Water Cherenkov Detector Research Articles

The energy spectrum of cosmic rays above 6 PeV as measured at the Pierre Auger Observatory

Since 2004, the Pierre Auger Collaboration has measured, with unprecedented precision, one of the most important features of ultra-high-energy cosmic rays, the energy spectrum. Located in the Southern hemisphere, the Observatory comprises an array of 1660 water-Cherenkov detectors covering 3000 km² overlooked by 27 fluorescence telescopes. Five sets of measurements have been used to reconstruct the spectrum from 6 PeV to beyond 100 EeV. The highest-energy events, recorded by surface detectors on a 1500 m triangular grid, are reconstructed differently depending on their inclination (vertical events below 60° and inclined ones above 60°). A cross-check of these measurements is made using ‘hybrid events’, in which there are simultaneous detections at both the fluorescence detectors and at least one surface detector. Events with energies below 3 EeV have been studied using a nested array with a spacing of 750 m and using the Cherenkov light recorded with three high-elevation telescopes. In this contribution, updated methods for the reconstruction of all events are discussed, together with their associated uncertainties. A combination of all data sets is reported to construct a spectrum above 6 PeV to the highest energies. A detailed discussion of the spectral features will be presented.

ESTUDIO DE LA ESTABILIDAD DE UN DETECTOR CHERENKOV DE AGUA EN EL MARCO DE LA COLABORACIÓN LAGO

The LAGO project (Latin American Giant Observatory) is primarily focused on studying Gamma Ray Bursts (GRBs) as well as studying high energy astroparticles, space weather and atmospheric radiation at ground level. The technique used in LAGO is ground-based Water Cherenkov Detectors (WCD), by using “individual particle” detection. The present work shows the performance and stability of the WCD prototype at the Cosmic Ray Laboratory of the Universidad Mayor de San Andrés (UMSA) based in Cota Cota La Paz, Bolivia (∼ 3400 m a.s.l.). The detector was studied taking into account four parameters: the independence of the data by testing goodness-of-fit to Poisson distribution, the behavior of the signals area-peak ratio and finally the behavior of the charge histogram and the muon half-life time was experimentally measured obtaining a value of about 2.2 ± 0.2 µs. Data collected of the muon half-life study was carried out over four month period.

Detection of the 4.4-MeV gamma rays from 16O(ν, ν′)16O(12.97 MeV, 2−) with a water Cherenkov detector in supernova neutrino bursts

Abstract We first discuss and determine the isospin mixing of the two 2− states (12.53 MeV and 12.97 MeV) of the16O nucleus using inelastic electron scattering data. We then evaluate the cross section of 4.4-MeV γ rays produced in the neutrino neutral-current (NC) reaction 16O(ν, ν′)16O(12.97 MeV, 2−) in a water Cherenkov detector at a low energy, below 100 MeV. The detection of γ rays for Eγ > 5 MeV from the NC reaction 16O(ν, ν′)16O(Ex > 16 MeV, T = 1) with a water Cherenkov detector in supernova neutrino bursts has been proposed and discussed by several authors previously. In this article, we discuss a new NC reaction channel from 16O(12.97 MeV, 2−) producing a 4.4-MeV γ ray, the cross section of which is more robust and even larger at low energy (Eν < 25 MeV) than the NC cross section from 16O(Ex > 16 MeV, T = 1). We also evaluate the number of such events induced by neutrinos from supernova explosion which can be observed by the Super-Kamiokande, an Earth-based 32-kton water Cherenkov detector.

Rocks, water, and noble liquids: Unfolding the flavor contents of supernova neutrinos

Measuring core-collapse supernova neutrinos, both from individual supernovae within the Milky Way and from past core collapses throughout the Universe (the diffuse supernova neutrino background, or DSNB), is one of the main goals of current and next generation neutrino experiments. Detecting the heavy-lepton flavor (muon and tau types, collectively ${\ensuremath{\nu}}_{x}$) component of the flux is particularly challenging due to small statistics and large backgrounds. While the next galactic neutrino burst will be observed in a plethora of neutrino channels, allowing us to measure a small number of ${\ensuremath{\nu}}_{x}$ events, only upper limits are anticipated for the diffuse ${\ensuremath{\nu}}_{x}$ flux even after decades of data taking with conventional detectors. However, paleo detectors could measure the time-integrated flux of neutrinos from galactic core-collapse supernovae via flavor-blind neutral current interactions. In this work, we show how combining a measurement of the average galactic core-collapse supernova flux with paleo detectors and measurements of the DSNB electron-type neutrino fluxes with the next-generation water Cherenkov detector Hyper-Kamiokande and the liquid noble gas detector DUNE will allow to determine the mean supernova ${\ensuremath{\nu}}_{x}$ flux parameters with precision of order ten percent. Realizing this potential requires both the cosmic supernova rate out to $z\ensuremath{\sim}1$ and the integrated Galactic supernova rate over the last $\ensuremath{\sim}1\text{ }\text{ }\mathrm{Gyr}$ to be established at the $\ensuremath{\sim}10%$ level.

First measurements of periodicities and anisotropies of cosmic ray flux observed with a water-Cherenkov detector at the Marambio Antarctic base

A new water-Cherenkov radiation detector, located at the Argentine Marambio Antarctic Base (64.24S-56.62 W), has been monitoring the variability of galactic cosmic ray (GCR) flux since 2019. One of the main aims is to provide experimental data necessary to study interplanetary transport of GCRs during transient events at different space/time scales. In this paper we present the detector and analyze observations made during one full year. After the analysis and correction of the GCR flux variability due to the atmospheric conditions (pressure and temperature), a study of the periodicities is performed in order to analyze modulations due to heliospheric phenomena. We can observe two periods: (a) 1 day, associated with the Earth’s rotation combined with the spatial anisotropy of the GCR flux; and (b) ∼ 30 days due to solar impact of stable solar structures combined with the rotation of the Sun. From a superposed epoch analysis, and considering the geomagnetic effects, the mean diurnal amplitude is ∼ 0.08% and the maximum flux is observed in ∼ 15 h local time (LT) direction in the interplanetary space. In such a way, we determine the capability of Neurus to observe anisotropies and other interplanetary modulations on the GCR flux arriving at the Earth.

Emission characteristics of gadolinium ions in a water Cherenkov detector

Abstract To observe supernova relic neutrino events, 13 tons of gadolinium sulfate octahydrate (Gd2(SO4)3·8H2O), corresponding to 0.01% Gd solution, was dissolved in the Super-Kamiokande water Cherenkov detector in 2020. The aim is to improve the detection efficiency of neutrons from inverse β decay involving electron antineutrinos. However, Gd3+ ions can be excited by the Cherenkov light from cosmic muons, and the subsequent emission at 312 nm is a possible background (BG) source for Cherenkov signal detection. In this work, we constructed an experimental setup based on time-resolved laser-induced luminescence spectroscopy to investigate the emission characteristics of Gd3+ ions in water. The excitation laser wavelength was tuned in the range of 245–255 nm, and large resonant peaks were observed at 246.2 nm and 252.3 nm with measured emission lifetimes of around 3 ms. Good linearity was observed between Gd concentration and emission intensity for these two wavelengths, indicating that our setup is useful for remote monitoring of Gd concentration. According to the simulation results using our spectroscopic data and reference values, the Gd3+ emission BG rate from cosmic muons is expected to be 10−1 counts/μs or less, which seems small but not negligible.

The ARTI framework: cosmic rays atmospheric background simulations

ARTI is a complete framework designed to simulate the signals produced by the secondary particles emerging from the interaction of single, multiple, and even from the complete flux of primary cosmic rays with the atmosphere. These signals are simulated for any particle detector located at any place (latitude, longitude and altitude), including the real-time atmospheric, geomagnetic and detector conditions. Formulated through a sequence of codes written in C++, Fortran, Bash and Perl, it provides an easy-to-use integration of three different simulation environments: MagnetoCosmics, CORSIKA and Geant4. These tools evaluate the geomagnetic field effects on the primary flux and simulate atmospheric showers of cosmic rays and the detectors’ response to the secondary flux of particles. In this work, we exhibit the usage of the ARTI framework by calculating the total expected signal flux at eight selected sites of the Latin American Giant Observatory: a cosmic ray Observatory all over Latin America covering a wide range of altitudes, latitudes and geomagnetic rigidities. ARTI will also calculate the signal flux expected during the sudden occurrence of a gamma-ray burst or the flux of energetic photons originating from steady gamma sources. It also compares these fluxes with the expected background when they are detected in a single water Cherenkov detector deployed in a high-altitude site. Furthermore, by using ARTI, it is possible to calculate in a very precise way the expected flux of high-energetic muons and other secondaries at the ground level and to inject them through geological structures for muography applications.

Mechanical design of water cherenkov test experiment (WCTE) at CERN

The Water Cherenkov Test Experiment (WCTE) is an experiment proposed at CERN to measure the response of a Water Cherenkov Detector for charged particles such as π±, p+, e±, etc. The data obtained from WCTE will be used in future neutrino experiments. WCTE consists of a sealed cylindrical tank filled with ultrapure water. 128 multi-PhotoMultiplier Tubes (mPMTs) are mounted on a cylindrical support structure facing inwards to map out the Cherenkov radiation with high granularity. This work presents the mechanical design and analysis of the support structure for WCTE. It is designed to sustain the load of 128 mPMTs, arrangement of Photogrammetry system Cameras & lights and Calibration arm without significant change in the position / geometry of the structure. SS304 is identified as a suitable material to ensure the compatibility with the ultrapure water and Gd-loaded water. The structure is robust against stresses during handling and subsequent transport with and without water.

Mechanical design of multi-PMTs for IWCD

Approximately 500 multi-PMTs (mPMTs) will be used as the photosensors for the Intermediate Water Cherenkov Detector (IWCD), a new near detector for the approved Hyper-Kamiokande experiment that will be built by 2025. The IWCD mPMT design has nineteen 3” PMTs enclosed in a water-tight pressure vessel, along with the associated electronics. The 3” PMTs provide excellent spatial imaging of the neutrino-induced Cherenkov light ring. This work will focus on the mechanical design of the mPMT vessel. In particular, design of the acrylic dome, use of optical gel to couple the dome to the PMTs, assembly procedures of dome and PMT sub-assembly (including the necessary jigs / fixtures), design of water-tight feed-through & plans for testing and results from several mPMT prototypes.

Status of the AugerPrime upgrade of the Pierre Auger Observatory

The Pierre Auger Observatory consists of a detector system to study ultrahigh-energy cosmic rays. These cosmic rays can be detected only through the observation of extensive air showers. The hybrid detection of air showers at the Observatory is based on the Surface Detector (SD) – an array of about 1660 water-Cherenkov detectors, and the Fluorescence Detector, with 27 telescopes overlooking the SD. Recently, an upgrade of the Observatory was initiated, called AugerPrime. The main purpose of the upgrade is to improve the mass composition sensitivity of the SD through precise measurements of the muonic and electromagnetic components of extensive air showers. For this purpose, additional scintillator and radio detectors are being installed on top of SD stations. The upgrade also includes updated SD electronics, and underground muon detectors. In this talk, the motivation and the current status of the upgrade will be reviewed.

Extracting the best physics sensitivity from T2HKK: A study on optimal detector volume

T2HK is an upcoming long-baseline experiment in Japan which will have two water Cherenkov detector tanks of 187 kt volume each at a distance of 295 km from the source. An alternative project, T2HKK is also under consideration where one of the water tanks will be moved to Korea at a distance of 1100 km. The flux at 295 km will cover the first oscillation maximum and the flux at 1100 km will mainly cover the second oscillation maximum. As physics sensitivity at the dual baseline relies on variation in statistics, the dependence of systematic uncertainty, the effect of the second oscillation maximum and matter density, 187 kt detector volume at 295 km and 187 kt detector volume at 1100 km may not be the optimal configuration of T2HKK. Therefore, in this work we have tried to optimize the ratio of the detector volume at both the locations by studying the interplay between the above mentioned parameters. For the analysis of neutrino mass hierarchy, octant of ${\ensuremath{\theta}}_{23}$ and $CP$ precision, we have considered two values of ${\ensuremath{\delta}}_{CP}$ as 270\ifmmode^\circ\else\textdegree\fi{} and 0\ifmmode^\circ\else\textdegree\fi{} and for $CP$ violation we have considered the value of ${\ensuremath{\delta}}_{CP}=270\ifmmode^\circ\else\textdegree\fi{}$. These values are motivated by the current best-fit values of this parameter as obtained from the experiments T2K and $\mathrm{NO}\ensuremath{\nu}\mathrm{A}$. Interestingly we find that if the systematic uncertainty is negligible then the T2HK setup i.e., when both the detector tanks are placed at 295 km gives the best results in terms of hierarchy sensitivity at ${\ensuremath{\delta}}_{CP}=270\ifmmode^\circ\else\textdegree\fi{}$, octant sensitivity, $CP$ violation sensitivity, and $CP$ precision sensitivity at ${\ensuremath{\delta}}_{CP}=0\ifmmode^\circ\else\textdegree\fi{}$. For current values of systematic errors, we find that neither T2HK, nor T2HKK setup is giving better results for hierarchy, $CP$ violation and $CP$ precision sensitivity. The optimal detector volume which is in the range between 255 kt to 345 kt at 1100 km gives better results in the above mentioned parameters.

The Mercedes water Cherenkov detector

The concept of a small, single-layer water Cherenkov detector, with three photomultiplier tubes (PMTs), placed at its bottom in a 120^{circ } star configuration (Mercedes Water Cherenkov Detector) is presented. The PMTs are placed near the lateral walls of the stations with an adjustable inclination and may be installed inside or outside the water volume. To illustrate the technical viability of this concept and obtain a first-order estimation of its cost, an engineering design was elaborated. The sensitivity of these stations to low energy Extensive Air Shower (EAS) electrons, photons and muons is discussed, both in compact and sparse array configurations. It is shown that the analysis of the intensity and time patterns of the PMT signals, using machine learning techniques, enables the tagging of muons, achieving an excellent gamma/hadron discrimination for TeV showers. This concept minimises the station production and maintenance costs, allowing for a highly flexible and fast installation. Mercedes Water Cherenkov Detectors (WCDs) are thus well-suited for use in high-altitude large gamma-ray observatories covering an extended energy range from the low energies, closing the gap between satellite and ground-based measurements, to very high energy regions, beyond the PeV scale.

Neutron tagging following atmospheric neutrino events in a water Cherenkov detector

We present the development of neutron-tagging techniques in Super-Kamiokande IV using a neural network analysis. The detection efficiency of neutron capture on hydrogen is estimated to be 26%, with a mis-tag rate of 0.016 per neutrino event. The uncertainty of the tagging efficiency is estimated to be 9.0%. Measurement of the tagging efficiency with data from an Americium-Beryllium calibration agrees with this value within 10%. The tagging procedure was performed on 3,244.4 days of SK-IV atmospheric neutrino data, identifying 18,091 neutrons in 26,473 neutrino events. The fitted neutron capture lifetime was measured as 218±9 μs.

Using machine learning to improve neutron identification in water Cherenkov detectors.

Water Cherenkov detectors like Super-Kamiokande, and the next generation Hyper-Kamiokande are adding gadolinium to their water to improve the detection of neutrons. By detecting neutrons in addition to the leptons in neutrino interactions, an improved separation between neutrino and anti-neutrinos, and reduced backgrounds for proton decay searches can be expected. The neutron signal itself is still small and can be confused with muon spallation and other background sources. In this paper, machine learning techniques are employed to optimize the neutron capture detection capability in the new intermediate water Cherenkov detector (IWCD) for Hyper-K. In particular, boosted decision tree (XGBoost), graph convolutional network (GCN), and dynamic graph convolutional neural network (DGCNN) models are developed and benchmarked against a statistical likelihood-based approach, achieving up to a 10% increase in classification accuracy. Characteristic features are also engineered from the datasets and analyzed using SHAP (SHapley Additive exPlanations) to provide insight into the pivotal factors influencing event type outcomes. The dataset used in this research consisted of roughly 1.6 million simulated particle gun events, divided nearly evenly between neutron capture and a background electron source. The current samples used for training are representative only, and more realistic samples will need to be made for the analyses of real data. The current class split is 50/50, but there is expected to be a difference between the classes in the real experiment, and one might consider using resampling techniques to address the issue of serious imbalances in the class distribution in real data if necessary.

AugerPrime: The Pierre Auger Observatory upgrade

After more than 15 years of successful data taking, the Pierre Auger Observatory started a major upgrade, called AugerPrime, whose main aim is the collection of new information about the primary mass of ultrahigh-energy cosmic rays, besides adding new indications on hadronic interactions at energies much larger than those reached in human-made accelerators. The upgrade program includes: the installation of a plastic scintillator detector (SSD) and of a radio antenna on top of each water-Cherenkov detector (WCD) of the surface array, an extension of the dynamic range of measurement through an additional small photomultiplier tube inside the WCD, new electronics to process signals from the WCD and the SSD with higher sampling frequency and enhanced resolution in amplitude. An array of underground scintillator detectors will measure the muonic component of lower energy extensive air showers. After presenting the motivations for upgrading the Observatory, an overview of the detectors is provided, together with the deployment and commissioning status.

A Novel RID Algorithm of Muon Trajectory Reconstruction in Water Cherenkov Detectors

Cosmic rays that strike the top of the Earth’s atmosphere generate a shower of secondary particles that move toward the surface with relativistic speeds. Water Cherenkov detectors (WCDs) on the ground can detect charged muons, which are one of the many particles generated in the shower, with the Cherenkov imaging technique. A large number of these muons travel in WCD tanks near the speed of light in a vacuum, faster than the speed of light in water, and so trigger isotropic Cherenkov radiation, which is detected by the photomultiplier tubes (PMTs) placed inside the tanks. When the radial component of the speed of the muon toward a PMT drops from superluminal to subluminal, the PMT records Cherenkov light from an optical phenomenon known as relativistic image doubling (RID), which causes two Cherenkov images of the same muon to appear suddenly, with both images moving in geometrically opposite directions on the original muon track. The quantities associated with the RID effect can be measured experimentally with a variety of detector types and can be used to find various points on the original trajectory of the muon. In this paper, a detailed study of reconstructing the trajectory of a muon entering a WCD using the RID technique has been presented. It is found that the measurements of standard RID observables enables a complete reconstruction of the trajectory of the muon to a high degree of accuracy with less than 1% error.

Improvement in light collection of a photomultiplier tube using a wavelength-shifting plate

Large-volume water-Cherenkov neutrino detectors are a light-starved environment, as each interaction produces only ∼50−100 photons per MeV. As such, maximizing the light collection efficiency of the detector is vital to performance. Since Cherenkov emission is heavily weighted towards the near UV, one method to maximize overall detector light collection without increasing the number of photomultiplier tubes is to couple each tube to a wavelength-shifting plastic plate, thus shifting photon wavelengths to a regime better suited to maximize photomultiplier efficiency and potentially detecting photons that miss the photocathode. To better understand the behavior of such plates, a scan of a rectangular wavelength-shifting plate was performed, and the results were used to calculate the overall percentage improvement in light collection that could be expected for individual PMTs in a large water-Cherenkov detector. Measurements of a 15.1 in. by 11.5 in. wavelength-shifting plate using a 365 nm LED were found to increase overall light collection at the photomultiplier tube by 7.4±0.7%. A simulation tuned to reproduce these results was used to predict the behavior of a wavelength shifting plate exposed to Cherenkov spectrum light and found increases in light collection that were linear with edge length, assuming square geometries. These results demonstrate the potential of wavelength-shifting plates to increase the overall light collection efficiency in a large detector.

Maximum Likelihood Reconstruction of Water Cherenkov Events With Deep Generative Neural Networks.

Large water Cherenkov detectors have shaped our current knowledge of neutrino physics and nucleon decay, and will continue to do so in the foreseeable future. These highly capable detectors allow for directional and topological, as well as calorimetric information to be extracted from signals on their photosensors. The current state-of-the-art approach to water Cherenkov reconstruction relies on maximum-likelihood estimation, with several simplifying assumptions employed to make the problem tractable. In this paper, we describe neural networks that produce probability density functions for the signals at each photosensor, given a set of inputs that characterizes a particle in the detector. The neural networks we propose allow for likelihood-based approaches to event reconstruction with significantly fewer assumptions compared to traditional methods, and are thus expected to improve on the current performance of water Cherenkov detectors.

New model of intranuclear neutron-antineutron transformations in O816

There has been much work in recent years pertaining to viability studies for the intranuclear observation of neutron-antineutron transformations. These studies begin firstly with the design and implementation of an event generator for the simulation of this rare process, where one hopes to retain as much of the underlying nuclear physics as possible in the initial state, and then studying how these effects may perturb the final state observable particles for detector efficiency studies following simulated reconstruction. There have been several searches for intranuclear neutron-antineutron transformations, primarily utilizing the ${}^{16}_{8}$O nucleus, and completed within large underground water Cherenkov detectors such as Super-Kamiokande. The latest iteration of a generator is presented here for use in such an experiment. This generator includes several new features, including a new radial (position) annihilation probability distribution and related intranuclear suppression factor for ${}^{16}_{8}$O, as well as a highly general, modern nuclear multifragmentation model with photonic de-excitations. The latter of these may allow for improved identification of the signal using large underground detectors such as Super-Kamiokande and the future Hyper-Kamiokande, potentially increasing the overall signal efficiencies of these rare searches. However, it should be noted that certain fast photonic de-excitations may be washed out by $\pi^0$ decays to photons. These new features implemented in these $\bar{n}$\isotope[15][8]{O} simulations increase the overall physical realism of the model, and are easily portable to other future searches such as to extranuclear $\bar{n}{}^{12}_{6}$C for the ESS NNBAR experiment, as well as intranuclear $\bar{n}{}^{39}_{18}$Ar used in DUNE.

Neutrino tagging: a new tool for accelerator based neutrino experiments

This article describes a new experimental method for accelerator based neutrino experiments called neutrino tagging. The method consists in exploiting the neutrino production mechanism, the decay, to kinematically reconstruct the neutrino properties from the decay incoming and outgoing charged particles. The reconstruction of these particles relies on the recent progress and on-going developments in silicon particle detector technology. A detailed description of the method and achievable key performances is presented, together with its potential benefits for short and long baseline experiments. Then, a novel configuration for long baseline experiments is discussed in which a tagged beam would be employed together with mega-ton scale natural deep water Cherenkov detectors. The coarseness of this type of detectors is overcome by the precision of the tagging and, conversely, the rate limitation imposed by the tagging is outweighed by the size of the detector. These mutual benefits result in an affordable design for next generations of long baseline experiments. The physics potential of such experiments is quantified using the Protvino to KM3NeT/ORCA setup as a case study for which an unprecedented sensitivity to the leptonic CP violation could be achieved.