Tag Archives: 2014

Farewell to ICARUS

The ICARUS experiment just left Hall B at the Gran Sasso underground laboratory in Italy for its journey to CERN in Switzerland. We had designed the XENON1T water tank a bit smaller than originally planned to allow ICARUS to move past. Everything went smoothly, but it was a tight fit…

ICARUS moving

Part of the ICARUS experiment is hanging from the large crane in Hall B of the Gran Sasso underground lab, tightly squeezing past the XENON1T water tank on its way to CERN.

We wish our colleagues all the best with the future of their experiment. Read the full story of this move at interactions.org.

Muon Veto Construction

The cryostat of the XENON1T experiment is surrounded by an huge and fascinating detector: the Muon Veto. In order to understand what it is, let us remember why we are building an experiment underground. Over our heads, a lot of particles are constantly produced by primary cosmic rays. Secondary particles can provide contamination for low background experiments, such as XENON1T. For this reason, one has to build such experiments in a place where most of these particles cannot penetrate. Only high-energy particles, like muons, and weakly interacting particles, like dark matter, can cross many kilometres of rock. Even though muons can be distinguished from dark matter due to their electric charge, they can also produce neutrons, which mimic dark matter signals. It is therefore very important to properly identify muons and reject their associated signals. This is the main task of the Muon Veto system.

The Muon Veto exploits the peculiarity of very fast muons to induce photons (sometimes thousands of them!) when crossing a layer of water. It is composed by a big cylindrical water tank, about 10m high and 9.6m diameter. Roughly 4m of water, surrounding the inner detector, provide an additional passive shield from the environmental radioactivity, reaching a factor 100 of background suppression. The water tank is equipped with 84 water proof Photo-Multiplier-Tubes (PMTs), which behave like super-sensitive single-pixel cameras. Before mounting the PMTs, we have subjected them to high pressure and water tests, in order to simulate the water tank conditions. Moreover, we have measured their most important properties and classified in different setups. The inner part of the water tank is covered by a reflective foil, which with 99% reflectivity looks like a perfect mirror. Its purpose is to keep the photons inside the tank until they reach the PMTs. A quick estimate can give us an idea about the importance of the foil: in absence of the reflective foil, a single photon would be collected only in 0.001% of the cases.

Last September 2013, the Muon Veto group, constituted by Bologna, LNGS-Torino and Mainz colleagues, had put the first stone towards the assembly of the XENON1T experiment. The water tank, constructed from the top, was at that time only few meters high. The inner part of the roof was then easy to reach and allowed us to attach the reflective foil in few days. It was a very delicate job.

Examination of the foil reflectivity

Examination of the foil reflectivity: Where the protective layer has been removed, it just looks like a mirror…

In the following months the construction of other parts of XENON1T developed very fast (see previous blog entries) and after one year of intermittent work, this October 2014 the Muon Veto group travelled to the water tank and meet all together. We continued carefully attaching the reflective foil, cladding the complete, huge water tank from the inside.

The next important step was to mount the PMTs to the roof and wall of the water tank. In order to allow the path from the farthest PMTs to the electronic room outside the tank, one had to deal with 30m of high voltage and signal cables for each PMT. Mounting the PMT was the most sensitive step, because these detectors are very delicate and any mistake could result in permanent damage. For this reason, we used appropriate white Mickey Mouse gloves and a lot of caution. The high accuracy of these detectors can be well understood by considering that a PMT can perfectly distinguish a single photon, while the threshold for the human eyes is around hundred photons.

PMTs mounted on the roof and covered with mechanical protections

PMTs mounted on the roof of the water tank, and still covered with their mechanical protections.

Later on, the two independent PMT calibration systems were mounted. They allow us to obtain, when necessary, a response of the PMTs even when the water tank is closed. The first calibration system consists in a set of optical fibers with one end connected to a PMT and the other end to a blue LED pulser, outside the water tank. The optical fibers are able to transmit all the incoming light via total internal reflection. In fact, when you illuminate one side, light travels through the 30m of fiber and gets out entirely from the other side, looking like some peculiar Christmas lights. The second calibration system is made of four diffuser balls submerged in the water, which can illuminate all the 84 PMTs simultaneously. Thanks to a wise choice of materials, this handmade system is capable of transmitting light homogeneously in all directions. For calibration purposes, it is useful that all PMTs receive the same amount of light. The diffuser ball looks like a very uniform blue bulb when it is turned on in a dark room.

PMT and relative optical fiber mounted on the wall of the water tank

PMT and relative optical fiber mounted on the wall of the water tank. Most of the reflective foil still has a protective layer on.

After one month of hard work now, in November 2014, we completed the main part of the Muon Veto installation. All this work has been concluded successfully thanks to a strongly motivated team that has seen years of preparation finally getting realized.

Top view of the water tank

Top view of the water tank. The XENON1T cryostat is already mounted together with the cryogenic pipe. The reflective foil is still covered in a protective layer.

Xenon Storage and Recovery System Installed

Building a detector which uses thousands of kilograms of xenon in liquid phase poses many serious technological challenges. Details that may appear trivial at small scales become a challenge when we go towards high masses. The storage of xenon is maybe the most evident example. One option is to keep xenon in several standard gas bottles, another option is to have a very large tank to store it. Both solutions imply keeping xenon in gaseous phase. To get an idea of the dimensions of the problem, we have to think that storing about 4000 kg of xenon at standard pressure would require a volume as big as the XENON1T water tank! Moreover, we would like to have something more than a simple storage vessel, namely a “bottle”, with its own cooling system, capable of keeping xenon already in liquid phase. We also wanted to have liquid xenon continuously purified during its storage, so that we could have ultra pure xenon available at any time for the detector. Finally we wanted to use this storage also as an efficient recovery system: for any reason, due to a maintenance or even an emergency, we wanted to be able to transfer xenon from the detector into this storage system in few hours. Can all these requirements be met by a single smart system? Yes, and we have built such a system for XENON1T. We call it ReStoX (Recovery and Storage of Xenon) and it has been successfully installed in the LNGS Laboratory on August 13th, 2014. It’s a beautiful and shiny double insulated stainless steel sphere, capable of containing up to 7 tons of xenon. Seven? Yes, because ReStoX is ready to store much more than what XENON1T will require for the first science phase expected to last a couple of years starting in 2015.

ReStoXInstalledInLNGSReStoX installed in the ground floor of the service building of XENON1T

The system was conceived by a team of experts from Columbia University and Subatech Laboratory, and initially designed in collaboration with Air Liquide. It was patented by them in 2012. The design was later changed in many important details and much improved, thanks to the contributions of Karl Giboni and Jean-Marie Disdier. The construction was assigned to the Italian company Costruzioni Generali (CG), located near Milano, which not only built it in record time (about half a year from the design to the installation) but also improved it with technological solutions to make it the biggest and most reliable liquid xenon storage ever conceived. ReStoX exists thanks to the main contribution of Columbia University and with contributions of Subatech Laboratory and Mainz University.

ReStoXComponentsReStoX (in the center) and some of its components

ReStoX has been built with two redundant and complementary cooling systems, both of them based on liquid nitrogen, so that ReStoX is able to work even in case of black-out. One is based on a circuit surrounding the inner sphere, so powerful to be even capable of freezing xenon in a short time, and another one is internal, capable of regulating the xenon pressure with high precision.

And what if we run out of liquid nitrogen? No problem. ReStoX is very strong and with its 3.4 cm thick inner sphere is capable of keeping xenon safely even in gaseous phase if necessary, withstanding about 70 bar of pressure. Not bad for a “bottle”, isn’t it?

Cable Installation in the Cryogenic Pipe

The XENON1T detector sits in the center of a large water tank. All the signal and high voltage cables for the photosensors in the time projection chamber are guided by a pipe that goes from outside where the computers are located—through the tank to the detector. This stainless steel pipe was produced by ALCA, a company located near Vicenza in Italy.

More than 900 cables, each 10 meter long, had to be inserted into a 10 centimeter diameter pipe. Before the installation the cables were prepared at the University of Zurich. We developed two types of connector made out of PTFE and copper; one for the high voltage cables, one for the signal cables. These connectors satisfy the stringent requirement on radioactive cleanliness. Each holds 24 cables into one bunch. These connectors were mounted on both sides such that they can be easily connected to the detector itself inside the water tank and to other cables, leading to the electronics outside of the water tank. After bunching the cables they had to be cleaned and packed carefully to protect them from pollution during the transportation to ALCA.

Custom made HV connector with Kapton single wires

Custom made HV connector with Kapton single wires

At ALCA, each bunch was unpacked and one after the other inserted into the pipe for which we fixed each to a steal pulling wire. After all bunches were successfully inserted, both ends of the pipe were closed with caps, because the pipe had to be pumped in order to remove substances like water or alcohol that remained in the cable bunches from the cleaning process.

Installation of the signal cables under clean conditions.

Installation of the signal cables under clean conditions.

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.


First axion results from the XENON100 experiment

E. Aprile et al. (XENON100), First Axion Results from the XENON100 Experiment, Physical Review D 90, 062009 (2014) and arXiv:1404.1455.

Is it better a dark matter WIMP or the Imp from GoT? I don’t know, but I would rather advice you to not forget the axions from GUT – Grand Unification Theories. Axions, if they exist, could solve several yet unsolved problems in understanding our Universe and in the description of the forces that govern the subatomic world. The axions have been postulated by Roberto Peccei and Helen Quinn in 1977 to explain the discrepancy between theory and observation in Quantum Chromodynamics for what concern the Charge-Parity Violation. They could be an excellent dark matter candidate and solve at the same time the CPV problem. What does this mean?

In the Standard Model of particle physics, the fundamental force that regulates the interaction among the quarks is called the Strong Force. Let me remind you that the quarks are thought to be the fundamental constituent of the hadrons, among which we have the nucleons, i.e. the protons and neutrons which made the atoms. We know that the quarks come with a colour. To be clear, this colour is just a conventional name without implying that quarks are literally red, green or blue. It’s just a way to distinguish different kinds of quarks. Because of these colours, the quantum theory formalism that describes the quarks gets the name of chromo: Quantum Chromo Dynamics or QCD.

Now, in the Standard Model we have another force, called the Weak Force. This Weak Force is responsible of the decay of the nuclei; and whenever a neutrino is involved. Why do we care about Weak Interaction if the axons deal with Strong one? This is because of the CP symmetry violation.

Already in 1964 it was found that the Weak Interaction violates the CP symmetry. The fundamental particles may come with a charge (C), like the electron, and with a parity (P), which can be seen as a spatial symmetry. Like the human face which is symmetric (although not perfectly symmetric) between left and right. Before 1964 it was expected that by changing the charge of a particle (performing a so called charge conjugation) you get something different from what you had at the beginning: a positron is not an electron, but it is its charged-conjugated partner. The same thing was expected to happen with the parity conjugation: imagine to put a particle in front of a mirror, the mirrored particle won’t be the same as the original one.

However, it was believed that if you combine these two transformations (if you make a CP conjugation) you obtain the same situation as the one present at the beginning of the process. Well, in 1964, it was proven that this is not the case for the Weak Interactions, that is to say: Weak Interactions violate the CP symmetry. Nowadays we understand this process better and we can precisely describe this violation within the Standard Model of particle physics.

This CP symmetry violation, although perfectly fine with the Standard Model, has not been observed in the Strong Interaction. Imagine that you see a leaf that is about to fall from a branch, but never falls. The fall is predicted by the gravity, but it doesn’t happen. There must be something wrong! Or maybe we must be missing something. Like, the leaf being stuck to the branch. So, what is it happening to the Strong Interactions? Why haven’t we yet observed the CP violation in the Strong sector of the Standard Model?

We don’t know… yet. To solve this problem, Peccei and Quinn have introduced this new particle, the axion, that takes away the CP violation in the Strong Interaction processes, restoring the symmetry. It is like preventing the leaf to fall, and making the violation invisible. Why is this important for us?

Simple: now that the Higgs boson has been discovered and we have a clearer idea on how the particles acquire the mass they have, we are still unable to explain why we are living in a matter-dominated universe rather than an antimatter-dominated one. The definition of what is matter and what is antimatter is a purely human artifact: the two options, matter or antimatter universes, would be completely indistinguishable in terms of the laws of nature. The only difference you might experience is that instead of switching on the light letting the electrons flowing, you would do the same using positrons instead. So why the Nature has chosen the matter (electron) instead of the antimatter (positron)?

We think that the solution lies in understanding the CP violation. And the axion is one of the keystones in the building of this cathedral. There are several experimental groups searching for these particles, and many theoretical physicists are working on various axion models (oscillating between predictions and readjustment, once experimental results get published).

Concerning the experimental searches, it was recently realized that the dark matter detectors (like CDMS, EDELWEISS or xenon-based instruments) can be particularly suitable for such a challenge. About one year ago, we understood that XENON100 could play in the world championship of this competition, maybe winning the AC (not the America’s Cup, but the Axion’s Cup). So we have involved ourselves in this venture.

Supported by several theoretical models (also arising from Grand Unification Theories) we expect the axions to interact with the normal matter by coupling  either to photons, nucleon or electrons. By normal baryonic matter we mean the building blocks that constitute the Universe to which we naturally interacts. Everything you see, everything you touch is normal baryonic matter. Also XENON100 is made only of baryonic matter.

With it we could test the axion-electron coupling. This means that to explore the existence of this very elusive particles, we tried to observe the probability of an axion to kick out an electron from the xenon atoms (see the figure below). This process is called the axio-electric effect.

The axio-electric effect

The axio-electric effect converts an axion A into an electron e-, in the presence of either a nucleus Z+ or another electron e-.

The axio-electric effect is very similar to the photo-electric effect (whose discovery won Albert Einstein the Nobel Prize of Physics in 1921), with a crucial difference though: in our case instead of a photon we consider an axion hitting the electron and ionizing the xenon target. The axio-electric effect was first introduced and formalized by A. Derevianko and others in the late 1990s. What happen when an axion hits our xenon target?

It generates a small spark, which is immediately detected by the photomultiplier tubes, which continuously monitor the situation inside XENON100. XENON100 particularly good in discovering the axions through this effect. The secret lies in the cleanliness of the detector. XENON100 is definitively one of the cleanest places of the Universe. In which sense? Everything that is surrounding us is radioactive, emits radiation which continuously hits us: when you wash your hands you receive quite some amount of radiation, particularly if the washbasin is made of ceramic, because of the cobalt contained in the ceramic. This radiation is completely harmless for your body so we never worry about it. But in contrast, if you put the same amount of ceramic inside XENON100, the whole experiment would be spoiled! Hence, every single component has been carefully selected and the detector is operated in such a way that everything that generates a spark in its interior can be considered as good signal, and not some spurious radiation.


To give you an idea of the cleanliness of the XENON100 detector: imagine that you could sit inside the inner part of the XENON detector (wear the proper clothes, since the temperature is about -100 degrees). That place is so radiation-clean that you will have to wait for about a day between one low-energy event and another. All this means that if we see some light we have quite a good chance that this light is coming from something interesting — such as axions.

We have carefully run our experiment for more than a year, taking care of it like a sacred cow. We then skimmed the data that we collected during that time. At the end of the skimming procedure we have found no evidences of axions, as shown below.


What you see in the plot is the following: on the y-axis we show the coupling of the axion with the electron, i.e. a way to describe the probability they interact with the electrons; on the x-axis we shod the hypothetical mass of the axion. Since we don’t know either the coupling nor the mass, we have to plot them in such a graph, in order to check where they like to live (for a given mass the corresponding coupling and vice-versa). In these so-called exclusion plots, we show different experiments (whose names you can find on the plot) which have excluded certain phase space: each point [coupling, mass] above the line for a particular experiment has been rejected, and if the axion exist, it can be only be in the region below these lines. For example, it is highly impossible that an axion in the galaxy can have a mass of 2 keV and a coupling to the electrons 1E-11 (i.e. one in eight hundredth of millionth), since these characteristic have been excluded by CoGeNT, CDMS, EDELWEISS and more recently by XENON100. An axion with a mass of 2 keV and a coupling of 1E-13 is still possible: we haven’t been able to search for that yet. You can think of it like fishing: we try to go deeper and deeper with our fishing rods in different places of the lake. You can immediately see that the XENON100 has reached the deepest level in this search with respect to the other fishermen.

It has taken 40 years before finding the Higgs boson. The hunt for the axion has just started. We are out in front for tracking down these fundamental, elusive particles.