Search for Supernova Relic Neutrinos
Supernova remnant in the universe
In February of 1987, the Kamiokande detector detected the world’s first neutrinos from a supernova burst. (See more) Since then, no supernova explosion has occurred in or near our galaxy, so we have not observed any neutrinos from a supernova burst since then.
Supernova explosions in our galaxy may be fairly rare, but supernovae themselves are not. On average, there is one neutrino-generating supernova burst somewhere in the universe each second. The neutrinos emitted from all of these supernova bursts since the onset of stellar formation have suffused the universe. We refer to this thus-far unobserved flux as the Diffuse Supernova Neutrino Background [DSNB], also known as the “relic” supernova neutrinos.
The detection of the supernova relic neutrinos enables us to investigate the history of star formation, a key factor in cosmology, nucleosynthesis, and stellar evolution. Furthermore, the study of supernova bursts, which produce and disperse elements heavier than helium, is vital to understanding many aspects of the present physical universe.
New method to detect supernova relic neutrinos
The flux of the supernova relic neutrinos is expected to be several tens per square centimeter per second. It is very weak compared to the flux of higher energy neutrinos: six million per square centimeter per second. Theoretical models vary, but as many as five supernova relic neutrinos per year above 10 MeV are expected to interact in Super-Kamiokande. However, in order to separate the rare DSNB signals from the much more common solar neutrinos and other backgrounds, we need a new detection method.
Supernova bursts generate all types of neutrinos, however anti-electron neutrinos are most observable in a water Cherenkov detector like Super-Kamiokande. About 80% of the detectable supernova neutrino events are inverse beta interactions: an anti-electron neutrino interacts with a proton, ending up with a positron and a neutron in the final state. Super-K can detect the relativistic positron because it emits Cherenkov light. But to identify the signal as coming from an anti-electron neutrino, we need to detect not only the positron but also the neutron.
We are considering dissolving a 0.2% concentration of a gadolinium compound in Super-Kamiokande in order to detect the neutron. The cross section of gadolinium to capture neutrons is very large, and the gadolinium then emits a cascade of observable gamma rays after the capture reaction. The coincident detection of a positron’s Cerenkov light, followed shortly thereafter in roughly the same place by a shower of gamma rays, will serve to positively identify inverse beta reactions in the detector.
Once we add gadolinium to Super-Kamiokande, we expect to record up to 20 supernova relic neutrino signals – with almost no background – after five years data. This will be the world’s first observation of the DSNB. The same coincidence technique will also allow Super-K to make a very high statistics measurement of the anti-electron neutrino flux and spectrum from all of Japan’s nuclear power reactors, yielding the world’s most accurate determination of the mixing parameters connecting the first two generations of neutrinos.
Proof-of-Principle Experiment is underway
Super-Kamiokande continues its precise observation of solar, atmospheric, and man-made neutrinos, so it is necessary to confirm that adding the gadolinium will not negatively affect other neutrino observations. Therefore, we have excavated a new experimental hall for dedicated gadolinium R&D studies; it is located near the Super-Kamiokande detector in the Kamioka mine. The gadolinium test facility will consist of a 200 ton stainless steel tank containing 240 50-cm photomultiplier tubes (the same type as in Super-K), DAQ electronics, calibration equipment, a water attenuation length measurement device, and a selective water filtration system needed to keep the water clean yet retain the dissolved gadolinium in solution.
The new experimental hall’s excavation was completed in late 2009, and the laboratory space itself was ready for occupancy in early 2010. The main tank will be constructed by the middle of 2010, and the rest of the equipment will be installed during the remainder of the year. We shall then begin the experimental program to conclusively verify that the gadolinium-loaded water’s attenuation length remains high (more than 70 meters) and that the gadolinium solution does not damage the components of the tank. These studies – including long-term running to evaluate system stability – are expected to take about three years. By the end of this period we expect to have demonstrated that gadolinium is appropriate for use in Super-Kamiokande.