Precision Measurement of the n-235U fission cross section using the “Alpha-Gamma” Absolute Cold Neutron Flux Measurement Device

The Alpha-Gamma device at the National Institute of Standards and Technology utilizes the interaction of neutrons with a totally absorbing $^{10}$B target to precisely measure the flux of a monochromatic neutron beam. This measurement provides a calibration of the $^{6}$Li(n,$\alpha$)$^{3}$H based flux monitor used in the NIST neutron lifetime experiment to better than 0.1% and is now being utilized in novel, 0.2% level, measurements of the $^{235}$U neutron-induced fission and $^6$Li cross sections in an effort to provide systematically independent determinations of these important quantities. The results of recent and ongoing measurements will be presented, and planned operations will be discussed.