XENONnT INFRASTRUCTURE

Ever wondered how the XENON infrastructure looks like?

The XENON Dark Matter Project is hosted by the INFN Gran Sasso National Laboratory (LNGS). It is the largest underground laboratory  with a worldwide research facility where particle physics, cosmology and astrophysics meet. LNGS offers the most advanced underground infrastructures in terms of dimensions, complexity and completeness.

Located between L’Aquila and Teramo, at about 120 km from Rome in central Italy, the underground structures are on one side of the 10 km long highway tunnel which crosses the Gran Sasso massif (towards Rome).  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 on the picture on the right.

The 1400 m  rock thickness above the Laboratory represents a natural coverage reducing the cosmic ray flux by one million times. Additionally, the neutron flux in Hall A-C is about thousand times lower than on the surface due to the very small amount of uranium and thorium of 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 which hosts almost all the xenon handling systems and a water tank that contains three nested detectors: nVeto, µVeto and 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 effciency.  

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

The XENONnT experiment underground at LNGS.
The XENONnT experiment underground at LNGS.

Service Building

Radon Removal System - Distillation Column

Rn-222 is the main source of background for the dark matter WIMP search. Radon is continuously emanated from the detector materials into the target material xenon. Its half-life of 3.8 d allows for a homogeneous mixing within the active detection volume. The beta decay of Pb-214 within the Rn-222 decay chain cannot be tagged by time coincidences and thus, can mimic a dark matter interaction.

A high-flow cryogenic distillation column is used to significantly reduce the radon background by using liquid xenon inlet and outlet. As radon is in its liquid form at liquid xenon temperatures (-100°C), it is trapped in the liquid xenon at the column's bottom where it decays away. Radon-free xenon can be extracted at the column's top, but only in its gaseous form. 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 radon-free xenon is extracted from the radon distillation column in gaseous from with the compressor. It is a four-cylinder magnetically-coupled piston pump. Each cylinder contains a piston that is magnetically coupled to a external magnetic ring. This ring is moved up and down via an external drive. Like that, only the piston that can be made from ultra-clean materials, is in contact with the ultra-pure xenon.

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 xenon into a xenon-xenon-based heat exchanger at the bottom of the radon distillation column acting as a heat-pump.

Cryogenics System

The cryogenic system maintains the xenon in the cryostat in liquid form, at a constant temperature and pressure, and so for years without interruption. It achieves this using pulse tube refrigerators and liquid nitrogen cooling systems, vacuum insulation, and a host of redundancies.

Gaseous Purification System

The gaseous purifcation system is responsible for pumping and distributing high purity xenon to every part of the experiment. Continuous recirculation of the xenon gas removes the electronegative impurities improving the detector performance.

Internal Calibration Source Box

External solid radioactive calibration sources are inefficient in multi-tonne liquid xenon detectors due to the high stopping power of liquid xenon for radiation. The radiation simply cannot penetrate into the center of the detector. 

For this reason, internal gaseous calibration sources are employed, in our case Kr-83m, Rn-220 and Ar-37. These are usually 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 re-circulation cylce. From here, the sources are transported directy into the detection volume via the cryogenic system, where they can homogenously distribute in the entire volume.

Data Acquisition System

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

Computing

During one hour of science run data taking, the DAQ processes 200-1000* GB of raw data, which is processed straight away to monitor the detector. Later on, this data is re-analyzed to search for dark matter. During this process, the data is transferred across the Atlantic and to other parts of Europe, where high-performance computing through an open science grid re-process the data using our analysis framework straxen.


*calibrations

Muon and Neutron Veto Acquisition System

Part of the readout system is dedicated to the the muon and neutron veto. The three constituents are integrated into a single DAQ and can be operated both independently and 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. Alarm conditions (e.g., parameter out of range, equipment failure, connection loss, etc.) are notified by email and SMS.

Operation Room

The XENON collaboration has always two members on shift at LNGS to monitor the global system's stability, the data taking as well as to perform regular detector calibration measurements. In addition, several subsystem experts are available at LNGS and from remote to keep everything stable.

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 distillation system towers into the operation room on the middle 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 sphere is cooled down with liquid nitrogen to liquid xenon temperatures and below, while an insulation vacuum is established between the inner and outer sphere to avoid heat losses.

The system is rated for high pressures up to 70 bar and can store up to 7.6 tonnes of xenon either in gas, liquid or solid phase, preserving the high purity of the xenon.

In case of emergency, the xenon from the detector can be recovered safely and controlled in liquid phase within a few hours.

Liquid Purification System

The liquid purification system continuously extracts xenon directly from the bottom of the cryostat and purifies it while keeping its liquid form using custom-designed filters.

Its main purpose is to reduce oxygen and other electronegative traces and by that to enhance the survival probablity of free charges created in particle interactions.

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 measurements, from which raw xenon can be filled to the system.

The xenon purity inside each of the over 200 bottles was measured with a commercial gas chromatograph or a residual gas analyzer before adding the raw xenon to the global system.

A liquid nitrogen bath can be utilized to cool down two aluminum bottles to recover 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 muon veto represents the most outer part of the XENONnT experiment. It tags muons, thanks to the Cherenkov light emitted in the passage of these particles through the water contained in the tank.

Photons generated in the Cerenkov process are then collected in the Photomultiplier Tubes which signal, analyzed in coincidence with the TPC one, is used for the rejection of muon events in the experimental active volume.

Thanks to this active veto technique, the background due to cosmic muons can be greatly reduced.

Neutron Veto

The neutron veto is contained in the muon one. Neutrons, mostly generated by materials radio-impurities, after having undergone a moderation process, have a high probability to be captured by hydrogen nuclei of the water molecules. Subsequently to the neutron capture a 2 MeV gamma is emitted and triggers a neutron veto PMT signal.

By studying in coincidence signals from the neutron veto and the TPC, a great fraction of events in the Xenon active volume due to neutron interaction can be rejected.

This greatly help in discriminating (and so rejecting) neutron background events that otherwise would have constitute an irreducible background for the WIMP signal searches.

Time Projection Chamber

In the innermost part of the water tank there is the TPC. The TPC encloses the both the active volume of the experiment, which is the liquid xenon, and the xenon gas phase which is exploited for the creation of the S2 scintillation signals generated after electrons extraction.

Learn more.

Photomultiplier Tubes

A total of 494 photomultiplier tubes (PMTs), distributed in two arrays, record the scintillation light of the liquid xenon. These sensors were developed to operate stably at the liquid xenon cryogenic temperature (at -100C), and are built out of materials carefully selected for lowest intrinsic radioactivities.

Calibration System

The water tank hosts the calibration system that exposes both TPC and neutron veto to wide range of radioactive sources.

Beyond the sources directly diffuses in the TPC (Kr-83m, Rn-220 and Ar-37) the system is equipped with one I-belt, highlighted in blue and used to lower a Tungsten collimator with Y-Be photo-neutron source, two U-tubes, shown in red and green colors and used to guide small external sources both high energy gamma and neutron sources, and one L-shaped beam pipe, shown in pink and designed to provide a collimated beam of neutrons reaching the TPC with a 20 degree angle.

Outside the Watertank - Gd Plant

Gadolinium plant

The water inside the XENON water tank will be eventually doped with Gd Sulfate, to enhance the capability to capture and tag the dangerous neutrons, which can induce a signal similar to that of a WIMP, and so reduce effectively the background in the TPC.
 
The so-called GdPlant consists of a Chiller, two skids with pumps and filters where the Gd Sulfate is separated, cleaned and the unified back inside a 2 m3 mixing tank.

Outside the Watertank - ReStoX-II

ReStoX-II

The new 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 system is capable of storing the full xenon  amount even at room temperature, beneficial in case of an extended shutdown of the experiment.