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.

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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.

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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.
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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.

E. Aprile et al. (XENON Collaboration), Exclusion of Leptophilic Dark Matter Models using XENON100 Electronic Recoil Data, Science 2015 vol. 349 no. 6250 pp. 851, and Search for Event Rate Modulation in XENON100 Electronic Recoil Data, Physical Review Letters 115, 091302 (2015) and arxiv.1507.07748

The annual modulation signature

Although we believe that Dark Matter is Out There, we are completely oblivious to the impact of Dark Matter on our daily lives. On the human scale Dark Matter is nearly impossible to detect, the faintest whisper of the galaxy. The vast majority of the time Dark Matter particles pass right through us as if we don’t exist.

It is hypothesized, however, that we may be able to tune our ears to hear the unique song of Dark Matter here on Earth. Doing so successfully would constitute direct proof that Dark Matter exists.

Rather than the swelling symphony that you might expect from the most abundant matter in the Universe, this song will be a random melody, plucked out in individual notes. The tempo of these notes, that is the rate of events in a Dark Matter detector, should vary over the course of one year.

Evidence suggests that both the Sun and the Earth are enveloped by the Dark Matter halo of the Milky Way. As the Earth’s velocity relative to the Sun varies over its one-year orbit, so does it’s velocity relative to the Dark Matter. This should result in the so-called “WIMP wind” that blows harder in June, and softer in December.

This variation itself becomes the song of Dark Matter, repeating every year like clockwork – the annual modulation signature.

 

Illustration of the expected “WIMP wind” due to the motion of the Sun relative to the DM halo of the Milky Way. Figure from arXiv:1209.3339

Illustration of the expected “WIMP wind” due to the motion of the Sun relative to the DM halo of the Milky Way. Figure from arXiv:1209.3339

XENON100 was the first instrument using liquified xenon that was able to search for such a signature. The liquid xenon that fills the detector emits light when particles interact with it. We take pictures of the light with extremely sensitive devices, and use them to identify the energy and type of interaction. We took data with this detector from February 2011 to March 2012, long enough to observe more than one full cycle of the Dark Matter annual modulation.

What will Dark Matter events look like?

In XENON100, more than one type of event is identifiable. The type depends on whether Dark Matter interacts with the nuclei of the atoms in the detector, or with the electrons surrounding these nuclei. Typically, we assume the interactions of Dark Matter are with the nuclei.

For our newest study, we considered the possibility that Dark Matter instead interacts with the electrons in XENON100, and looked for an annual modulation signature.

One challenge of such a study is that many things can potentially make the rate of events in the detector vary in time, for example random noise in the instrument itself or the decay of radioactive particles. We examined all these possibilities carefully, and determined to what extent they might affect the rate of events in the detector.

The results of our study show some evidence for a rate of events varying periodically over the course of roughly one year, or perhaps longer. This slight change in rate – about half of the average rate in the detector, which is itself very small – can not yet be explained. There’s a one in a thousand chance that it is just a statistical fluke.

Before you go extolling the news from the rooftops, however, take note that our observation is not what we would naively expect from Dark Matter.

Our data shows that the rate of multiple-scatter events (interactions with more than one atom) varies almost as much as that of single-scatter events. Since Dark Matter interacts extremely rarely, we would never expect it to cause multiple-scatter events. In addition, the date of the peak rate in our detector does not match up with what we expect due to the motion of the Earth through the Dark Matter halo.

New perspective on an old claim of Dark Matter discovery

The DAMA/LIBRA collaboration has observed an annual modulation signal in their NaI detectors for more than a decade. They claim that it can be interpreted as a direct detection of Dark Matter. Meanwhile, many experiments that are more sensitive than DAMA/LIBRA (including XENON100) have found no comparable evidence of Dark Matter interacting with atomic nuclei.

However, given the fact that the NaI detectors are unable to differentiate between different types of events, one way to resolve this tension between the different experiments is if the interactions in DAMA/LIBRA are with the electrons.

Although our study shows that XENON100 sees some hint of a signal varying over long periods, the size of that signal is still much smaller than what we would expect to see if we were, in fact, detecting the same signal as DAMA/LIBRA. Thus, we find that it is extremely unlikely to be the case that DAMA/LIBRA observes an annual modulation due to interactions with electrons. The data from XENON100 exclude this possibility with a statistical significance of 4.8σ, corresponding to a probability of about one in a million.

Best-fit amplitude and phase of annual modulation signal in XENON100 from a profile likelihood study. Expected signal from DAMA/LIBRA and expected phase from the standard Dark Matter halo overlaid for comparison.

Best-fit amplitude and phase of annual modulation signal in XENON100 from a profile likelihood study. Expected signal from DAMA/LIBRA and expected phase from the standard Dark Matter halo overlaid for comparison.

Our study answers an important question about how to interpret the DAMA/LIBRA annual modulation signal, but raises many more. Why haven’t we discovered the annual modulation of Dark Matter? What causes the annual modulation in DAMA/LIBRA? What causes the slight variation of rate in XENON100?

More data has since been taken by XENON100 that will hopefully allow the last question to be answered. As to the nature of Dark Matter, well, we will have to keep listening.

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.

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.