Gamma attenuation lengths in water and lead
For gamma radiation absorberd, the attenuation length is the inverse of the attenuation coefficient. The shorter the length, the faster the attenuation. Less than a millimetre of lead is enough to halve the number of gamma rays with an energy below 200 keV, whereas it would take over 5cm of water to achieve the same effect. Lead is less effective, however, when it comes to gamma rays with energies over 1 MeV: 1cm of lead is needed as opposed to 10com of water. At these high energies, the attenuation lengths are proportional to densities: lead is 11.6 times denser than water.