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 subsequent 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.
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.
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.
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.
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.
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.
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.
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.
On 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.
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.