Large Liquid Scintillator Research Articles

Study of Using Machine Learning for Level 1 Trigger Decision in JUNO Experiment

A study on the use of a machine learning algorithm for the level 1 trigger decision in the JUNO experiment ispresented. JUNO is a medium baseline neutrino experiment in construction in China, with the main goal of determining the neutrino mass hierarchy. A large liquid scintillator (LS)volume will detect the electron antineutrinos issued from nuclear reactors. The LS detector is instrumented by around 20000 large photomultiplier tubes. The hit information from each PMT will be collected into a center trigger unit for the level 1 trigger decision. The current trigger algorithm used to select a neutrino signal event is based on a fast vertex reconstruction. We propose to study an alternative level 1 (L1) trigger in order to achieve a similar performance as the vertex fitting trigger but with less logic resources by using firmware implemented machine learning model at the L1 trigger level. We treat the trigger decision as a classification problem and train a Multi-Layer Perceptron (MLP)model to distinguish the signal events with an energy higher than a certain threshold from noise events. We use JUNO software to generate datasets which include 100K physics events with noise and 100K pure noise events coming from PMT dark noise.For events with energy higher than 100 keV, the L1 trigger based on the converged MLP model can achieve an efficiency higher than 99%. After the training performed on simulations,we successfully implemented the trained model into a Kintex 7FPGA. We present the technical details of the neural network development and training, as well as its implementation in the hardware with the FPGA programming. Finally the performance of the L1 trigger MLP implementation is discussed.

Improving the energy uniformity for large liquid scintillator detectors

It is challenging to achieve high precision energy resolution for large liquid scintillator detectors. Energy non-uniformity is one of the main obstacles. To surmount it, a calibration-data driven method was developed previously to reconstruct event energy in the JUNO experiment. In this paper, we investigated the choice of calibration sources thoroughly, optimized the calibration positions and corrected the residual detector azimuthal asymmetry. All these efforts lead to a reduction of the energy non-uniformity near the detector boundary, from about 0.64% to 0.38%. And within the fiducial volume of the detector it is improved from 0.3% to 0.17%. As a result the energy resolution could be further improved.

Neutral-current background induced by atmospheric neutrinos at large liquid-scintillator detectors. II. Methodology for in situ measurements

Future large liquid-scintillator (LS) detectors are competitive with and complementary to the water-Cherenkov detectors on the searches for diffuse supernova neutrino background and nucleon decay. In a companion paper, we have performed a systematic calculation of the neutral-current (NC) background induced by atmospheric neutrino interactions on $^{12}\mathrm{C}$ nuclei in LS detectors, which are expected to be crucially important for the experimental searches for the diffuse supernova neutrino background and nucleon decay. In this paper, we perform a systematic study on the measurement of the NC background and evaluate the associated uncertainties. We first exploit the characteristics of the NC background, in particular, the multiplicities of neutrons and pions, and the possible association with unstable residual nuclei. It turns out that the neutron multiplicity distribution is very powerful to discriminate among different models. Then, we develop a maximum-likelihood method to allow an in situ measurement of the NC interactions with a triple-coincidence signature. Finally, a data-driven approach is proposed to evaluate the uncertainty of the NC background in the search for the diffuse supernova neutrino background. We conclude that future large LS experiments like the Jiangmen Underground Neutrino Observatory will be able to make a unique contribution to the worldwide dataset to improve the prediction of atmospheric neutrino NC interactions on $^{12}\mathrm{C}$.

Neutral-current background induced by atmospheric neutrinos at large liquid-scintillator detectors. I. Model predictions

The experimental searches for diffuse supernova neutrino background and proton decay in next-generation large liquid-scintillator (LS) detectors are competitive with and complementary to those in the water-Cherenkov detectors. In this paper, we carry out a systematic study of the dominant background induced by atmospheric neutrinos via their neutral-current (NC) interactions with the $^{12}{\rm C}$ nuclei in the LS detectors. The atmospheric neutrino fluxes at the location of Jiangmen Underground Neutrino Observatory (JUNO) are used, as the JUNO detector is obviously a suitable representative for future LS detectors. Then, we implement the sophisticated generators \texttt{GENIE} and \texttt{NuWro} to simulate the neutrino interactions with the carbon nuclei, and the package \texttt{TALYS} to deal with the deexcitations of final-state nuclei. Finally, the event rates for the production of additional nucleons, $\gamma$'s, $\alpha$'s, pions and kaons are obtained and categorized, and the systematic uncertainty of the NC background represented by a variety of data-driven nuclear models is estimated. The implications of the NC background from atmospheric neutrinos for the detection of diffuse supernova neutrino background and proton decay are also discussed.

Particle identification at MeV energies in JUNO

JUNO is a multi-purpose neutrino experiment currently under construction in Jiangmen, China. It is primarily aiming to determine the neutrino mass ordering. Moreover, its 20 kt target mass makes it an ideal detector to study neutrinos from various sources, including nuclear reactors, the Earth and its atmosphere, the Sun, and even supernovae. Due to the small cross section of neutrino interactions, the event rate of neutrino experiments is limited. In order to maximize the signal-to-noise ratio, it is extremely important to control the background levels. In this paper we discuss the potential of particle identification in JUNO, its underlying principles and possible areas of application in the experiment. While the presented concepts can be transferred to any large liquid scintillator detector, our methods are evaluated specifically for JUNO and the results are mainly driven by its high optical photon yield of 1,200 photo electrons per MeV of deposited energy. In order to investigate the potential of event discrimination, several event pairings are analysed, i.e. α/β, p/β, e+/e−, and e−/γ. We compare the discrimination performance of advanced analytical techniques based on neural networks and on the topological event reconstruction keeping the standard Gatti filter as a reference. We use the Monte Carlo samples generated in the physically motivated energy intervals. We study the dependence of our cuts on energy, radial position, PMT time resolution, and dark noise. The results show an excellent performance for α/β and p/β with the Gatti method and the neural network. Furthermore, e+/e− and e−/γ can partly be distinguished by means of neural network and topological reconstruction on a statistical basis. Especially in the latter case, the topological method proved very successful.

In Memoriam: Marvin A. Van Dilla: 1919-2019.

© 2020 International Society for Advancement of Cytometry.

Slow fluors for effective separation of Cherenkov light in liquid scintillators

The timing and spectral characteristics of four highly efficient, slow fluors are presented for liquid scintillator solutions using linear alkylbenzene (LAB) as the primary solvent. The mixtures exhibit high light yields, but with rise times of several ns or more and decay times on the order of tens of ns. Consequently, such liquid scintillator mixtures can be used for effective separation of Cherenkov and scintillation components based on timing in large scale liquid scintillation detectors. Such a separation, showing high light yield and directional information, is demonstrated here on a bench-top scale for electrons with energies extending below 1 MeV. This could have significant consequences for the future development of such detectors for measurements of solar neutrinos and neutrinoless double beta decay (0νββ) as well as providing good directional information for elastic scattering events from supernovae neutrinos and reactor anti-neutrinos, amongst others.

Search for reactor neutrino directionality using a LAB-based Gd-loaded liquid scintillation detector

Recently, several interesting phenomena related to neutrinos have been found using reactor neutrinos and we have learned much about the properties of neutrinos. Ultrapure water or liquid scintillator is generally used to detect neutrinos. Since a detector filled with water can use the Cherenkov light emitted in a conical shape based on the direction of a charged particle, the direction of the incident neutrinos can be reconstructed relatively well. However, in a detector filled with the liquid scintillator, the reactor neutrino signal can be reconstructed using information from the Inverse Beta Decay (IBD) process. In this case, it is not easy to determine the direction of an incident neutrino because light is emitted in all directions. In this study, we briefly investigated the possibility of the directional reconstruction of reactor neutrinos in the Linear Alkyl Benzene (LAB)-based Gadolinium (Gd)-loaded liquid scintillator using a Monte Carlo simulation technique. The LAB was used in place of existing, conventional liquid scintillation detector solution. It has a high flash point and is environmentally friendly. The purpose of this study was to show a practical and interesting application of neutrino direction reconstruction for reactor neutrinos that could be used in future large liquid scintillator neutrino detectors.

Status and the perspectives of the Jiangmen Underground Neutrino Observatory (JUNO)

One of the remaining undetermined fundamental aspects in neutrino physics is the determination of the neutrino mass hierarchy, i.e. discriminating between the two possible orderings of the mass eigenvalues, known as Normal and Inverted Hierarchies. The Jiangmen Underground Neutrino Observatory (JUNO), a 20 kt Liquid Scintillator Detector currently under construction in the South of China, can determine the neutrino mass hierarchy and improve the precision of three oscillation parameters by one order of magnitude. Moreover, thanks to its large liquid scintillator mass, JUNO will also contribute to study neutrinos from non-reactor sources such as solar neutrinos, atmospheric neutrinos, geoneutrinos, supernova burst and diffuse supernova neutrinos. Furthermore, JUNO will also contribute to nucleon decay studies. In this work, I will describe the status and the perspectives of the JUNO experiment.

Reduction of radioactive impurities from the liquid scintillator by using a Metal scavenger

The KamLAND2-Zen experiment is a future plan to perform the search for neutrinoless double-beta decay (0vββ) by using a large liquid-scintillator (LS) based detector, i.e., KamLAND2. The KamLAND detector will be upgraded by increasing the light yield of scintillation. One of the upgrade plans consists of using a new linear alkylbenzene (LAB) based LS. We have developed the LAB-LS radio-purification method by using a metal scavenger. The scavenging process resulted in a Pb-removal efficiency of 90.1±1.0% at most, and LS performance parametrics, such as light yield, light transparency, amount of radioactive impurities remaining, and the concentration of components were observed to be stable after scavenging. Therefore, it can be asserted that scavenging is a useful method for LS purification.

Background analysis for the SNO+ experiment

SNO+ is a large multi-purpose liquid scintillator based experiment, with the main physics goal of searching for the neutrinoless double-beta decay of 130Te. The first of the three SNO+ phases started in May 2017, with the detector filled with ultra-pure water. The deep underground location (6000 m.w.e.) and the low background levels allowed new physics searches, together with the measurement of the 8B solar neutrino flux in the 5-15 MeV energy range. Recently, SNO+ has began the transition to the scintillator phase. The double-beta decay phase is expected to start in early 2020, when the ultra-pure liquid scintillator will be loaded with 3.9 tonnes of natural tellurium, for a half-life sensitivity larger than 2×1026 years in 5 years of data taking. This article focuses on the low background measurement of the initial water phase.

Jinping Neutrino Experiment: a Status Report

A large liquid scintillator detector is being discussed and actively developed in China JinPing underground Laboratory. We envision a 5kt detector at 7000-water-meter-equivalent overburden to target terrestrial, solar and supernovae neutrinos. A 1t prototype detector has been installed on-site in 2017. Liquid scintillator performance, radioactive and cosmogenic backgrounds, simulation and analysis pipelines have been carefully investigated. We report the status of the Jinping Neutrino Experiment project and milestones of R&D studies.

The Status of JUNO

The Jiangmen Underground Neutrino Observatory (JUNO) is a multi-purpose underground experiment and the largest liquid scintillator (LS) detector going for neutrino mass hierarchy, precise measurement of neutrino oscillation parameters and other rare processes which include but not limited to solar neutrino, geo-neutrino, supernova neutrinos and the diffuse supernova neutrinos background. The 20” PMT system with ∼18000 high quantum efficiency phototubes, including Hamamatsu 20” and newly developed NNVT MCP 20” tubes, is the key component of JUNO experiment for better energy resolution, good detector response etc. This article reports the status of the JUNO experiment.

The Jiangmen Underground Neutrino Observatory

The Jiangmen Underground Neutrino Observatory (Juno) is a large and high precision liquid scintillator detector under construction in the south of China. With its 20 kt target mass, it aims to achieve an unprecedented 3% energy resolution at 1 MeV. Its main goal is to study the disappearance of reactor antineutrino to determine the neutrino mass ordering, and to precisely measure the mixing parameters θ12, Δm122, and Δmee2. It also aims to detect neutrinos emitted from radioactive processes taking place within the inner layers of the Earth (geoneutrinos), as well as neutrinos produced during rare supernova bursts. Neutrinos emitted in solar nuclear reactions could also be observed, if stringent radiopurity requirements on the scintillator are met. This manuscript provides some highlights of Juno's Physics Programme, and describes the detector design, as well as the ongoing detector R&D.

Luminous signals of inelastic dark matter in large detectors

We study luminous dark matter signals in models with inelastic scattering. Dark matter χ1 that scatters inelastically off elements in the Earth is kicked into an excited state χ2 that can subsequently decay into a monoenergetic photon inside a detector. The photon signal exhibits large sidereal-daily modulation due to the daily rotation of the Earth and anisotropies in the problem: the dark matter wind comes from the direction of Cygnus due to the Sun's motion relative to the galaxy, and the rock overburden is anisotropic, as is the dark matter scattering angle. This allows outstanding separation of signal from backgrounds. We investigate the sensitivity of two classes of large underground detectors to this modulating photon line signal: large liquid scintillator neutrino experiments, including Borexino and JUNO, and the proposed large gaseous scintillator directional detection experiment CYGNUS. Borexino's (JUNO's) sensitivity exceeds the bounds from xenon experiments on inelastic nuclear recoil for mass splittings delta underset{sim }{>} 240 (180) keV, and is the only probe of inelastic dark matter for 350 keV underset{sim }{<}delta underset{sim }{<} 600 keV. CYGNUS's sensitivity is at least comparable to xenon experiments with ~ 10m3 volume detector for delta underset{sim }{<} 150 keV, and could be substantially better with larger volumes and improved background rejection. Such improvements lead to the unusual situation that the inelastic signal becomes the superior way to search for dark matter even if the elastic and inelastic scattering cross sections are comparable.

Multi-site event discrimination in large liquid scintillation detectors

Simulation studies have been carried out to explore the ability to discriminate between single-site and multi-site energy depositions in large scale liquid scintillation detectors. A robust approach has been found that is predicted to lead to a significant statistical separation for a large variety of event classes, providing a powerful tool to discriminate against backgrounds and break important degeneracies in signal extraction. This has particularly relevant implications for liquid scintillator searches for neutrinoless double beta decay ($0\nu\beta\beta$) from $^{130}$Te and $^{136}$Xe, where it is possible for a true $0\nu\beta\beta$ signal to be distinguished from most radioactive backgrounds (including those from cosmogenic production) as well as unknown gamma lines from the target isotope.

Optical properties and pulse shape discrimination in siloxane-based scintillation detectors

The possibility to detect fast neutrons as a distinct signal from that one of γ-rays background is surely of great importance for several topics, spanning from homeland security to radiation monitoring in nuclear physics research plants. Nowadays, Helium-3 based detectors are extremely expensive, while the use of large volume liquid scintillators presents serious concerns related to spillage risks and waste disposal. A very attractive alternative is the use of commercially available solid scintillators, which exploits an aromatic polymer matrix entrapping very high loadings of primary dye, thereby enabling the use of pulse shape analysis (PSA) to discriminate between fast neutrons and γ-rays. In this work, we analyse in detail the optical features of a solid scintillator composed by polymethylphenylsiloxane (PMPS) as base polymer loaded with moderate amounts of 2,5-diphenyloxazole (PPO). Furthermore, fluorescence decay kinetics have been correlated to the observed pulse shape discrimination capabilities of this radiation and thermally resistant scintillator, whose performances have been discussed in terms of conformational features and excimers formation revealed by the optical analyses.

Sensitivity to neutrino-antineutrino transitions for boron neutrinos

Neutrino-antineutrino conversion is an important new physics process. The observation of this phenomenon could indicate total lepton number violation and potential CPT-violation. Searching for the appearance of electron antineutrinos from solar neutrinos from B8 decay allows us to hunt for this rare process, although it can also be explained by other mechanisms or hypotheses. This analysis examines the capabilities of observing neutrino-antineutrino transition from B8 unoscillated solar neutrinos using different liquid scintillator detector configurations. High energy reactor neutrinos and atmospheric neutrinos are the two dominant background sources. Large volume liquid scintillator detectors with deep underground shielding, placed far away from reactors and with capabilities of pulse shaped discrimination will significantly increase the search sensitivity. It is demonstrated that for the next generation of large liquid scintillator detectors being planned or under construction, the sensitivity to the average probability of neutrino-antineutrino transitions can reach 10−6, which is an order of magnitude better than the current best experimental limits.

Foraging for dark matter in large volume liquid scintillator neutrino detectors with multiscatter events

We show that dark matter with a per-nucleon scattering cross section $\gtrsim 10^{-28}~{\rm cm^2}$ could be discovered by liquid scintillator neutrino detectors like BOREXINO, SNO+, and JUNO. Due to the large dark matter fluxes admitted, these detectors could find dark matter with masses up to $10^{21}$ GeV, surpassing the mass sensitivity of current direct detection experiments (such as XENON1T and PICO) by over two orders of magnitude. We derive the spin-independent and spin-dependent cross section sensitivity of these detectors using existing selection triggers, and propose an improved trigger program that enhances this sensitivity by two orders of magnitude. We interpret these sensitivities in terms of three dark matter scenarios: (1) effective contact operators for scattering, (2) QCD-charged dark matter, and (3) a recently proposed model of Planck-mass baryon-charged dark matter. We calculate the flux attenuation of dark matter at these detectors due to the earth overburden, taking into account the earth's density profile and elemental composition, and nuclear spins.

A new method of energy reconstruction for large spherical liquid scintillator detectors

Liquid scintillators are commonly used to detect low energy neutrinos from the reactors, sun, and earth. It is a challenge to reconstruct deposited energies for a large liquid scintillator detector. For detectors with multiple optical mediums such as JUNO and SNO+, the prediction of the propagation of detected photons is extremely difficult due to mixed optical processes such as Rayleigh scattering, refraction and total reflection at their boundaries. Calibration based reconstruction methods consume impractical time since a large number of calibration points are required in a giant detector. In this paper, we propose a new model-independent method to reconstruct deposited energies with minimum requirements on the calibration system. This method is validated with JUNO's offline software. Monte Carlo studies show that the energy non-uniformity can be controlled below 1%, which is crucial for JUNO to achieve 3% energy resolution.