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.
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.
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.
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.