A new search for the diffuse supernova neutrino background (DSNB) flux has been conducted at Super-Kamiokande (SK), with a 22.5 × 2970-kton•day exposure from its fourth operational phase IV. The new analysis improves on the existing background reduction techniques and systematic uncertainties and takes advantage of an improved neutron tagging algorithm to lower the energy threshold compared to the previous phases of SK. This allows for setting the world's most stringent upper limit on the extraterrestrial νe flux, for neutrino energies below 31.3 MeV. The SK-IV results are combined with the ones from the first three phases of SK to perform a joint analysis using 22.5 × 5823 kton•days of data. This analysis has the world's best sensitivity to the DSNB νe flux, comparable to the predictions from various models. For neutrino energies larger than 17.3 MeV, the new combined 90% C.L. upper limits on the DSNB νe flux lie around 2.7 cm −2 •sec −1 , strongly disfavoring the most optimistic predictions. Finally, potentialities of the gadolinium phase of SK and the future Hyper-Kamiokande experiment are discussed.

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To examine the effect of so-called "concentration polarization" on the performance of a membrane reactor with a highly hydrogen-permeable membrane, methane steam reforming was conducted, using a Pd/Ag membrane with a thickness of a few micrometers. First, the relation between the methane conversion and the hydrogen recovery was experimentally examined, and the relation was compared with that predicted by a rather simple simulation that assumes the instant achievement of equilibrium. When the hydrogen recovery was smallest, the experimental results agreed well with the simulation results. With increasing reaction pressure, the experimental methane conversion became lower than the simulated conversion. These results suggest that the reaction is limited by reduced hydrogen removal, because of the concentration polarization. The influence of concentration polarization was confirmed by the comparison of the experimental results of hydrogen permeation from a mixture of H 2 and N 2 with the simulation results based on the plug-flow model. It then was experimentally attempted to reduce the concentration polarization by changing the configuration of the reactor. The methane conversion was successfully improved using reactors that had narrower inner diameters and baffle plates, probably because of the reduction in concentration polarization. It was concluded that the reactor configuration was quite essential to make the best use of a membrane reactor with a highly permeable membrane.

Core-collapse supernovae are among the most magnificent events in the observable universe. They produce many of the chemical elements necessary for life to exist and their remnants-neutron stars and black holes-are interesting astrophysical objects in their own right. However, despite millennia of observations and almost a century of astrophysical study, the explosion mechanism of core-collapse supernovae is not yet well understood.