Neutrinos are one of the fundamental particles that
constitute the universe. These elementary particles are
created when an unstable atomic nucleus undergoes
beta decay. They are produced in great numbers by
nuclear fusion reactions in the core of the Sun or
from supernova explosions (and also in nuclear reactors).
Neutrinos were thought to be one of the candidates for
dark matter and were referred to as “ghostly particles,”
because they have no electric charge and their masses
are so tiny that they are extremely difficult to observe.
Researchers all over the world have joined in the
search for neutrinos, the key to solving the mystery of
the universe, with the Center for Underground Physics
(Director Yeongduk Kim) taking the lead.
Detector and data acquisition
facility installed inside tendon
gallery of Hanbit Nuclear Power
Plant in Yeong-gwang
Three types of neutrinos have been discovered.
However, the existing three flavors of neutrinos
cannot account for the reactor neutrino
anomaly recently discovered in experiments
conducted a short distance from reactors.
Hence, the Center has started a new experiment
to find the unknown fourth neutrino.
The three neutrino flavors are the electron
neutrino, muon neutrino, and tau neutrino (each
corresponding to an antineutrino with an opposite
lepton number). Neutrinos can change from
one type to another, a phenomenon called
neutrino oscillation. The measure of how
much one neutrino type oscillates to another is
called “neutrino oscillation parameters.” Since
neutrinos come in three flavors, three neutrino
oscillation parameters can be measured.
In 2012, the final yet-to-be observed neutrino
oscillation parameter was measured at the
Reactor Experiment for Neutrino Oscillation
(RENO) in Korea and the Daya Bay experiment in
China. With the measurement, all three neutrino
oscillation parameters of currently discovered
neutrinos were revealed. This represented
remarkable progress in understanding neutrino
oscillation.
However, several neutrino oscillation anomalies
have been reported. The reactor neutrino
anomaly is especially difficult to account for
with oscillation of the three neutrinos, as the
number of measured antineutrinos was 6%
lower than the predicted value in experiments
measuring reactor neutrinos.
Researchers in the field have proposed a
hypothesis on the existence of an unknown
fourth neutrino to resolve the reactor neutrino
anomaly. This new neutrino is called the “sterile
neutrino,” as it is expected to not interact
via weak force. A new short baseline reactor
neutrino experiment is needed to identify
the existence of sterile neutrinos, as previous
reactor antineutrino experiments have been
low-accuracy and error-prone.
The Center leads the Neutrino Experiment
for Oscillation at Short Baseline (NEOS) at the Hanbit Nuclear Power Plant in Yeonggwang.
NEOS detects electron antineutrinos
created during reactor operations using a
detector installed 27 meters from the reactor.
This experiment was the first among the short
baseline reactor neutrino experiments currently
being conducted all over the world.
Meanwhile, NEOS must overcome the
problem of background events caused by
cosmic rays. Background events mainly occur
due to neutrons and muons generated in the
atmosphere which fall to the earth’s surface or
by gamma rays generated near the detector. In
a short baseline reactor neutrino experiment,
the detector must be installed as close as
possible to the reactor, so the detector must be
installed above ground. Therefore, whether
or not the experiment succeeds depends on
minimizing background conditions that hinder
the experiment.
PSD performances of the LABbased
liquid scintillator (left)
and the newly developed liquid
scintillator (right).
The two lines
represent neutrons and gamma
rays from the top, respectively.
The NEOS detector is composed of several
layers of shielding that block or eliminate
background events. The outermost layer is
polyethylene, under which the detector that
prevents the interference of muons with a
plastic scintillator and a photomultiplier tube is
installed. Lead blocks gamma rays, and lastly,
borated polyethylene is used to block neutrons.
In addition, background events can also be
eliminated using the properties of the liquid
scintillator, which is used as a target for
neutrinos. This process is called pulse shape
discrimination (PSD). Pulse shapes emitted
when particles react with the scintillator differ
according to particle type. Particles may be
distinguished using this difference.
Linear Alkyl Benzene, or LAB, has mainly
been used as a solvent for liquid scintillators.
LAB is safe to handle due to its high flash
point, nontoxic and therefore environmentally
friendly, and has good transparency. However,
a drawback of LAB-based liquid scintillators
is their low PSD performance. For this reason,
the Center developed a liquid scintillator
with groundbreaking improvement in PSD
performance.
The Center achieved higher PSD performance by
adding a small amount of diisopropylnaphthalene
(DIN)-based liquid scintillator to LAB-based
liquid scintillator. Mixing in the relatively
expensive DIN-based liquid scintillator by 10%
rendered great effects. The Center expects
that this new development will greatly reduce
background events, as the PSD of the newly
developed liquid scintillator can eliminate more
than 99% of signals caused by neutrons of
more than 2 MeV (megaelectron volt; 1 MeV is the
kinetic energy of an electron as it accelerates through an
electric potential difference of one million volts).
The new liquid scintillator is expected to provide
a clue to solve the reactor neutrino anomaly. The
NEOS detector was installed at the Hanbit
Nuclear Power Plant in Yeong-gwang in July
2015, and started operating from August 2015.
It is scheduled to collect data for approximately
six months.
Researchers all over the world are focusing on
whether the Center will succeed in discovering
the fourth neutrino holding the key to solving
the mystery of the universe.