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v t e

36°25′32.6″N 137°18′37.1″E / 36.425722°N 137.310306°E / 36.425722; 137.310306^[1]

Super-Kamiokande (abbreviation of Super-Kamioka Neutrino Detection Experiment, also abbreviated to Super-K or SK; Japanese: スーパーカミオカンデ) 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.^[2][3] 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 1,000 m (3,300 ft) underground in the Mozumi Mine in Hida's Kamioka area.^[4][5] It consists of a cylindrical stainless steel tank that is 41.4 m (136 ft) tall and 39.3 m (129 ft) in diameter holding 50,220 metric tons (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 36.2 m (119 ft) in height and 33.8 m (111 ft) in diameter, and outer detector (OD) which consists of the remaining tank volume. Mounted on the superstructure are 11,146 photomultiplier tubes (PMT) 50 cm (20 in) in diameter that face the ID and 1,885 20 cm (8 in) PMTs that face the OD. There is a Tyvek and blacksheet barrier attached to the superstructure that optically separates the ID and OD.^{[citation needed]}

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.^{[citation needed]}

History

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 detect whether proton decay exists, one of the most fundamental questions of elementary particle physics.^{[6][7][8][9][10]}

The detector, named KamiokaNDE for Kamioka Nucleon Decay Experiment, was a tank 16.0 m (52 ft) in height and 15.6 m (51.2 ft) in width, containing 3,058 metric tons (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 would be 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 US portion of the proposal, which was primarily to build the OD system, was approved by the US Department of Energy in 1993 for $3M. In addition, the US has also contributed about 2000 20 cm PMTs recycled from the IMB experiment.^[11]

Super-Kamiokande started operation in 1996 and announced the first evidence of neutrino oscillation in 1998.^[12] 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 (costing about $3000 each^[13]) in the Super-Kamiokande detector imploded, apparently in a chain reaction or cascading failure, as the shock wave from the concussion of each imploding tube cracked its neighbours. The detector was partially restored 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 work was completed in June 2006, whereupon the detector was renamed Super-Kamiokande-III. This phase of the experiment collected data from October 2006 till August 2008. At that time, significant upgrades were made to the electronics. After the upgrade, the new phase of the experiment has been referred to as Super-Kamiokande-IV. SK-IV collected data on various natural sources of neutrinos, as well as acted as the far detector for the Tokai-to-Kamioka (T2K) long baseline neutrino oscillation experiment.

SK-IV continued until June 2018. After that, the detector underwent a full refurbishment during Autumn of 2018. On 29 January 2019 the detector resumed data acquisition.^[14]

In 2020 the detector was upgraded for the SuperKGd project by adding a Gd salt to the ultrapure water in order to enable detection of antineutrinos from supernova explosions.^[15]

Detector

The Super-Kamiokande (SK) is a Cherenkov detector used to study neutrinos from different sources including the Sun, supernovae, the atmosphere, and accelerators. It is also used to search for proton decay. The experiment began in April 1996 and was shut down for maintenance in July 2001, a period known as "SK-I". Since an accident occurred during maintenance, the experiment resumed in October 2002 with only half of its original number of ID-PMTs. In order to prevent further accidents, all of the ID-PMTs were covered by fiber-reinforced plastic with acrylic front windows. This phase from October 2002 to another closure for an entire reconstruction in October 2005 is called "SK-II". In July 2006, the experiment resumed with the full number of PMTs and stopped in September 2008 for electronics upgrades. This period was known as "SK-III". The period after 2008 is known as "SK-IV". The phases and their main characteristics are summarised in table 1.^[16]

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.^[16]

The SK system was upgraded in September 2008 in order to maintain the stability in the next decade and improve the throughput of the data acquisition systems, QTC-based electronics with Ethernet (QBEE).^[17] 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).^[18] Each QTC input has three gain ranges "Small", "Medium" and "Large" – the resolutions for each are shown in Table.^[16]

For each range, analog to digital conversion is conducted separately, but the only range used is that with the highest resolution that is not being saturated. The overall charge dynamic range of the QTC is 0.2–2500 pC, five times larger than the old . The charge and timing resolution of the QBEE at the single photoelectron level is 0.1 photoelectrons and 0.3 ns respectively, both are better than the intrinsic resolution of the 20-in. PMTs used in SK. The QBEE achieves good charge linearity over a wide dynamic range. The integrated charge linearity of the electronics is better than 1%. The thresholds of the discriminators in the QTC are set to −0.69 mV (equivalent to 0.25 photoelectron, which is the same as for SK-III). This threshold was chosen to replicate the behavior of the detector during its previous ATM-based phases.^[16]

SuperKGd

Gadolinium was introduced into the Super-Kamiokande water tank in 2020 in order to distinguish neutrinos from antineutrinos that arise from supernova explosions.^[15][19] This is known as the SK-Gd project (other names include SuperKGd, SUPERK-GD, and similar names).^[20] In the first phase of the project, 1.3 tons of a Gd salt (gadolinium sulfate octahydrate, Gd(SO₄)₃⋅(H₂O)₈) 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.^[15][19]

Nuclear fusion in the Sun and other stars turns protons into neutrons with the emission of neutrinos. Beta decay in the Earth and in supernovas turns neutrons into protons with the emission of anti-neutrinos. The Super-Kamiokande detects electrons knocked off a water molecule producing a flash of blue Cherenkov light, and these are produced both by neutrinos and antineutrinos. A rarer instance is when an antineutrino interacts with a proton in water to produce a neutron and a positron.^[21]

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.^[19] The brightness of the first flash allows physicists to distinguish between low energy antineutrinos from the Earth and high energy antineutrinos from supernovas. In addition to observing neutrinos from distant supernovas, the Super-Kamiokande will be able to set off an alarm to inform astronomers around the world of the presence of a supernova in the Milky Way within one second of it occurring.

The biggest challenge was whether the detector's water could be continuously filtered to remove impurities without removing the gadolinium at the same time. A 200-ton prototype called EGADS with added gadolinium sulfate was installed in the Kamioka mine and operated for years. It finished operation in 2018 and showed that the new water purification system would remove impurities while keeping the gadolinium concentration stable. It also showed that gadolinium sulfate would not significantly impair the transparency of the otherwise ultrapure water, or cause corrosion or deposition on existing equipment or on the new valves that will later be installed in the Hyper-Kamiokande.^[20][21]

Water tank

The outer shell of the water tank is a cylindrical stainless-steel tank 39 m in diameter and 42 m in height. The tank is self-supporting, with concrete backfilled against the rough-hewn stone walls to counteract water pressure when the tank is filled. The capacity of the tank exceeds 50 kilotonnes of water.^[11]

PMTs and associate structure

The basic unit for the ID PMTs is a "supermodule", a frame which 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, supermodules 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 supermodules. 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 3 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 Mineguard. Mineguard is a spray-applied polyurethane membrane developed for use as a rock support system and radon gas barrier in the mining industry.^[11]

The average geomagnetic field is about 450 mG and is inclined by about 45° with respect to the horizon at the detector site. This presents a problem for the large and very sensitive PMTs which prefer a much lower ambient field. The strength and uniform direction of the geomagnetic field could systematically bias photoelectron trajectories and timing in the PMTs. To counteract this 26 sets of horizontal and vertical Helmholtz coils are arranged around the inner surfaces of the tank. With these in operation the average field in the detector is reduced to about 50 mG. The magnetic field at various PMT locations were measured before the tank was filled with water.^[11]

A standard fiducial volume of approximately 22.5 kilotonnes is defined as the region inside a surface drawn 2.00 m from the ID wall to minimize the anomalous response caused by natural radioactivity in the surrounding rock.

Monitoring system

Online monitoring system

An online monitor computer located in the control room reads data from the DAQ host computer via an FDDI link. It provides shift operators with a flexible tool for selecting event display features, makes online and recent-history histograms to monitor detector performance, and performs a variety of additional tasks needed to efficiently monitor status and diagnose detector and DAQ problems. Events in the data stream can be skimmed off and elementary analysis tools can be applied to check data quality during calibrations or after changes in hardware or online software.^[11]

Realtime supernova monitor

To detect and identify such bursts as efficiently and promptly as possible Super-Kamiokande is equipped with an online supernova monitor system. About 10,000 total events are expected in Super-Kamiokande for a supernova explosion at the center of the Milky Way Galaxy. Super-Kamiokande can measure a burst with no dead-time, up to 30,000 events within the first second of a burst. Theoretical calculations of supernova explosions suggest that neutrinos are emitted over a total time-scale of tens of seconds with about a half of them emitted during the first one or two seconds. The Super-K will search for event clusters in specified time windows of 0.5, 2 and 10 s.^[11] Data are transmitted to realtime SN-watch analysis process every 2 min and analysis is completed typically in 1 min. When supernova (SN) event candidates are found, ${\displaystyle {R_{\text{mean}}}}$ is calculated if the event multiplicity is larger than 16, where ${\displaystyle {R_{\text{mean}}}}$ is defined as the average spatial distance between events, i.e.

${\displaystyle {R_{\text{mean}}}={\frac {\sum _{i=1}^{{N_{\text{multi}}}-1}\sum _{j=i+1}^{N_{\text{multi}}}|{r_{\text{i}}}-{r_{\text{j}}}|}{{N_{\text{multi}}}{C_{\text{2}}}}}}$

Neutrinos from supernovae interact with free protons, producing positrons which are distributed so uniformly in the detector that ${\displaystyle {R_{\text{mean}}}}$ for SN events should be significantly larger than for ordinary spatial clusters of events. In the Super-Kamiokande detector, Rmean for uniformly distributed Monte Carlo events shows that no tail exists below ${\displaystyle {R_{\text{mean}}}}$⩽1000 cm. For the "alarm" class of burst, the events are required to have ${\displaystyle {R_{\text{mean}}}}$⩾900 cm for 25⩽${\displaystyle {N_{\text{multi}}}}$⩽40 or ${\displaystyle {R_{\text{mean}}}}$⩾750 cm for ${\displaystyle {N_{\text{multi}}}}$>40. These thresholds were determined by extrapolation from SN1987A data.^[11][22] 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 is to reconstruct the direction to supernova. By neutrino–electron scattering, ${\displaystyle \nu _{\text{x}}+e^{-}\to \nu _{\text{x}}+e^{-}}$, a total of 100–150 events are expected in case of a supernova at the center of the Milky Way Galaxy.^[11] The direction to supernova can be measured with angular resolution

${\displaystyle \delta \theta \sim {30^{\circ } \over {\sqrt {N}}}}$

where N is the number of events produced by the ν–e scattering. The angular resolution, therefore, can be as good as δθ~3° for a supernova at the center of the Milky Way Galaxy.^[11] In this case, not only time profile and the energy spectrum of a neutrino burst, but also the information on direction of supernova can be provided.

Slow control monitor and offline process monitor

There is a process called the "slow control" monitor, as part of the online monitoring system, watches the status of the HV systems, the temperatures of electronics crates and the status of the compensating coils used to cancel the geomagnetic field. When any deviation from norms is detected, it will alert physicists to prompt to investigate, take appropriate action, or notify experts.^[11]

To monitor and control the offline processes that analyze and transfer data, a sophisticated set of software was developed. This monitor allows non-expert shift physicists to identify and repair common problems to minimize down time, and the software package was a significant contribution to the smooth operation of the experiment and its overall high lifetime efficiency for data taking.^[11]

Research

Solar neutrino

The energy of the Sun comes from the nuclear fusion in its core where a helium atom and an electron neutrino are generated by 4 protons. These neutrinos emitted from this reaction are called solar neutrinos. Photons, created by the nuclear fusion in the center of the Sun, take millions of years to reach the surface; on the other hand, solar neutrinos arrive at the earth in eight minutes due to their lack of interactions with matter. Hence, solar neutrinos make it possible for us to observe the inner Sun in "real-time" that takes millions of years for visible light.^[23]

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

${\displaystyle 4p\to {}^{4}\!He+2e^{+}+2\nu _{e}+26.73}$ 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. Zdroj:https://en.wikipedia.org?pojem=Super-Kamiokande
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