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

The Alpha-Gamma device at NIST 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 a novel measurement of the $^{235}$U neutron-induced fission cross section at the 0.2\% level in an effort to provide a systematically independent measurement greatly needed in the global determination of this quantity. The results of recent and ongoing measurements will be presented, and planned operations will be discussed.