For the record, here is the correct math for attenuation of high energy photons with a nice explanation from NIST: https://physics.nist.gov/PhysRefData/XrayMass

For the record, here is the correct math for attenuation of high energy photons with a nice explanation from NIST: https://physics.nist.gov/PhysRefData/XrayMassCoef/chap2.html https://physics.nist.gov/PhysRefData/XrayMassCoef/chap2.html The coefficient in the exponential is 0.0161/mm for these data, so a 100 mm thick layer will stop 80% of their measured spectrum. A 200 mm thick layer will stop 96%. The same relationship applies for higher energy gammas as well, but the coefficents will be energy dependent.