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.