ReStoX is an original cryogenic system designed for experiments that make use of high quantities of liquid xenon. It allows to store the total amount of xenon in gaseous, liquid or solid phase and to fill it into the detector vessel under high purity conditions. The system is crucial in case of emergencies that might require a fast recovering of the whole xenon present in the detector. ReStoX is currently being used by the XENON1T experiment and a future upgrade for XENONnT has already started.
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.
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.
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.
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.
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.
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.
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?
The Krypton Problem
One of the many advantages of using xenon as a dark matter target is that xenon has no naturally occurring long-lived radioactive isotopes. However, when xenon is distilled from air, about 1 krypton atom per billion xenon atoms is also gathered. A very small fraction of these krypton atoms, only one in one hundred billion, are the radioactive isotope 85-Kr.
The decay of 85-Kr releases an electron which can then scatter in the xenon detector. These electronic recoil events can potentially obscure even rarer signals from interactions with dark matter. Thus, for dark matter detectors using liquid xenon, the krypton needs to be removed. This is done by passing the xenon through a cryogenic distillation column specifically designed for removing krypton.
After going through the krypton column, the xenon is very clean. For XENON100, there are only ~10 krypton atoms per trillion xenon atoms. Finding one of those krypton atoms is like picking out one single star from the entire Milky Way galaxy. XENON1T has 10 times even less krypton in the xenon.
Measuring the Krypton Contamination
Measuring such a tiny amount of krypton is not trivial. One way is to look for the decay signature of 85-Kr using the XENON detector itself. However, due to its relatively long half life (~11 years), it takes many months to get an accurate estimate with this method. So, how do we measure the tiny amount of krypton relatively quickly and accurately?
An atom trapping device has has been developed by the group at Columbia University to do exactly that (see E. Aprile, T. Yoon, A. Loose, L. W. Goetzke, and T. Zelevinsky, “An atom trap trace analysis system for measuring krypton contamination in xenon dark matter detectors”, Rev. Sci. Instrum., 84, 093105 (2013), arXiv:1305.6510). The method, called Atom Trap Trace Analysis (ATTA), was originally developed at Argonne National Lab for the purpose of radioactive dating. It has been adapted to measure samples of xenon gas taken directly from the XENON detectors.
All ATTA devices have the same operating principle: traditional laser cooling and trapping techniques are employed to selectively cool and trap the element of interest present in the sample. The trapped atoms emit light which is detected by a photo detector, in our case an avalanche photodiode. The trapped atoms can thus be counted. The Columbia ATTA device is designed to be sensitive to single trapped atoms, since for clean samples the average number of krypton atoms in the trap at any given time is close to zero.
The rate at which the atoms are loaded into the trap is the number we are after. The device is calibrated carefully in order to find the trapping efficiency, i.e. the fraction of krypton atoms that get trapped and counted successfully. Multiplying the measured loading rate for a given sample by the known trapping efficiency gives the total number of krypton atoms flowing through the system. Finally, measuring how many xenon atoms flow through the system at the same time allows the krypton fraction to be calculated. The entire measurement can be completed in one working day.
The Columbia ATTA device allows the xenon used in XENON1T to be assayed for krypton contamination quickly and accurately, thus ensuring that krypton levels are safe before beginning a dark matter run, and during the run itself. And it looks pretty cool, too!
The XENON dark matter experiment is installed underground at the Laboratory Nazionali del Gran Sasso of INFN, Italy. A 62 kg liquid xenon target is operated as a dual phase (liquid/gas) time projection chamber to search for interactions of dark matter particles.
An interaction in the target generates scintillation light which is recorded as a prompt signal (called S1) by two arrays of photomultiplier tubes (PMTs) at the top and bottom of the chamber. In addition, each interaction liberates electrons, which are drifted by an electric field to the liquid-gas interface with a speed of about 2 mm/μs. There, a strong electric field extracts the electrons and generates proportional scintillation which is recorded by the same photomultiplier arrays as a delayed signal (called S2). The time difference between these two signals gives the depth of the interaction in the time-projection chamber with a resolution of a few mm. The hit pattern of the S2 signal on the top array allows to reconstruct the horizontal position of the interaction vertex also with a resolution of a few mm. Taken together, our experiment is able to precisely localize events in all three coordinates. This enables the fiducialization of the target, yielding a dramatic
reduction of external radioactive backgrounds due to the self-shielding capability of liquid xenon.
In addition, the ratio S2/S1 allows to discriminate electronic recoils, which are the dominant
background, from nuclear recoils, which are expected from Dark Matter interactions. And of course, the more energy a particle deposits in the detector, the brighter both S1 and S2 signals are, hence allowing us to reconstruct the particle’s deposited energy as well.
Dark matter has been discovered. We know from measurements of the relic abundance of light elements that were generated just minutes after the Big Bang that the known, baryonic, matter is not sufficient to explain the energy-matter density of the Universe today. A cold dark matter component has been measured from the incredibly accurate observations of the Cosmic Microwave Background, which was emitted just 300,000 years after the Big Bang. And dark matter must exist in order to turn the tiny fluctuations in the Cosmic Microwave Background into the huge density fluctuations that are observed in the Universe today.
Gravitational lensing and dispersion measurements of galaxy clusters, the largest bound systems that have been observed, show that dark matter is the dominating mass component. Detailed studies of half a dozen or so merging galaxy clusters have clearly ruled out possible alternative explanations involving modifications of the gravitational law, and are now starting to probe the properties of dark matter itself. We also know that dark matter exists in our own galaxy, the Milky Way, which shows rotational velocities that are independent of radius at high radii, just as in any other spiral galaxy we observe. This flat rotation curve is clearly inconsistent with that expected from Kepler’s laws but is naturally explained by the fact that galaxies are immersed in a halo of dark matter that dominates their mass.
Taken together, we have discovered dark matter with independent measurements spanning vast time scales from a few minutes after the Big Bang all the way to today, and at length scales from the Cosmos as a whole to individual galaxies. Yet, what dark matter is made out of remains entirely unknown. Thus, research into the nature of dark matter is of utmost importance to our view of the Cosmos. It is pursued with a variety of diverse approaches that test dark matter interactions with other known particles, with itself, and at a range of different energies.
Dark matter can be expected to have couplings, albeit weak, to standard matter, so that it can be searched for with laboratory experiments. This direct search for dark matter is pursued with a variety of complementary technologies and experiments. The XENON project in particular is one of the most sensitive direct searches for dark matter.
In XENON100, we observe individual electrons and describe this signal together with its applications in a dedicated publications:
E. Aprile et al. (XENON100), Observation and applications of single-electron charge signals in the XENON100 experiment, J. Phys. G: Nucl. Part. Phys. 41 (2014) 035201, available via arXiv:1311.1088.
Our detector is so sensitive that we can detect individual photons and individual electrons. A description of the response of the detector to the latter is published here: