Tag Archives: 2016

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.