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.
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.
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?