A STUDY OF THE 235U(n,f) CROSS SECTION IN THE 3 TO 30 MeV ENERGY REGIONNational Bureau of Standards, Gaithersburg, MD 20899, U.S.A. P.W. Lisowski and J.L. Ullmann Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. N.W. Hill Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A. The 235U(n,f) cross section is often considered the most favorable of the neutron cross section standards. Above a few MeV, however, the uncertainties are still unacceptably high, except for the 14 MeV region. In an effort to improve the accuracy of this cross section, measurements have been initiated at the new target 4 facility at the Los Alamos National Laboratory (LANL) 800 MeV proton accelerator. This target provides an intense source of high energy neutrons at the 20 meter flight path used in the present study. The fission reaction rate is determined with a fast parallel plate ionization chamber designed at LANL while the neutron fluence is being measured with an annular proton recoil telescope whose properties were carefully studied earlier at the National Bureau of Standards (NBS). This detector provides satisfactory performance for neutron energies up to about 30 MeV. Possible use of this detector at higher neutron energies where the recoil proton range exceeds the solid state detector thickness is being investigated. The measurements provide the shape of the 235U(n,f) cross section relative to the hydrogen scattering cross section. The data will be normalized to the very accurately known values at 14 MeV. Experimental tests, diagnostic studies and preliminary cross section determinations have been completed and will be reported. KEYWORDS: annular proton recoil telescope, fission, fission chamber, fluence, neutron, standard, 235U(n,f) |