The Neutron Capture Flux Detectors for The BL3 Experiment

The BL3 Experiment aims to measure the neutron lifetime with an uncertainty of 0.3 s and investigate any systematic effects of an in-flight measurement. The lifetime is given by tau = Nn/Np, where Nn is the neutron count rate. A reaction with a Li6 target, whose cross-section varies as 1/v, is used to measure the capture flux of the neutrons. Our goal is to have all target deposits, collimators, and detectors held in a stable geometry that is measurable within 100 um to ensure the relative error in tau is less than 1e-5.due to corrections for nonuniform detection efficiency.. We use a Keyence XM-5000 coordinate measuring machine to measure the as-built vacuum chamber and 3D-print a vacuum-compatible alignment support to position the detector within this tolerance each time the chamber is assembled on the beamline.