Large Liquid Scintillator Research Articles

Search for charged excited states of dark matter with KamLAND-Zen

Particle dark matter could belong to a multiplet that includes an electrically charged state. WIMP dark matter (χ0) accompanied by a negatively charged excited state (χ−) with a small mass difference (e.g. < 20 MeV) can form a bound-state with a nucleus such as xenon. This bound-state formation is rare and the released energy is O(1−10) MeV depending on the nucleus, making large liquid scintillator detectors suitable for detection. We searched for bound-state formation events with xenon in two experimental phases of the KamLAND-Zen experiment, a xenon-doped liquid scintillator detector. No statistically significant events were observed. For a benchmark parameter set of WIMP mass mχ0=1 TeV and mass difference Δm=17 MeV, we set the most stringent upper limits on the recombination cross section times velocity 〈σv〉 and the decay-width of χ− to 9.2×10−30cm3/s and 8.7×10−14 GeV, respectively at 90% confidence level.

Optimized α/β pulse shape discrimination in Borexino

Borexino could efficiently distinguish between α and β radiation in its liquid scintillator by the characteristic time profile of its scintillation pulse. This α/β discrimination, first demonstrated on the ton scale in the counting test facility prototype, was used throughout the lifetime of the experiment between 2007 and 2021. With this method, the α events are identified and subtracted from the solar neutrino events similar to β. This is particularly important in liquid scintillators, as the α scintillation is strongly quenched. In Borexino, the prominent Po210 decay peak was a background in the energy range of electrons scattered from Be7 solar neutrinos. Optimal α/β discrimination was achieved with a , with a higher ability to leverage the timing information of the scintillation photons detected by the photomultiplier tubes. An event-by-event, high efficiency, stable, and uniform pulse shape discrimination was essential in characterizing the spatial distribution of background in the detector. This benefited most Borexino measurements, including solar neutrinos in the pp chain and the first direct observation of the CNO cycle in the Sun. This paper presents key milestones in α/β discrimination in Borexino as a term of comparison for current and future large liquid scintillator detectors. Published by the American Physical Society 2024

The status of the back-end card for the JUNO experiment

The Jiangmen Underground Neutrino Observatory (JUNO) aims to determine the neutrino mass hierarchy by detecting antineutrinos from nuclear reactors using a large liquid scintillator volume. The detector employs approximately 20,000 20-inch photomultiplier tubes powered and read out by two electronics readout systems: underwater and above water. The back-end card (BEC) is a crucial component of the latter and links 7,000 underwater electronics boxes to the trigger system. 180 BECs have been installed and tested at the JUNO site, including self-tests and combined tests. This paper presents the current status of the BEC.

First attempt of directionality reconstruction for atmospheric neutrinos in a large homogeneous liquid scintillator detector

The directionality information of incoming neutrinos is essential to atmospheric neutrino oscillation analysis since it is directly related to the oscillation baseline length. Large homogeneous liquid scintillator detectors, while offering excellent energy resolution, are traditionally very limited in their capabilities of measuring event directionality. In this paper, we present a novel directionality reconstruction method for atmospheric neutrino events in large homogeneous liquid scintillator detectors based on waveform analysis and machine learning techniques. We demonstrate for the first time that such detectors can achieve good direction resolution and potentially play an important role in future atmospheric neutrino oscillation measurements. Published by the American Physical Society 2024

Flavor identification of atmospheric neutrinos in JUNO with machine learning

The Jiangmen Underground Neutrino Observatory (JUNO) is designed to determine neutrino mass ordering (NMO) using a large liquid scintillator detector located in southern China. While JUNO's NMO sensitivity mostly comes from reactor neutrinos, atmospheric neutrino oscillations in JUNO can provide complimentary sensitivity via matter effects, and enhance its overall sensitivity in a joint analysis. Neutrino flavor identification is crucial to atmospheric neutrino oscillation measurements, but is traditionally a very difficult task in liquid scintillator detectors such as JUNO. In this paper, we present a novel method for the flavor identification of atmospheric neutrinos in JUNO with machine learning techniques. This method takes features from photomultiplier tube waveforms as inputs, and shows promising results with JUNO simulation. This method could also be applied to other liquid scintillator detectors, potentially benefiting other future atmospheric neutrino oscillation experiments.

A laserball calibration device for the SNO+ scintillator phase

Located 2 km underground in SNOLAB, Sudbury, Canada, SNO+ is a large scale liquid scintillator experiment that primarily aims to search for neutrinoless double beta decay. Whilst SNO+ has light and radioactive calibration sources external to the inner volume, an internally deployed optical source is necessary for the full characterization of the detector model. A laser diffuser ball developed for SNO has previously demonstrated to be an effective optical calibration device for both SNO and SNO+ water phase. Since the introduction of liquid scintillator for SNO+, the material compatibility, cleanliness, and radiopurity requirements of any materials in contact with the internal medium have increased. Improving on the original SNO laserball design, a new laserball calibration device has been developed for the SNO+ scintillator phase with the goal of measuring the optical properties of the detector and performing routine PMT gain and timing calibrations. Simulations have been written to model the diffusion properties to optimise optical and temporal performance for calibration. Prototype laserballs have been built and characterised, demonstrating sub-ns timing resolution and a quasi-isotropic light distribution.

JUNO sensitivity on proton decay p → ν K + searches* *We are grateful for the ongoing cooperation from the China General Nuclear Power Group. This study was supported by the Chinese Academy of Sciences, the National Key R&D Program of China, the CAS Center for Excellence in Particle Physics, Wuyi University, and the Tsung-Dao Lee Institute of Shanghai Jiao

The Jiangmen Underground Neutrino Observatory (JUNO) is a large liquid scintillator detector designed to explore many topics in fundamental physics. In this study, the potential of searching for proton decay in the mode with JUNO is investigated. The kaon and its decay particles feature a clear three-fold coincidence signature that results in a high efficiency for identification. Moreover, the excellent energy resolution of JUNO permits suppression of the sizable background caused by other delayed signals. Based on these advantages, the detection efficiency for the proton decay via is 36.9% ± 4.9% with a background level of events after 10 years of data collection. The estimated sensitivity based on 200 kton-years of exposure is years, which is competitive with the current best limits on the proton lifetime in this channel and complements the use of different detection technologies.

Data-driven simultaneous vertex and energy reconstruction for large liquid scintillator detectors

Data-driven simultaneous vertex and energy reconstruction for large liquid scintillator detectors

Energy reconstruction for large liquid scintillator detectors with machine learning techniques: aggregated features approach

Large-scale detectors consisting of a liquid scintillator target surrounded by an array of photo-multiplier tubes (PMTs) are widely used in the modern neutrino experiments: Borexino, KamLAND, Daya Bay, Double Chooz, RENO, and the upcoming JUNO with its satellite detector TAO. Such apparatuses are able to measure neutrino energy which can be derived from the amount of light and its spatial and temporal distribution over PMT channels. However, achieving a fine energy resolution in large-scale detectors is challenging. In this work, we present machine learning methods for energy reconstruction in the JUNO detector, the most advanced of its type. We focus on positron events in the energy range of 0–10 MeV which corresponds to the main signal in JUNO – neutrinos originated from nuclear reactor cores and detected via the inverse beta decay channel. We consider the following models: Boosted Decision Trees and Fully Connected Deep Neural Network, trained on aggregated features, calculated using the information collected by PMTs. We describe the details of our feature engineering procedure and show that machine learning models can provide the energy resolution sigma = 3% at 1 MeV using subsets of engineered features. The dataset for model training and testing is generated by the Monte Carlo method with the official JUNO software.

Verification of the calibration method for the boundary effect in JUNO

Jiangmen Underground Neutrino Observatory (JUNO) is constructed to probe multiple physics goals, including determining the neutrino mass ordering (MO), measuring solar neutrinos, detecting supernova neutrinos, etc. A large Liquid Scintillator (LS) detector with a high energy resolution is being built to determine the neutrino MO within 6-year of data taking. To accomplish this goal, a calibration complex has been designed. Concerning the energy leakage near the LS boundary, a Guide Tube Calibration System (GTCS) will be deployed on the outer surface of the JUNO central detector to help better understand the boundary effect. A fitting algorithm has been presented during the design of GTCS considering the overlap of full-energy peak and Compton tail. In this paper, we verify the fitting algorithm experimentally, using a radioactive source in the Daya Bay experiment. The maximum bias of full-energy peak for the experiment at Daya Bay is less than 1.0% even if a change of ±20.0% in acrylic density was assumed. Besides, the tuned acrylic density is applied in JUNO GTCS simulation. The maximum uncertainty estimation of GTCS energy response in total is 0.52%, satisfying the JUNO GTCS requirement which is 1.0%.

Status of the Jiangmen Underground Neutrino Observatory

The Jiangmen Underground Neutrino observatory (JUNO) experiment uses a large liquid scintillator detector to detect electron antineutrinos issued from nuclear reactors at a distance of 53 km. Its main goal is to determine the neutrino mass hierarchy and precisely measure some of the oscillation parameters. The detector will be located 700 m underground and will consist of 20 ktons of liquid scintillator contained in a 35 m diameter acrylic sphere, instrumented with 17612 (20-inch) and 25600 (3-inch) photomultiplier tubes (PMT), respectively. It will achieve an unprecedented 3% energy resolution (at 1 MeV). The objective is to detect 100000 events after 6 years of data taking. Two independent veto systems are foreseen to reduce the different backgrounds. A 40 kton ultrapure water Cherenkov pool instrumented with 2400 20-inch PMTs surrounds the central detector. It will tag events coming from outside the neutrino target. It will also act as a passive shielding for neutrons and gammas. In addition, a muon tracker will be installed on top of the detector (top muon veto) in order to tag cosmic muons and validate the muon track reconstruction.

Improvement of machine learning-based vertex reconstruction for large liquid scintillator detectors with multiple types of PMTs

Improvement of machine learning-based vertex reconstruction for large liquid scintillator detectors with multiple types of PMTs

Large-energy single hits at JUNO from atmospheric neutrinos and dark matter

Large liquid scintillator detectors, such as JUNO, present a new opportunity to study neutral current events from the low-energy end of the atmospheric neutrinos, and possible new physics signals due to light dark matter. We carefully study the possibility of detecting ``large-energy singles'' (LES), i.e., events with visible scintillation energy $&gt;15\text{ }\text{ }\mathrm{MeV}$, but no other associated tags. For an effective exposure of $20\text{ }\text{ }\mathrm{kton}\ensuremath{-}\mathrm{yr}$ and considering only Standard Model physics, we expect the LES sample to contain $\ensuremath{\sim}40$ events from scattering on free protons and $\ensuremath{\sim}108$ events from interaction with carbon, from neutral-current interactions of atmospheric neutrinos. Backgrounds, largely due to $\ensuremath{\beta}$ decays of cosmogenic isotopes, are shown to be significant only below 15 MeV visible energy. The LES sample at JUNO can competitively probe a variety of new physics scenarios, such as boosted dark matter and annihilation of galactic dark matter to sterile neutrinos.

Alternative design for large scale liquid scintillator detectors

The construction of large-scale liquid scintillator detectors is complicated by the need to separate the scintillation region from photomultiplier tubes (PMTs) due to their intrinsic radioactivity. This is generally done using acrylic or nylon barriers, whose own activity can also lead to substantial reduction of the fiducial detection volume for a number of low energy (MeV) studies, and whose construction becomes increasingly difficult and expensive for larger detector volumes. Here we present an initial simulation study of an alternative geometric construction known as SLIPS (Stratified LIquid Plane Scintillator), which aims to avoid such physical barriers entirely by instead using layers of lipophobic liquids to separate PMTs from the scintillation region.

Novel Detector Assembly for Neutron/Gamma-Ray Discrimination Applications Based on Large-Sized Scintillators Coupled to Large Area SiPM Arrays

In this work, we propose a prototype of a compact detector assembly capable of discriminating between neutrons and gamma rays, by performing pulse shape analysis. The device takes advantage of recent improvements in silicon photomultiplier (SiPM) technology and the excellent neutron/gamma-ray discrimination capabilities of new plastics and standard liquid scintillators. Our work is focused on the read-out of small-, medium-, and large-sized organic scintillators with large-area SiPM arrays ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4 \times 4$ </tex-math></inline-formula> NUV and RGB-SiPM arrays, 17 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$ </tex-math></inline-formula> 17 mm). In particular, we used EJ-276 (and its old version EJ-299) and EJ-276G plastic scintillators of several sizes (ranging from 20- to 50-mm diameter) and two large liquid scintillator cells (EJ-301 and EJ-309, 50 mm diameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 50$ </tex-math></inline-formula> mm thickness). Moreover, we show the comparison of neutron/gamma-ray discrimination performance between SiPM and standard photomultiplier read-out configurations. The presented innovative device could be used in different fields, first in fundamental nuclear physics experiments, as an ancillary neutron detector for the active target at the selective production of exotic species (SPES) facility at the Legnaro National Laboratories in Italy. Additionally, it could be used in radiation monitoring applications, as a fast neutron/gamma-ray counter to be embedded in unmanned vehicles for radiation monitoring.

Prospects for searching new Double Beta Decay physics with KamLAND-Zen 800

Two-neutrino double beta decay (2υββ) is a rare second-order weak radioactive decay process. It has been observed in neutrinoless double beta decay (0υββ) search experiments. Precise observation of 2υββ is essential to reduce the theoretical uncertainty in the calculation of nuclear matrix elements required to obtain the effective Majorana mass from a lifetime of 0υββ. Also, 2υββ itself is interesting because new physics could be hiding in the energy spectrum, such as for example, Majoron emission mode 0υββ and 2υββ with neutrino self-interaction. KamLAND-Zen 800 is an experiment to search for 0υββ of 136Xe with a large ultra-pure liquid scintillator detector, KamLAND. KamLAND-Zen 800 has been observing 745 kg of xenon gas at 91% enriched in 136Xe since 2019, providing a high statistics 2υββ decay sample. We describe the potential for new physics searches in 2υββ with KamLAND-Zen 800.

Data analysis strategy used for the detection of CNO solar neutrinos with Borexino

Borexino is a large liquid scintillator experiment located at the underground INFN Laboratori Nazionali del Gran Sasso, in Italy. It was designed and built with the primary goal of real-time detection of low energy solar neutrinos, and in more than ten years of data taking it has measured all the neutrino fluxes produced in the proton-proton chain, i.e. the main fusion process accounting for 99% of the energy production in the Sun. Recently, after improvements and developments in both hardware and software, Borexino has provided the first observation of solar neutrinos emitted from the subdominant Carbon-Nitrogen-Oxygen (CNO) fusion cycle. All the crucial steps of the analysis strategy adopted to disentangle the signal of CNO neutrinos from backgrounds present in the detector will be described in this article.

20-inch photomultiplier tube timing study for JUNO

Jiangmen Underground Neutrino Observatory (JUNO) is a large liquid scintillator neutrino detector now under construction at Jiangmen, Guangdong, China for determination of neutrino mass ordering with 3% energy resolution at 1 MeV, a precise measurement of neutrino oscillation parameters, and other neutrino physics. The central detector is made up of a 35.4-meter diameter acrylic sphere which contains 20 kton of liquid scintillator and is surrounded by about 18k 20-inch photomultiplier tubes (PMTs). The PMTs performance is one of the JUNO’s key successes to reach the high resolution goal. In this study, the PMT characteristic and its timing related responses were determined via the PMT generated signals, extracted from the PMT in a scanning station system. About 2,400 of micro-channel plate PMTs (MCP-PMTs) and dynode PMTs were analyzed for their responses with LED source such as rise time, fall time, transit time spread (TTS), gain, etc., which relate to photon incident on different positions of PMT’s glass surface. Furthermore, we also observed the fluctuation of PMT performance under magnetic field which can decrease the PMT photon detection efficiency (PDE).

Unambiguously Resolving the Potential Neutrino Magnetic Moment Signal at Large Liquid Scintillator DetectorsSupported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA10010800). DLX and ZPY are grateful to the Double First Class Start-up Fund (WF220442603) provided by Shanghai Jiao Tong University. DLX also thanks support from the CAS Center for Excellence in Particle Physics

Non-vanishing electromagnetic properties of neutrinos have been predicted by many theories beyond the Standard Model, and an enhanced neutrino magnetic moment can have profound implications for fundamental physics. The XENON1T experiment recently detected an excess of electron recoil events in the 1–7 keV energy range, which can be compatible with solar neutrino magnetic moment interaction at a most probable value of μν = 2.1 × 10−11 μ B. However, tritium backgrounds or solar axion interaction in this energy window are equally plausible causes. Upcoming multi-tonne noble liquid detectors will test these scenarios more in depth, but will continue to face similar ambiguity. We report a unique capability of future large liquid scintillator detectors to help resolve the potential neutrino magnetic moment scenario. With O(100) kton⋅year exposure of liquid scintillator to solar neutrinos, a sensitivity of μν < 10−11 μ B can be reached at an energy threshold greater than 40 keV, where no tritium or solar axion events but only neutrino magnetic moment signal is still present.

The study of active geomagnetic shielding coils system for JUNO

The Jiangmen Underground Neutrino Observatory (JUNO) is a large liquid scintillator (LS) detector for neutrino mass ordering and other neutrino physics research. The detector uses large-size 20 inches photomultiplier tubes to detect photons from a liquid scintillator. The large PMTs are sensitive and easily affected such that the detection efficiency loses about 60% under the geomagnetic field intensity (∼500 mG). It has a significantly negative effect on the detector performance, and a compensation system is necessary for geomagnetic field shielding. As permalloys are easily rusted in water, a better way for the geomagnetic shielding is to apply an active compensation coils system. The simulations show that a set of 32 circular coils can meet the experiment requirement. The residual magnetic field is less than 0.05 G in the Central Detector Photomultiplier Tube (CD-PMT) region (38.5–39.5 m in diameter). A prototype coil system with a 1.2 m was built to validate the simulation and the design. The measured data of prototype and simulation results are consistent with each other, and geomagnetic field intensity is effectively reduced by coils, verifying the shielding coils system design for JUNO. This study is expected to provide practical guidance for the PMT magnetic field shielding for future large-scale detector designs.