KamLAND: neutrinos from heaven and Earth
Japanese Kamiokande underground detector played a leading role in the
study of neutrinos produced via cosmic rays and also helped to pioneer
the subject of neutrino astronomy. With Kamiokande now having given way
to Superkamiokande, the Kamioka mine becomes the scene of a new
A new large neutrino detector is currently
being constructed by an international collaboration that includes
Hungary, Japan and the US in the underground site that used to be the
home of the pioneering Kamiokande experiment. Called KamLAND (Kamioka
Liquid scintillator Anti-Neutrino Detector), it will be the largest
low-energy antineutrino detector ever built and will study a wide range
of science, spanning particle physics, geophysics and astrophysics.
The principal goal will be to investigate the possibility of
neutrino oscillations by studying the flux and energy spectra of
neutrinos produced by Japananese commercial nuclear reactors.
Approximately one-third of all Japanese electrical power (which is
equivalent to 130 GW thermal power) is produced by nuclear reactors and
KamLAND is centrally located on the main island of Honshu, therefore
the experiment is exposed to a very large flux of low-energy
antineutrinos, which are mainly produced at a distance of between 150
and 200 kilometres. The broad energy spectrum of antineutrinos emitted
by the neutron-rich fission fragments of a reactor has a maximum at
around 3.5 MeV.
With two kinds of neutrinos,
the oscillation probability depends on one mixing parameter and on the sine of Dm2 L/En,
where L is the "baseline" traversed by the neutrinos,
EnDm2 is the mass squared difference. Measurements would constrain Dm2 and the mixing parameter.
Since for a given baseline the
oscillation probability depends on the neutrino energy, different
neutrino energies will have different oscillation behaviour. Hence, in
general, oscillations would distort and suppress the detected
spectrum. With an expected (non-oscillating) rate of 700 antineutrinos
detected per year, three years of data-taking would be a hundred-fold
improvement over existing neutrino mass measurements.
More important is that the experiment's sensitivity covers
one of the possible solutions of the "solar neutrino puzzle". While
solar and atmospheric neutrinos have provided the first signals for
neutrino oscillations, terrestrial experiments, with both source and
detector well quantified, are required to investigate and understand
such oscillations in detail.
While Minos in the US, K2K in Japan and a possible CERNGran
Sasso beam are designed to investigate the mass region of interest for
atmospheric neutrinos, KamLAND is the first attempt to study the solar
neutrino puzzle "in the laboratory". This has a historical precedent in
meson physics, where the first investigations were driven by
cosmic-ray experiments using balloons or on high mountains, while
subsequent discoveries and systematic study came via accelerators and
KamLAND will, for the first time, also be able to detect
antineutrinos from uranium and thorium beta decays inside the Earth.
While the present experimental data on this topic are derived from
sparse and shallow samplings, KamLAND will provide a global
measurement. About half of the heat produced in our planet is believed
to be produced by such decays, therefore this measurement will be of
considerable geophysical interest.
In addition, comparisons with similar measurements from
Borexino should make it possible to calculate the ratio of uranium to
thorium in the crust and mantle of the Earth. This is because Borexino
is surrounded by thick continental crust, while KamLAND is located at
the edge of the Asian plate and, hence, over about half of its
angular coverage it receives antineutrinos from beneath the Pacific
A natural continuation of the
KamLAND programme will include the
direct observation of beryllium-7 and
boron-8 solar neutrinos by detecting recoil energy in neutrinoelectron scattering processes.
While Borexino is particularly optimized for the
study of beryllium-7 solar neutrinos,
larger mass of KamLAND opens up the
low-energy measurement of the
rarer boron-8 neutrinos.
(The high-energy component of such neutrinos will be covered by the
Sudbury (SNO) water-Cherenkov detectors.)
The direct detection of solar neutrinos
involves single ionization events (as opposed to double in the case of
reactor and terrestrial antineutrinos), thus the quality of these
measurements will depend on the extent to which the radioactive
contamination (and hence the background) can be controlled. Here the
kiloton mass of the detector is a major asset, allowing the active
scintillator volume to be separated from the external components by a
very thick layer of inert shielding oil.
Should the background be higher than expected, an additional
layer of scintillator will be used as shielding, with a software cut
on the event position, while retaining a very large fiducial volume.
Simulations show that the the KamLAND background will be dominated by
the internal radioactivity of the scintillator, and by the radon
produced mainly in the photomultiplier glass and carried to the centre
of the detector by convection.
The first phase of running will show conclusively whether more effort is needed for precise observations of the
solar neutrino fluxes to be made.
Such a staged approach minimizes expense and
effort in the
notoriously tricky field of detector background.
Searches for supernova neutrinos,
solar antineutrinos and,
neutrinoless double beta decay will complete the
The heart of KamLAND will be a spherical volume containing a kiloton of very-high-purity liquid scintillator.
Unlike water-Cherenkov detectors that can only detect relatively high-energy particles,
KamLAND's scintillator has enough sensitivity to detect fractions of a mega electron volt,
possibility of background-free low-energy electron-type antineutrino detection.
This would be achieved by observing both the
neutron produced by the
inverse beta decay capture of antineutrinos by protons.
Fake events resulting from natural radioactivity and
cosmic-ray backgrounds are reduced by different layers of shielding,
careful selection of construction materials and
The active scintillator volume is housed in a 2.5 m thick layer of
ultrapure mineral oil that shields it from external neutron and gamma
radiation. Scintillation light is picked up by an array of about 2000
specially developed photomultipliers achieving 3.5 ns time resolution
using a very large (17 inch diameter) photocathode. While good time
resolution is essential to localize events within the fiducial volume,
an important feature is the novel approach of its electronics, thus
providing a complete history of the signals from each tube preceding
and following triggered events. This will be invaluable in suppressing
backgrounds, either using the scintillator pulse shapes produced by
different types of particle or by studying correlations in radioactive
decay chains over a broad timescale range.
An advanced system of buffering will ensure no deadtime up to several kilohertz during several 1 s bursts.
The active scintillator is separated from the
buffer oil by a very thin layer of transparent plastic a 13 m diameter weather balloon! This is a critical component of the
allowing scintillation light to reach the
but blocking the
radon from uranium contamination in external materials.
The 3000 tons of liquid scintillator,
buffer oil and
photomultipliers are contained and
supported by an 18 m diameter stainless-steel sphere.
The volume between the
cylindrical cavity in the
rock is flooded with water in which cosmic-ray muons are detected by their Cherenkov light.
The read-out of this veto detector is provided by the
old Kamiokande photomultipliers.
By recycling and upgrading existing facilities, a new
"superdetector" will rise from the "ashes" of Kamiokande for a modest
investment. KamLAND's schedule foresees the exploration of neutrino
physics, geophysics and astrophysics from the beginning of 2001.
Article 13 of 21.
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