Jiangmen Underground Neutrino Observatory Central Detector Research Articles

Research on the optical properties and effect of the transparent epoxy for the JUNO central detector

The Central Detector (CD) of the Jiangmen Underground Neutrino Observatory (JUNO) has a large acrylic sphere containing 20,000 tons of liquid scintillator (LS) and is immersed in high purity water. The CD consists of 263 acrylic spherical panels and two chimneys, which are bonded together by mass polymerization. Transmittance and stress are critical factors for the CD, with transmittance affecting JUNO's energy resolution and stress affecting the structural integrity of the detector. The bonding areas of the CD are of particular concern, leading to the development of specialized measurement systems for acrylic stress and transmittance. Stress measurements revealed the presence of higher than expected levels of stress in the bonding areas, requiring assessments and protective measures. The application of a transparent epoxy on the bonding areas was proposed as an effective structural protection measure, although it may reduce the transmittance of these areas. Consequently, the optical properties of the transparent epoxy, such as refractive index and absorbance at 420 nm, were studied using acrylic samples to analyze the effect of the epoxy application on the transmittance performance of the CD. These results are detailed and discussed in this article.

Developing a [formula omitted]Bq/m3 level 226Ra concentration in water measurement system for the Jiangmen Underground Neutrino Observatory

The Jiangmen Underground Neutrino Observatory (JUNO), a 20 kton multi-purpose low background Liquid Scintillator (LS) detector, was proposed primarily to determine the neutrino mass ordering. To suppress the radioactivity from the surrounding rocks and tag cosmic muons, the JUNO central detector is submerged in a Water Cherenkov Detector (WCD). In addition to being used in the WCD, ultrapure water is used in LS filling, for which the 226Ra concentration in water needs to be less than 50 μBq/m3. To precisely measure the 226Ra concentration in water, a 6.0 μBq/m3226Ra concentration in water measurement system has been developed. In this paper, the detail of the measurement system as well as the 226Ra concentration measurement result in regular EWII ultrapure water will be presented.

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%.

The study of acrylic bonding gap stress for the JUNO Central Detector

The Central Detector (CD) of Jiangmen Underground Neutrino Observatory (JUNO) has a huge acrylic spherical vessel, which will contain 20000 tons of liquid scintillator, being immersed in pure water, endure about 3 × 107 N of buoyancy. It is composed of 263 pieces of acrylic spherical panels bonded by the mass polymerization. Acrylic stress is critical to the structural safety of the JUNO CD, and it is required not to be greater than 3.5 MPa. The specialized measurement system for acrylic stress of the JUNO CD has been designed and realized, which supports non-destructive and quantitative measurement. The measurement system was applied to the main acrylic spherical panels without bonding gap first to verify stress level, which met JUNO's requirement. The bonding quality of acrylic spherical panels is critical, and the stress of bonding gap is concerned. The stresses of acrylic bonding gaps were measured using acrylic samples to help study the bonding techniques. The stress optical coefficient of acrylic bonding gap was calibrated and different bonding methods were studied and compared. We found that the objective function of numerical fitting for acrylic bonding gap needs modification when the measured optical path difference is large, which was analyzed and discussed in this article.

Reconstruction of a muon bundle in the JUNO central detector

The Jiangmen Underground Neutrino Observatory (JUNO) is a multi-purpose neutrino experiment. One of the main goals is to determine the neutrino mass ordering by precisely measuring the energy spectrum of reactor antineutrinos. For the detection of reactor antineutrinos, cosmogenic backgrounds, such as 9Li/8He and fast neutrons induced by cosmic muons, should be rejected carefully by applying muon veto cuts, which require good muon track reconstruction. With a 20-kton liquid scintillator detector, the simulation shows the proportion of muon bundles (muon multiplicity ≥ 2) to be approximately 8% in JUNO, whereas its reconstruction has been rarely discussed in previous experiments. This study proposes an efficient algorithm for muon track reconstruction based on the charge response of a photomultiplier tube array. This is the first reconstruction of muon bundles in a large-volume liquid scintillator detector. In addition, the algorithm shows good performance and potential for reconstruction for both a single muon and double muons (muon multiplicity = 2). The spatial resolution of a single-muon reconstruction was 20 cm, and the angular resolution was 0.5°. For double-muon reconstruction, the spatial and angular resolutions could be 30 cm and $$1.0^\circ$$ , respectively. Moreover, this paper also discusses muon classification and the veto strategy.

JUNO Detector Design & Status

The Jiangmen Underground Neutrino Observatory (JUNO) is a next-generation liquid-scintillator reactor neutrino experiment under construction in Southern China. It is a multi-purpose experiment with a wide range of applications in neutrino physics, ranging from a neutrino mass ordering (NMO) determination, the detection of solar, geo-, atmospheric neutrinos, and Supernova neutrinos, to the measurement on particular oscillation parameters with sub-percentage precision. The JUNO central detector (CD) is designed to reach an energy resolution of 3% at 1MeV. The detector is being installed in a recently excavated experimental hall, 50 m wide and located under 700 m of granite overburden (1800 m.w.e.). The CD contains a 35.4-meter diameter acrylic vessel filled with 20-kt of LAB-based liquid scintillator, making it the largest liquid-scintillator detector of its kind in the world. The scintillation light will be read-out by 17612 20” photomultiplier tubes (PMTs) and 25600 3” PMTs, reaching a photocathode coverage higher than 75%. A water pool filled with ultrapure water equipped with 2,400 PMTs surrounds the CD; on top of the water pool, a 3-layer plastic scintillator tracker completes the JUNO veto system for cosmic muons detection. JUNO construction will be completed in 2022. This talk presents the detector design and status of the experiment.

The stress measurement system for the JUNO Central Detector acrylic panels

The Jiangmen Underground Neutrino Observatory (JUNO) Central Detector (CD) is a huge acrylic spherical vessel containing 20,000 tons of liquid scintillator; the sphere is composed of 263 pieces of acrylic spherical panels bonded by the mass polymerization. The operation life time of the JUNO CD is 20 years. To ensure the structural safety during the JUNO CD life time, the acrylic stress of CD is required not to be greater than 3.5 MPa. The stresses of acrylic spherical panels are required to be measured during the installation on-site; unfortunately there is no suitable commercial measurement equipment that can meet JUNO's requirements. Therefore, a measurement setup based on photo-elastic principle and spectrometric methods was designed, developed and tested for on-site measurements. The measurement system performs accurate calibration of stress-optical coefficient of acrylic in JUNO, and gives reliable results of acrylic stresses. The measurement system has been tested in the Taixing Donchamp Acrylic Ltd mechanical workshop, and the achieved results meet the JUNO's requirements. The measurement principle, the system components, and the tooling design are introduced in the paper. Moreover, the calibration of stress-optical coefficient of the acrylic and measurements results on JUNO acrylic spherical panels are discussed in the following.

Optimization of the rod forces in the reticular support structure of JUNO central detector

The structure of Jiangmen Underground Neutrino Observatory (JUNO) central detector is a complex structure subjected to a complex environment. The inner continuous acrylic spherical shell is connected to the outer reticulated stainless-steel (SS) shell through SS rods, and all these structural components are supported by an SS supporting frame. The inner shell is subjected to considerable buoyancy since the detection liquid inside the inner shell has less density than the outside water. Optimization of load path of such an underground structure subjected to buoyancy and pressure loads is difficult due to these complex situations, and in this paper, we tackle the problem of finding the optimal load path to minimize the rod forces. We adopt three types of design variables relevant to the load path, i.e., rod locations, chimney liquid level, and disc spring stiffness. Specifically, B-spline function is adopted to map the rod locations so as to significantly reduce the number of variables, and the chimney liquid level that influences the pressure distribution is taken as a new type of design variable. Subsequently, a numerical method combining the finite element analysis and a derivative-free optimization algorithm is proposed. Based on the optimization results, influences of the design variables, including their types, number, and initial values, on the optimal solutions are discussed in depth. The results suggest that the chimney liquid level and disc spring stiffness have a global effect of shifting rod force distribution, while distribution of rod forces strongly depends on the rod locations. The proposed method is shown to be effective to find an optimal load path of a complex spherical shell structure under various nonlinear constraints.

Calibration strategy of the JUNO experiment

We present the calibration strategy for the 20 kton liquid scintillator central detector of the Jiangmen Underground Neutrino Observatory (JUNO). By utilizing a comprehensive multiple-source and multiple-positional calibration program, in combination with a novel dual calorimetry technique exploiting two independent photosensors and readout systems, we demonstrate that the JUNO central detector can achieve a better than 1% energy linearity and a 3% effective energy resolution, required by the neutrino mass ordering determination.

Research on the measurement of connecting bars’ axial force of JUNO central detector

The Jiangmen Underground Neutrino Observatory (JUNO) is a multipurpose neutrino experiment designed to determine neutrino mass hierarchy and precisely measure oscillation parameters and study the solar neutrino, supernova neutrino, geo-neutrino, etc. JUNO’s central detector (CD) has 20 kilo-ton liquid scintillator as target mass, which is contained by a huge acrylic sphere with the inner diameter of 35.4 m, and the acrylic sphere is supported by a stainless steel structure through 590 connecting bars. Part of the connecting bars bear pull force and the other bars bear push force. There is a direct relationship between the stress of connecting bars and that of acrylic sphere. For the installation process of the CD, the pretightening force and axial force of the connecting bars should be monitored with accuracy, and for the filling process and running condition, the precise measurement of axial force can indicate the safety of structure of the CD. Statistical method was used to evaluate the performance of measurement schemes, and 4-fiber Bragg grating measurement scheme was determined to be the final scheme, which can get the measurement uncertainty better than 0.7 kN. Performance of different measurement schemes are discussed in detail, and some related finite element analysis and evaluation method are also introduced in this paper.

The measurement system of acrylic transmittance for the JUNO central detector

The Jiangmen Underground Neutrino Observatory (JUNO) has 20 kilo-ton liquid scintillator as target mass contained by a huge acrylic sphere to carry out the research on neutrino physics. Acrylic transmittance is considered as an important optical performance of the JUNO Central Detector as it has a direct relationship to the number of photons detected by the JUNO Central Detector and further affects the energy resolution of JUNO. The transmittance requirement of acrylic spherical panel for the JUNO Central Detector is exceeding 96% at the wavelength of 420 nm in ultrapure water and a specific measurement system was built and studied in Institute of High Energy Physics for this purpose, as the existing equipment cannot perform the sample test in the liquid and support the long sample length of 120 mm. After improving the technology of thermoforming, the transmittances of the acrylic spherical panels meet the requirement of JUNO, which guarantees the batch production of acrylic panels on time. The design of the measurement system, the constitution of hardware, the development of control and data acquisition software are introduced, and the measurement uncertainty of the system and the transmittance results of the acrylic samples from the spherical panel are discussed in this article.

Capability of detecting low energy events in JUNO central detector

The Jiangmen Underground Neutrino Observatory (JUNO) is an experimental project designed to determine the neutrino mass ordering and probe the fundamental properties of the neutrino oscillations. The JUNO central detector is a spherical liquid scintillator detector with a diameter of 35.4 m and equipped with two independent photomultiplier (PMT) systems, approximately 18,000 20 inch PMTs and 25,000 3 inch PMTs. A trigger threshold of 0.5 MeV can be easily achieved by using a common multiplicity trigger and can meet the requirements for measuring the neutrino mass ordering. However, it is essential to further reduce the trigger threshold for detecting solar neutrinos and supernova neutrinos. Based on the 20 inch PMT system, a sophisticated trigger scheme is proposed to achieve a low energy threshold by reducing the rate of events due to radioactivity contaminants and dark noise coincidence. With the new trigger scheme, the event rate of the central detector from different types of sources have been carefully studied by using a detailed detector simulation. It shows that the trigger threshold can be reduced to 0.2 MeV, or even 0.1 MeV, if the concentration of 14C in liquid scintillator can be well controlled.

Study on the Fracture Properties of the PMMA Structure for the JUNO Central Detector

Polymethyl methacrylate (PMMA) is increasingly used in building structures nowadays. PMMA materials utilized in structures have different fracture property compared with those used in aircrafts or biomedical equipment. Single-edge-notch bending (SENB) tests were firstly carried out at various temperatures (−40°C, −20°C, 0°C, 20°C, and 40°C) to determine the KIC values of base PMMA materials and connected areas. The crack-resisting capacity of PMMA plate is subsequently studied. The fracture property of the PMMA joint for the Jiangmen Underground Neutrino Observatory (JUNO) central detector is investigated. The results show that base PMMA material has higher KIC values than connected area. The KIC of base PMMA material is lowest at 20°C and highest at −20°C, while that of connected area is lowest at 40°C and highest at −40°C. For the PMMA joint of the JUNO detector, the cracks perpendicular to the X axis are more disadvantageous than those perpendicular to the Z axis. The stress intensity factors (SIFs) at the crack front of the embedded crack decrease with the increase of embedded depth. Due to the presence of two parallel surface or embedded cracks, the SIFs at the crack front decrease.

The design of the small prototype for the central detector of JUNO

The Jiangmen Underground Neutrino Observatory (JUNO) is a multipurpose neutrino experiment designed to determine neutrino mass hierarchy, precisely measure oscillation parameters and study solar neutrinos, supernova neutrinos and geo-neutrinos. The JUNO Central Detector 20 kton liquid scintillator target mass is contained by a huge acrylic sphere with a 35.4 m inner diameter, supported by a stainless steel structure, and the sphere is eventually submerged in pure water. Before the JUNO Central Detector is built, a small prototype has been designed and static loading experiments will be carried out to verify the consistency of the finite-element calculations and static loading experiments and test the subsystems performances, such as the monitoring system and liquid scintillator filling system. The small prototype is composed of an acrylic sphere with a 3 m inner diameter and an aluminum alloy support structure. In this article, the structure of the small prototype is briefly described. A detailed simulation study using finite element analysis is conducted to account for liquid-filled condition, temperature variation and existence of high load (due to vacuum pumping) and demonstrate a satisfying mechanical performance of the small prototype. The experimental plan about the prototype on the basis of the simulation will also be mentioned.

Muon tracking with the fastest light in the JUNO central detector

The Jiangmen Underground Neutrino Observatory (JUNO) is a multi-purpose neutrino experiment designed to measure the neutrino mass hierarchy using a central detector (CD), which contains 20 kton liquid scintillator (LS) surrounded by about 18,000 photomultiplier tubes (PMTs), located 700~m underground. The rate of cosmic muons reaching the JUNO detector is about 3~Hz and the muon induced neutrons and isotopes are major backgrounds for the neutrino detection. Reconstruction of the muon trajectory in the detector is crucial for the study and rejection of those backgrounds. This paper will introduce the muon tracking algorithm in the JUNO CD, with a least squares method of PMTs' first hit time (FHT). Correction of the FHT for each PMT was found to be important to reduce the reconstruction bias. The spatial resolution and angular resolution are better than 3~cm and 0.4~degree, respectively, and the tracking efficiency is greater than 90\% up to 16~m far from the detector center.

Bearing Capacities of the Structure and Joint of JUNO Central Detector

The Jiangmen Underground Neutrino Observatory (JUNO) central detector will be placed underground to detect neutrinos. In order to achieve the feasible scheme for JUNO, the structural scheme of an acrylic ball supported by a double-layer stainless steel latticed shell is designed and modeled using ABAQUS software. The bearing capacity of the structure under working condition is investigated and influences of external factors are analyzed. For the purpose of studying the load-bearing behavior of the joint of acrylic and stainless steel in this scheme, tests of three joint specimens are conducted and the results are compared with finite element (FE) predictions. It is concluded that the structure is safe and reliable under the effects of external factors. The bearing capacity of the joint is at least 2 times as large as the design load and the stress on the acrylic is limited within 10MPa.

Fast muon simulation in the JUNO central detector**Supported by Strategic Priority Research Program of Chinese Academy of Sciences (XDA10010900) and National Natural Science Foundation of China (11405279, 11575224)

The Jiangmen Underground Neutrino Observatory (JUNO) is a multi-purpose neutrino experiment designed to measure the neutrino mass hierarchy using a central detector (CD), which contains 20 kton liquid scintillator (LS) surrounded by about 17000 photomultiplier tubes (PMTs). Due to the large fiducial volume and huge number of PMTs, the simulation of a muon particle passing through the CD with the Geant4 toolkit becomes an extremely computation-intensive task. This paper presents a fast simulation implementation using a so-called voxel method: for scintillation photons generated in a certain LS voxel, the PMT’s response is produced beforehand with Geant4 and then introduced into the simulation at runtime. This parameterisation method successfully speeds up the most CPU consuming process, the optical photon’s propagation in the LS, by a factor of 50. In the paper, the comparison of physics performance between fast and full simulation is also given.

Simulation of natural radioactivity backgrounds in the JUNO central detector**Supported by the Strategic Priority Research Program of Chinese Academy of Sciences (XDA10010900), CAS Center for Excellence in Particle Physics (CCEPP), National Natural Science Foundation of China (NSFC), Chinese Academy of Sciences (CAS) Large-Scale Scientific Facility Program, Joint Large-Scale Scientific Facility Funds of NSFC and CAS (U1332201)

The Jiangmen Underground Neutrino Observatory (JUNO) is an experiment proposed to determine the neutrino mass hierarchy and probe the fundamental properties of neutrino oscillation. The JUNO central detector is a spherical liquid scintillator detector with 20 kton fiducial mass. It is required to achieve a energy resolution with very low radioactive background, which is a big challenge to the detector design. In order to ensure the detector performance can meet the physics requirements, reliable detector simulation is necessary to provide useful information for the detector design. A simulation study of natural radioactivity backgrounds in the JUNO central detector has been performed to guide the detector design and set requirements for the radio-purity of the detector materials. The accidental background induced by natural radioactivity in the JUNO central detector is 1.1/day. The result is satisfied for the experiment.

Application of an acrylic vessel supported by a stainless-steel truss for the JUNO central detector

After the success of the Daya Bay experiment, the Jiangmen Underground Neutrino Observatory (JUNO) was launched to measure neutrino-mass hierarchy and oscillation parameters and to study other neutrino physics. Its central detector is set for antineutrinos from reactors, the Earth, the atmosphere, and the Sun. The main requirements of the central detector are containment of 20 kt of liquid scintillator, as the target mass, and 3% energy resolution. It is about a ball-shape detector of 38.5 m with ∼75% coverage of PMT on its inner surface. The design of such a huge detector is a big challenge because it must meet the requirements for several different types of physics measurement and possess the feasibility and reliability in its structure and engineering, all at reasonable time and cost. One option for the JUNO central detector is a hyper-scale acrylic ball submerged in the water to shield the background. This paper proposes a structural scheme for such an acrylic ball that is supported by a stainless-steel truss, inspired by point-supported glass-curtain walls in civil engineering. The preliminary design of the scheme is completed and verified by finite element (FE) method using ABAQUS. FE analysis shows that the scheme can control the stress level of the acrylic ball within the limit of 5 to 10 MPa, in accordance with the demand of the design objective of the central detector. The scheme is of outstanding global stability and allows various choices on local connections. We prove that the scheme is of good feasibility and should be a reasonable option for the central detector.