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

 

[raining Pete]

1. The reason hydraulic materials, such as water, are good at shielding against neutrons is due to their ability to slow down and absorb the neutrons. This is because water is made up of hydrogen atoms, which have a low atomic mass and therefore a higher probability of interacting with the neutrons. On the other hand, high Z materials like lead have a higher atomic mass and therefore a lower probability of interacting with the neutrons. This means that lead would not be as effective in slowing down and absorbing the neutrons, making it a less efficient shielding material.

2. The cross section vs. energy curve in nuclear reactions is influenced by a number of factors such as the nuclear properties of the target material and the energy of the incident particles. In some cases, a higher cross section may lead to a higher reaction rate, but this is not always the case. In fact, at higher energies, the probability of a reaction occurring decreases, leading to a decrease in the cross section. This is why the graph shows a larger cross section at lower energies. The use of gold in neutron activation is due to its specific nuclear properties and its ability to efficiently capture neutrons at low energies.