The XENON Detection Principle

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.

Schema of XENON

Schema of the XENON experiment: any particle interaction in the liquid xenon (blue) yields two signals: a prompt flash of light, and a delayed charge signal. Together, these two signals give away the energy and position of the interaction as well as the type of the interacting particle. (Schema: The XENON collaboration/Rafael Lang)

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.

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.

XENON1T First Light

Today XENON1T has seen its first light:

firstlightThis is literally the first event recorded by the detector in that is is a single photon that was detected by one of the photomultipliers and recorded by the whole XENON1T data acquisition setup. What you can see from the picture is that our noise is indeed very low compared to the smallest possible signal – that of a single photon! And this is even without any fine-tuning of our electronics yet.

The detector is still empty and we are checking the photomultipliers one by one before making first background measurements. Filling with liquid xenon will happen as soon as those tests are concluded successfully.

Assembly of PMT arrays

Interactions of particles with liquid xenon are detected by the observation of scintillation light. To observe even the smallest particle recoil-energies, the “eyes” of the experiment have to be able to detect single photons. In XENON1T, this is realized by photomultiplier tubes (PMTs) which convert the incoming photons to a measurable charge signal. In total 248 PMTs are used in the experiment, split in two arrays of 127 PMTs at the top and 121 at the bottom of the time projection chamber. It is of uttermost importance for the performance  of XENON1T that each PMTs works within the specifications and has a stable performance during the dark matter search campaign.

Each PMT was cooled down and tested typically three times at the Max-Planck-Institut für Kernphysik in Heidelberg (MPIK). In this way the materials are exposed to thermal stress before the final assembly. This ensures that only PMTs which reach the high requirements for our dark matter search are built in the detector. In addition, a selection of PMTs were operated in liquid xenon at the Universtiy of Zurich (UZH) to test not only their long term stability but also to check for leaks by an analysis of PMT afterpulsing. The assembly of the PMT arrays started in the clean room at MPIK. The arrays were designed at UCLA and are composed of copper plates for stability and Teflon for an optimized reflectivity of ultraviolet photons. Before the assembly of all components, all materials have been individually treated with dedicated cleaning procedures to reduce radioactive contaminations at the surfaces.

The distribution of PMTs in the arrays were optimized to achieve a maximal light collection and, hence, a low energy threshold. It is worth mentioning that the bottom array has an exceptional large average quantum efficiency of 36.7 %. Before installation, the PMTs were equipped with a custom-made low-radioactive base provided by UZH. The picture below shows the PMTs including its bases and cables during the installation in the clean room.
802_5218

The assembly of all PMTs was accomplished within two weeks and the arrays were shipped from Heidelberg to Gran Sasso in custom-built transport boxes to ensure safe passage. Furthermore, these boxes enable a light tight environment to be able to test each signal of the PMT in its final position and configuration. All tubes showed satisfactory signals in the oscilloscope upon arrival to LNGS.  The picture below shows the complete assembled bottom (left) and top (right) arrays. On top, the picture shows the top array from the side which will be facing the liquid xenon target.
Composition

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.

Gran Sasso Lab on Google Street View

The Gran Sasso laboratory that hosts the XENON1T experiment is the largest underground laboratory in the world. More than a dozen different experiments make use of the low background from cosmic radiation that you get when you go more than a mile deep underground. You can virtually walk around the lab using the Street View from Google Maps.

The lab also offers public tours, just get in touch with them directly if you want to walk around in person.