We have measured the neutron lifetime using a superconducting electron spectrometer and a pulsed beam of cold neutrons. Spatially defined neutron bunches are completely contained within the spectrometer's active volume while the /3-decay rate is measured. The flux is determined from the radioactivity of nearly totally absorbing thick samples of cobalt and gold exposed to the neutron beam. We obtain r" =876 ± 21 sec.PACS numbers: 13.30.Ce, 14.20.Dh, 23.40.Bw The measured neutron lifetime provides information about fundamental parameters, in particular the vector and axial-vector weak-coupling constants, Gv and GA> Knowing these quantities is crucial for an understanding of the weak interaction and there are important consequences for cosmology and astrophysics. x At present Gv is best determined from nuclear /?-decay experiments and the experimentally determined neutron lifetime is used to infer GA/GVIn previous experiments electrons or protons were detected from neutrons decaying in continuous thermal or cold beams. Published lifetimes 2 vary between 877 and 937 sec, with errors between 8 and 18 sec. The discrepancies are much larger than the stated errors. Until recently, precise values for GA/GV came only from the experimental lifetime, but the inconsistencies introduce a large systematic uncertainty. Fortunately, a recent measurement of the neutron p asymmetry determines GA/GV better than the lifetime measurements, mitigating the difficult choice between inconsistent lifetimes. 3 Nevertheless, it is important to resolve the lifetime discrepancies. The combination of very precise neutron-decay experiments will eventually allow us to determine both GA and Gv without resorting to more complex nuclear systems.We measured the neutron lifetime with a new method using a pulsed neutron beam. This pulsed beam passes through an "in-beam" electron spectrometer with large active volume. We detect decay electrons during short time intervals of length At ~ 0.5 msec, when neutron bunches are fully contained within the spectrometer and all neutron decays with electron energies above threshold have the same detection probability. The neutron lifetime r n is obtained from the decay law AN n /At = -N n /T n . The integrated number of neutrons N n comes from measurements of the y activity, n r =N n /r r , of thick samples of high-purity 5 Co and 197 Au exposed to the beam. The neutron-decay rate ANjAt is obtained from the measured p rate n p =Np/At. Thus the experimental method is summarized by the simple relation Tn^tyiriy/np) which relates the neutron lifetime to the well-known nuclear lifetimes x y .Compared to continuous-beam lifetime measurements our method does not require precise knowledge of the effective length of the spectrometer or the neutron velocity distribution. In many previous experiments sensitivity to the neutron velocity spectrum is avoided by the measurement of the neutron-capture flux directly with thin detectors containing 3 He or 10 B. However, this procedure relies on the exactness of the l/v dependenc...

Both the neutron lifetime and the neutron β-decay asymmetry have recently been measured with high accuracy. A joint evaluation of these measurements gives values for the weak-coupling constants gV and gA. We investigate implications of these measurements for the conserved vector current hypothesis, the unitarity of the weak quark mixing matrix, hyperon decay and SU(3)-symmetry, the existence of right-handed currents, the solar-neutrino problem, and big-bang cosmology.

We have experimentally studied the electron-positron pairs coming from the decay of the unbound deuteron first excited state following cold-neutron capture on hydrogen. The measured branching ratio for pair emission, re, /T, = 3.44( 14) X is in excellent agreement with internal-pair-emission theory. This result is a strong constraint on theories which suggest the existence of short-lived pseudoscalar particles, such as the "viable axion," heavy enough to decay to electron-positron pairs.

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