arXiv subjects

Y. H. Kim

Publications and source records attributed to Y. H. Kim.

At least 91 records · Page 5Linked to original sources

arXiv2018

Background model for the NaI(Tl) crystals in COSINE-100

The COSINE-100 dark matter search experiment is an array of NaI(Tl) crystal detectors located in the Yangyang Underground Laboratory (Y2L). To understand measured backgrounds in the NaI(Tl) crystals we have performed Monte Carlo simulations using the Geant4 toolkit and developed background models for each crystal that consider contributions from both internal and external sources, including cosmogenic nuclides. The background models are based on comparisons of measurement data with Monte Carlo simulations that are guided by a campaign of material assays and are used to evaluate backgrounds and identify their sources. The average background level for the six crystals (70 kg total mass) that are studied is 3.5 counts/day/keV/kg in the (2-6) keV energy interval. The dominant contributors in this energy region are found to be $^{210}$Pb and $^3$H.

astro-ph.IM↗

arXiv2018

Direct dark matter search by annual modulation with 2.7 years of XMASS-I data

An annual modulation signal due to the Earth orbiting around the Sun would be one of the strongest indications of the direct detection of dark matter. In 2016, we reported a search for dark matter by looking for this annual modulation with our single-phase liquid xenon XMASS-I detector. That analysis resulted in a slightly negative modulation amplitude at low energy. In this work, we included more than one year of additional data, which more than doubles the exposure to 800 live days with the same 832 kg target mass. When we assume weakly interacting massive particle (WIMP) dark matter elastically scattering on the xenon target, the exclusion upper limit for the WIMP-nucleon cross section was improved by a factor of 2 to 1.9$\times$10$^{-41}$cm$^2$ at 8 GeV/c$^2$ at 90\% confidence level with our newly implemented data selection through a likelihood method. For the model-independent case, without assuming any specific dark matter model, we obtained more consistency with the null hypothesis than before with a $p$-value of 0.11 in the 1$-$20 keV energy region. This search probed this region with an exposure that was larger than that of DAMA/LIBRA. We also did not find any significant amplitude in the data for periodicity with periods between 50 and 600 days in the energy region between 1 to 6 keV.

astro-ph.CO↗

arXiv2018

Improved search for two-neutrino double electron capture on $^{124}$Xe and $^{126}$Xe using particle identification in XMASS-I

We conducted an improved search for the simultaneous capture of two $K$-shell electrons on the $^{124}$Xe and $^{126}$Xe nuclei with emission of two neutrinos using 800.0 days of data from the XMASS-I detector. A novel method to discriminate $γ$-ray/$X$-ray or double electron capture signals from $β$-ray background using scintillation time profiles was developed for this search. No significant signal was found when fitting the observed energy spectra with the expected signal and background. Therefore, we set the most stringent lower limits on the half-lives at $2.1 \times 10^{22}$ and $1.9 \times 10^{22}$ years for $^{124}$Xe and $^{126}$Xe, respectively, with 90% confidence level. These limits improve upon previously reported values by a factor of 4.5.

nucl-ex↗

arXiv2018

Generic features of the neutron-proton interaction

We show that fully aligned neutron-proton pairs play a crucial role in the low-energy spectroscopy of nuclei. with valence nucleons in a high-j orbital. Their dominance is valid in nuclei with valence neutrons and protons in different high-j orbitals as well as in N = Z nuclei, where all nucleons occupy the same orbital. We demonstrate analytically this generic feature of the neutron-proton interaction for a variety of systems with four valence nucleons interacting through realistic, effective forces. The dominance of fully aligned neutron-proton pairs results from the combined effect of (i) angular momentum coupling and (ii) basic properties of the neutron-proton interaction.

nucl-th↗

arXiv2018

Initial Performance of the COSINE-100 Experiment

COSINE is a dark matter search experiment based on an array of low background NaI(Tl) crystals located at the Yangyang underground laboratory. The assembly of COSINE-100 was completed in the summer of 2016 and the detector is currently collecting physics quality data aimed at reproducing the DAMA/LIBRA experiment that reported an annual modulation signal. Stable operation has been achieved and will continue for at least two years. Here, we describe the design of COSINE-100, including the shielding arrangement, the configuration of the NaI(Tl) crystal detection elements, the veto systems, and the associated operational systems, and we show the current performance of the experiment.

physics.ins-det↗

arXiv2018

Identification of $^{210}$Pb and $^{210}$Po in the bulk of copper samples with a low-background alpha particle counter

We established a method to assay $^{210}$Pb and $^{210}$Po contaminations in the bulk of copper samples using a low-background alpha particle counter. The achieved sensitivity for the $^{210}$Pb and $^{210}$Po contaminations reaches a few mBq/kg. Due to this high sensitivity, the $^{210}$Pb and $^{210}$Po contaminations in oxygen free copper bulk were identified and measured for the first time. The $^{210}$Pb contaminations of our oxygen free copper samples were 17-40 mBq/kg. Based on our investigation of copper samples in each production step, the $^{210}$Pb in oxygen free copper was understood to be a small residual of an electrolysis process. This method to measure bulk contaminations of $^{210}$Pb and $^{210}$Po could be applied to other materials.

physics.ins-det↗

arXiv2017

Muon detector for the COSINE-100 experiment

The COSINE-100 dark matter search experiment has started taking physics data with the goal of performing an independent measurement of the annual modulation signal observed by DAMA/LIBRA. A muon detector was constructed by using plastic scintillator panels in the outermost layer of the shield surrounding the COSINE-100 detector. It is used to detect cosmic ray muons in order to understand the impact of the muon annual modulation on dark matter analysis. Assembly and initial performance test of each module have been performed at a ground laboratory. The installation of the detector in Yangyang Underground Laboratory (Y2L) was completed in the summer of 2016. Using three months of data, the muon underground flux was measured to be 328 $\pm$ 1(stat.)$\pm$ 10(syst.) muons/m$^2$/day. In this report, the assembly of the muon detector and the results from the analysis are presented.

physics.ins-det↗

arXiv2017

Search for solar Kaluza-Klein axion by annual modulation with the XMASS-I detector

In theories with the large extra dimensions beyond the standard 4-dimensional spacetime, axions could propagate in such extra dimensions, and acquire Kaluza-Klein (KK) excitations. These KK axions are produced in the Sun and could solve unexplained heating of the solar corona. While most of the solar KK axions escape from the solar system, a small fraction is gravitationally trapped in orbits around the Sun. They would decay into two photons inside a terrestrial detector. The event rate is expected to modulate annually depending on the distance from the Sun. We have searched for the annual modulation signature using $832\times 359$ kg$\cdot$days of XMASS-I data. No significant event rate modulation is found, and hence we set the first experimental constraint on the KK axion-photon coupling of $4.8 \times 10^{-12}\, \mathrm{GeV}^{-1}$ at 90% confidence level for a KK axion number density of $\bar{n}_\mathrm{a} = 4.07 \times 10^{13}\, \mathrm{m}^{-3}$, the total number of extra dimensions $n = 2$, and the number of extra dimensions $δ= 2$ that axions can propagate in.

hep-ex↗

arXiv2016

A measurement of the time profile of scintillation induced by low energy gamma-rays in liquid xenon with the XMASS-I detector

We report the measurement of the emission time profile of scintillation from gamma-ray induced events in the XMASS-I 832 kg liquid xenon scintillation detector. Decay time constant was derived from a comparison of scintillation photon timing distributions between the observed data and simulated samples in order to take into account optical processes such as absorption and scattering in liquid xenon. Calibration data of radioactive sources, $^{55}$Fe, $^{241}$Am, and $^{57}$Co were used to obtain the decay time constant. Assuming two decay components, $τ_1$ and $τ_2$, the decay time constant $τ_2$ increased from 27.9 ns to 37.0 ns as the gamma-ray energy increased from 5.9 keV to 122 keV. The accuracy of the measurement was better than 1.5 ns at all energy levels. A fast decay component with $τ_1 \sim 2$ ns was necessary to reproduce data. Energy dependencies of $τ_2$ and the fraction of the fast decay component were studied as a function of the kinetic energy of electrons induced by gamma-rays. The obtained data almost reproduced previously reported results and extended them to the lower energy region relevant to direct dark matter searches.

physics.ins-det↗

arXiv2016

Simulations of background sources in AMoRE-I experiment

The first phase of the Advanced Mo-based Rare Process Experiment (AMoRE-I), an experimental search for neutrinoless double beta decay (0$νββ$) of $^{100}$Mo in calcium molybdate (CMO) crystal using cryogenic techniques, is in preparation at the YangYang underground laboratory (Y2L) in South Korea. A GEANT4 based Monte Carlo simulation was performed for background estimation in the first-phase the AMoRE-I detector and shield configuration. Background sources such as $^{238}$U, $^{232}$Th, $^{40}$K, $^{235}$U, and $^{210}$Pb were simulated from inside the crystals, surrounding materials, outer shielding walls of the Y2L cavity. The estimated background rate in the region of interest was found to be $< 1.5 \times 10^{-3}$ counts/keV/kg/yr (ckky). The effects of random coincidences between background and two-neutrino double beta decay of $^{100}$Mo were estimated as a potential background source and its estimated rate was $< 2.3 \times 10^{-4}$ ckky.

physics.ins-det↗

arXiv2016

Direct dark matter search by annual modulation in XMASS-I

A search for dark matter was conducted by looking for an annual modulation signal due to the Earth's rotation around the Sun using XMASS, a single phase liquid xenon detector. The data used for this analysis was 359.2 live days times 832 kg of exposure accumulated between November 2013 and March 2015. When we assume Weakly Interacting Massive Particle (WIMP) dark matter elastically scattering on the target nuclei, the exclusion upper limit of the WIMP-nucleon cross section 4.3$\times$10$^{-41}$cm$^2$ at 8 GeV/c$^2$ was obtained and we exclude almost all the DAMA/LIBRA allowed region in the 6 to 16 GeV/c$^2$ range at $\sim$10$^{-40}$cm$^2$. The result of a simple modulation analysis, without assuming any specific dark matter model but including electron/$γ$ events, showed a slight negative amplitude. The $p$-values obtained with two independent analyses are 0.014 and 0.068 for null hypothesis, respectively. we obtained 90\% C.L. upper bounds that can be used to test various models. This is the first extensive annual modulation search probing this region with an exposure comparable to DAMA/LIBRA.

astro-ph.CO↗

arXiv2016

Search for two-neutrino double electron capture on $^{124}$Xe with the XMASS-I detector

Double electron capture is a rare nuclear decay process in which two orbital electrons are captured simultaneously in the same nucleus. Measurement of its two-neutrino mode would provide a new reference for the calculation of nuclear matrix elements whereas observation of its neutrinoless mode would demonstrate lepton number violation. A search for two-neutrino double electron capture on $^{124}$Xe is performed using 165.9 days of data collected with the XMASS-I liquid xenon detector. No significant excess above background was observed and we set a lower limit on the half-life as $4.7 \times 10^{21}$ years at 90% confidence level. The obtained limit has ruled out parts of some theoretical expectations. We obtain a lower limit on the $^{126}$Xe two-neutrino double electron capture half-life of $4.3 \times 10^{21}$ years at 90% confidence level as well.

nucl-ex↗

arXiv2016

Search for solar axions with CsI(Tl) crystal detectors

The results of a search for solar axions from the Korea Invisible Mass Search (KIMS) experiment at the Yangyang Underground Laboratory are presented. Low-energy electron-recoil events would be produced by conversion of solar axions into electrons via the axio-electric effect in CsI(Tl) crystals. Using data from an exposure of 34,596 $\rm kg \cdot days$, we set a 90 \% confidence level upper limit on the axion-electron coupling, $g_{ae}$, of $1.39 \times 10^{-11}$ for an axion mass less than 1 keV/$\rm c^2$. This limit is lower than the indirect solar neutrino bound, and fully excludes QCD axions heavier than 0.48 eV/$\rm c^2$ and 140.9 eV/$\rm c^2$ for the DFSZ and KSVZ models respectively.

hep-ex↗

arXiv2016

Understanding internal backgrounds of NaI(Tl) crystals toward a 200~kg array for the KIMS-NaI experiment

The Korea Invisible Mass Search (KIMS) collaboration has developed low-background NaI(Tl) crystals that are suitable for the direct detection of WIMP dark matter. With experience built on the KIMS-CsI programs, the KIMS-NaI experiment will consist of a 200~kg NaI(Tl) crystal array surrounded by layers of shielding structures and will be operated at the Yangyang underground laboratory. The goal is to provide an unambiguous test of the DAMA/LIBRA's annual modulation signature. Measurements of six prototype crystals show progress in the reduction of internal contaminations of radioisotopes. Based on our understanding of these measurements, we expect to achieve a background level in the final detector configuration that is less than 1~count/day/keV/kg for recoil energies around 2~keV. The annual modulation sensitivity for the KIMS-NaI experiment shows that an unambiguous 7$σ$ test of the DAMA/LIBRA signature would be possible with a 600~kg$\cdot$year exposure with this system.

physics.ins-det↗

arXiv2016

A CaMoO4 Crystal Low Temperature Detector for the AMoRE Neutrinoless Double Beta Decay Search

We report the development of a CaMoO4 crystal low temperature detector for the AMoRE neutrinoless double beta decay (0ν\b{eta}\b{eta}) search experiment. The prototype detector cell was composed of a 216 g CaMoO4 crystal and a metallic magnetic calorimeter. An over-ground measurement demonstrated FWHM resolution of 6-11 keV for full absorption gamma peaks. Pulse shape discrimination was clearly demonstrated in the phonon signals, and 7.6 σ of discrimination power was found for the α and \b{eta}/γ separation. The phonon signals showed rise-times of about 1 ms. It is expected that the relatively fast rise-time will increase the rejection efficiency of two-neutrino double beta decay pile-up events which can be one of the major background sources in 0ν\b{eta}\b{eta} searches.

physics.ins-det↗

arXiv2015

Technical Design Report for the AMoRE $0νββ$ Decay Search Experiment

The AMoRE (Advanced Mo-based Rare process Experiment) project is a series of experiments that use advanced cryogenic techniques to search for the neutrinoless double-beta decay of \mohundred. The work is being carried out by an international collaboration of researchers from eight countries. These searches involve high precision measurements of radiation-induced temperature changes and scintillation light produced in ultra-pure \Mo[100]-enriched and \Ca[48]-depleted calcium molybdate ($\mathrm{^{48depl}Ca^{100}MoO_4}$) crystals that are located in a deep underground laboratory in Korea. The \mohundred nuclide was chosen for this \zeronubb decay search because of its high $Q$-value and favorable nuclear matrix element. Tests have demonstrated that \camo crystals produce the brightest scintillation light among all of the molybdate crystals, both at room and at cryogenic temperatures. $\mathrm{^{48depl}Ca^{100}MoO_4}$ crystals are being operated at milli-Kelvin temperatures and read out via specially developed metallic-magnetic-calorimeter (MMC) temperature sensors that have excellent energy resolution and relatively fast response times. The excellent energy resolution provides good discrimination of signal from backgrounds, and the fast response time is important for minimizing the irreducible background caused by random coincidence of two-neutrino double-beta decay events of \mohundred nuclei. Comparisons of the scintillating-light and phonon yields and pulse shape discrimination of the phonon signals will be used to provide redundant rejection of alpha-ray-induced backgrounds. An effective Majorana neutrino mass sensitivity that reaches the expected range of the inverted neutrino mass hierarchy, i.e., 20-50 meV, could be achieved with a 200~kg array of $\mathrm{^{48depl}Ca^{100}MoO_4}$ crystals operating for three years.

physics.ins-det↗

arXiv2015

Pulse-shape discrimination between electron and nuclear recoils in a NaI(Tl) crystal

We report on the response of a high light-output NaI(Tl) crystal to nuclear recoils induced by neutrons from an Am-Be source and compare the results with the response to electron recoils produced by Compton scattered 662 keV $γ$-rays from a $^{137}$Cs source. The measured pulse-shape discrimination (PSD) power of the NaI(Tl) crystal is found to be significantly improved because of the high light output of the NaI(Tl) detector. We quantify the PSD power with a quality factor and estimate the sensitivity to the interaction rate for weakly interacting massive particles (WIMPs) with nucleons, and the result is compared with the annual modulation amplitude observed by the DAMA/LIBRA experiment. The sensitivity to spin-independent WIMP-nucleon interactions based on 100 kg$\cdot$year of data from NaI detectors is estimated with simulated experiments, using the standard halo model.

astro-ph.IM↗

arXiv2014

Tests on NaI(Tl) crystals for WIMP search at the Yangyang Underground Laboratory

Among the direct search experiments for WIMP dark matter, the DAMA experiment observed an annual modulation signal interpreted as WIMP interactions with 9.2$σ$ significance. However, this result is contradictory with other direct search experiments reporting null signals in the same parameter space allowed by the DAMA observation, necessitating clarification of the origin of the modulation signal observed using the NaI(Tl) crystals of the DAMA experiment independently. Here, we report the first results of NaI(Tl) crystal measurement at the Yangyang Underground Laboratory to grow ultra-low-background NaI(Tl) crystal detectors.

astro-ph.IM↗