Precision neutron flux measurement using the Alpha-Gamma device

Abstract

The Alpha-Gamma device at the National Institute of Standards and Technology (NIST) utilizes neutron capture on a totally absorbing 10B deposit to measure the absolute neutron flux of a monochromatic cold neutron beam. Gammas produced by the boron capture are counted using high purity germanium detectors, which are calibrated using a well-measured 239Pu alpha source and the alpha-to-gamma ratio from neutron capture on a thin 10B target. This device has been successfully operated and used to calibrate the neutron flux monitor for the BL2 neutron lifetime experiment at NIST. It is also being used for a measurement of the 6Li(n,t)4He cross section. We shall present its principle of operation along with the current and planned projects involving the Alpha-Gamma device, including the recalibration of the U.S. national neutron standard NBS-1 and (n,f) cross section measurements of 235U.

Progress on the BL2 beam measurement of the neutron lifetime

“…What was worse, the evaluation of this particular cross section is based upon measurements that span a wide neutron energy range and that include no direct measurements made at thermal energies. In order to eliminate this reliance on tabulated data from other experiments, we developed the Alpha-Gamma method [9,10,29], a brief overview of which is given here. The Alpha-Gamma device is a totally absorbing neutron detector based on neutron absorption by 10 B.…”