Measurement of UCN Energy Spectrum of He-II Spallation UCN source

Ultra cold neutron (UCN) is a very low energy neutron ($\sim$200neV), which can be confined in a material bottle. UCN can be used for experiments on fundamental physics such as precise measurements of neutron EDM. A higher intensity UCN source is required to do such precise measurements. We have developed a He-II spallation UCN source at RCNP, Osaka Univ.. In this source, fast neutrons obtained by spallation reactions are moderated by RT D$_2$O and 10K solid D$_2$O, then scattered by phonons in superfluid helium (He-II) to be UCN. The obtained UCN density was 15 UCN/cm$^3$. We measured the energy spectrum of UCN at the exit of this source by storing them in a cylindrical bottle in order to understand the performance of this source. A polyethylene disk installed at the height {\it h} in this bottle absorbs UCN with energy larger than {\it mgh}. The UCN energy spectrum was deduced by differentiating the UCN counts detected as a function of the disk height {\it h}. The obtained spectrum is well reproduced by the Monte Carlo simulation.