Improvements to the neutron detector for the BL3 experiment

The goal of the BL3 Experiment is to measure the neutron in-flight decay lifetime with an absolute precision of 0.3 s. This is needed to resolve the discrepancy between previous in-flight decay measurements versus experiments using trapped UCNs. One source of systematic uncertainty arises from measuring the neutron fluence through the penning trap. To improve the neutron count measurement, a novel "caustic" aperture design can be used to provide perfect uniform detection efficiency across the entire beam profile. I will share analytic solutions for uniform detection efficiency in 2D and numerical methods for designing 3D apertures for circular detectors.