The thickness of lead needed to stop half of the neutrinos in a beam is about 3000lightyears (which you may take as 3.0×1019m).Calculate the fraction of neutrinos which would be stopped by 100m of water assuming that the attenuation coefficients for water and lead are about the same (which they’re not). You may need to use the approximation e^x≈1 x provide x has a small magnitude.