We describe an electron spectrometer designed for a precision measurement of the neutron β-asymmetry with spin-polarized ultracold neutrons. The spectrometer consists of a 1.0-Tesla solenoidal field with two identical multiwire proportional chamber and plastic scintillator electron detector packages situated within 0.6-Tesla fieldexpansion regions. Select results from performance studies of the spectrometer with calibration sources are reported.
Abstract. Precision measurements in neutron beta decay serve to determine the coupling constants of beta decay and allow for several stringent tests of the standard model. This paper discusses the design and the expected performance of the Nab spectrometer.Keywords: neutron beta decay; weak interactions PACS: 23. 23.40.Bw A program of precision studies of neutron beta decay is planned with the Nab spectrometer. Nab will use the state of the art neutron beamline (FNPB) at the new Spallation Neutron Source (SNS) in Oak Ridge, TN. The expected results will play a critical role in resolving longstanding discrepancies in the neutron decay world data set, and will allow an extraction of V ud , the upper left element of the Cabbibo-Kobayashi-Maskawa matrix. The redundancy inherent in the standard model description of the neutron beta decay process allows uniquely sensitive checks of the model's validity and limits [1,2,3,4], with strong implications in astrophysics [5].The aim of the Nab collaboration is to determine a, the neutrino electron correlation coefficient, and b, the Fierz parameter, in the decay of the free neutron [6]. The a coefficient can be obtained through a measurement of the electron energy E e , and the momentum p p of the corresponding proton. In zeroth order approximation, the a arXiv:1209.4663v1 [nucl-ex]
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