Concrete for example - you need less concrete to obtain the same lead equivalent for photon energy 500 keV than for 200 keV. What is the reason for this?
It's a property of lead rather than a property of the concrete.https://archive.org/details/jresv38n6p665
it's in this article for example
I asked this question as I was reading about shieldings in PET departments, where they suggest lead/concrete ratio 12-15, while ratio for 150 keV X-Ray is 80... (These are example regulations from my country).