  To solve the ''neutron lifetime puzzle,'' where measured neutron lifetimes
differ depending on the measurement methods, an experiment with pulsed neutron
beam at J-PARC is in progress. In this experiment, neutrons are bunched into
40-cm lengths using a spin flip chopper (SFC), where the statistical
sensitivity was limited by the aperture size of the SFC. The SFC comprises
three sets of magnetic supermirrors and two resonant spin flippers. In this
paper, we discuss an upgrade to enlarge the apertures of the SFC. With this
upgrade, the statistics per unit time of the neutron lifetime experiment
increased by a factor of 2.8, while maintaining a signal-to-noise ratio of
250-400 comparable to the previous one. Consequently, the time required to
reach a precision of 1 s in the neutron lifetime experiment was reduced from
590 to 170 days, which is a significant reduction in time. This improvement in
statistic will also contribute to the reduction of systematic uncertainties,
such as background evaluation, fostering further advancements in the neutron
lifetime experiments at J-PARC.
