Average Neutron Total Cross Sections in the 3- to 12-Mev Region

An experiment is described for measuring average neutron total cross sections over the 3- to 12-Mev energy region. The neutron source is a collimated beam of fast neutrons emerging from the Los Alamos Fast Reactor, and the detector is a proton recoil ionization chamber which serves as a neutron spectrometer. The average energy spread of the measurements is around 10 percent, and the over-all accuracy of the cross-section determinations is ±{}10 percent or better. Total cross-section results are presented for H, Be, C, O, Al, Si, S, Fe, Cu, Zr, Pb, Bi, and U. Neighboring elements above Fe are observed to exhibit regularities in their total cross-section patterns.