Super-Kamiokande
Super-Kamiokande 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 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 of ultrapure water. The tank volume is divided by a stainless steel superstructure into an inner detector region, which is in height and in diameter, and outer detector which consists of the remaining tank volume. Mounted on the superstructure are 11,146 photomultiplier tubes 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
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 of pure water and about 1,000 photomultiplier tubes attached to its inner surface. The detector was upgraded, starting in 1985, to allow it to observe solar neutrinos. As a result, the detector 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.
Super-Kamiokande started operation in 1996 and announced the first evidence of neutrino oscillation in 1998. 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 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.
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 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.
In 2020, the detector was upgraded for the [|SuperKGd] project by adding a gadolinium salt to the ultrapure water in order to enable the detection of antineutrinos from supernova explosions.
Detector
The Super-Kamiokande 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.
SK-IV upgrade
In the previous phases, the ID-PMTs processed signals by custom electronics modules called analog timing modules. Charge-to-analog converters and time-to-analog converters are contained in these modules that had dynamic range from 0 to 450 picocoulombs 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 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. The QBEE provides high-speed signal processing by combining pipelined components. These components are a newly developed custom charge-to-time converter in the form of an application-specific integrated circuit, a multi-hit time-to-digital converter, and field-programmable gate array. Each QTC input has three gain ranges "Small", "Medium", and "Large" – the resolutions for each are shown in Table.
| Range | Measuring region | Resolution |
| Small | 0–51 pC | 0.1 pC/count |
| Medium | 0–357 pC | 0.7 pC/count |
| Large | 0–2500 pC | 4.9 pC/count |
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. This threshold was chosen to replicate the behavior of the detector during its previous ATM-based phases.
SuperKGd
was introduced into the Super-Kamiokande water tank in 2020 in order to distinguish neutrinos from antineutrinos that arise from supernova explosions. This is known as the SK-Gd project. In the first phase of the project, 1.3 tons of a Gd salt were added to the ultrapure water in 2020, giving 0.02% of the salt. This amount is about a tenth of the planned final target concentration.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.
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. 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.