This a special photomultiplier tube built in the mid 1980s by the Hamamatsu Corporation, designed to detect proton decay predicted by the Grand Unified Theory. The tube, one of the original array of 1000 that surrounded an immense underground water reservoir, was designed to detect a flash of Cerenkov radiation caused by the decay process. But when no interactions were found in over a year of operation, the instrument was upgraded to be sensitive enough to detect the interaction of a neutrino and an electron in the mass of water molecules in the chamber. Renamed Kamiokande II, this detector recorded the passage of eleven neutrinos, members of a vast storm resulting from the eruption of the brightst supernova in the past 400 years (SN 1987A) that took place in the large Magellanic Cloud. This opened up a new form of observaitonal astronomy called today "neutrino astronomy," an event which has since been recognized by the award of the Nobel Prize for physics to the principal scientist heading the project, M. Koshiba.

During normal operations the array of 1000 tubes is periodically checked for vacuum leakage, and this one was found to be defective and so was decommissioned in 1990. The leakage is most evident in the evaporation of the antimony coating on the inside of the glass, which renders its inner workings visible. The tube was selected by the curator during a site visit in 199to the mine in Japan that houses the detector. It was donated to NASM in 1997 by the Institute for Cosmic Ray Research of the University of Tokyo and prepared for shipment by Hamamatsu.