If a thickness of 7.35 mm of lead can absorb half of the 1 MeV gamma photons that enter the lead, then what fraction of gamma rays will be absorbed by twice this thickness (i.e. 14.7 mm)?

A: Since half of the gamma photons are absorbed by 7.35 mm Pb, then that is the “half value layer”. A second half value layer placed after the first one would again absorb half of the photons that emerge from the first slab of lead. This would leave half of a half, that is, one quarter emerging from the second half value layer. Hence ¾ have been absorbed.