An Apparatus for Absolute Neutron Flux Measurement

A fully-absorbing neutron detector is being developed to measure the absolute flux (s$^{-1}$) of a cold neutron beam at the level of 0.1\,\%. The device will be used to calibrate a neutron flux monitor used in an in-beam neutron lifetime measurement performed at NIST ($\tau_{n} = \left(886.3 \pm 3.4\right)$ s). The precision of the measurement was limited by the uncertainty in the efficiency of the neutron flux monitor (0.3\,\%). The flux monitor operates by counting charged particles produced when neutrons impinge on a ${}^{6}$Li (or ${}^{10}$B) foil. Its efficiency was calculated from the cross section, the solid angle subtended by the charged particle detectors, and the amount of neutron-absorbing material present on the foil. Successful calibration would reduce the neutron lifetime uncertainty to approximately 0.25\,\%. In addition, using the measured solid angle and amount of material on the deposit, a new experimental value for the ${}^{6}$Li or (${}^{10}$B) capture cross section will be obtained. Details of the apparatus and the measurement technique along with the status of the experiment will be discussed.