arXiv subjects

J. G. Kim

Publications and source records attributed to J. G. Kim.

11 recordsLinked to original sources

arXiv2024

Measurement of reactor antineutrino oscillation amplitude and frequency using 3800 days of complete data sample of the RENO experiment

We report an updated neutrino mixing angle of $θ_{13}$ obtained from a complete data sample of the RENO experiment. The experiment has measured the amplitude and frequency of reactor anti-electron-neutrinos ($\barν_{e}$) oscillations at the Hanbit nuclear power plant, Younggwang, Korea, since August 2011. As of March 2023, the data acquisition was completed after a total of 3800 live days of detector operation. The observed candidates via inverse beta decay (IBD) are 1,211,995 (144,667) in the near (far) detector. Based on an observed energy-dependent reactor neutrino disappearance, neutrino oscillation parameters of $θ_{13}$ and $\lvertΔm_{ee}^2\rvert$ are precisely determined as $\sin^{2}2θ_{13}=0.0920_{-0.0042}^{+0.0044}(\text{stat.})_{-0.0041}^{+0.0041}(\text{syst.})$ and $\lvertΔm_{ee}^2\rvert=\left[2.57_{-0.11}^{+0.10}(\text{stat.})_{-0.05}^{+0.05}(\text{syst.})\right]\times10^{-3}~\text{eV}^{2}$. Compared to the previous RENO results published in Ref.~\cite{PhysRevLett.121.201801}, the precision is improved from 7.5\% to 6.4\% for $\sin^{2}2θ_{13}$ and from 5.2\% to 4.5\% for $\lvertΔm_{ee}^2\rvert$. The statistical error of the measurement has reached our goal and is hardly improved with additional data-taking.

hep-ex↗

arXiv2023

Pulse shape discrimination using a convolutional neural network for organic liquid scintillator signals

A convolutional neural network (CNN) architecture is developed to improve the pulse shape discrimination (PSD) power of the gadolinium-loaded organic liquid scintillation detector to reduce the fast neutron background in the inverse beta decay candidate events of the NEOS-II data. A power spectrum of an event is constructed using a fast Fourier transform of the time domain raw waveforms and put into CNN. An early data set is evaluated by CNN after it is trained using low energy $β$ and $α$ events. The signal-to-background ratio averaged over 1-10 MeV visible energy range is enhanced by more than 20% in the result of the CNN method compared to that of an existing conventional PSD method, and the improvement is even higher in the low energy region.

physics.ins-det↗

arXiv2022

Search for sterile neutrino oscillation using RENO and NEOS data

We present a reactor model independent search for sterile neutrino oscillation using 2\,509\,days of RENO near detector data and 180 days of NEOS data. The reactor related systematic uncertainties are significantly suppressed as both detectors are located at the same reactor complex of Hanbit Nuclear Power Plant. The search is performed by electron antineutrino\,($\overlineν_e$) disappearance between six reactors and two detectors with baselines of 294\,m\,(RENO) and 24\,m\,(NEOS). A spectral comparison of the NEOS prompt-energy spectrum with a no-oscillation prediction from the RENO measurement can explore reactor $\overlineν_e$ oscillations to sterile neutrino. Based on the comparison, we obtain a 95\% C.L. excluded region of $0.1<|Δm_{41}^2|<7$\,eV$^2$. We also obtain a 68\% C.L. allowed region with the best fit of $|Δm_{41}^2|=2.41\,\pm\,0.03\,$\,eV$^2$ and $\sin^2 2θ_{14}$=0.08$\,\pm\,$0.03 with a p-value of 8.2\%. Comparisons of obtained reactor antineutrino spectra at reactor sources are made among RENO, NEOS, and Daya Bay to find a possible spectral variation.

hep-ex↗

arXiv2022

Measurement of cosmogenic $^9$Li and $^8$He production rates at RENO

We report the measured production rates of unstable isotopes $^9$Li and $^8$He produced by cosmic muon spallation on $^{12}$C using two identical detectors of the RENO experiment. Their beta-decays accompanied by a neutron make a significant contribution to backgrounds of reactor antineutrino events in precise determination of the smallest neutrino mixing angle. The mean muon energy of its near (far) detector with an overburden of 120 (450) m.w.e. is estimated as 33.1 +- 2.3 (73.6 +- 4.4) GeV. Based on roughly 3100 days of data, the cosmogenic production rate of $^9$Li ($^8$He) isotope is measured to be 44.2 +- 3.1 (10.6 +- 7.4) per day at near detector and 10.0 +- 1.1 (2.1 +- 1.5) per day at far detector. This corresponds to yields of $^9$Li ($^8$He), 4.80 +- 0.36 (1.15 +- 0.81) and 9.9 +- 1.1 (2.1 +- 1.5) at near and far detectors, respectively, in a unit of 10$^{-8}$ $μ^{-1}$ g${^-1}$ cm${^2}$. Combining the measured $^9$Li yields with other available underground measurements, an excellent power-law relationship of the yield with respect to the mean muon energy is found to have an exponent of $α$ = 0.75 +- 0.05.

nucl-ex↗

arXiv2021

Measurement of Reactor Antineutrino Flux and Spectrum at RENO

The RENO experiment reports measured flux and energy spectrum of reactor electron antineutrinos\,($\overlineν_e$) from the six reactors at Hanbit Nuclear Power Plant. The measurements use 966\,094\,(116\,111)\,$\overlineν_e$ candidate events with a background fraction of 2.39\%\,(5.13\%), acquired in the near\,(far) detector, from August 2011 to March 2020. The inverse beta decay (IBD) yield is measured as (5.852$\,\pm\,$0.124$) \times 10^{-43}$\,cm$^2$/fission, corresponding to 0.941\,$\pm$ 0.019 of the prediction by the Huber and Mueller (HM) model. A reactor $\overlineν_e$ spectrum is obtained by unfolding a measured IBD prompt spectrum. The obtained neutrino spectrum shows a clear excess around 6\,MeV relative to the HM prediction. The obtained reactor $\overlineν_e$ spectrum will be useful for understanding unknown neutrino properties and reactor models. The observed discrepancies suggest the next round of precision measurements and modification of the current reactor $\overlineν_e$ models.

hep-ex↗

arXiv2020

Pulse Shape Discrimination of Fast Neutron Background using Convolutional Neural Network for NEOS II

Pulse shape discrimination plays a key role in improving the signal-to-background ratio in NEOS analysis by removing fast neutrons. Identifying particles by looking at the tail of the waveform has been an effective and plausible approach for pulse shape discrimination, but has the limitation in sorting low energy particles. As a good alternative, the convolutional neural network can scan the entire waveform as they are to recognize the characteristics of the pulse and perform shape classification of NEOS data. This network provides a powerful identification tool for all energy ranges and helps to search unprecedented phenomena of low-energy, a few MeV or less, neutrinos.

physics.ins-det↗

arXiv2020

Search for Sub-eV Sterile Neutrino at RENO

We report a search result for a light sterile neutrino oscillation with roughly 2200 live days of data in the RENO experiment. The search is performed by electron antineutrino ($\overlineν_e$) disappearance taking place between six 2.8 GW$_{\text{th}}$ reactors and two identical detectors located at 294 m (near) and 1383 m (far) from the center of reactor array. A spectral comparison between near and far detectors can explore reactor $\overlineν_e$ oscillations to a light sterile neutrino. An observed spectral difference is found to be consistent with that of the three-flavor oscillation model. This yields limits on $\sin^{2} 2θ_{14}$ in the $10^{-4} \lesssim |Δm_{41}^2| \lesssim 0.5$ eV$^2$ region, free from reactor $\overlineν_e$ flux and spectrum uncertainties. The RENO result provides the most stringent limits on sterile neutrino mixing at $|Δm^2_{41}| \lesssim 0.002$ eV$^2$ using the $\overlineν_e$ disappearance channel.

hep-ex↗

arXiv2019

Observation of Reactor Antineutrino Disappearance Using Delayed Neutron Capture on Hydrogen at RENO

The Reactor Experiment for Neutrino Oscillation (RENO) experiment has been taking data using two identical liquid scintillator detectors of 44.5 tons since August 2011. The experiment has observed the disappearance of reactor neutrinos in their interactions with free protons, followed by neutron capture on hydrogen. Based on 1500 live days of data taken with 16.8 GW$_{th}$ reactors at the Hanbit Nuclear Power Plant in Korea, the near (far) detector observes 567690 (90747) electron antineutrino candidate events with a delayed neutron capture on hydrogen. This provides an independent measurement of $θ_{13}$ and a consistency check on the validity of the result from n-Gd data. Furthermore, it provides an important cross-check on the systematic uncertainties of the n-Gd measurement. Based on a rate-only analysis, we obtain sin$^{2}$2$θ_{13}$= 0.087 $\pm$ 0.008 (stat.) $\pm$ 0.014 (syst.).

hep-ex↗

arXiv2018

Measurement of Reactor Antineutrino Oscillation Amplitude and Frequency at RENO

The RENO experiment reports more precisely measured values of $θ_{13}$ and $|Δm_{ee}^2|$ using $\sim$2\,200 live days of data. The amplitude and frequency of reactor electron antineutrino ($\overlineν_e$) oscillation are measured by comparing the prompt signal spectra obtained from two identical near and far detectors. In the period between August 2011 and February 2018, the far (near) detector observed 103\,212 (850\,666) electron antineutrino candidate events with a background fraction of 4.7\% (2.0\%). A clear energy and baseline dependent disappearance of reactor $\overlineν_e$ is observed in the deficit of the measured number of $\overlineν_e$. Based on the measured far-to-near ratio of prompt spectra, we obtain $\sin^2 2 θ_{13} = 0.0896 \pm 0.0048({\rm stat}) \pm 0.0048({\rm syst})$ and $|Δm_{ee}^2| =[2.68 \pm 0.12({\rm stat}) \pm 0.07({\rm syst})]\times 10^{-3}$~eV$^2$.

hep-ex↗

arXiv2010

Coexisting Magnetic Order and Cooperative Paramagnetism in the Stuffed Pyrochlore Tb_{2+x}Ti_{2-2x}Nb_xO_7

Neutron scattering and magnetization measurements have been performed on the stuffed pyrochlore system Tb2+xTi2-2xNbxO7. We find that despite the introduction of chemical disorder and increasingly antiferromagnetic interactions, a spin glass transition does not occur for T >= 1.5 K and cooperative paramagnetic behavior exists for all x. For x = 1, Tb3NbO7, an antiferromagnetically ordered state coexisting with cooperative paramagnetic behavior is seen without applying any external fields or pressure, a situation advantageous for studying this cooperative behavior.

cond-mat.str-el↗

arXiv2002

Studies of Electron Avalanche Behavior in Liquid Argon

Electron avalanching in liquid argon is being studied as a function of voltage, pressure, radiation intensity, and the concentrations of certain additives, especially xenon. The avalanches produced in an intense electric field at the tip of a tungsten needle are initiated by ionization from a moveable americium (241Am) gamma ray source. Photons from xenon excimers are detected as photomultiplier signals in coincidence with the current pulse from the needle. In pure liquid argon the avalanche behavior is erratic, but the addition of even a small amount of xenon (>100ppm) stabilizes the performance. Similar attempts with neon (30%) as an additive to argon have been unsuccessful. Tests with higher energy gamma rays (57Co) yield spectra and other performance characteristics quite similar to those using the 241Am source. Two types of signal pulses are commonly observed: a set of pulses that are sensitive to ambient pressure, and a set of somewhat smaller pulses that are not pressure dependent.

hep-ex↗