Question about neutron shielding and resonance integral

[aznsaiyan1029]

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.

 

[Astronuc]

For neutron shielding, is one wishing to shield fast or thermal neutrons.

Bascially hydrogen and light elements are good shields for fast neutrons because the with each collising the fast neutron loses a good fraction of it's kinetic energy. If a neutron has a collision with a proton, it could lose almost all of it's energy, assuming the collision is perfectly elastic.

Higher Z materials are used for gamma interaction, through the Compton and photo-electric effects, and pair-production for gamma-rays of energy > 1.022 MeV.

High Z materials would simply scatter neutrons with little energy loss, and if they absorb neutrons, then they also become a gamma source (n, gamma reaction) as well as probably producing a beta emitting radionuclide.

In terms of absorption, having a high cross-section is desirable.

Does one have a particular set of data in mind?

See - http://www.nndc.bnl.gov/sigma/index.jsp?as=197&lib=endfb7.0&nsub=10
Total elastic cross-section Au-197 - http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=4518&mf=3&mt=2&nsub=10