XENONnT Infrastructure

Ever wondered what the XENON infrastructure looks like?

The XENON Dark Matter Project is hosted by the INFN Gran Sasso National Laboratory (LNGS). It is one of the largest underground laboratories in the world, and a globally renowned research facility where particle physics, cosmology, and astrophysics meet. LNGS offers the most advanced underground infrastructures with spacious experimental halls, vehicle accessibility, and auxiliary facilities.

Located between L’Aquila and Teramo (about 120 km northeast of Rome) in central Italy, the underground structures are on the eastbound (toward Rome) side of the 10 km long highway tunnel that crosses the Gran Sasso massif. The underground complex consists of three huge experimental halls named Hall A, Hall B and Hall C (each 100 m long, 20 m wide and 18 m high). XENON is located in the middle of Hall B shown in the picture.

The 1400 m of rock above the Laboratory provides natural coverage, reducing the cosmic ray flux by one in a million. Additionally, the neutron flux is about a thousand times lower than on the surface due to the very small amounts of uranium and thorium in the Dolomite calcareous rock of the mountain. Both are crucial for the background reduction in our dark matter experiment.

The experiment’s infrastructure can be split into a service building that hosts almost all of the xenon handling systems, and a water tank that contains three nested detectors: the nVeto, the µVeto and the TPC.

In the surrounding areas outside the water tank we have the main xenon storage system ReStoX-II and a water purification plant that also enables us to mix in Gadolinium sulfate to enhance the nVeto’s efficiency.

Explore below and learn more about the specific subsystem buy hovering over the hotspots.

Service Building

Radon Removal System - Distillation Column

Rn-222, another noble element, is the main source of background for our WIMP dark matter search. Radon is continuously emanated from the detector materials, and its half-life of 3.8 days allows it to mix homogeneously with the xenon. The beta decay of Pb-214 within the Rn-222 decay chain can mimic a dark matter interaction.

A high-flow cryogenic distillation column is used to significantly reduce the radon background in the liquid xenon. Since radon and xenon are both liquids at -100°C, radon becomes trapped in the liquid xenon at the bottom of the column, where it decays away. Radon-free xenon gas is extracted at the column's top. Custom-made heat exchangers are installed in combination with a heat-pump-like compressor to liquefy the xenon again before sending it back to the detector.

Radon Removal System - Compressor

The compressor extracts radon-free xenon from the radon distillation column as a gas. It is a four-cylinder magnetically-coupled piston pump, meaning that each cylinder contains an ultra-clean piston that is magnetically coupled to an external magnetic ring. This ring is moved up and down via an external driver, so the xenon inside remains pure and clean in the sealed pump.

The four cylinders are operated in shifted phase to achieve a maximum performance in terms of flow and compression. The pump is used to push the clean xenon into a heat exchanger at the bottom of the radon distillation column, driving the cryogenic distillation inside with its heat and reliquifying the clean xenon.

Cryogenics System

The cryogenic system maintains the xenon in liquid form, at a constant temperature and pressure, for years without interruption. The system is vacuum insulated, and achieves stable running conditions using pulse tube refrigerators and liquid nitrogen cooling systems.

Gaseous Purification System

The gaseous xenon purification system is responsible for pumping and distributing high-purity xenon to every part of the experiment. Continuous recirculation of the xenon gas through hot zirconium getters removes electronegative impurities, improving the detector performance.

Internal Calibration Source Box

Applying radioactive calibration sources to the outside of the detector is inefficient in multi-tonne liquid xenon detectors due to the high stopping power of liquid xenon. While this self-shielding is good for searching for dark matter, the calibration radiation simply cannot penetrate into the center of the detector.

For this reason, internal calibration sources are employed. We dissolve Kr-83m, Rn-220, and Ar-37 in our xenon for calibration. Typically, calibration sources are noble gases that decay away fast or can be removed with purification techniques after a calibration.

The radioactive sources are contained inside a box connected to the gas purification system, where they can be injected into the recirculation cycle. From here, the sources are transported directly into the detection volume via the cryogenic system, where they diffuse into the entire volume.

TPC Data Acquisition System

Small signals coming from the TPC are amplified and digitized through a data acquisition system (DAQ) made of commercially available CAEN hardware accompanied by open-source and custom-developed software. The XENONnT DAQ is a triggerless system, reading every signal that exceeds the digitization threshold of each photosensor channel.

Computing

During one hour of science data taking, the DAQ processes 200+ GB of raw data, which is processed immediately to monitor the three detectors' performances. This data will be re-analyzed to search for dark matter. For this process, the data is transferred to other parts of Europe and across the Atlantic to the United States, where we use high-performance computing capabilities with an open science grid to reprocess the data according to our analysis framework straxen.

nVeto and µVeto Data Acquisition System

Parts of the readout system of amplifiers and digitizers are dedicated to the the muon and neutron veto detectors. The three constituent detectors' data are integrated into a single DAQ, but can be operated independently or as a unified system.

Slow Control Server

The various XENON subsystems are operated and monitored by a slow control system which is based on industry-standard process control. It ensures the detectors run stably and smoothly. Alarm conditions (such as parameters out of stable range, equipment failures, and communication errors) create automated notifications by email and SMS.

Operation Room

The XENON collaboration requires two members to be on shift at LNGS at all times to monitor the global system's stability and the data quality. Shifters also perform regular detector calibration measurements. Additionally, several subsystem experts are available at LNGS and through remote connections to keep everything running smoothly.

The operation room is where shifter and system experts meet underground to discuss, prepare, and perform different detector operations.

Krypton Distillation Column

The top part of the 5.5m tall krypton distillation system towers into the operation room through a cutout in the floor. Go to the ground floor to learn more about it.

ReStoX-I: Recovering and Storage of XENON

The ReStoX-I system is a double-walled stainless steel sphere, where the inner, vacuum-sealed volume can be cooled down with liquid nitrogen to condense and store xenon.

The system is rated for high pressures up to 70 bar and can store up to 7.6 tonnes of xenon, preserving its high purity. In case of emergency, the xenon from the detector can be recovered safely into ReStoX-I within a few hours.

Liquid Purification System

The liquid purification system continuously extracts xenon directly from the bottom of the cryostat and purifies it with custom-designed filters while maintaining its liquid phase.

Its main purpose is to reduce oxygen and other electronegative impurity traces, thus enhancing the survival probability of free electrons created in particle interactions so they contribute to measured S2 signals. The system is able to clean the 8.6 tonnes in the detector once every 0.9 days.

Gas Bottle Rack

The gas bottle rack features up to eight bottle connections, all equipped with weight sensors. Here, new xenon can be filled into the system.

The xenon purity inside each bottle (of which we had over 200!) was measured with a commercial gas chromatography instrument or a residual gas analyzer before adding the bottle's xenon to the global system.

A liquid nitrogen bath can be applied to two aluminum bottles to cryogenically recouperate xenon from other systems.

Krypton Distillation Column

The isotope Kr-85 is a β-emitter with a half-life of 10.76 yr and is a background that can mimic a dark matter interaction. Therefore, it needs to be removed before a dark matter search can start.

It is anthropogenically produced in uranium and plutonium fission and is released in the atmosphere by nuclear weapon tests and nuclear reprocessing plants. Since xenon is extracted from air by fractional distillation, a small portion of natural krypton, including Kr-85, is contained within the xenon, typically at the level of ppm (parts per million).

Since krypton is in its gaseous form at liquid xenon temperature (-100°C), cryogenic distillation can be used to separate krypton from xenon. Krypton is enhanced in the gas at the top, while the krypton-free xenon can be extracted from the bottom of the system.

Inside the Watertank

Muon Veto

The μVeto is the outermost part of the XENONnT experiment. It tags muons based on the Cherenkov light emitted by these energetic charged particles as they travel through the water.

The Cerenkov light is collected by the photomultiplier tubes, and the signals, analyzed in coincidence with the TPC, are used for the rejection of muon events from the TPC science data. Thanks to this active veto technique, the background due to cosmic muons can be greatly reduced.

Neutron Veto

The nVeto is contained in the μVeto. Neutrons, mostly generated by materials' radioactive impurities, have a high probability to be captured by the hydrogen nuclei of the water molecules. A 2 MeV gamma is emitted with neutron captures and triggers the nVeto PMTs.

By studying coincident signals from the nVeto and the TPC, most neutron-nucleus scatter events in the xenon active volume can be rejected. This greatly helps in rejecting neutron background events that otherwise would constitute an irreducible background to WIMP dark matter searches.

Time Projection Chamber

The actual TPC sits in the center of the water tank. It encompasses the active xenon volume of the experiment, including the small layer of xenon gas which is exploited for the creation of S2 scintillation signals.

Learn more.

Photomultiplier Tubes

A total of 494 photomultiplier tubes (PMTs), distributed in two arrays, record the scintillation light of the liquid xenon in the TPC. These sensors were developed to operate stably at liquid xenon cryogenic temperatures (-100C) with high quantum efficiency for xenon scintillation photons. They are built out of materials carefully selected to have the lowest intrinsic radioactivity.

Calibration System

The water tank hosts the calibration system that exposes both the TPC and nVeto to a wide range of externally-applied radioactive sources.

Beyond the sources directly diffused in the TPC (Kr-83m, Rn-220 and Ar-37) the system is equipped with infrastructure to bring radioactive sources close to the cryostat. The I-belt, highlighted in blue, is used to lower a Tungsten collimator with a Y-Be photo-neutron source to the airbox, in orange, which reduces the calibration neutrons lost in the water. Two U-tubes, shown in red and green, are used to guide high energy gamma and neutron sources (particularly AmBe) to places around the detector. The L-shaped beam pipe, shown in purple is designed to provide a collimated beam of neutrons from a deuterium-deuterium neutron generator into the TPC at a 20 degree angle.

Cryogenic Pipe

Coming soon.

Cryostat

Coming soon.

Support Structure

Coming soon.

Outside the Watertank - Gd Plant

Gadolinium plant

The water inside the XENON water tank will be doped with Gd Sulfate, enhancing its capability to capture and tag neutrons. Neutrons can induce a signal similar to that of a WIMP in the TPC, so a light signal recorded in the nVeto can be used to reject coincident nuclear recoils from that neutron in the TPC and lower this major background to WIMP searches.

This Gd Plant consists of a chiller and two skids with pumps and filters where the Gd Sulfate and water are separated and cleaned. They are recombined inside a 2 m³ mixing tank before reentering the water tank.

Outside the Watertank - ReStoX-II

ReStoX-II

The newest Recovery and Storage System for XENON, ReStoX-II, is able to store up to 10 tonnes of xenon. It has a direct connection with ReStoX-I as well as the detector to recover the xenon in case of an emergency. The inside of the insulated tower contains a large surface heat exchanger and can be cooled with liquid nitrogen to rapidly freeze xenon and recuperate it out of the system.

The system is capable of storing the entire supply of xenon, even at room temperature, in case of an extended shutdown of the experiment.

Nitrogen exhaust

Many systems use liquid nitrogen for cooling and maintaining the xenon in a liquid state. The expended nitrogen gas (colder than 0 °C!) is warmed to room temperature with a heat exchanger and vented here. Oxygen sensors and the powerful ventilation system of LNGS maintain worker safety. Water in the air in Hall B condenses and freezes on the exhaust piping.