The BL3 experiment at NIST

Neutron beta decay provides input to the primordial abundance of 4He in Big Bang Nucleosynthesis, and is a theoretically clean test of CKM unitarity unencumbered by nuclear structure effects. However, the experimental determination of the neutron lifetime is plagued by a 8.7 second (4 sigma) discrepancy between in-flight decay and trapped ultra-cold neutron measurements. The goal of the BL3 experiment is to examine this discrepancy by improving the in-flight decay determination to 0.3 seconds, examining various systematic effects which might explain the difference. Enhancements in this experiment involve a larger, improved proton trap to measure the absolute neutron decay rate, and a neutron detector with improved detection uniformity and absolute calibration to measure the neutron fluence of the beam. Both new detectors were designed to take advantage of the high-flux NG-C beamline at the NIST Center for Neutron Research. I will describe the in-flight decay measurement technique and some innovations distinguishing the BL3 experiment from other measurements using this method.