Operation and performance of the ICARUS-T600 cryogenic plant at Gran Sasso underground Laboratory
M. Antonello, P. Aprili, B. Baibussinov, F. Boffelli, A. Bubak, E. Calligarich, N. Canci, S. Centro, A. Cesana, K. Cieślik, D.B. Cline, A.G. Cocco, A. Dabrowski, A. Dermenev, J.M. Disdier, A. Falcone, C. Farnese, A. Fava, A. Ferrari, D. Gibin, S. Gninenko, A. Guglielmi, M. Haranczyk, J. Holeczek, A. Ivashkin, M. Kirsanov, J. Kisiel, I. Kochanek, J. Lagoda, S. Mania, A. Menegolli, G. Meng, C. Montanari, S. Otwinowski, P. Picchi, F. Pietropaolo, P. Plonski, A. Rappoldi, G. L. Raselli, M. Rossella, C. Rubbia, P. R. Sala, A. Scaramelli, E. Segreto, F. Sergiampietri, D. Stefan, R. Sulej, M. Szarska, M. Terrani, M. Torti, F. Varanini, S. Ventura, C. Vignoli, H.G. Wang, X. Yang, A. Zalewska, A. Zani, K. Zaremba
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Operation and performance of the ICARUS-T600cryogenic plant at Gran Sasso undergroundLaboratory
M. Antonello a , P. Aprili a , B. Baibussinov b , F. Boffelli c , A. Bubak d , E. Calligarich c ,N. Canci a , S. Centro b , A. Cesana e , K. Cie ´slik f , D.B. Cline g , A.G. Cocco h ,A. Dabrowska f , A. Dermenev i , J.M. Disdier p , A. Falcone c , C. Farnese b , A. Fava b ,A. Ferrari j , D. Gibin b , S. Gninenko i , A. Guglielmi b , M. Haranczyk f , J. Holeczek d ,A. Ivashkin i , M. Kirsanov i , J. Kisiel d , I. Kochanek d , J. Lagoda k , S. Mania d ,A. Menegolli c , G. Meng b , C. Montanari c , S. Otwinowski g , P. Picchi l , F. Pietropaolo b ,P. Plonski m , A. Rappoldi c , G. L. Raselli c , M. Rossella c , C. Rubbia a , j , P. R. Sala n ,A. Scaramelli n , E. Segreto a , F. Sergiampietri o , D. Stefan n , R. Sulej k , M. Szarska f ,M. Terrani e , M. Torti c , F. Varanini b , S. Ventura b , C. Vignoli a , ∗ , H.G. Wang g , X. Yang g ,A. Zalewska f , A. Zani c , K. Zaremba m a INFN - Laboratori Nazionali del Gran Sasso, Assergi, Italy b Università di Padova e INFN, Padova, Italy c Università di Pavia e INFN, Pavia, Italy d Institute of Physics, University of Silesia, Katowice, Poland e INFN e Politecnico di Milano, Milano, Italy f H.Niewodnicza´nski Institute of Nuclear Physics, Kraków, Poland g Department of Physics, UCLA, Los Angeles, USA h Università Federico II di Napoli e INFN, Napoli, i Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia j CERN, Geneva, Switzerland k National Centre for Nuclear Research, Otwock, ´Swierk, Poland l INFN Laboratori Nazionali di Frascati, Frascati, Italy m Institute for Radioelectronics, Warsaw Univ. of Technology, Warsaw, Poland n INFN Sezione di Milano, Milano, Italy o Università di Pisa e INFN, Pisa, Italy p Luca Scarcia Company, Italy ∗ Corresponding author; e-mail: [email protected] – 1 – a r X i v : . [ phy s i c s . i n s - d e t ] A p r BSTRACT : ICARUS T600 liquid argon time projection chamber is the first large mass electronicdetector of a new generation able to combine the imaging capabilities of the old bubble chamberswith the excellent calorimetric energy measurement. After the three months demonstration runon surface in Pavia during 2001, the T600 cryogenic plant was significantly revised, in terms ofreliability and safety, in view of its long-term operation in an underground environment. The T600detector was activated in Hall B of the INFN Gran Sasso Laboratory during spring 2010, whereit was operated without interruption for about three years, taking data exposed to the CERN toGran Sasso long baseline neutrino beam and cosmic rays. In this paper the T600 cryogenic plant isdescribed in detail together with the commissioning procedures that lead to the successful operationof the detector shortly after the end of the filling with liquid Argon.Overall plant performance andstability during the long-term underground operation are discussed. Finally, the decommissioningprocedures, carried out about six months after the end of the CNGS neutrino beam operation, arereported.K
EYWORDS : Large detector systems for particle and astro-particle physics; Ultra-pure nobleliquids; Liquid Argon Detectors; Time Projection Chambers. ontents
1. Introduction 12. Overview of the ICARUS T600 liquid argon TPC 2
3. Commissioning 14
4. Cryogenic plant operation and performance 195. Decommissioning 246. Conclusions 257. Acknowledgements 25
1. Introduction
The ICARUS T600 liquid argon time projection chamber (LAr-TPC) [1] is the largest LAr imagingdetector ever built with a total argon mass of ∼
760 t. It was located at the INFN Gran Sassounderground Laboratory (LNGS) with a coverage of 1400 meters of rock. The operational principleof the LAr-TPC [2] is based on the possibility, in highly purified LAr, to transport free electronsfrom ionizing tracks practically undistorted by a uniform electric field over macroscopic distances.A suitable set of electrodes (wires) placed at the end of the drift path continuously sense andrecord the signals induced by the drifting electrons. This provides simultaneous projective viewsof the same event, allowing precise three-dimensional imaging capability [3] and high resolutioncalorimetric measurements. The design and assembly of the ICARUS-T600 LAr-TPC relied onindustrial support and represents the application of concepts matured in laboratory tests to the ktonscale.The T600 was smoothly and safely operated from May 2010 to June 2013, taking data on theCERN to Gran Sasso (CNGS) neutrino beam with extremely high argon purity, stability and detec-tor live-time [1]. It also acted as underground observatory recording cosmic ray and atmospheric– 1 –eutrino events. The T600 detector has a unique role since it is presently the largest physics gradeoperational LAr-TPC and it will remain so for several years to come. It represents the state of theart and it marks a milestone in the practical realisation of any future larger scale LAr detector. Thesuccessful operation of the ICARUS T600 LAr-TPC, which allowed to perform a sensitive searchfor ν µ → ν e oscillations [4, 5], demonstrates the enormous potential of this detection technique.The procedures that brought to the activation and operation of the T600 cryogenic plant in a dif-ficult underground environment open the way to larger detector masses (up to tens of ktons) asforeseen by several new neutrino and rare event physics projects.The ICARUS T600 detector was moved to CERN in 2014 for overhauling. It is expected tobe put again in operation at FNAL exposed to the short Booster neutrino beam [6], for a definitiveclarification of the observed neutrino anomalies [7, 8, 4] hinting at the presence of a new “sterile”neutrino state. It will also collect a large sample ( ≥ ) of neutrino interactions from the NuMIbeam in the few GeV energy domain relevant to the future long base-line experiment [9], allowingdetailed study of any event topology and precise tuning of the reconstruction tools. The foreseenLAr program may also pave the way to ultimate realization of the multi kton detector with theprecise measurement of the visible energies of both hadron and electron showers and of the muonmomentum determination [9, 10, 11, 12, 13].In this paper the T600 cryogenic plant is described in details together with all the undergroundinfrastructures, as well as the main phases of ICARUS T600 commissioning, which allowed col-lecting cosmic ray and CNGS neutrino events soon after the detector filling with ultra-pure LAr.Steady state operation is reported focusing on the stability and reliability of the plant. The decom-missioning phase is also described.
2. Overview of the ICARUS T600 liquid argon TPC
The ICARUS T600 plant [14] consists of a large cryostat split into two identical, adjacent “mod-ules” with internal dimensions 3.6 (width) × × and filled with ∼ ∼ D = 500 V/cm)towards the TPC anode made of three parallel wire planes, 3 mm apart, facing the drift volume. Atotal of 53248 wires are deployed, with 3 mm pitch, oriented on each plane at a different angle (0 ◦ ,+60 ◦ , -60 ◦ ) with respect to the horizontal direction. Wires are made of AISI 304V stainless steelwith a diameter of 150 µ m and maximum length of 9.42 m for the horizontal wires or 3.77 m for theinclined ones. The wire-frame mechanics is based on the innovative concept of variable geometrydesign consisting in movable and spring-loaded frames to set the proper tension of the wires afterinstallation for precise detector geometry and planarity, to compensate for possible over-stress dur-ing the cooling-down and liquid argon filling phases and to counteract the flexibility of the frame.This design demonstrated its reliability, since none of the wires broke and no damages at the wirechamber structure occurred during the 2001 test run, the transport of the two modules from PaviaINFN Laboratory to LNGS, the installation movements on site, the commissioning phase at LNGSand all the successive operations. – 2 –y appropriate voltage biasing, the first two wire planes (“Induction-1” and “Induction-2”)provide signals in non-destructive way; finally the ionization charge is collected and measuredon the last plane (“Collection”). The relative time of each ionization signal, combined with theelectron drift velocity information (v D ∼ µ s), provides the position of the track alongthe drift coordinate. Combining the wire coordinate on each plane at a given drift time, a three-dimensional image of an ionizing event can be reconstructed with the remarkable resolution ofabout 1 mm .A special feed-through flange for the wire signals has been adopted in the T600 detector.It is based on multilayer printed circuit boards where the electrical contacts are ensured by blindholes realized staggering successive printed circuit boards (PCB) layers. The absence of through-going holes ensures perfect tightness for Ultra High Vacuum (UHV) applications. This design wassuccessfully tested in the WARP experiment [15] at LNGS showing high reliability. A set of 96flanges, holding 576 channels each, was installed on the T600 during detector reassembly at LNGS.The read-out electronic chain was designed to allow continuous read-out, digitization andindependent waveform recording of signals from each wire of the TPC. Organized in 96 cratesplaced on top of the detector cryostat, it provided wire biasing, hosted the front-end amplifiersand performed 16:1 channel multiplexing and 10-bit ADC digitization at 400 ns sampling timeper channel. The electronic noise achieved with the custom designed low noise front-end was ∼ ∼ ∼ ∼ O, O , CO ) must be kept at a very low concentration level (less than 0.1 ppb), to allow “unper-turbed” drift of ionization electrons from the point of production to the wire planes. To this purposethe ICARUS Collaboration developed a successful technique based on the use of commercial fil-ters, carefully selected among the many availabilities on the market and operated directly on liquidand the adoption of ultra high vacuum materials and techniques. The commissioning proceduresincluded an initial vacuum phase followed at regime by the continuous argon re-circulation andpurification of both the liquid bulk and the top gas phase.Additional details on the T600 detector design and construction can be found in [14]; initialdetector operation and performance at LNGS exposed to CNGS neutrino beam and cosmic rays aredescribed in [1]. The ICARUS T600 plant and its dedicated technical infrastructures were installed in the Northern-end side of the Hall B of the underground LNGS Laboratory (Fig. 1, Fig. 2). The final design of the Proprietary technology from INFN. – 3 –pparatus was strongly affected by strict requirements on efficiency, safety, seismic constraints andreliability for long time operation in a confined underground experimental area at 1400 m depth.Further severe prescriptions arose from the specific position of the Laboratory along a 10.5 km longhighway tunnel with the access in the middle of the tunnel. Additional restrictions came from theLNGS Laboratory location inside a National Park and from the proximity of a public aqueduct.
Figure 1.
The ICARUS T600 plant in the Northern side of the Hall B at LNGS. Ladders to access theintermediate and the top levels are visible in the front together with the 3 m high wall that separates the T600area from the rest of Hall B. Cabinets with the readout electronics are visible at the intermediate level. Atthe top level, the two 30 m cryogenic storage tanks and some additional electronic equipments (HV supply,PMT electronics, trigger system, etc.) are also visible. Most of the cryogenic equipment (nitrogen and argonpumps, cryo-coolers, etc.) is located on the rear side of the T600, at the Northern end of the Hall B. The T600 plant occupied about 10 ×
22 m surface, while the whole “ICARUS Area” inthe Hall B was 15 × . This area contained a dedicated service structure surrounding theT600 (about 11 ×
36 m ) organized in three levels (floor level, 5 and 10 m height) in order tomaximize the use of available space in the semi-cylindrical hall. It hosted two 30 m horizontalcryogenic liquid storages (top level), the nitrogen re-liquefaction apparatus (rear floor) and most ofthe ICARUS auxiliary systems. The ICARUS Area was served by the two Hall B cranes (40 t and5 t). The apparatus was placed on 28 dampers to fulfill the seismic requirements of the Gran Sassoregion. As a reference, the 5.8 Richter magnitude earthquake occurred in the L’Aquila region in2009 did not produce any appreciable effects on the T600 plant. The ICARUS Area, adequatelyequipped with a fine grid of sensors to promptly detect any presence of liquids, oxygen deficiencyand temperature decrease, was separated from the rest of the Hall B by means of a 3 m high wallfor safety reasons. A capillary fast extraction aspiration from ground to protect plant and personnel– 4 – est and East modules Figure 2.
Schematic view of the whole ICARUS T600 plant in the Hall B at LNGS. in case of cryogenic liquid and cold gas spillages, as well as many other safety sensors such as fireand smoke monitors were also installed. The ICARUS Control Room, which hosted all the dataacquisition systems and the remote controls of the cryogenic plant and of the detector, was locatedon the Southern side of the Hall B.Most of the infrastructures and auxiliary systems were specifically developed for the T600 op-eration at LNGS, such as redundant power supply sources and distribution systems, uninterruptedpower supply for cryogenic plant and detector control systems, upgraded and redundant coolingwater system and the nitrogen re-liquefaction plant.
To achieve the physics goals of the ICARUS experiment, the T600 design had to fulfill severalstrict requirements in terms of detector mechanics precision and stability, electronic noise, argonpurity and cryogenic plant performance and reliability. The main technical specifications for theT600 cryogenic plant were:- an extremely high liquid argon purity in terms of residual contamination of electronegativemolecules such as water, oxygen and carbon dioxide (better than 0.1 part per billion) to allowionization electrons to drift over long distances (meters);- a fast cooling from room to liquid Ar temperature to minimize outgassing and obtain a goodinitial liquid argon purity while guaranteeing a temperature difference within each detector– 5 –omponent within prescribed limits ( ∆ T <
50 K on the wire-chamber structures, ∆ T < ∆ T < .Safety requirements for underground run implied major changes and improvements with re-spect to the operational conditions carried out during the surface test in the Pavia INFN Laboratoryin 2001 [14]. In particular new solutions for cooling and insulation systems were adopted [22]. Thefinal T600 design at LNGS was further conditioned by the choice of building major componentsoutside the tunnel to work in a more effective and comfortable way and to reduce interferenceswith other activities and experiments running underground. Schematic views of the T600 plantat LNGS are shown in Fig. 2 and Fig. 3. In the rest of the paper we describe in details only therelevant innovations of the cryogenic plant while for the unchanged components we refer to [14].The two T600 modules were independent from the point of view of LAr containment andpurification plants (Fig. 4) while the nitrogen cooling system and the thermal insulation were com-mon to both. The two modules were aluminum parallelepiped containers, each one with an internalvolume of 275 m and 4.2 h × × external dimensions, had the maximum sizeallowed for transportation into the LNGS underground laboratory. In the following they will bereferred as West module - the one already commissioned in 2001 in Pavia and widely character-ized [23, 24, 25, 26, 27, 28] - and East module (the CNGS beam was arriving from the North,Fig. 2). Each cold vessels was realized with 15 mm thick aluminum honeycomb panels linked to – 6 – igure 3. Schematic view of the T600 cryostat with representation of the electronic racks and gaseousargon re-circulation systems on the top, the liquid nitrogen circulation pumps and liquid argon re-circulationsystems in the rear side. The major safety equipments are visible: the vent line with the electrical heaterconnected to all the possible exhausts and the collecting pipes from the T600 modules and insulation vesselsafety disks to the six passive heater system.
Figure 4.
Vertical cross section of the ICARUS T600 cryostat.
Al skins and to extruded profiles at the borders with the external and internal skins acting as dou-ble cryogenic containment for safety. This unconventional vessel solution was adopted mainly for– 7 –ightness request for transportability, rigidity to stand stresses during the evacuation phase and theoverall LAr and detector weight during steady state [14].The T600 thermal insulation was a single vessel surrounding the two modules and designed tobehave as an additional container for safety in case of cryogenic liquid spillages. Insulation vesselwalls, with the exception of the roof, were made of metallic boxes filled with insulating honeycombpanels (0.4 m thick of Nomex
T M or equivalent material) and super-insulation layers placed on theinternal (cold) surface [22]. The outer skins of the boxes were made of stainless steel while the innerand side skins were of Pernifer
T M to avoid thermal shrinking, Each compartment was designedto be operated in vacuum to reduce gas conduction and convection (limited by the honeycombcell geometry but still present due to residual gas) while radiative losses were suppressed by thesuper-insulation layers, resulting in an overall heat load around 10 W/m . They were intended tobe evacuated only once down to 10 − mbar and then kept in static vacuum conditions by meansof getter pumps. However various difficulties introduced by the large wall dimensions togetherwith an excessive outgassing of the internal honeycomb surface (mainly water) were faced duringthe construction and mounting of the insulation vessel preventing to reach the nominal designparameters . As a consequence, during cryostat operation, insulation walls were maintained underdynamic evacuation and their internal pressure was continuously monitored for safety reasons.Holes for pipes and chimneys for cables of the detector signals were all located on the ceilingof the insulation vessel matching the position of all feed-throughs on the two module ports. Specialbellows mounted around all the crossing tubes ensured the proper tightness of the box. An externalmetallic cage placed on an anti-seismic shock-absorber system reinforced the insulation box guar-anteeing stress distribution in case of internal overpressure and proper performance for residualaccelerations during earthquake events. It supported all the weight of the electronic racks presenton the T600 top.A cooling shield for nitrogen circulation was placed between the insulation vessel and thealuminum containers to intercept the residual heat losses through the insulation walls thus avoidingthe boiling of the LAr bulk. Circulation of 2-phase nitrogen, with its latent heat, was chosen insteadof under-cooled single phase liquid with much lower specific heat as used in Pavia test run. Thisnew solution allowed using lower power circulation pumps and guaranteed fast cooling-down phasewhile guaranteeing thermal gradients within specification. Moreover it forced de-stratification ofLAr during normal operation maintaining uniform and stable temperature in the LAr bulk. Forsafety reasons the cooling shield was designed to operate also with liquid nitrogen circulationdriven by gravity without the necessity of a dedicated circulation pump thus ensuring the coolingeven in case of emergency situations such as the total lack of power supply.Each T600 module was equipped with two gas and one liquid re-circulation systems to purifygas top and liquid bulk respectively, in order to be used to reach and maintain the required LArpurity for the physics run. A schematic view of all the argon and nitrogen cryogenic circuits isshown in Fig. 5.The gas re-circulation system was intended to purify the internal gas phase since the initialfilling phase when the outgassing rate was still present (it decreases exponentially with temperature) A mechanical instability problem occurred during evacuation of the Northern insulation wall, causing some dominoeffects. The North wall was repaired and its internal honeycomb replaced on-site with four closed-cell Divinycell TM – 8 – + LAr purifier LN2 from purif. GAr 1LN2 from purif. GAr 2LN2 storage tank Electrical heater LArGAr purif. 1 GAr purif. 2GN2 LN2
Figure 5.
Schematic view of the ICARUS T600 cryostat with argon and nitrogen circuits including theimplementation of the system to operate in full gravity-driven mode even for GAr re-condensers. and to act as detector pressure stabilizer during steady state operation. The gas re-circulationunits collected Ar gas (GAr) from the chimneys hosting the read-out cables and the feed-throughflanges. GAr on the T600 top is warm and dirtier with respect to the liquid, as it is in contact withhygroscopic plastic cables and it could be polluted by possible small leaks due to the presence ofseveral joints on each chimney. The gas was re-condensed and then dropped into a liquid nitrogencooled Oxysorb TM filter placed below the re-condenser. Finally the purified LAr flowed back intothe LAr bulk just below the liquid/gas interface. The condenser was fed with liquid nitrogen at thetemperature required for efficient re-condensation of the argon gas, by means of forced circulation.The argon re-circulation rate was normally kept at the maximum rate of 25 GAr Nm /h/unit.The re-circulation in liquid phase was instead devoted to massively purify LAr and to reachand maintain the highest purity level after the cryostat filling and, in addition, to rapidly restoreargon purity in case of accidental pollution during the detector operation. Each LAr re-circulationsystem extracted LAr at about 2 m below the surface on the 4 m height sides of the T600 mod-ules and injected it on the opposite side, 20 m apart, close to module floor, through a horizontalpierced pipe that ensured a uniform distribution over the vessel width. Each system was equippedwith an immersed cryogenic pump (ACD CRYO AC-32 centrifugal pump) placed inside an in-dependent dewar. From the pumps reservoir, the circulated LAr went through a battery of fourOxysorb/Hydrosorb TM filter cartridges (connected in parallel) and was then re-injected into the de-tector volume. Each set of filters had a nominal O absorption capacity exceeding 200 normalliters, sufficient to purify a module starting from standard commercial liquid argon (O concen-tration ≈ ≈ /h, resulting from the pumpthroughput and the filter battery impedance and corresponding to a full volume re-circulation inabout six days. Liquid nitrogen was used to cool the pump vessel, purifier cartridges and all the Artransfer lines. – 9 –hree Barber Nichols external motor centrifugal pumps BNCP-51B-000 model were installedto circulate liquid nitrogen inside the T600 cryostat circuits: one was dedicated to the cooling shieldand one to the argon re-circulation systems, while the third one was redundant and ready to startin case of need. Each pump was located inside an independent cryostat fed by gravity from thenitrogen phase separator connected to the main liquid nitrogen storage on the top of the supportingstructure. One more extra spare pump was present on site.The two-phase nitrogen returning from cryostat screens and from the Ar gas and liquid Arre-circulation system cooling was sent back to the two 30 m liquid nitrogen storages on thetop of the ICARUS service structure. For safety reasons connected with the long term operationin underground the T600 nitrogen circuit was designed to work in closed loop by means of adedicated nitrogen re-liquefaction system. Emergency operation in open circuit was also possibleand easily handled in case of prolonged stops of the system provided that liquid nitrogen reservoirwas maintained through periodic refill by trucks.The nitrogen re-liquefaction system was dimensioned to cover the nominal total cold powerrequired for the whole ICARUS T600 plant with at least 50% margin, determined by the designheat load through the insulation (including the heat input through joints, cryostat feet and cables),the foreseen nitrogen consumption for the cooling screen (included the circulation pump and dis-tribution lines) and the Ar gas and liquid Ar re-circulation-purification systems.The implemented system consisted of twelve Stirling Cryogenics BV SPC-4 (4-cylinder)cryo-coolers , based on the “reverse Stirling thermodynamic cycle”, delivering 4.1 kW of coldpower each at 84 K with an efficiency of 10.4% (each unit requires 45 kW power for its electricalmotor). This system was organized in 3 skids, each one composed by 4 cryo-coolers and one 1 m reservoir for liquid nitrogen (LN ) (Fig. 6).During steady state the liquid nitrogen tank pressure was kept stable by the re-liquefaction sys-tem: the typical working pressure was ≈ LN storage tanks was re-condensed in the 1 m reservoirs and theninjected in one of the two 30 m storage tanks by means of two redundant cryogenic transfer pumps(Barber Nichols external motor centrifugal pump BNCP-68-M1 model).All units operated independently, automatically switching on/off to keep the nitrogen pressureat any given set-point thus delivering the actual cold power needed by the system. This designprovided large flexibility in delivering cold power because of its intrinsic factorization; in additionit featured further advantages such as less critical maintenance stops (one unit at the time) withoutinterference with the continuous cooling demand, electrical consumption minimization, easy plantexpansion. The ICARUS T600 detector was equipped with several intrinsic safety systems mainly dedicated In steady state conditions both the 30 m reservoirs were dedicated to nitrogen storage (filled up to about 80 %),while during commissioning one was used for liquid argon. At the time of the commissioning only ten cryo-coolers were installed. The system was later upgraded to guaranteelarger redundancy. – 10 – igure 6.
Schematic view of the ICARUS T600 LN re-liquefaction system plant composed by the 12cryo-generators installed on the Hall B floor and the two storage tanks on the top of the ICARUS supportingstructure. A dedicated 2 t crane was installed above the skids to ease cryocooler maintenance. to prevent and eventually confine possible liquid and gas spillages:- each T600 module was protected by two ADAREG T M magnetic disks opening at an internalrelative pressure of 0.45 mbar and closing when the relative pressure drops below 0.4 mbar;three manual valves were also installed on three different feed-throughs chimneys. Theycould be operated manually to lower the pressure;- the volume between the insulation vessel and the two T600 modules was monitored by tem-perature sensors and protected by two safety magnetic disks and one safety valve;- all the magnetic disks outlets were connected to a system of two batteries of three passiveheaters (filled by half-rings of Steatite with high thermal exchanging surface) that bring theexhausted argon to room temperature (Fig. 3). A blower and a 10 kW electrical heater wereinstalled to warm-up the saturated battery while the other one was in operation;- the aluminum honeycomb walls of the T600 modules were monitored and protected againstpressure increase;- redundant safety valves for all the cryogenic tanks and pipes were installed;- all the possible points of exhaust (safety valves and rupture disks) were collected togetherinto a single vent line to convoy gas to the ventilation extraction port; this vent line wasprovided with an electrical/pneumatic control to vent at a fixed set point;- a 50 kW electrical heater (regulated by the temperature at the exit) to warm-up all the coldgas exhaust was put after the discharge valve before the extraction port of the ventilationsystem in the Northern side of the Hall B, see Fig. 3. The discharge valve plus the electricalheater were used to vent warm nitrogen from the two 30 m tanks in case of emergency whenthe nitrogen re-liquefaction system was off (open-loop operation) or to vent the transfer lineduring the liquid refill of the storage vessels.– 11 –he whole T600 plant (argon purification and nitrogen circulation systems) and the nitrogencryo-coolers system, were provided with two local and independent control systems. Both systemswere based on widely employed industrial devices (Allen-Bradley for the T600 cryogenic plant andHitachi for the cryo-coolers). A high level of redundancy was achieved with automatic interventionto guarantee the maximum operational continuity. All the plant parameters of the T600 cryostatwere handled by two redundant PLCs (Programmable Logic Controllers) and the critical nodesrelated to safety were also connected to a third PLC. Automatic process control was developed inorder to promptly react to any parameter change or emergency. A common interface based on aSCADA server (iFix Intellution installed on an industrial PC) was also available for higher level,remote, supervision and control system. to record and store all the relevant parameters and eventsand issue alarms and automatic notifications. It was interfaced with the general LNGS undergroundSafety Control Room.The ICARUS Area was equipped with several safety sensors and devices, also monitored withthe same SCADA supervision system:- 13 temperature sensors (PT100 heads with range from -50 o C to +150 o C) located near thefloor all around the T600 plant to detect temperature decrease connected to accidental coldgas or cryogenic liquid leaks;- 13 oxygen sensors (Draeger-Politron II-O ) located near the floor all around the T600 cryo-stat and the nitrogen re-liquefaction plant, 3 on the other two supporting structure levels andother 4 near the ICARUS Control Room and the cryogenic liquid downloading station, topromptly detect an oxygen concentration decrease due to accidental cold gas or liquid leaks;- an on-line smoke monitoring system and aspiration ports for each electronic rack;- an on-line smoke monitoring system in the ICARUS Area for fast fire detection;- a closed circuit TV system composed by 9 cameras located all around the T600 plant.All the possible emergency situations were extensively studied and several scenarios wereidentified and classified in terms of risk level. Significant spillage of cold liquid and/or gas wasdefined as the most critical scenario (cryogenic emergency). Dedicated infrastructures with highredundancy were implemented for the whole apparatus:- Hall B air extraction system, to create appropriate differential pressure between the Hall Band the rest of the underground Laboratory. During normal operation, about 7,000 m /h airwere inlet in the Southern end of the Hall B and extracted (from bottom and top) in the Northend side, maintaing a slight overpressure with respect to the rest of the Laboratory. In case of“cryogenic emergency” the extraction system was set to create in the Hall B a lower pressurewith respect to the rest of the Laboratory;- an emergency extraction system made of capillary pipes extracting gas from the bottom levelof ICARUS area and connected to the main Hall B air extraction system. It protected the per-sonnel and the plant in case of cryogenic liquid and cold gas spillages; in this configurationthe other extraction ports of the Hall B are closed and the only extraction way is through thisemergency system; – 12 – a redundant electrical plant (including a spare source and double distribution line) consistingof a 850 kW electrical cabinet dedicated to the T600 plant and of a second distribution systempowered by an independent LNGS cabinet and distributing electrical power to the T600 usingan alternative geometrical path in order to enhance reliability;- an uninterrupted power supply (UPS) for control and safety systems, detector relevant com-ponents and for the ICARUS control room;- a water cooling system to cool the re-liquefaction plant equipped with redundant pumps toenhance water pressure (5 bar). The typical required water quantity per unit was 60 l/min;- a redundant compressed air system to actuate pneumatic or electro-pneumatic valves. Toenhance redundancy an extra compressor ready to start and several nitrogen gas bottles wereinstalled;- a manually activated diesel generator covering the base electrical power needs of the cryo-genic plant (for valves, sensors, controls, electrical heaters for argon and nitrogen exhaust,nitrogen pumps, insulation vacuum pumps). The behavior of the two T600 modules was continuously monitored during the whole critical phasesof evacuation, cooling and filling with liquid Ar, through the survey of dedicated sensors, installedin each module for this purpose:- 8 sensors to measure the module inner wall displacement during the vacuum phase and 10potentiometric linear sensors to monitor the mechanical behavior of the insulation walls un-der differential pressure and thermal gradients;- 4 sensors to control the rotation speed and the adsorbed current of the 4 turbo-molecularpumps used during the vacuum phase;- 2 internal pressure sensors and one external pressure sensor;- 30 platinum resistors (Pt1000 type in West T600 module and Pt10000 type in the East one)for internal temperature measurement and 40 external platinum resistors Pt1000 located onthe outer insulation surface to monitor thermal losses;- 14 capacitive position meters to measure the movement of the springs, that compensate thethermal contraction of the wires during the cooling phase;- 16 continuous level sensors to monitor the liquid Ar filling and 20 carbon resistors, acting aslevel probes, to monitor the final part of the liquid Ar filling and precisely set the final level.The acquisition and storage of the slow control signals was based on two National Instruments compact Field Point modules, one for each T600 module. – 13 – . Commissioning The commissioning of the ICARUS T600 cryogenic plant started at the beginning of 2010 fol-lowing the same approach adopted in the successful surface test run in 2001 [14], with an extraattention to maximize safety and minimize interference with other underground activities. Theprocedure consisted in four main subsequent phases: (i) detector volume evacuation, (ii) cryostatcool-down, (iii) liquid Ar filling and GAr purification/recirculation start-up, (iv) LAr-TPC detectorcommissioning and LAr purification/recirculation start-up. The most critical phases were remotelyoperated and controlled from the ICARUS Control Room located in the Southern side of Hall B.A dedicated area to unload cryogenic liquids from trucks was set-up in the Hall B in corre-spondence of the LNGS Truck Tunnel. Vacuum jacked liquid nitrogen and liquid argon transferlines were installed from the unloading station to the two storage tanks on top of the ICARUSservice structure at a distance of about 80 m.
The adopted strategy to ensure an acceptable initial LAr purity relied on the cryostat evacuationdown to a residual pressure of about 10 − ÷ − mbar to perform an appropriate out-gassing of allthe internal walls and detector materials and to remove air pockets in the inner detector structures.To this purpose, each T600 module was equipped with four identical remotely controlledpumping groups, mounted on four UHV-CF200 flanges on the T600 insulation top. Each pumpingsystem consisted of a 24 m /h primary Varian Dry Scroll DS600 pump, a 1000 l/s Varian Turbo-V1001 Navigator pump and three electro-pneumatic gate valves (two on UHV-CF200 flanges, oneon UHV-CF35 flange), which allowed to intercept the pumping group, isolate the inner volumesand start vacuum phase with only primary pumps. A safety valve was mounted in parallel to eachdry scroll pump to prevent air return in case of power failure.Before evacuation, the tightness of both T600 modules was tested to a moderate internal overand under- relative pressure in order to find and eventually repair major leaks. The commission-ing procedure continued with the vacuum pumping of all the volumes to be filled with argon bothin liquid and gaseous phase (the main volumes, the purifiers, the argon transfer lines and the re-circulation units) in order to remove air and other pollutants. In order to limit vacuum load onlyto the external skin of the cold vessels panels, the aluminum honeycomb structures of the coldvessels walls were evacuated by means of rotary vane pumps and their pressure was continuouslymonitored.The evacuation of the two modules was performed in sequence. In both cases the effective timeto reach 0.2 mbar was approximately 30 hours. The turbo-molecular pumps were then switched onto proceed with the high vacuum phase. For both modules a systematic search and repair of leaksresulted in a sudden improvement of vacuum level. Fig. 7 shows the pressure evolution in the twomodules during the pumping phase as a function of the effective pumping time.During the whole vacuum phase a continuous monitoring of the mechanical deformations ofthe inner walls was carried out by the eight position meters (see Sec. 2.4). As expected from sim-ulations and from the Pavia experience, the walls deformation increased linearly with decreasing Ambient pressure in the tunnel is about 900 mbar as it is located at the height of about 1000 m above the see level. – 14 – A b s o l u t e p r ess u r e ( m b a r ) Elapsed time (hours)
Jan 6, 2010
Elapsed time (hours)
Jan 6, 2010Leak repair -5 WEST module EAST module
Figure 7.
Pressure on the West (left) and East (right) modules as a function of the pumping time. The spikesin the plot are due to interventions devoted to leak repair. The large step at about 300 h in the West moduleis due to the repair of a major leak. Peaks are due to the stop of one of the four turbo-molecular pumps. pressure and reached a maximum of about 35 mm at the center of the longest vertical walls in bothmodules.The target pressure of 10 − mbar was reached in less than three weeks in both modules; thenvacuum pumping was continued for a period of three months before starting the cooling phase.The final equilibrium pressure was 4.5 · − mbar (3.8 · − mbar) for the West (East) module.These values correspond to a global leak rate of about 6 · − mbar l/s (4 · − mbar l/s),dominated by internal outgassing as verified by measuring the residual gas composition with massspectrometers installed on two of the top flanges. The measurements showed a 1 ÷
10 relativecontent of air to water, showing that outgassing was the dominant source of the residual gas whichwas expected to freeze on the internal surfaces during the cooling-down phase; contributions fromother components were negligible.
The Stirling plant was commissioned in advance and it had been successfully operating for one yearwhen the cooling phase started. During this period, a series of tests was performed, simulatingdifferent working situations, transient phases and cold power requests up to a maximum of 36kW and demonstrating the correct behavior of the system in agreement with specifications. Failuretests were also successfully performed, including lack of water cooling, power-cuts, liquid nitrogentransfer pump stop.Immediately after vacuum pumping was stopped, the two T600 modules were loaded withultra-pure gas argon (Ar N60: < O, < , < ) at 100 mbaroverpressure, to minimize back-diffusion of air from residual leaks. Then LN circulation startedinside the cooling screens using nitrogen from the 30 m storage tank. Both forced and gravitydriven circulations were successfully tested and operated. A constant overpressure of 100 mbarwas maintained by means of continuous injection of purified gas argon in both modules. Cryostatinternal pressure, cryostat wall displacement, temperature gradients on the wire chambers, insula-tion external temperatures and displacement were monitored.During the cooling phase, all cryo-coolers were fully active and able to handle most of the– 15 –itrogen evaporation, which was partially exceeding the re-condensation power only during thebeginning of the cooling of the 100 ton mass of the metallic containers. The residual nitrogenvapor was warmed-up through the 50 kW electrical heater and safely evacuated from Hall B viathe ventilation system. The cooling phase lasted about eight days, reaching 90 K at an average rateof about -1 K/h. The cooling was slowed-down when required to keep the internal temperaturegradients on the wire chambers within specifications (50 K). Fig. 8 shows the temperature trend onthe wire chambers structures of the West and East modules; the wiggles on the cooling trends aredue to the stops of the liquid nitrogen circulation that were used to keep the thermal gradients withinspecifications. The total LN consumption was only 55,800 l, to be compared with an estimate of200,000 l required in case of full open loop. Figure 8.
Internal temperature trend on the wire chambers structures in the West (top) and East (bottom)modules as a function of time along the whole cooling phase. The values of two temperature probes (out of15) for each wires chamber are shown, one on the top and one on the bottom of the structure.
After the conclusion of the cooling phase, when the stabilization of the system was achievedand with nitrogen re-condensation system off, the power consumption (insulation losses, plus feet,pipes, cables, chimneys heat input) was determined to be 24 kW in total (corresponding to about sixactive Stirling units), well within the capability of the re-liquefaction system with all the cryogenicplant activated. The specific contribution to the heat losses of the thermal insulation was estimatedto be ∼
20 kW as derived from the temperature differences with respect to ambient air measured– 16 –n the outer skin of the insulation panels (see Fig. 9).The insulation bottom panel resulted within specifications with an internal working pressureof the order of 10 − mbar. T e m p e r a t u r e ( K ) Elapsed Time (hours)
Apr 16, 2010
Figure 9.
Temperature trend of Pt1000 probes located on the outer skin of the insulation vessel: South (red),West (blue and green), East (black and pink). Temperature values stabilization were 282 K ÷
284 K whilethe difference with ambient value was about -7 K.
To ensure filling without interruptions, one of the 30 m tanks was used as LAr storage buffer,continuously feeding the purification cartridges of re-circulation systems of the two modules.A dedicated cryo-cooler (Stirling Cryogenics BV 1-cylinder SPC-1 500 with a nominal coldpower of 1 kW at 77 K) was installed on the liquid argon tank only for the commissioning phasewith the aim of stabilizing the argon pressure by means of re-condensation.Each liquid argon delivery (13 m ), certified to be within purity specifications (H O ≤ ≤ ≤ vessel. An additional buffer tank(1.3 m ) was installed at the liquid argon unloading station, to keep the 80 m long LAr transfer linecold and to preserve argon quality between two consecutive deliveries.A filter composed by a standard Oxysorb T M /Hydrosorb
T M cartridge with high purificationcapability was put at the outlet of the 30 m vessel, to avoid early saturation of the T600 mainpurification cartridges. The liquid argon quality was monitored on-line at the 30 m storage inletand downstream this filter by means of a gas chromatograph. On average, a typical contaminationof 30 ppb in oxygen and 100 ppb in nitrogen after the additional filter was measured. Initial nitrogen content in LAr was not removed by means of the ICARUS purification system. Even if nitrogenis not electro-negative, it has to be maintained as low as possible as it affects the scintillation light production, that isfundamental for internal trigger and timing purposes as explained in the following. – 17 –o minimize argon pollution due to outgassing from surfaces, T600 filling was performed inthe shortest possible time, while avoiding to reach the opening pressure of the magnetic safetydisks. Before starting the filling phase, all transfer lines and storage vessels were accurately purgedwith high quality argon, until the residual oxygen and water content was lower than 1 ppm.In order to intercept residual outgassing impurities, the four Gar recirculation/purification unitswere put into operation at the beginning of the filling, at a rate slightly exceeding the nominal valueof 25 Nm /hour per unit. To guarantee an internal overpressure, the cryostat filling was started withultra-pure Ar gas followed by injection of the first 10,000 liters of liquid argon in West module. Asimilar procedure was followed for the East cryostat.Few days after, as required to fully thermalize the inner detector structures, the continuousliquid argon filling in both modules started with an overall rate of about 2 m /h. The whole fillinglasted about two weeks and was carried out without the need of opening the cryostat exhaust valves.The final level of liquid argon was precisely set at 3825 ± Elapsed Time (hours) L eve l ( mm ) Elapsed Time (hours)
WEST module EAST module
Figure 10.
Liquid argon level during filling inside West (left) and East (right) modules.
After the completion of the cryogenic plant commissioning, the T600 detector steady state workingconditions were reached in few days. Soon after, the TPC in the West module was activated byturning on the high voltage biasing system (- 75 kV at the cathode), the data acquisition and PMTtrigger system: the first ionization track was immediately recorded and visualized [1].As already mentioned, the detector cooling and filling procedures did not produce any signif-icant effect on the internal detector structures, the TPC wires and the PMTs. The initial electronicnoise level was in agreement with expectations, without detectable microphonic effect due to thecryogenic plant operation. – 18 –n May 28 th τ ele was surprisinglymeasured with cosmic muon tracks to exceed 600 µ s, uniform in the whole sensitive volume, cor-responding to a liquid argon residual contamination of about 0.5 ppb of O equivalent. Soon after,the East module was activated showing very similar initial performance. The liquid recirculationsystems of both modules were also turned on, leading to the steady increase of the LAr purity. Figure 11.
First CNGS neutrino interaction observed in the ICARUS T600 detector.
4. Cryogenic plant operation and performance
In the first few months of T600 operation, the whole cryogenic plant was tested, all the regulationsfine-tuned and the related parameters were then stabilized. A complete set of tests on possiblefailures of the apparatus and emergency events were satisfactorily carried out with the automaticintervention of the dedicated backup systems:- stop of LN circulation pumps, replaced by redundancy or gravity driven operation;- complete lack of electrical power, replaced by UPS and/or emergency diesel generator;- lack of compressed air, replaced by operation with nitrogen gas bottles supply;- failure of the main ICARUS PLC control system, recovered with automatic activation ofredundancies;- cold gas exhaust at the vent to check electrical heater functionality, temperature and oxygentrend at the exit. – 19 –urther redundancy and safety systems were afterward implemented including a gravity drivenLN /GAr heat exchange system to re-condense the GAr phase, an emergency power line, a fullypneumatic control system to operate valves, to specifically handle the emergency situations of totallack of power in the underground Laboratory.With all the implemented upgrades and the high intrinsic redundancy, the cryogenic plant wasoperated safely and reliably during the whole period of steady state T600 run even in case of se-vere emergency situations without stopping the data taking. The control and supervision systemsdemonstrated to be extremely efficient allowing adopting a smooth surveillance strategy based onan on-call group of experts intervening underground in case of need. As a result, the few recordedemergency situations, all to be ascribed to external power cuts, were rapidly and successfully han-dled.Major attention was dedicated to study the reliability and verify proper redundancy of theinvolved dynamic components, such as cryo-coolers and pumps, as typically represent the mostcritical part of a working plant.The nitrogen re-liquefaction system demonstrated to efficiently cover the maximum T600 coldpower request with flexibility and margin. The system design redundancy allowed to perform peri-odic maintenance to substitute some worn components (in the 3,000 ÷ ∼ ∼ < few ppt/day O equivalent) in both modules were found.The ACD AC-32 immersed pumps resulted to be less reliable than expected due to excessivebearing case damages that caused frequent system faults with consequent purity drop. A preciseand fast intervention procedure was adopted for the LAr pump substitution with a spare one. Onaverage the time interval between two consecutive faults was about 2,000 h.In spite of the LAr recirculation stops, the impurity concentration was maintained below 0.1ppb all over the detector run (see Fig. 13). Similar purity trend were observed in both the T600– 20 – A c t i ve C r y o - C oo l e r U n i t s Date (mmm/dd/2010)
Figure 12.
Number of active cryo-coolers during the first months of the T600 operation. Some stops of thecooling system are present, due to power cuts or to failure tests. modules. Electron lifetime values of the order of 7-8 ms were reached in both modules, correspond-ing to an impurity content of few tens of ppt that imply a maximum attenuation of free electrons of ∼ transfer linesleading to a significant upgrade of the liquid argon recirculation system achieved in the last fewmonths of detector operation [30]. One of the ACD AC-32 pumps was substituted with a new Bar-ber Nichols BNHEP-23-000 model similar to the other Barber Nichols pumps used on the liquidnitrogen circulation that showed much longer lifetime between ordinary maintenance cycles. Thenew pump, characterized by magnetic coupling and vacuum housing, was installed inside a newdedicated vacuum insulated cryostat. A LAr/LN heat exchanger was added up-stream of the pumpaspiration to under cool liquid argon and to ensure mono-phase liquid state. A Venturi flow-meterwas inserted down-stream of the pump output, profiting of the mono-phase argon to measure theflux. After the new pump was switched on, the electron lifetime started increasing at a rate fasterthan before. At the end of the ICARUS data taking an electron lifetime exceeding 15 ms still risingwas measured corresponding to 20 parts per trillion of O -equivalent contamination and an attenu-ation length of 25 meters, a milestone for any future project involving liquid argon TPC [29]. Theseresults demonstrated the effectiveness of the single phase LAr-TPC detectors paving the way to the– 21 –
200 400 600 800 1000 1200 0 200 400 600 800 1000 1200
June 30, 2010 June 30, 2010
Elapsed time (days)
WEST Module EAST Module e - n e g a t i v e i m p u r i t y c o n c e n t r a t i o n ( ppb O e q u i v . ) F r ee e l e c t r o n L i f e t i m e ( m s ) Recirculation pump stop for maintenance Recirculation pump stop for maintenanceNew recirculation pump
Figure 13.
Evolution of the concentration of electronegative impurities concentration in the West (left) andEast (right) modules as a function of the elapsed time for more than two years of operation of the T600detector. The corresponding free electron lifetime is shown on the right axis. construction of huge detectors with longer drift distances. With the achieved purity level only 23%of the signal attenuation is expected at 5 m from the wire planes.The oxygen contamination contents inferred by the electron lifetime measurements all overthe T600 detector run were perfectly compatible with the specifications for untouched scintillationlight production and transport [31] . Together with oxygen, also the nitrogen concentration had tobe continuously monitored as it was demonstrated that concentrations of few ppms of N stronglyquench scintillation light [32] and it couldn’t be removed by the ICARUS T600 filtering system. Asa consequence a custom set-up based on a commercial mass spectrometer (Pfeiffer QMG 220) wasspecifically developed to measure nitrogen contamination in Ar. A sample of the T600 gas phasewas periodically analyzed and N concentration was always found below the 1 ppm sensitivity ofthe instrument according to evidence from the PMT signals.Beside LAr purity, other cryogenic parameters affecting the LAr-TPC performance were ac-curately monitored along the whole detector operation. In particular, the internal temperature,directly connected to the electron drift velocity, was found stable and uniform to better than 0.25 K(Fig. 14). This was confirmed by the observed stability of the internal absolute pressure (Fig. 15)in spite of the previously mentioned stops and accidents. O contamination in LAr leads to the attenuation of both the free electron charge (via attachment process) and thescintillation light (via quenching and absorption mechanisms). The request on O concentration to avoid scintillationlight reduction is much less stringent than that for electron attachment (effects are visible for concentrations above 0.5ppm). – 22 – T e m p e r a t u r e ( K ) T e m p e r a t u r e ( K ) Elapsed Time (Days)
WEST moduleEAST module
Figure 14.
Trend of the internal temperatures measured in three different vertical positions (bottom, middleheight, top) in the two modules recorded in two periods of the detector live time, one at the beginning of therun, in 2010, and the second in 2013, close to the end of the run. A b s o l u t e P r ess u r e ( m b a r ) Elapsed Time (Days)
EAST ModuleWEST ModuleAmbient Pressure
Dec 19, 2012
Figure 15.
Absolute internal pressure in the two modules during a period of about two months between theend of 2012 and the beginning of 2013. The internal pressure was kept uniform and stable within ≈
10 mbar. – 23 – . Decommissioning
The preparation for the T600 decommissioning was started during the last months of detectoroperation. In particular an emptying skid was installed to host an immersed LAr pump, togetherwith an intermediate buffer to speed up the emptying process. The decommissioning process startedon June 27 th th and took about one month proceed-ing with the help of a heating system to speed-up the process circulating warm nitrogen gasinside T600 cooling screens while keeping the thermal gradients within the ∆ T max < 50 Kspecification to prevent thermal shock on wire chambers (Fig. 16).3. The T600 detector dismantling started in September 2013 and lasted about 15 months. It wasfinalized to the cryostat opening to extract the TPC detectors as a whole including the lightdetection system, cabling and ancillary equipments, placed into dedicated boxes specificallydesigned for the transport to CERN. The cryogenic plant, DAQ and trigger electronic systemswere recuperated and separately sent to CERN. Jul/26 Jul/31 Aug/5 Aug/10 Aug/15 Aug/20 Aug/25 Aug/30 Sep/4
BottomIntermediateTop T e m p e r a t u r e ( K ) Jul/26 Jul/31 Aug/5 Aug/10 Aug/15 Aug/20 Aug/25 Aug/30 Sep/4
Date (2013)Date (2013)
BottomIntermediateTop
WEST module EAST module
Figure 16.
Temperature trends on the wire-chamber structure all over the T600 warm-up phase. – 24 – . Conclusions
The ICARUS T600 LAr-TPC, installed at LNGS, is the biggest liquid argon detector ever realizedand represents so far the state of the art of the liquid argon TPC technology. Industry partnershipwas crucial to perform a scaling-up of the technology from the laboratory prototypal scale to the ktmass scale.The commissioning at LNGS was successfully and safely performed during the first half of2010. The detector smoothly reached optimal working conditions and took cosmic and CNGSneutrino beam data with extremely high liquid argon purity and high detector live-time, performingeven beyond expectations. The obtained results demonstrated, as reported in several publishedpapers, the effectiveness of the single phase LAr-TPC detectors paving the way to the constructionof huge detectors with longer drift distances.The three years safe and stable operation in the severe underground environment conditionwas an important achievement for LAr-TPC technique demonstrating the technology is matureand scalable to several kton mass as required by future projects. Lessons learned from the plantoperation, accidental events and plant improvements will be useful for future developments.
7. Acknowledgements
The ICARUS Collaboration acknowledges the fundamental support of INFN and, in particular, ofthe LNGS Laboratory, all the staff and its Directors, to the construction and operation of the exper-iment. Moreover the authors thank LNGS Safety and Prevention Service, the Research and Tech-nical Divisions, and in particular the Experiment Support Service, the LNGS cryogenic group andthe Exercise and Maintenance Service for their contribution to the commissioning and operationof the T600 apparatus. The collaboration recognizes the fundamental involvement of the industrialcompanies Air Liquide, Stirling Cryogenics BV and Luca Scarcia, which contributed in the real-ization, operation and maintenance of the cryogenic plant. A special thank to Marco Brugnolli,Arnaldo Di Cesare and Donatello Ciccotti. The authors warmly thank the Electronics Service ofINFN Pavia, in particular M.C. Prata, for the design and realization of the slow control electronicsand of the vacuum system remote controls. The Polish groups acknowledge the support of the Na-tional Science Center, Harmonia (2012/04/M/ST2/00775) and Preludium (2011/03/N/ST2/01971)funding schemes.
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