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Liquid Level Measurement in the XENON1T TPC

Knowing the exact level of the interface between the liquid and the gaseous phase in the XENON1T TPC is crucial for the operation of the detector, and very important to understand its response. Reason for this is the so-called S2 signal, which is the second signal one measures after an event happens in the detector. It originates from electrons, which are produced when a particle scatters off the xenon, and which rise up in the electric field of the TPC until they reach the liquid-gas interface. There, an even stronger electric field is extracting them from the liquid and accelerates them towards the top of the detector. The field is strong enough that, while drifting through the xenon gas, the electrons hit xenon atoms on their way, exciting each of them to emit an ultraviolet photon. A single electron will thus produce an amplified signal of up to 300 photons, of which about 20 will be ultimately detected.

The proportional scintillation light produced by this electron avalanche is detected by the top PMT array of the detector. The size of the resulting signal is proportional to the number of electrons produced. The meshes which apply the electric fields in the detector are at fixed positions. Hence, a lower or higher level of the liquid-gas interface has direct influence on the drift length of the extracted electrons in the gas and thus a direct influence on the size of the S2 signal. The size of the S2 signal in turn is a very important parameter which is used in many different ways in the data analysis. So a very good understanding is required of where the liquid level is.

To get that information, we have designed special instruments to measure the liquid level inside the TPC. Those levelmeters work capacitively, which means that they are basically hollow capacitors, which change their capacitance proportional to the level they are immersed in liquid xenon. In normal operation mode, the system is in a thermal equilibrium, so there are no changes in the liquid level. The TPC is designed in a way that one can manually adjust the liquid-gas interface to a higher or lower level. This dynamic range of the XENON1T TPC is about 5mm. Hence the levelmeters are of similar height.

The capacitance of a capacitor increases with the area of its electrodes. To achieve the highest possible capacitance change from the lower end of the capacitor to its upper end, a detailed simulation has been performed at the University of Mainz in Germany for different shapes and sizes of capacitors. It turned out that a triple-plate capacitor of 61mm length and 10mm height is the best compromise of having a large capacitance change per unit height, while still being small enough to enable a point-like measurement of the level in the TPC. The three plates of the capacitor are 0.5mm thick and are separated 1mm from each other. To prevent the capacitors from the large electric fields surrounding them, they are shielded by a copper cage. In addition, since the levelmeters are very close to the detector, they are made out of high purity copper to prevent introducing additional radioactive backgrounds. The levelmeters change their capacitance by ~1pF per mm that is filled with liquid xenon. This translates to a resolution of an amazing ~3µm to measure the liquid xenon level! Four of those devices are distributed around the TPC. This gives us the possibility to level the detector in µm precision. The capacitor signals are read out via a pair of 15m long coax cables and an electronic circuit that is connected to the slow control system of XENON1T.

DSCF1953_resized_cut DSCF1952_resized_cut
The short levelmeters for the XENON1T experiment. Three capacitor plates inside a copper cage provide a precise measurement of the liquid level inside.

Another use case for levelmeters is the monitoring of the filling process of the cryostat. In order to do this, two 1.4m long double-walled stainless steel cylindrical capacitors are located at the outside of the TPC, covering its full height. As for the short levelmeters, the long ones also work in a way that their capacitance is changing according to how high the liquid xenon rose inside them. Here, the compromise between having a large capacitance change per height value versus very small space requirements had to be made. The diameter of the outer conductor was designed to be 6mm, for the inner conductor to be 3mm. This leads to a capacitance change of 0.10 pF/mm and enables a resolution of ~30µm for measuring the absolut level of liquid xenon in the TPC.

The XENON1T levelmeters are well designed sensors by its own and have been developed over more than one year. After production in June 2015, they are shipped to LNGS, where they will do their job over the next years during the run-time of the XENON1T detector.

Muon Veto Construction

The cryostat of the XENON1T experiment is surrounded by an huge and fascinating detector: the Muon Veto. In order to understand what it is, let us remember why we are building an experiment underground. Over our heads, a lot of particles are constantly produced by primary cosmic rays. Secondary particles can provide contamination for low background experiments, such as XENON1T. For this reason, one has to build such experiments in a place where most of these particles cannot penetrate. Only high-energy particles, like muons, and weakly interacting particles, like dark matter, can cross many kilometres of rock. Even though muons can be distinguished from dark matter due to their electric charge, they can also produce neutrons, which mimic dark matter signals. It is therefore very important to properly identify muons and reject their associated signals. This is the main task of the Muon Veto system.

The Muon Veto exploits the peculiarity of very fast muons to induce photons (sometimes thousands of them!) when crossing a layer of water. It is composed by a big cylindrical water tank, about 10m high and 9.6m diameter. Roughly 4m of water, surrounding the inner detector, provide an additional passive shield from the environmental radioactivity, reaching a factor 100 of background suppression. The water tank is equipped with 84 water proof Photo-Multiplier-Tubes (PMTs), which behave like super-sensitive single-pixel cameras. Before mounting the PMTs, we have subjected them to high pressure and water tests, in order to simulate the water tank conditions. Moreover, we have measured their most important properties and classified in different setups. The inner part of the water tank is covered by a reflective foil, which with 99% reflectivity looks like a perfect mirror. Its purpose is to keep the photons inside the tank until they reach the PMTs. A quick estimate can give us an idea about the importance of the foil: in absence of the reflective foil, a single photon would be collected only in 0.001% of the cases.

Last September 2013, the Muon Veto group, constituted by Bologna, LNGS-Torino and Mainz colleagues, had put the first stone towards the assembly of the XENON1T experiment. The water tank, constructed from the top, was at that time only few meters high. The inner part of the roof was then easy to reach and allowed us to attach the reflective foil in few days. It was a very delicate job.

Examination of the foil reflectivity

Examination of the foil reflectivity: Where the protective layer has been removed, it just looks like a mirror…

In the following months the construction of other parts of XENON1T developed very fast (see previous blog entries) and after one year of intermittent work, this October 2014 the Muon Veto group travelled to the water tank and meet all together. We continued carefully attaching the reflective foil, cladding the complete, huge water tank from the inside.

The next important step was to mount the PMTs to the roof and wall of the water tank. In order to allow the path from the farthest PMTs to the electronic room outside the tank, one had to deal with 30m of high voltage and signal cables for each PMT. Mounting the PMT was the most sensitive step, because these detectors are very delicate and any mistake could result in permanent damage. For this reason, we used appropriate white Mickey Mouse gloves and a lot of caution. The high accuracy of these detectors can be well understood by considering that a PMT can perfectly distinguish a single photon, while the threshold for the human eyes is around hundred photons.

PMTs mounted on the roof and covered with mechanical protections

PMTs mounted on the roof of the water tank, and still covered with their mechanical protections.

Later on, the two independent PMT calibration systems were mounted. They allow us to obtain, when necessary, a response of the PMTs even when the water tank is closed. The first calibration system consists in a set of optical fibers with one end connected to a PMT and the other end to a blue LED pulser, outside the water tank. The optical fibers are able to transmit all the incoming light via total internal reflection. In fact, when you illuminate one side, light travels through the 30m of fiber and gets out entirely from the other side, looking like some peculiar Christmas lights. The second calibration system is made of four diffuser balls submerged in the water, which can illuminate all the 84 PMTs simultaneously. Thanks to a wise choice of materials, this handmade system is capable of transmitting light homogeneously in all directions. For calibration purposes, it is useful that all PMTs receive the same amount of light. The diffuser ball looks like a very uniform blue bulb when it is turned on in a dark room.

PMT and relative optical fiber mounted on the wall of the water tank

PMT and relative optical fiber mounted on the wall of the water tank. Most of the reflective foil still has a protective layer on.

After one month of hard work now, in November 2014, we completed the main part of the Muon Veto installation. All this work has been concluded successfully thanks to a strongly motivated team that has seen years of preparation finally getting realized.

Top view of the water tank

Top view of the water tank. The XENON1T cryostat is already mounted together with the cryogenic pipe. The reflective foil is still covered in a protective layer.

Xenon Storage and Recovery System Installed

Building a detector which uses thousands of kilograms of xenon in liquid phase poses many serious technological challenges. Details that may appear trivial at small scales become a challenge when we go towards high masses. The storage of xenon is maybe the most evident example. One option is to keep xenon in several standard gas bottles, another option is to have a very large tank to store it. Both solutions imply keeping xenon in gaseous phase. To get an idea of the dimensions of the problem, we have to think that storing about 4000 kg of xenon at standard pressure would require a volume as big as the XENON1T water tank! Moreover, we would like to have something more than a simple storage vessel, namely a “bottle”, with its own cooling system, capable of keeping xenon already in liquid phase. We also wanted to have liquid xenon continuously purified during its storage, so that we could have ultra pure xenon available at any time for the detector. Finally we wanted to use this storage also as an efficient recovery system: for any reason, due to a maintenance or even an emergency, we wanted to be able to transfer xenon from the detector into this storage system in few hours. Can all these requirements be met by a single smart system? Yes, and we have built such a system for XENON1T. We call it ReStoX (Recovery and Storage of Xenon) and it has been successfully installed in the LNGS Laboratory on August 13th, 2014. It’s a beautiful and shiny double insulated stainless steel sphere, capable of containing up to 7 tons of xenon. Seven? Yes, because ReStoX is ready to store much more than what XENON1T will require for the first science phase expected to last a couple of years starting in 2015.

ReStoXInstalledInLNGSReStoX installed in the ground floor of the service building of XENON1T

The system was conceived by a team of experts from Columbia University and Subatech Laboratory, and initially designed in collaboration with Air Liquide. It was patented by them in 2012. The design was later changed in many important details and much improved, thanks to the contributions of Karl Giboni and Jean-Marie Disdier. The construction was assigned to the Italian company Costruzioni Generali (CG), located near Milano, which not only built it in record time (about half a year from the design to the installation) but also improved it with technological solutions to make it the biggest and most reliable liquid xenon storage ever conceived. ReStoX exists thanks to the main contribution of Columbia University and with contributions of Subatech Laboratory and Mainz University.

ReStoXComponentsReStoX (in the center) and some of its components

ReStoX has been built with two redundant and complementary cooling systems, both of them based on liquid nitrogen, so that ReStoX is able to work even in case of black-out. One is based on a circuit surrounding the inner sphere, so powerful to be even capable of freezing xenon in a short time, and another one is internal, capable of regulating the xenon pressure with high precision.

And what if we run out of liquid nitrogen? No problem. ReStoX is very strong and with its 3.4 cm thick inner sphere is capable of keeping xenon safely even in gaseous phase if necessary, withstanding about 70 bar of pressure. Not bad for a “bottle”, isn’t it?

Nitrogen Tanks Installed

We use liquid and gaseous nitrogen for a variety of things: Liquid nitrogen is used to initially liquefy the xenon and to keep the xenon cold in case of power failures. Gaseous nitrogen is mainly used as a blanket on top of the water inside the muon veto in order to keep radioactive radon gas out. Our two nitrogen storage tanks have been delivered, installed, and tested:

Nitrogen Tanks

Dr. Marcello Messina (from Columbia University) and Dr. Domenico Franco (from Zürich University) underground in front of the two XENON nitrogen storage tanks.


The XENON Detection Principle


Schema of XENON




Like Mushrooms

The XENON1T is shielded from ambient radioactivity by a large water tank that is equipped as a muon veto. The tank has a diameter of 10 meters and is 10 meters high. It is constructed from top to bottom and went up in the Gran Sasso underground laboratory within less than a month:


Construction Started

The XENON1T experiment has been approved by the INFN executive committee to be built in Hall B of the underground laboratory Laboratori Nazionali del Gran Sasso (LNGS) near Assergi, Italy. The experiment is designed to perform a search for Dark Matter with a sensitivity that is more than two orders of magnitude better than the current best sensitivities in the field.

XENON at Gran Sasso

Drawing of the XENON experiment at the Gran Sasso underground laboratory. Left the water shielding with the cryostat, on the right the service building with the electronics and xenon handling systems.

XENON1T will contain more than 3000kg of liquid xenon that are instrumented as a two-phase (liquid/gas) time projection chamber. The cryostat is housed in a water tank ten meters high and ten meters in diameter, shown on the left in the picture. This water tank shields the experiment from ambient radioactivity. A three-story service building, shown on the right in the picture, houses the systems required for handling, cooling and purification of the xenon as well as electronics and computing required for data taking. First filling with liquid xenon is expected in 2014.