Neutron β decay with Ultracold neutrons

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The aim of this research work was to carry out a β decay experiment with ultracold neutrons (UCN). High density UCN are highly desirable for the UCN experiment. We have studied UCN production in superfluid helium (He-II) with spallation neutrons. Based on this study, we built a spallation neutron source, thermal and cold neutron moderators with heavy water and a He-II cryostat for the UCN production. We succeeded in the UCN production and observation in the new generation UCN source. We confirmed the UCN observation by placing a complete reflector for UCN in front of a UCN detector and changing the temperature of He-II. UCN counts varied as it was expected. We studied a UCN detector efficiency and then obtained a UCN density in a experimental bottle. The value was 1.4 UCN/cm^3 for UCN energies less than 100 neV at a proton beam power of 156 W. The UCN density is competitive to UCN density at Grenoble which is largest in the world. We expect the UCN density will be larger than 10000 UCN/cm^3 by improvement of the proton beam power, the He-II ctyostat and aneutron moderator reflector configuration. We published the result in Phys. Rev. Lett.Ultracold NeutronSymmetry