Question

This is in a nuclear radiation lab.
(gamma rays from Cs 137 passing through lead.

initial radiation (with one plate) 500cpm

final radiation intensity with 2 plates 300cpm

thickness of 1 absorbing plate in centimeters 0.635

I need to figure out the penetration factor and i need to figure the thickness required to block 99 percent of the gamma rays. I'm getting .40 and 1.257cm respectively. Please tell me what it actually should be and how to figure it out, i'm pretty certain it is wrong.