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 core collapse supernova (ccSN) somewhere in the universe each second.  The neutrinos emitted from all of these ccSN 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 understand many aspects of the present 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.


New method to detect supernova relic neutrinos

 

Supernova bursts generate all types of neutrinos, however, because of its larger cross section, anti-electron neutrinos are the most copiously detected neutrinos 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 wants to continue 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 consists of a 200 ton stainless steel tank containing 240 50-cm photomultiplier tubes (227 50-cm tubes are of the same type as in Super-K and 13 of them are prototypes for Hyper-Kamiokande), 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.

テスト用200トンタンク
The gadolinium test facility

 

The new laboratory for the gadolinium test facility was completed in March 2010, and the main 200-ton tank was completed in June 2010. Performance tests of the innovative selective water circulation system, both with and without dissolved gadolinium sulfate in the main tank, were carried out from 2011 through early 2013. In August 2013, 240 photomultiplier tubes were installed in the tank and connected to a DAQ system, turning the 200-ton test tank into an operational detector.

In early 2014 the water filtration system was upgraded, and starting in November of that year the operational 200-ton detector was loaded in steps with gadolinium. By late April 2015 we had reached the target concentration of 0.2% gadolinium sulfate by mass.

タンク内部
Inside of the main 200-ton tank

We have since confirmed that the water attenuation length in the fully Gd-loaded test detector is maintained at the same level as the attenuation length of the ultrapure water in Super-Kamiokande. Other calibration efforts have allowed us to observe the gamma ray cascades emitted by the dissolved gadolinium following neutron capture, and to confirm that our simulations of this new water Cherenkov technology are accurate and reliable. We are now very close to the goal of this proof-of-principle experiment.

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