I have two nuclear engineer related questions, please bear with me. 1. Why is hydraulic material (like water) is good to shield against neutrons? Why not high Z material like lead? 2. In cross section vs. energy of the nuclei, the curve of the graph undergoes 1/E relationship until it hits the resonance integral, then the value of the cross section just keep going down as the energy increase. This may sound stupid, but isn't it always better to have a higher cross section since bigger cross section leads to more reaction rate right? The graph really confuses me since the graph shows bigger cross section in lower energy. For example, the gold element used in neutron activation is like that. Let me rephrase if there is anything unclear (I have bad English), thanks.