Tag Archives: XENON1T

XeSAT2017: Online krypton and radon removal for the XENON1T experiment

This talk by Michael Murra (slides) was presented at the XeSAT2017 conference in Khon Kaen, Thailand, from 3. – 7. April 2017.

The  main background for the XENON1T experiment are the intrinsic contaminants krypton and radon in the xenon gas. Instead of purifying the xenon once before starting the science run we were able to operate our distillation column in a closed loop with the XENON1T detector system running during its commissioning phase. This resulted into reducing the krypton concentration quickly below 1 ppt (parts per trillion, 1 ppt = 10^(-12) mol/mol) without emptying and refilling of the detector.

In addition, the column was operated in the same closed loop in inverse mode in order to reduce Rn-222 by about 20% during the first science run.

This so-called online removal for both noble gases along with the working principle of the distillation system are presented within this talk.

Search for Two-Neutrino Double Electron Capture of Xenon-124 with XENON100

Besides the hunt for dark matter particles, the XENON detectors can be used to search for many other rare processes. One interesting case arises from one of the xenon isotopes itself, namely 124Xe, which is slightly abundant in natural xenon (0.1%). While it is considered stable since its direct decay into 124I is energetically forbidden, there is a rare process in nature, so far only indirectly observed, which would lead to a decay of 124Xe into the isotope 124Te. This requires, in the most probable case, the simultaneous capture of two electrons from the closest atomic shell turning two protons into two neutrons. Since this happens rarely, the corresponding half-life is predicted to be as large as 1022 years, which overshoots the lifetime of the universe by some 12 orders of magnitude. Nevertheless, as the XENON detectors are built for the rare event detection of dark matter particles, they are also very well suited for a search of such a rare process. What would one expect to be the trace of such a decay within the detector? Although the nuclear reaction

124Xe + 2e124Te* + 2νe

would suggest that neutrinos are the signal to search for, as they are a direct product of the decay with a total energy of 2.8MeV, their weak interaction cross-section makes them not detectable. But there are two electrons now missing from the atom’s shell, which is usually from the closest one (K-shell). So there are two “holes” left at an energetically favored position. In a cascade-like process, electrons from upper shells are now dropping down, filling these holes. This releases their former higher binding energy of a characteristic value in the form of secondary particles such as X-rays or Auger electrons. These particles cascade is releasing a summed energy of 64 keV, which is the signature we expect to see with our detector.

Looking for this signal in our well-known XENON100 data from 2011 to 2012 with 225 live days of exposure, we found no signal excess above our background. This way, a lower limit on the half-life of the decay with a value of 6.5×1020 years could be determined using a Bayesian analysis approach. This is close to the optimistic theoretical predictions, but a bit less sensitive than the XMASS detector, which estimated the half-life to be larger than 4.7 x 1021 years.
However, the results from XENON100 can be seen as the preparation for the next step, XENON1T. As XENON1T has about 2kg of 124Xe in its two-ton active xenon target (a factor of 70 more compared to the 29g used in XENON100) it will be more sensitive to this rare decay. Moreover, the background in XENON1T is a factor of 30 smaller in the region of interest. After only five live days of measurement it is thus expected to explore regions no experiment has explored before, and after 2 live years of measurement, we can probe half lives up to 6 x 1022 years (see Fig.1). It has to be emphasized that this data comes for free while searching for dark matter particles, since both searches require the same settings.

hp

Figure 1: Expected sensitivity of XENON1T as a function of live time in days. The aimed duration for the dark matter search is marked at 2 ton years, which would translate into
two years of measurement using 1 ton of the detector mass as a fiducial volume. After 5
days new parameter space is explored.

The XENON1T detector is also prepared to search for competing decay modes of the double electron capture, as it has an improved response to high energy signals. The so far unobserved emission of two positrons and two neutrinos as well as a mixture with one positron emitted and one electron captured simultaneously. While any detection of these decay modes would certainly lead to a deeper understanding of standard nuclear physics another possible decay branch could open the door to physics beyond the Standard Model: The neutrinoless double beta decay. If this hypothetical mode, where no neutrinos would be emitted, would be detected, it would reveal that they are their own anti-particles and annihilate in this process of double beta decay. This would prove the violation of lepton number conservation and, additionally, it could tell something about the mass of neutrinos, which is known to be very small (<eV) but is not determined today. Unfortunately, the expected life time of these decays given by theoretical calculations is even larger than for the process with the emission of two neutrinos.

First Signals in the XENON1T Time Projection Chamber

While the functionality of each of the 248 PMTs had been tested during the different commissioning stages of the XENON1T dark matter detector, the signal detection with both PMT arrays and the full data acquisition system remained to be tested. For this, and for the LED_event1_cutsubsequent calibration of the time projection chamber (TPC), an LED illumination system has been set up with 3 individual channels, each branching out into six optical fibers distributed in a circumference around the TPC. Light shining through the fibers is collected by the PMTs, whose output signals are then magnified by a factor 10 with operational amplifiers and digitized with fast analog-to-digital converters.

The figure on the right shows the first detection of blue LED light by the XENON1T PMT arrays. A time delay between the LEDs has been set, resulting in the three peaks seen in the top panel, which correspond to the combined waveforms of all PMTs. The bottom panel shows the signals detected by each individual channel.

On March 17th, the TPC was filled with warm xenon gas for the first time, allowing to acquire the first scintillation signals with the detector. For these measurements, only the PMTs have been biased and no electric drift field was applied. The figure below shows the detection of an event occurring between the so-called screening mesh in front of the top PMT array and the photosensors (see the January 19 post for details on the TPC structure) and constitutes the first detection of an S2-like signal in XENON1T. The left panels show the hit pattern on the top and bottom arrays, while the right top and bottom panels display the summed waveform and the individual PMT hits, respectively.

first_s2

Water Tank Filling

We started to fill the water tank:

In this view from the top, the cryostat with the actual detector is visible on the left. Photomultiplier tubes of the water Cherenkov muon veto are seen at the bottom and side of the water tank, to the right of the image.

In this view from the top, the cryostat with the actual detector is visible on the left. Photomultiplier tubes of the water Cherenkov muon veto are seen at the bottom and side of the water tank, to the right of the image.

The water acts as a passive shielding against external radioactivity. In addition, using the photomultipliers that can be seen towards the right of the picture, the water acts as an active muon detector. Muons may induce events in the xenon detector that may mimic dark matter signals. We therefore turn a blind eye (“veto”) for a short time whenever a muon travels through the water tank.

Assembly and installation of the XENON1T time projection chamber

In October 2015 the assembly of the XENON1T time projection chamber (TPC) began in the above-ground cleanroom at LNGS. After methodical cleaning to remove impurities and etch away radioactive surface contamination, all of the necessary components to build the new instrument were ready. A small team of scientists with the help of a few technicians steadily constructed the first of the next generation of TPCs for dark matter direct detection.

Insertion of fiber optic cables through the PTFE panels of the field cage.

Insertion of fiber optic cables through the PTFE panels of the field cage.

First the field cage was assembled by mounting the teflon (PTFE) support pillars between top and bottom rings and inserting the 74 copper field-shaping rings (see the October 5 post for details). The approximately 1 meter high by 1 meter diameter structure was assembled on a special table to allow access from the top and inside of the cage to install reflector panels and resistor chains and to insert fiber optic cables. Weaving of one of the 24 fiber optic cables around the top ring of the TPC and through a 250 μm hole in the PTFE panel is shown in the image to the right. The fibers will be used to uniformly distribute light inside the TPC for PMT calibration. One can also see in the image two sets of high voltage chains (diagonal strips inside the copper rings)  that run vertically along the field cage. A chain consists of 73 resistors (5 GΩ each) that bridge neighbouring rings, allowing for an optimal electric field of 1 kV/cm.  In parallel to the field cage construction, the top PMT array (see the October 29 post for more details) was installed inside the TPC diving bell.

topstack

Field cage from above after installation of gate and anode electrodes.

Next the cathode, anode and gate electrodes that provide radially-uniform electric fields across the TPC and the screening meshes that protect the PMTs from the high electric field were installed. The electrodes consist of wires or hexagonal meshes (grids) stretched across stainless steel rings. The bottom screening mesh, cathode, and small PTFE reflectors were assembled onto the bottom PMT array while still in its transport box. To assemble the “top stack”, shown in the image to the right, the gate grid was gently lifted and affixed onto the top TPC ring, followed by the anode grid, with 5 mm insulating spacers in between the two grids. The xenon liquid/gas interface will reside between these two electrodes. Then the small PTFE reflector panels were assembled and the protective mesh for the PMTs was placed on top. Levelmeters that measure by capacitance the height of the liquid xenon were installed onto the top TPC ring. At this point the field cage was ready to be mounted inside the bell.

TopPMTarray_lb2CR

Top PMT array and reflector panels as seen from below the field cage.

The striking image to the left shows the top PMT array as seen from the bottom of the field cage after mounting it to the bell. One can even see the ghost-like images of PMTs reflected in the polished surfaces of the PTFE panels! The graininess of the array in the photo comes from the three mesh layers of the top stack. In the days that followed, the bottom PMT array, with cathode, was mounted to the field cage, and monitoring devices such as temperature sensors and diagnostic PMTs were installed. Finally, the TPC was wrapped and secured to prepare for its big move underground.

TPC mounted inside water tank.

TPC (high-voltage feedthrough side) mounted to the top dome inside the water tank.

On November 4th the TPC was transported into Hall B and wheeled inside the water tank for installation. Using a set of 3 winches from the top dome of the water tank, the delicate instrument, now close to 500 kg, was slowly and carefully raised from the bottom of the tank, through an opening in the cleanroom floor, and up to the dome of the tank. At this point integration of the TPC with other XENON1T subsystems, such as the DAQ and cryogenics systems, began. The high voltage feedthrough, piping for liquid xenon, and cabling for PMTs, fiber optics, sensors, and electrodes were connected. After many visual and mechanical checks, electrical tests, and a final cleaning, the stainless steel vessel that will contain the liquid xenon was lifted and sealed to enclose the TPC. The instrument is now ready for the next phase of XENON1T commissioning.

Lowering the radioactivity of the XENON1T photosensors

E. Aprile et al (XENON Collaboration), Lowering the radioactivity of the XENON1T photosensors, arXiv:1503.07698, Eur. Phys. J. C75 (2015) 11, 546.

The XENON1T experiment employs 242 photomultiplier tubes (PMTs) in the time projection chamber, arranged into two circular arrays. Because the overall background goal of the detector is incredibly low, with less than 1 expected event in a tonne of liquid xenon and one full year of data, the PMTs must be made out of ultra-pure materials. These materials were selected for their content in traces of 238-U, 232-Th, 40-K, 60-Co, 137-Cs and other long-lived radionuclides.

The XENON collaboration joined efforts with Hamamatsu to produce a photosensor that meets the strict requirements of our experiment. The sensor is a 3-inch diameter tube that operates stably at -100 C and at a pressure of 2 atmospheres. It has a high quantum efficiency, with a mean around 35%, for the xenon scintillation light at 178 nm and 90% photon collection efficiency.

PMT_schematicsThe sensor, shown schematically in the left picture, features a VUV-transparent quartz window, with a low-temperature bi-alkali photocathode deposited on it. A 12-dynode electron multiplication system ensures a signal amplification of ~3 millions, which is a crucial feature to detect the tiny signals induced by the rare collisions of dark matter particles with xenon nuclei.

Before the tubes were ready to be manufactured, the construction materials were inspected with gamma-ray spectroscopy and glow-discharge mass spectroscopy (GDMS). For the former, we employed the world’s most sensitive high-purity germanium detectors, GeMPI and Gator, operated deep underground at the Gran Sasso Laboratory. GDMS can detect trace impurities in solid samples and the results were compatible with those from germanium screening. We measured many samples to select the final materials for the PMT production. As an example, specific 226-Ra activities around or below 0.3 mBq/PMT were seen in most of the inspected materials. Such an activity corresponds to 3 x 10-4 226-Ra decays per second and tube, or about 26 decays per day.

BarChart_blogThe relative contribution of the selected materials to the trace contaminations in U, Th, K, Co and Cs of the final product, seen in the left picture, also tells us how to improve further sensor versions for the XENONnT upgrade. Most of the nuclides in the 238-U and 232-Th chains, especially dangerous for their emission of alpha particles, that can the produce fast neutrons in (alpha,n) reactions, are located in the ceramic stem of the tube. In consequence, finding a new material to replace the ceramic might drastically improve the background expectations.

pmts_gatorOnce the final production started, and the tubes were delivered in several batches to our collaboration, they were measured in the Gator detector. Its inner chamber can accommodate 15 PMTs at a time, as seen in the left picture. Each batch was screened for about 15 days, and theobserved activities were mostly consistent from batch to batch. For all measured PMTs, we obtain contaminations in uranium and thorium below 1 mBq/PMT. While 60-Co was at the level of 0.8 mBq/PMT, 40-K dominates the gamma activity with about 13 mBq/PMT. The information from screening was considered in the final arrangement of the PMTs in the XENON1T arrays. PMTs with somewhat higher activities are placed in the outer rings, where they are more distant from the central, fiducial xenon region of the detector.

The average activities per PMT of all trace isotopes served as input contaminations to a full Monte Carlo simulation of the expected backgrounds in XENON1T. The results show that the PMTs will provide about 1% and 6% of the total electronic and recoil background of the experiment, respectively. We can therefore safely conclude that the overall radioactivity of the sensors is sufficiently low, and they will certainly not limit the dark matter sensitivity of the XENON1T experiment.

First assembly and cold tests of the XENON1T time projection chamber

The XENON1T TPC is the largest of its kind, being about 1 m high and 1 m in diameter. It is to house more than 2 tons of xenon in liquid form, and consists of two photomultiplier (PMT) arrays, a field cage, Teflon reflectors, top and bottom support rings and electrode grids. The field cage is made of Teflon pillars that support 74 copper field shaping rings, connected via two resistor chains. The field shaping rings, optimised via detailed electrostatic field simulations, have rounded edges and are to ensure a highly uniform drift field for electrons over the whole volume of the TPC, designed to be 1 kV/cm. The inner surfaces of the Teflon reflectors are shiny, polished with a special diamond tool, to maximally reflect the 178 nm scintillation photons, and thus to optimise the overall light yield of the dark matter detector.

During a few sunny weeks in September 2015, a major part of the TPC, including the two support rings, the field cage, the reflectors and the bottom PMT array (without PMTs, consisting of a large copper and two Teflon structures), was carefully assembled in a high bay laboratory on Campus Irchel at the University of Zurich. The main goals were to rehearse the assembly procedure before the final installation work under clean room conditions, to discover and fix any potential small imperfections, and to slowly cool down the entire structure to -100 C, the planned operational temperature of the detector.

TPC_assembly_UZH

The picture shows members of the XENON team at the University of Zurich, immersed in the assembly work. The copper field shaping rings, a few connecting resistors, the Teflon pillars, the top and bottom support rings as well as the empty PMT array can be seen. Because the final top support ring, made out of stainless steel, was not yet available at this time, an aluminium mock up version was used.

The tests proceeded smoothly, apart from a minor design issue with the reflectors, that was carefully fixed by the Zurich workshop team within a few days. After all parts were assembled, and the reflectors, which are long, interlocking Teflon panels, were inserted into their final positions, the TPC was lifted with a crane with the help of a support structure attached to the top aluminium ring, as seen in the second picture. It was first moved to the side, then slowly immersed into a large, empty stainless steel dewar that could easily house the entire TPC.

Now the cold tests could finally start. The temperature inside the dewar was lowered over more than 14 hours to -100 C, and kept stable within 2%. Besides the slow rate of cooling down, a uniform temperature across the TPC was essential to prevent any non-uniform contractions of materials. This was achieved with cold nitrogen gas, four fans and two heaters placed on the bottom of the dewar, below a heavy aluminium support plate. It was monitored with 10 sensors, placed at various heights: 4 on the Teflon pillars, 4 in the middle of the TPC, inside the nitrogen gas, and 2 on the bottom of the dewar. As expected, the whole TPC had contracted by about 1.4% once it reached the final, low temperature. After a slow warm up period to room temperature, the initial dimensions were regained, and no structural damages could be observed.

TPC_lifting_UZHOn a foggy, cold morning at the end of September, the whole structure was disassembled again. The components parted in various directions: the PMT array to MPIK Heidelberg where the PMTs are to be installed, the Teflon structures to Münster where they will be cleaned in a dedicated facility, and the copper rings directly to the Gran Sasso laboratory. All parts will be thoroughly cleaned using dedicated recipes for each type of material, to avoid radioactive impurities on, or just below the surfaces, making it into the detector. They will finally come together in a clean room above ground at Gran Sasso, to be assembled into what will soon become the core of the XENON1T experiment.

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?