- Kamioka Observatory
The nihongo|Kamioka Observatory, Institute for Cosmic Ray Research|神岡宇宙素粒子研究施設|Kamioka Uchū Soryūshi Kenkyū Shisetsu is a
neutrino physics laboratory located underground in the Mozumi Mine of the Kamioka Mining and Smelting Co. near the Kamioka section of the city of Hida inGifu Prefecture ,Japan . A set of groundbreakingneutrino experiments have taken place at the observatory over the past twodecades . All of the experiments have been very large and have contributed substantially to the advancement ofparticle physics , in particular to the study ofneutrino astronomy andneutrino oscillation .Past experiments
KamiokaNDE
The first of the Kamioka experiments was named KamiokaNDE for Kamioka Nucleon Decay Experiment. It was a large
water Cerenkov detector designed to search forproton decay . To observe the decay of a particle with a lifetime as long as a proton an experiment must run for a long time and observe an enormous number of protons. This can be done most cost effectively if the target (the source of the protons) and the detector itself are made of the same material. Water is an ideal candidate because it is inexpensive, easy to purify, stable, and can detect relativistic charged particles through their production ofCerenkov radiation . A proton decay detector must be buried deep underground or in amountain because the background fromcosmic ray muons in such a large detector located on the surface of theEarth would be far too large. The muon rate in the KamiokaNDE experiment was about 0.4 Hz, roughly five orders of magnitude smaller than what it would have been if the detector had been located at the surface.cite web
first=Masayuki
last=Nakahata
authorlink=Masaayuki Nakahata
url=http://aapps.org/archive/bulletin/vol13/13_4/13_4_p07p12.pdf
title=Kamiokande and Super-Kamiokande
publisher=Association of Asia Pacific Physical Societies
accessdate=2006-07-11]The distinct pattern produced by cerenkov radiation allows for
particle identification , an important tool both understanding the potential proton decay signal and for rejecting backgrounds. The ID is possible because the sharpness of the edge of the ring depends on the particle producing the radiation.Electrons (and therefore alsogamma rays ) produce fuzzy rings due to themultiple scattering of the low mass electrons. Minimumionizing muons , in contrast produce very sharp rings as their heavier mass allows them to propagate directly.Construction of Kamioka Underground Observatory (the predecessor of the present Kamioka Observatory, Institute for Cosmic Ray Research,
University of Tokyo ) began in 1982 and was completed in April, 1983. The detector was acylindrical tank which contained 3,000 tons of pure water and had about 1,000 50 cm diameterphotomultiplier tubes (PMTs) attached to the inner surface. The size of the outer detector was 16.0 m in height and 15.6 m in diameter. The detector failed to observe proton decay, but set what was then the world's best limit on the lifetime of the proton. The name of the project, "kamiokande", means "bite into god" when pronounced in Japanese.Kamiokande-II
The Kamiokande-II experiment was a major step forward from KamiokaNDE, and made a significant number of important observations.
olar Neutrinos
In the 1930s,
Hans Bethe had postulated that the source of the sun's energy was fusion reactions in its core. While this theory was widely accepted for decades there was no way of observing the sun's core and directly testing thehypothesis . Ray Davis'sHomestake Experiment was the first to detectsolar neutrinos , strong evidence that the nuclear theory of the sun was correct. Over a period of decades the Davis experiment consistently observed only about 1/3 the number of neutrinos predicted by theStandard Solar Model s of hiscolleague and close friendJohn Bahcall . Because of the great technical difficulty of the experiment and its reliance on radiochemical techniques rather thanreal time direct detection manyphysicists were suspicious of his result.It was realized that a large water Cerenkov detector could be an ideal neutrino detector, for several reasons. First, the enormous volume possible in a water Cerenkov detector can overcome the problem of the very small cross section of the 5-15
MeV solar neutrinos. Second, water Cerenkov detectors offer real time event detection. This meant that Individual neutrino-electron interaction candidate events could be studied on an event-by-event basis, starkly different from the month-to-month observation required in radiochemical experiments. Third, in the neutrino-electron scattering interaction the electron recoils in roughly the direction that the neutrino was travelling (similar to the motion of billiard balls), so the electrons "point back" to the sun. Fourth, neutrino-electron scattering is an elastic process, so the energy distribution of the neutrinos can be studied, further testing the solar model. Fifth, the characteristic "ring" produced by Cerenkov radiation allows discrimination of the signal against backgrounds. Finally, since a water Cerenkov experiment would use a different target, interaction process, detector technology, and location it would be a very complementary test of Davis's results.It was clear that KamiokaNDE could be used to perform a fantastic and novel experiment, but a serious problem needed to be overcome first. The presence of radioactive backgrounds in KamiokaNDE meant that the detector had an
energy threshold of tens ofMeV . The signals produced by proton decay and atmospheric neutrino interactions are considerably larger than this, so the original KamiokaNDE detector had not needed to be particularly aggressive about its energy threshold or resolution. The problem was attacked in two ways. The participants of the KamiokaNDE experiment designed and built new purification systems for the water to reduce theradon background, and instead of constantly cycling the detector with "fresh" mine water they kept the water in the tank allowing the radon to decay away. A group from theUniversity of Pennsylvania joined thecollaboration and supplied newelectronics with greatly superior timing capabilities. The extra information provided by the electronics further improved the ability to distinguish the neutrino signal from radioactive backgrounds. One further improvement was the expansion of the cavity, and the installation of an instrumented "outer detector". The extra water provided shielding from gamma rays from the surrounding rock, and the outer detector provided aveto for cosmic ray muons.With the
upgrade s completed the experiment was renamed Kamiokande-II, and started data taking in 1985. The experiment spent several years fighting the radon problem, and started taking "production data" in 1987. Once 450 days of data had been accumulated the experiment was able to see a clear enhancement in the number of events which pointed away from sun over random directions. The directional information was thesmoking gun signature of solar neutrinos, demonstrating directly for the first time that the sun is a source of neutrinos. The experiment continued to take data for many years and eventually found the solar neutrino flux to be about 1/2 that predicted by solar models. This was in conflict with both the solar models and Davis's experiment, which was ongoing at the time and continued to observe only 1/3 of the predicted signal. This conflict between the flux predicted by solartheory and the radiochemical and water Cerenkov detectors became known as thesolar neutrino problem .Atmospheric neutrinos
The flux of atmospheric neutrinos is considerably smaller than that of the solar neutrinos, but because the reaction cross sections increase with energy they are detectable in a detector of Kamiokande-II's size. The experiment used a "ratio of ratios" to compare the
ratio of electron to muon flavor neutrinos to the ratio predicted by theory (this technique is used because manysystematic error s cancel each other out). This ratio indicated a deficit of muon neutrinos, but the detector was not large enough to obtain the statistics necessary to call the result a discovery. This result came to be known as the atmospheric neutrino deficit.upernova 1987A
The Kamiokande-II experiment happened to be running at a particularly fortuitous time, as a
supernova took place while the detector was online and takingdata . With the upgrades that had taken place the detector was sensitive enough to observe the thermal neutrinos produced bySupernova 1987A , which took place roughly 160,000light years away in theLarge Magellanic Cloud . The neutrinos arrived atEarth in February 1987, and the Kamiokande-II detector observed 11 events.Nucleon Decay
Kamiokande-II continued KamiokaNDE's search for proton decay and again failed to observe it. The experiment once again set a limit on the lifetime of the proton.
Nobel Prize
For his work directing the Kamioka experiments, and in particular for the first-ever detection of astrophysical neutrinos
Masatoshi Koshiba was awarded theNobel Prize in Physics in 2002.Raymond Davis Jr. andRiccardo Giacconi were co-winners of the prize.Current experiments
uper Kamiokande
By the 1990s particle physicists were starting to suspect that the solar neutrino problem and atmospheric neutrino deficit had something to do with
neutrino oscillation . The Super Kamiokande detector was designed to test the oscillation hypothesis for both solar and atmospheric neutrinos. The Super-Kamiokande detector is massive, even by particle physics standards. It consists of 50,000 tons of pure water surrounded by about 11,200 photomultiplier tubes. The detector was again designed as a cylindrical structure, this time 41.4 m tall and 39.3 m across. The detector was surrounded with a considerably more sophisticated outer detector which could not only act as a veto for cosmic muons but actually help in their reconstruction.Super-Kamiokande started data taking in 1996 and has made several important measurements. These include precision measurement of the solar neutrino flux using the elastic scattering interaction, the first very strong evidence for
neutrino oscillation , and a considerably more stringent limit on proton decay.uper Kamiokande-II
On
November 12 ,2001 , several thousand photomultiplier tubes in the Super-Kamiokande detector imploded, apparently in achain reaction as theshock 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 it was hoped would prevent another chain reaction from recurring. The data taken after the implosion is referred to as the Super Kamiokande-II data.uper Kamiokande-III
In July 2005, preparation began to restore the detector to its original form by reinstalling about 6,000 new PMTs. It was finished in June 2006. Data taken with the newly restored machine will be called the SuperKamiokande-III dataset.
KamLAND
The KamLAND experiment is a liquid scintillator detector designed to detect reactor
antineutrino s. KamLAND is a complementary experiment to theSudbury Neutrino Observatory because while the SNO experiment has good sensitivity to the solar mixing angle but poor sensitivity to the squared mass difference, KamLAND has very good sensitivity to the squared mass difference with poor sensitivity to the mixing angle. The data from the two experiments may be combined as long as CPT is a validsymmetry of ouruniverse . The KamLand experiment is located in the original KamiokaNDE cavity.K2K
The KEK To Kamioka experiment [cite web|url=http://neutrino.kek.jp/intro/k2k.html|title=Long Baseline neutrino oscillation experiment,from KEK to Kamioka (K2K)|accessdate=2008-09-10] used accelerator neutrinos to verify the oscillations observed in the atmospheric neutrino signal with a well controlled and understood beam. A neutrino beam was directed from the KEK accelerator to Super Kamiokande. The experiment found oscillation parameters which were consistent with those measured by Super-K.
Future experiments
T2K
The Tokai To Kamioka long baseline experiment is due to start in 2009. It will make a precision measurement of the atmospheric neutrino oscillation parameters and help discover or set a limit on the value of θ13. It will use a neutrino beam directed at the Super Kamiokande detector from the
Japanese Hadron Facility 50GeV proton synchrotron such that the neutrinos will travel a total distance of 295 km.Hyper Kamiokande
There are proposals to build a detector ten times larger than Super Kamiokande, and this project is known by the name Hyper Kamiokande. The discovery of proton decay could help to shed light on
supersymmetry or other "beyond the standard model" theories.ee also
*
MINOS
*Supernova Early Warning System References
External links
* [http://www-sk.icrr.u-tokyo.ac.jp/ The official Super-Kamiokande home page]
* [http://www.phys.washington.edu/~superk/ American Super-K home page]
* [http://www-sk.icrr.u-tokyo.ac.jp/sk/photo/index.html Pictures and illustrations]
* [http://www-sk.icrr.u-tokyo.ac.jp/cause-committee/1st/report-nov22e.pdf Official report on the Super-K accident (in PDF format)]
* [http://neutrino.kek.jp/jhfnu/ T2K website]
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