Tag Archives: commissioning

Distillation campaign for XENONnT finished

The up-coming XENONnT experiment utilizes a total of 8.3 tonnes of xenon to search for the ever elusive dark matter particles. In addition to the existing 3.3 tonnes of ultra-pure xenon from XENON1T, another 5 tonnes of xenon were purchased by the XENON collaboration. 

Before the new gas can be used for XENONnT, it needs to be purified. Besides oxygen, nitrogen and water that potentially absorb the light and charge signals in the detector, the radioactive noble gas Kr-85 within the xenon needs to be removed. Kr-85 is a man-made isotope created in nuclear bomb testing and nuclear fuel reprocessing. It makes up a fraction of 10-11 of the natural krypton (Kr-nat) abundance.

The commercially available xenon arrives with a Kr-nat in xenon concentration on the order of 10-6 (ppm, parts per million) to 10-9  (ppb, parts per billion) and needs to be purified down to a concentration of 0.1 x 10-12 (ppt, parts per trillion). To put this in relation: When purchased, an Olympic swimming pool filled with liquid xenon contains a 10 liter bucket of krypton. After purification, 200 Olympic swimming pools filled with liquid xenon contain together just one single droplet of krypton.

The purest xenon on Earth can be produced with the help of our Krypton Distillation Column located underground in the service building of XENON1T/nT as seen in the picture below. The purification method is based on the separation due to the different boiling points of xenon and krypton. While xenon is in its liquid form at -100°C, krypton, as the lighter atom, prefers to stay in its gaseous form. Like that, krypton is enriched at the top of our distillation tower from where it is removed and stored in a bottle as so-called “offgas”. The purified xenon can exit the distillation system at the bottom.

The picture shows the service building of XENON1T/nT. Bottles with new xenon, containing tiny trace amounts of the radioactive noble gas Kr-85, were connected to the “Bottle rack” (Blue-red-dashed line). The xenon is guided into the “Distillation column” to separate krypton from xenon. At the top, krypton-enriched xenon is extracted as “offgas” (red line), while at the bottom, the purified xenon is taken out (Blue line). Purified xenon is either stored inside “ReStoX-I” (Left side, blue line), the storage system of XENON1T, or in “ReStoX-II” (Right side, blue line), a newly installed storage system for XENONnT.

In total, over 100 bottles of freshly delivered xenon were installed in two bottle batches at the “Bottle rack”. Here, xenon samples from each batch were measured with a connected residual gas analyzer (RGA) system. Xenon from one of the bottle batches was continuously filled to the distillation system. Purified xenon was stored  either to the Recovery and Storage for XENON1T (ReStoX-I) (left side of the picture) or to the ReStoX-II system (right side of picture), a newly installed subsystem for XENONnT. ReStoX-II is a system designed to rapidly recover and safely store up to 10 tonnes of xenon, that will serve as an fast recovery system during operation of the XENONnT experiment as well as xenon storage previous to the start of the experiment.

The full distillation campaign was split into three phases starting from April 2019 and was finished in July 2019. Xenon samples were extracted to measure the purified xenon purity at MPIK Heidelberg with a rare gas mass spectrometer

As always in our collaboration, this operation too was an interplay between different groups: The bottle rack was installed by MPIK Heidelberg, the Distillation Column was operated by WWU Münster, and the ReStoX-I and -II systems were built and monitored by Columbia University in New York and Subatech-CNRS. The existing slowcontrol system was updated for the distillation campaigns by the Weizmann Institute of Science. Furthermore, local support was given by the group of INFN. Finally, to exchange bottles and to monitor the system 24/7, shifters from all over the collaboration supported the core distillation team.

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.

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.