is a neutrino observatory located under Mount Ikeno near the city of Hida, Gifu Prefecture, Japan. It is operated by the Institute for Cosmic Ray Research, University of Tokyo with the help of an international team. It is located 1,000 m (3,300 ft) underground in the Mozumi Mine in Hida's Kamioka area. The observatory was designed to detect high-energy neutrinos, to search for proton decay, study solar and atmospheric neutrinos, and keep watch for supernovae in the Milky Way galaxy.

Description

Super-K is located underground in the Mozumi Mine in Hida's Kamioka area. It consists of a cylindrical stainless steel tank that is tall and in diameter holding 50,220 tonnes (55,360 US tons) of ultrapure water. The tank volume is divided by a stainless steel superstructure into an inner detector (ID) region, which is in height and in diameter, and outer detector (OD) which consists of the remaining tank volume. Mounted on the superstructure are 11,146 photomultiplier tubes (PMT) in diameter that face the ID and 1,885 PMTs that face the OD. A Tyvek and blacksheet barrier attached to the superstructure optically separates the ID and OD.

A neutrino interaction with the electrons or nuclei of water can produce a charged particle that moves faster than the speed of light in water, which is slower than the speed of light in vacuum. This creates a cone of light known as Cherenkov radiation, which is the optical equivalent to a sonic boom. The Cherenkov light is projected as a ring on the wall of the detector and recorded by the PMTs. Using the timing and charge information recorded by each PMT, the interaction vertex, ring direction, and flavor of the incoming neutrino is determined. From the sharpness of the edge of the ring the type of particle can be inferred. The multiple scattering of electrons is large, so electromagnetic showers produce fuzzy rings. Highly relativistic muons, in contrast, travel almost straight through the detector and produce rings with sharp edges.

History

thumb|right|A model of KamiokaNDE

Construction of the predecessor of the present Kamioka Observatory, the Institute for Cosmic Ray Research, University of Tokyo began in 1982 and was completed in April 1983. The purpose of the observatory was to determine the existence of proton decay, one of the most fundamental questions of elementary particle physics.

The detector, named KamiokaNDE for Kamioka Nucleon Decay Experiment, was a tank in height and in width, containing 3,058 tonnes (3,400 US tons) of pure water and about 1,000 photomultiplier tubes (PMTs) attached to its inner surface. The detector was upgraded, starting in 1985, to allow it to observe solar neutrinos. As a result, the detector (KamiokaNDE-II) had become sensitive enough to detect ten neutrinos from SN 1987A, a supernova which was observed in the Large Magellanic Cloud in February 1987, and to observe solar neutrinos in 1988. The ability of the Kamiokande experiment to observe the direction of electrons produced in solar neutrino interactions allowed experimenters to directly demonstrate for the first time that the Sun was a source of neutrinos.

While making discoveries in neutrino astronomy and neutrino astrophysics, Kamiokande never detected a proton decay, the primary goal for its construction. The absence of any such observation pushed back the possible half-life of any potential proton decay far enough to eliminate some of the GUT models which allow for such a decay. Other models predict a longer half-life, with rarer decays.

To increase the chance of detecting such decays, a larger detector was needed. A higher sensitivity was also necessary to obtain a higher statistical confidence in other detections. This led to the design and construction of Super-Kamiokande, with fifteen times the volume of water and ten times as many PMTs as Kamiokande.

The Super-Kamiokande project was approved by the Japanese Ministry of Education, Science, Sports and Culture in 1991 for total funding of approximately $100 million. The American portion of the proposal, which was primarily to build the OD system, was approved by the United States Department of Energy in 1993 for $3 million. In addition, the United States has also contributed about 2000 20 cm PMTs recycled from the IMB experiment. This was the first experimental observation supporting the theory that the neutrino has non-zero mass, a possibility that theorists had speculated about for years. The 2015 Nobel Prize in Physics was awarded to Super-Kamiokande researcher Takaaki Kajita alongside Arthur McDonald at the Sudbury Neutrino Observatory for their work confirming neutrino oscillation.

On 12 November 2001, about 6,600 of the photomultiplier tubes imploded in a chain reaction, as the shock wave from the concussion of each imploding tube cracked its neighbours. The detector was partially restored from April to October 2002 by redistributing the photomultiplier tubes which did not implode, and by adding protective acrylic shells that are hoped will prevent another chain reaction from recurring (Super-Kamiokande-II).

In July 2005, preparations began to restore the detector to its original form by reinstalling about 6,000 PMTs. The reconstruction began in October 2005.

After Phase IV, the detector underwent a full refurbishment during Autumn of 2018, including sealing the tank against water leaks, cleaning, improving piping, and replacing failed PMTs. On 29 January 2019 the detector resumed data acquisition.

In 2020, the detector was upgraded for the SuperKGd project (phase SK-VI) by adding a gadolinium salt to the ultrapure water in order to enable the detection of antineutrinos from supernova explosions.

{| class="wikitable"

|+ Table 1

|-

! colspan="2" style="text-align: center;" |Phase !! SK-I !! SK-II !! SK-III !! SK-IV

|-

| Period || Start || 1996 Apr. || 2002 Oct.|| 2006 Jul. || 2008 Sep.

|-

| || End || 2001 Jul. || 2005 Oct. || 2008 Sep. || 2018 Jun.

|-

| Number of PMTs || ID || 11146 (40%) || 5182 (19%) || 11129 (40%) || 11129 (40%)

|-

| || OD || colspan="4" style="text-align: center;" |1885

|-

| colspan="2" style="text-align: center;" |Anti-implosion container || No || Yes || Yes || Yes

|-

| colspan="2" style="text-align: center;" |OD segmentation|| No || No || Yes || Yes

|-

| colspan="2" style="text-align: center;" |Front-end electronics || colspan="3" style="text-align: center;" |ATM (ID) || QBEE

|-

| || || colspan="3" style="text-align: center;" |OD QTC (OD)||

|}

SK-IV upgrade

In the previous phases, the ID-PMTs processed signals by custom electronics modules called analog timing modules (ATMs). Charge-to-analog converters (QAC) and time-to-analog converters (TAC) are contained in these modules that had dynamic range from 0 to 450 picocoulombs (pC) with 0.2 pC resolution for charge and from −300 to 1000 ns with 0.4 ns resolution for time. There were two pairs of QAC/TAC for each PMT input signal, this prevented dead time and allowed the readout of multiple sequential hits that may arise, e.g., from electrons that are decay products of stopping muons. The QBEE provides high-speed signal processing by combining pipelined components. These components are a newly developed custom charge-to-time converter (QTC) in the form of an application-specific integrated circuit (ASIC), a multi-hit time-to-digital converter (TDC), and field-programmable gate array (FPGA). Each QTC input has three gain ranges "Small", "Medium", and "Large" – the resolutions for each are shown in Table. This is known as the SK-Gd project (other names include SuperKGd, SUPERK-GD, and similar names). In the first phase of the project, 1.3 tons of a Gd salt (gadolinium sulfate octahydrate, ) were added to the ultrapure water in 2020, giving 0.02% (by mass) of the salt. This amount is about a tenth of the planned final target concentration.

Gadolinium has an affinity for neutrons and produces a bright flash of gamma rays when it absorbs one. Adding gadolinium to the Super-Kamiokande allows it to distinguish between neutrinos and antineutrinos. Antineutrinos produce a double flash of light about 30 microseconds apart, first when the neutrino hits a proton and second when gadolinium absorbs a neutron.

PMTs and associate structure

The basic unit for the ID PMTs is a "supermodule", a frame that supports a 3×4 array of PMTs. Supermodule frames are 2.1 m in height, 2.8 m in width, and 0.55 m in thickness. These frames are connected to each other in both the vertical and horizontal directions. Then the whole support structure is connected to the bottom of the tank and to the top structure. In addition to serving as rigid structural elements, super module simplified the initial assembly of the ID.

Each supermodule was assembled on the tank floor and then hoisted into its final position. Thus, the ID is in effect tiled with super module. During installation, ID PMTs were pre-assembled in units of three for easy installation. Each supermodule has two OD PMTs attached on its back side. The support structure for the bottom PMTs is attached to the bottom of the stainless-steel tank by one vertical beam per supermodule frame. The support structure for the top of the tank is also used as the support structure for the top PMTs.

Cables from each group of three PMTs are bundled together. All cables run up the outer surface of the PMT support structure, i.e., on the OD PMT plane, pass through cable ports at the top of the tank, and are then routed into the electronics huts.

The thickness of the OD varies slightly, but is on average about 2.6 m on top and bottom, and 2.7 m on the barrel wall, giving the OD a total mass of 18 kilotons. OD PMTs were distributed with 302 on the top layer, 308 on the bottom, and 1275 on the barrel wall.

To protect against low-energy background radiation from radon decay products in the air, the roof of the cavity and the access tunnels were sealed with a coating called Mine guard. Mine guard is a spray-applied polyurethane membrane developed for use as a rock support system and radon gas barrier in the mining industry.

The system will run special processes to check for spallation muons when burst candidates meeting "alarm" criteria and make a primary decision for further process. If the burst candidate passes these checks, the data will be reanalyzed using an offline process and a final decision will be made within a few hours. During the Super-Kamiokande I running, this never occurred.

One of the important capabilities for [Super-Kamiokande] is to reconstruct the direction to supernova. By neutrino–electron scattering, <math>\nu_\text{x} + e^- \to \nu_\text{x} + e^-</math>, a total of 100–150 events are expected in case of a supernova at the center of the Milky Way Galaxy.

In 1999, the Super-Kamiokande detected strong evidence of neutrino oscillation that successfully explained the solar neutrino problem. The Sun and about 80% of the visible stars produce their energy by the conversion of hydrogen to helium via

<math>4p \to {}^4\!He + 2e^+ + 2 \nu_e + 26.73</math> MeV

Consequently, stars are a source of neutrinos, including the Sun. These neutrinos primarily come through the p-p chain in lower masses, and for cooler stars, primarily through the CNO cycle of heavier masses.

In the early 1990s, particularly with the uncertainties that accompanied the initial results from Kamioka II and the Ga experiments, no individual experiment required a non-astrophysical solution of the solar neutrino problem. But in aggregate, the Cl, Kamioka II, and Ga experiments indicated a pattern of neutrino fluxes that was not compatible with any adjustment of the SSM. This in turn helped motivate a new generation of spectacularly capable active detectors. These experiments are Super-Kamiokande, the Sudbury Neutrino Observatory (SNO), and Borexino. Super-Kamiokande was able to detect elastic scattering (ES) events

:<math>\nu_x + e^- \to \nu_x + e^-</math>

which, due to the charged-current contribution to <math>\nu_e</math> scattering, has a relative sensitivity to <math>\nu_e</math> s and heavy-flavor neutrinos of ~7:1. Since the direction of the recoil electron is constrained to be very forward, the direction of the neutrinos is kept in the direction of the recoil electrons. Here, <math>\cos \theta_{Sun}</math> is provided, where <math>\theta_{Sun}</math> is the angle between the direction of recoil electrons and the Sun's position. This shows that the <math>{}^8\!B</math> solar neutrino flux can be calculated to be <math>2.40 \pm 0.03(stat.) {}_{-0.07}^{+0.08}\!(sys.) \times 10^6 cm^{-2} s^{-1}</math>. Comparing to the SSM, the ratio is <math>{Data \over SSM_{BP98=0.465 \pm 0.005(stat.) {}_{-0.013}^{+0.015}\!(sys.)</math>. The result clearly indicates the deficit of solar neutrinos.

Atmospheric neutrino

Atmospheric neutrinos are secondary cosmic rays produced by the decay of particles resulting from interactions of primary cosmic rays (mostly protons) with Earth atmosphere. The observed atmospheric neutrino events fall into four categories. Fully contained (FC) events have all their tracks in the inner detector, while partially contained (PC) events have escaping tracks from the inner detector. Upward through-going muons (UTM) are produced in the rock beneath the detector and go through the inner detector. Upward stopping muons (USM) are also produced in the rock beneath the detector, but stop in the inner detector.

The number of observed neutrinos is predicted uniformly regardless of the zenith angle. However, Super-Kamiokande found that the number of upward-going muon neutrinos (generated on the other side of the Earth) is half of the number of downward-going muon neutrinos in 1998. This can be explained by the neutrinos changing or oscillating into some other neutrinos that are not detected. This is called neutrino oscillation; this discovery indicates the finite mass of neutrinos and suggests an extension of the Standard Model. Neutrinos oscillate in three flavors, and all neutrinos have their rest mass. Later analysis in 2004 suggested a sinusoidal dependence of the event rate as a function of "Length/Energy", which confirmed the neutrino oscillations.

K2K Experiment

The K2K experiment was a neutrino experiment from June 1999 to November 2004. This experiment was designed to verify oscillations observed by Super-Kamiokande through muon neutrinos produced by interactions of protons accelerated at the KEK laboratory on a target. It provided the first positive measurement of neutrino oscillations in conditions where the neutrino flux could be measured before oscillations by a close detector. The Super-Kamiokande detector plays an important role in the experiment as the far detector. A second-generation experiment,T2K experiment, was designed as a follow-up of the K2K experiment.

T2K Experiment

The T2K (Tokai to Kamioka) experiment is a wide international collaboration with the goal of measuring all the unknown parameters of neutrino oscillations. T2K has made a search for oscillations from muon neutrinos to electron neutrinos, and announced the first experimental indications for them in June 2011. Super-Kamiokande is the "far detector".

Proton decay and neutron-antineutron oscillations

The proton is assumed to be absolutely stable in the Standard Model. However, the Grand Unified Theories (GUTs) predict that protons can decay into lighter energetic charged particles such as electrons, muons, pions, or others which can be observed. Super-Kamiokande published the most stringer limits for most of the proton decay channels:

  • for decay to a positron and a neutral pion (p → e⁺ + π⁰),
  • for decay to an muon antineutrino and a positive kaon (p → ν̄K<sup>+</sup>).

Furthermore, Super-Kamiokande published the most stringent limit on neutron-antineutron oscillations, establishing a limit to the neutron-antineutron nuclear lifetime of <math>T_{n\bar{n \geq 3.6 \cdot 10^{32}</math> yr at 90% confidence level, corresponding to a limit on the neutron-antineutron oscillation time of <math>\textstyle \tau_{n\bar n} \geq 4.7 \cdot 10^8 </math> s.

Purification

Water purification system

thumb|Water purification system schematic

Since 2002, the 50 kilotons of pure water have been continually reprocessed at a rate of about 30 tons per hour in a closed system. Now, raw mine water is recycled through the first step (particle filters and RO) for some time before other processes, which involve expensive expendables, are imposed. Initially, water from the Super-Kamiokande tank is passed through nominal 1&nbsp;μm mesh filters to remove dust and particles, which reduce the transparency of the water for Cherenkov photons and provide a possible radon source inside the Super-Kamiokande detector.

Results

In 1998, Super-K found first strong evidence of neutrino oscillation from the observation of muon neutrinos changed into tau-neutrinos.

SK has set limits on proton lifetime and other rare decays and neutrino properties. SK set a lower bound on protons decaying to kaons of 5.9 × 10<sup>33</sup> yr

In January 2023, from data collected during the 1996–2018 period, new limits were reported by Super-Kamiokande for sub-GeV dark matter excluding the dark matter–nucleon elastic scattering cross section between <math>10^{-33} cm^{2}</math> and <math>10^{-27} cm^{2}</math> with masses from <math>1\ MeV/c^2</math> to <math>300\ MeV/c^2</math>. and was featured in an episode of Cosmos: A Spacetime Odyssey.

In September 2018, the detector was drained for maintenance, affording a team of Australian Broadcasting Corporation reporters the opportunity to obtain 4K resolution video from within the detection tank.

The design of the room housing a super computer in the film Eagle Eye resembles the Super-Kamiokande detection tank.

See also

  • Masatoshi Koshiba
  • Yoji Totsuka
  • SNOLAB

References

  • The Super-Kamiokande Homepage
  • Super-Kamiokande experiment record on INSPIRE-HEP