The neutron lifetime has been measured by comparing the decay rate with the reaction rate of 3He nuclei of a pulsed neutron beam from the spallation neutron source at the Japan Proton Accelerator Research Complex (J-PARC). The decay rate and the reaction rate were determined by simultaneously detecting electrons from the neutron decay and protons from the 3He(n,p)3H reaction using a gas chamber, the working gas of which contains diluted 3He. The measured neutron lifetime was 898 ± 10stat+15-18 sys s.
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