm, the total thickness of lead required is approximately 1.70 mm.