Cryogenic Pipe Installed

The XENON1T detector sits in the center of a large water tank. All the signal and high voltage cables, pipes for liquid and gaseous xenon, vacuum piping and various other lines get there via one large pipe.

Installation of the cryogenic pipe inside the XENON1T water tank, July 2014

Installation of the cryogenic pipe inside the XENON1T water tank, July 2014

We have just finished the installation of this pipe. It’s actually a quite fascinating piece of engineering. In it, there are all the signal and high voltage cables for the photomultiplier tubes. There are pipes to recirculate the xenon for purification in the adjacent building, which are themselves inside a vacuum-insulated pipe that in turn runs inside this pipe. The large diameter pipe is also used to evacuate the cryostat, as well as the heat insulation of the cryostat. And it holds a bunch of extra cables and wires for various other sensors. So, it’s really much more than just a pipe. It’s the lifeline to the detector. And it’s pretty cramped:

Cable bunches

These are the signal wires, bunched together into a single pipe inside the cryogenic pipe. They are PTFE-insulated, low-radioactivity wires with custom-made connectors.

 

Support structure completed

Inside the XENON Water Tank, May 2014

Inside the XENON Water Tank, May 2014

After seven days of hard work the support structure for the XENON1T cryostat has been finished this month. The photo shows this support structure which sits inside the massive water tank. A total of 8.5 tons of steel with an ultra low radioactive background have been used for this construction. The cryostat vessel, weighting itself one tonne not counting the xenon, will hang inside this support structure. This work has been done by technicians and students from Nikhef and LNGS.

Measuring Kr Contamination with an Atom Trap

 

Prof. Elena Aprile and Graduate Student Luke Goetzke work on the ATTA system at Columbia University

Prof. Elena Aprile and Graduate Student Luke Goetzke work on the ATTA system at Columbia University

The Krypton Problem

One of the many advantages of using xenon as a dark matter target is that xenon has no naturally occurring long-lived radioactive isotopes. However, when xenon is distilled from air, about 1 krypton atom per billion xenon atoms is also gathered. A very small fraction of these krypton atoms, only one in one hundred billion, are the radioactive isotope 85-Kr.

The decay of 85-Kr releases an electron which can then scatter in the xenon detector. These electronic recoil events can potentially obscure even rarer signals from interactions with dark matter. Thus, for dark matter detectors using liquid xenon, the krypton needs to be removed. This is done by passing the xenon through a cryogenic distillation column specifically designed for removing krypton.

After going through the krypton column, the xenon is very clean. For XENON100, there are only ~10 krypton atoms per trillion xenon atoms. Finding one of those krypton atoms is like picking out one single star from the entire Milky Way galaxy. XENON1T has 10 times even less krypton in the xenon.

Measuring the Krypton Contamination

Measuring such a tiny amount of krypton is not trivial. One way is to look for the decay signature of 85-Kr using the XENON detector itself. However, due to its relatively long half life (~11 years), it takes many months to get an accurate estimate with this method. So, how do we measure the tiny amount of krypton relatively quickly and accurately?

An atom trapping device has has been developed by the group at Columbia University to do exactly that (see E. Aprile, T. Yoon, A. Loose, L. W. Goetzke, and T. Zelevinsky, “An atom trap trace analysis system for measuring krypton contamination in xenon dark matter detectors”, Rev. Sci. Instrum., 84, 093105 (2013), arXiv:1305.6510). The method, called Atom Trap Trace Analysis (ATTA), was originally developed at Argonne National Lab for the purpose of radioactive dating. It has been adapted to measure samples of xenon gas taken directly from the XENON detectors.

All ATTA devices have the same operating principle: traditional laser cooling and trapping techniques are employed to selectively cool and trap the element of interest present in the sample. The trapped atoms emit light which is detected by a photo detector, in our case an avalanche photodiode. The trapped atoms can thus be counted. The Columbia ATTA device is designed to be sensitive to single trapped atoms, since for clean samples the average number of krypton atoms in the trap at any given time is close to zero.

The rate at which the atoms are loaded into the trap is the number we are after. The device is calibrated carefully in order to find the trapping efficiency, i.e. the fraction of krypton atoms that get trapped and counted successfully. Multiplying the measured loading rate for a given sample by the known trapping efficiency gives the total number of krypton atoms flowing through the system. Finally, measuring how many xenon atoms flow through the system at the same time allows the krypton fraction to be calculated. The entire measurement can be completed in one working day.

The Columbia ATTA device allows the xenon used in XENON1T to be assayed for krypton contamination quickly and accurately, thus ensuring that krypton levels are safe before beginning a dark matter run, and during the run itself. And it looks pretty cool, too!

 

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.

Like Mushrooms

The XENON1T is shielded from ambient radioactivity by a large water tank that is equipped as a muon veto. The tank has a diameter of 10 meters and is 10 meters high. It is constructed from top to bottom and went up in the Gran Sasso underground laboratory within less than a month:

 

XENON is Big

All the xenon must somehow get to and from the detector through the water tank, as must signal and high voltage cables, various sensors, and we need large pipes to get a really good vacuum for cleaning the detector prior to filling. We use one large pipe for this lifeline of the detector, an aorta of sorts. Here is spokesperson Elena Aprile illustrating the huge scale of the XENON experiment.

Elena in the Tank

Spokesperson Elena Aprile behind the opening in the water tank through which all connections to the detector will be made. Picture credit: The XENON Collaboration.

Construction Started

The XENON1T experiment has been approved by the INFN executive committee to be built in Hall B of the underground laboratory Laboratori Nazionali del Gran Sasso (LNGS) near Assergi, Italy. The experiment is designed to perform a search for Dark Matter with a sensitivity that is more than two orders of magnitude better than the current best sensitivities in the field.

XENON at Gran Sasso

Drawing of the XENON experiment at the Gran Sasso underground laboratory. Left the water shielding with the cryostat, on the right the service building with the electronics and xenon handling systems.

XENON1T will contain more than 3000kg of liquid xenon that are instrumented as a two-phase (liquid/gas) time projection chamber. The cryostat is housed in a water tank ten meters high and ten meters in diameter, shown on the left in the picture. This water tank shields the experiment from ambient radioactivity. A three-story service building, shown on the right in the picture, houses the systems required for handling, cooling and purification of the xenon as well as electronics and computing required for data taking. First filling with liquid xenon is expected in 2014.