Absolute Neutron Fluence Measurements at the NIST Center for Neutron Research

Precise, absolute fluence measurements of cold and thermal neutron beams are of primary importance to beam-type determinations of the neutron lifetime, measurements of standard neutron cross sections, and the development of standards for neutron dosimetry. At the National Institute of Standards and Technology (NIST), a totally absorbing neutron detector based on absolute counting of the $^{10}$B(n,$\alpha_{1}$)$^{7}$Li reaction 478 keV gamma ray has been used to perform fluence measurements with a precision of 0.06\,\%. This detector has been used to improve the neutron fluence determination in the 2000 NIST beam neutron lifetime by a factor of five, significantly reducing the uncertainty in the lifetime result. Ongoing and possible future uses of the Alpha-Gamma device include 1) Calibration of the neutron fluence monitors that will be used in the upcoming NIST beam neutron lifetime measurement BL2; 2) The first direct, absolute measurement of the $^{6}$Li(n,t)$^{4}$He neutron cross section at sub-thermal neutron energy; 3) Measurements of the $^{10}$B(n,$\gamma$)$^{11}$B and $^{235}$U(n,f) neutron cross sections; 4) A re-calibration of the national neutron standard NBS-1. The apparatus, measurement technique, and applications will be discussed.