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Spallation ultracold-neutron production in superfluid helium.

The first ultracold-neutron (UCN) production in superfluid helium placed in a spallation neutron source is carried out. A UCN density of 0.7 UCN/cm(3), which can be used in experiments, is achieved for a proton-beam power of 78 W and a He-II temperature of 1.2 K. The present new UCN source is not limited by Liouville's theorem and extraction losses, which were serious problems in the previous sources. The present source has the possibility of extremely high-density UCN production compared with previous UCN sources.