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The Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) is an electron
antineutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass i ...
detector at the
Kamioka Observatory The is a neutrino and gravitational waves laboratory located underground in the Mozumi mine of the Kamioka Mining and Smelting Co. near the Kamioka section of the city of Hida in Gifu Prefecture, Japan. A set of groundbreaking neutrino experimen ...
, an underground neutrino detection facility in Hida, Gifu,
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
. The device is situated in a
drift mine Drift mining is either the mining of an ore deposit by underground methods, or the working of coal seams accessed by adits driven into the surface outcrop of the coal bed. A drift mine is an underground mine in which the entry or access is above ...
shaft in the old KamiokaNDE cavity in the
Japanese Alps The is a series of mountain ranges in Japan which bisect the main island of Honshu. The peaks that tower over central Honshu have long been the object of veneration and pilgrimage. These mountains had long been exploited by local people for raw m ...
. The site is surrounded by 53 Japanese commercial
nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from n ...
s. Nuclear reactors produce
electron antineutrino The electron neutrino () is an elementary particle which has zero electric charge and a spin of . Together with the electron, it forms the first generation of leptons, hence the name electron neutrino. It was first hypothesized by Wolfgang Pauli ...
s (\bar_e) during the decay of radioactive
fission product Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
s in the
nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing ...
. Like the intensity of light from a light bulb or a distant star, the isotropically-emitted \bar_e
flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport p ...
decreases at 1/R2 per increasing distance R from the reactor. The device is sensitive up to an estimated 25% of antineutrinos from nuclear reactors that exceed the
threshold energy In particle physics, the threshold energy for production of a particle is the minimum kinetic energy that must be imparted to one of a pair of particles in order for their collision to produce a given result. If the desired result is to produce a t ...
of 1.8 megaelectronvolts (MeV) and thus produces a signal in the detector. If neutrinos have mass, they may
oscillate Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulu ...
into
flavors Flavor or flavour is either the sensory perception of taste or smell, or a flavoring in food that produces such perception. Flavor or flavour may also refer to: Science * Flavors (programming language), an early object-oriented extension to ...
that an experiment may not detect, leading to a further dimming, or "disappearance," of the electron antineutrinos. KamLAND is located at an average flux-weighted distance of approximately 180 kilometers from the reactors, which makes it sensitive to the mixing of neutrinos associated with large mixing angle (LMA) solutions to the
solar neutrino problem The solar neutrino problem concerned a large discrepancy between the flux of solar neutrinos as predicted from the Sun's luminosity and as measured directly. The discrepancy was first observed in the mid-1960s and was resolved around 2002. The flu ...
.


KamLAND Detector

The KamLAND detector's outer layer consists of an 18 meter-diameter
stainless steel Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's resis ...
containment vessel with an inner lining of 1,879 photo-multiplier tubes (1325 17" and 554 20" PMTs). Photocathode coverage is 34%. Its second, inner layer consists of a -diameter
nylon Nylon is a generic designation for a family of synthetic polymers composed of polyamides ( repeating units linked by amide links).The polyamides may be aliphatic or semi-aromatic. Nylon is a silk-like thermoplastic, generally made from petr ...
balloon filled with a liquid
scintillator A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbed ...
composed of 1,000
metric tons The tonne ( or ; symbol: t) is a unit of mass equal to 1000  kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton (United States ...
of
mineral oil Mineral oil is any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum, as distinct from usually edible vegetable oils. The name 'mineral oil' by itself is imprecise, ...
,
benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen a ...
, and fluorescent chemicals. Non-scintillating, highly purified oil provides
buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...
for the balloon and acts as a buffer to keep the balloon away from the photo-multiplier tubes; the oil also shields against external radiation. A 3.2 kiloton cylindrical water
Cherenkov detector A Cherenkov detector (pronunciation: /tʃɛrɛnˈkɔv/; Russian: Черенко́в) is a particle detector using the speed threshold for light production, the speed-dependent light output or the speed-dependent light direction of Cherenkov radi ...
surrounds the containment vessel, acting as a
muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As ...
veto counter and providing shielding from
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our o ...
s and radioactivity from the surrounding rock.
Electron antineutrino The electron neutrino () is an elementary particle which has zero electric charge and a spin of . Together with the electron, it forms the first generation of leptons, hence the name electron neutrino. It was first hypothesized by Wolfgang Pauli ...
s () are detected through the Inverse beta decay reaction \bar_e + p \to e^+ + n, which has a 1.8 MeV \bar_e energy threshold. The prompt scintillation light from the
positron The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collid ...
(e^+) gives an estimate of the incident antineutrino energy, E_ = E_ + + 0.9 MeV , where E_is the prompt event energy including the
positron The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collid ...
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...
and the e^+e^-
annihilation In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total energy a ...
energy. The quantity <E_n> is the average neutron recoil energy, which is only a few tens of kiloelectronvolts (keV). The neutron is captured on hydrogen approximately 200 microseconds (μs) later, emitting a characteristic ray. This delayed-coincidence signature is a very powerful tool for distinguishing antineutrinos from backgrounds produced by other particles. To compensate for the loss in \bar_e flux due to the long baseline, KamLAND has a much larger detection volume compared to earlier devices. The KamLAND detector uses a 1,000-metric-ton detection mass, which is over twice the size of similar detectors, such as
Borexino Borexino is a particle physics experiment to study low energy (sub-MeV) solar neutrinos. The detector is the world's most radio-pure liquid scintillator calorimeter. It is placed within a stainless steel sphere which holds the photomultiplier ...
. However, the increased volume of the detector also demands more shielding from cosmic rays, requiring the detector be placed underground. As part of the Kamland-Zen double beta decay search, a balloon of scintillator with 320 kg of dissolved xenon was suspended in the center of the detector in 2011. A cleaner rebuilt balloon is planned with additional xenon. KamLAND-PICO is a planned project that will install the PICO-LON detector in KamLand to search for dark matter. PICO-LON is a radiopure NaI(Tl) crystal that observes inelastic WIMP-nucleus scattering. Improvements to the detector are planned, adding light collecting mirrors and PMTs with higher quantum efficiency.


Results


Neutrino oscillation

KamLAND started to collect data on January 17, 2002. First results were reported using only 145 days of data. Without neutrino oscillation, events were expected, however, only 54 events were observed. KamLAND confirmed this result with a 515-day data sample, 365.2 events were predicted in the absence of oscillation, and 258 events were observed. These results established antineutrino disappearance at high significance. The KamLAND detector not only counts the antineutrino rate, but also measures their energy. The shape of this energy spectrum carries additional information that can be used to investigate neutrino oscillation hypotheses. Statistical analyses in 2005 show the spectrum distortion is inconsistent with the no-oscillation hypothesis and two alternative disappearance mechanisms, namely the neutrino decay and de-coherence models. It is consistent with 2-neutrino oscillation and a fit provides the values for the Δm2 and θ parameters. Since KamLAND measures Δm2 most precisely and the solar experiments exceed KamLAND's ability to measure θ, the most precise oscillation parameters are obtained in combination with solar results. Such a combined fit gives \Delta = 7.9^_ \cdot 10^ \text^2 and \tan^2\theta = 0.40^_ , the best neutrino oscillation parameter determination to that date. Since then a 3 neutrino model has been used. Precision combined measurements were reported in 2008 and 2011: :\Delta m_^2 = 7.59 \pm 0.21 \cdot 10^ \, \text^2,\,\, \tan^2 \theta _ = 0.47^_


Geological antineutrinos (geoneutrinos)

KamLAND also published an investigation of geologically-produced antineutrinos (so-called
geoneutrinos A geoneutrino is a neutrino or antineutrino emitted in decay of radionuclide naturally occurring in the Earth. Neutrinos, the lightest of the known subatomic particles, lack measurable electromagnetic properties and interact only via the weak nucle ...
) in 2005. These neutrinos are produced in the decay of
thorium Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high ...
and
uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
in the Earth's crust and mantle. A few geoneutrinos were detected and these limited data were used to limit the U/Th radiopower to under 60TW. Combination results with Borexino were published in 2011, measuring the U/Th heat flux. New results in 2013, benefiting from the reduced backgrounds due to Japanese reactor shutdowns, were able to constrain U/Th radiogenic heat production to 11.2^_ TW using 116 \bar_e events. This constrains composition models of the bulk silicate Earth and agrees with the reference Earth model.


KamLAND-Zen Double Beta Decay Search

KamLAND-Zen uses the detector to study beta decay of 136Xe from a balloon placed in the scintillator in summer 2011. Observations set a limit for neutrinoless double-beta decay half-life of . A double beta decay lifetime was also measured: 2.38 \pm \pm *10^ yr, consistent with other xenon studies. KamLAND-Zen plans continued observations with more enriched Xe and improved detector components. An improved search was published in August 2016, increasing the half-life limit to , with a neutrino mass bound of 61–165 meV. The first KamLAND-Zen apparatus, KamLAND-Zen 400, completed two research programs, Phase I (2011 Oct. - 2012 Jun.) and Phase II (2013 Dec. - 2015 Oct.). The combined data of Phase I and II implied the lower bound 1.07 \times 10^ years for the neutrinoless double beta decay half-life. The KamLAND-Zen 400 operated from 2011 October to 2015 October and was then replaced by KamLAND-Zen 800. The second KamLAND-Zen experiment apparatus, KamLAND-Zen 800, with bigger balloon of about 750 kg of Xenon was installed in the KamLAND detector 10 May 2018. The operation was expected to start winter 2018-2019 with 5 years of expected operation. The KamLAND-Zen 800 experiment started data taking in January 2019 and first results were published in 2020. In March 2022 the KamLAND-Zen Collaboration using the KamLAND-Zen 800 published results about neutrinoless double-beta decay in Xe-136 using data collected between February 5, 2019 and May 8, 2021. No neutrinoless double-beta decay was observed, and the established lower bound for half-life was T > 2.3×10^ yr corresponding to upper limits on the effective Majorana neutrino mass of 36 – 156 meV. The KamLAND-Zen collaboration is planning to construct another apparatus, KamLAND2-Zen in the long term.


References


Further reading

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External links


KamLAND official website

KamLAND at Lawrence Berkeley National Laboratory (Berkeley Lab)KamLAND experiment
record on
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