emiT: An apparatus to test time reversal invariance in polarized neutron decay

We describe an apparatus used to measure the triple-correlation term (Dσn⋅pe×pν) in the beta decay of polarized neutrons. The D coefficient is sensitive to possible violations of time reversal invariance. The detector has an octagonal symmetry that optimizes electron–proton coincidence rates and reduces systematic effects. A beam of longitudinally polarized cold neutrons passes through the detector chamber, where a small fraction undergo beta decay. The final-state protons are accelerated and focused onto arrays of cooled semiconductor diodes, while the coincident electrons are detected using panels of plastic scintillator. Details regarding the design and performance of the proton detectors, beta detectors, and the electronics used in the data collection system are presented. The neutron beam characteristics, the spin-transport magnetic fields, and polarization measurements are also described.