Measurements of (n,α) cross-section of small samples using a lead-slowing-down-spectrometer

At the Los Alamos Neutron Science Center (LANSCE) a compensated ionization chamber (CIC) was placed in a lead slowing down spectrometer (LSDS) to measure the 6Li(n,α)3H cross-section as a feasibility test for further work. The LSDS consists of a 1.2 m cube of lead with a tungsten target in the center where spallation neutrons are produced when bombarded with pulses of 800 MeV protons. The resulting neutron flux is of the order of 1014 n/cm2 /s which allows the cross-section measurement of samples of the order of 10ʹs of nanograms. The initial experiment measured a 91 μg sample of natural lithium flouride. Cross-section measurements were obtained in the 0.1 eV–2 keV energy range. A 62 μg sample was placed in the chamber with a higher neutron beam intensity, and data was obtained in the 0.1–300 eV range. Adjustments in chamber dimensions and electronic configuration will improve gamma flash compensation at high beam intensity, decrease the dead time, and increase the energy range where data can be obtained. The intense neutron flux will allow the use of a smaller sample.