Shielding

Materials used for shielding detectors should minimize unwanted scattering in the sample well in neutron scattering experiments. This unwanted scattering from shielding materials is called albedo and is defined as the scattered neutron current divided by the incoming neutron current.

Materials with high absorption cross sections and low scattering cross sections make the best shielding materials. In this figure the albedo for 800 meV neutrons for four different detector angles is shown. Steel, which has the highest scattering cross section, exhibited the highest reflection albedo. Boron-10 which has the highest absorption cross section also exhibited the lowest reflection albedo.

This first chart shows the relative reflection albedos of polyethylene, 3% by weight boron borated polyethylene (pink), 4% by weight boron borated polyethylene (purple), boron carbide with epoxy and 4.5% by weight boron borated aluminum. As shown in the graph the borated aluminum and boron carbide had the lowest measured reflection albedo.

The second chart shows the reflection albedos of different combinations of materials. In comparing the boron carbide without cadmium lining to the boron carbide with cadmium lining, the reflection albedo is lower at shorter wavelengths for boron carbide by itself. At wavelengths longer than 0.45 angstroms, the boron carbide with cadmium lining exhibited lower albedo. This is due to the increase in absorption cross section of cadmium at lower wavelengths. When comparing the boron carbide with borated polyethylene to just boron carbide, we observed similar results. The polyethylene moderates high energy neutrons that are not detected to energies where they can be detected, thus, the measured albedo increases at shorter wavelengths. At longer wavelengths the polyethylene moderates the all ready detectable neutrons down to energies where they are absorbed more readily by the boron.

This project was conducted in collaboration with the Nuclear Engineering Department at the University of Illinois, Urbana-Champaign. W. Rothstein of the group of B. Heuser joined C. Hoffmann, G. Granroth, E. Iverson, J. Carpenter, R. Portes, J. Hammonds as research aid.