Design and implementation of the new scintillation light detection system of ICARUS T600
B. Ali-Mohammadzadeh, M. Babicz, W. Badgett, L. Bagby, V. Bellini, R. Benocci, M. Bonesini, A. Braggiotti, S. Centro, A. Chatterjee, A.G. Cocco, M. Diwan, A. Falcone, C. Farnese, A. Fava, D. Gibin, A. Guglielmi, W. Ketchum, U. Kose, A. Menegolli, G. Meng, C. Montanari, M. Nessi, F. Pietropaolo, A. Rappoldi, G.L. Raselli, M. Rossella, C. Rubbia, P. Sala, A. Scaramelli, F. Sergiampietri, M. Spanu, D. Torretta, M. Torti, F. Tortorici, F. Varanini, S. Ventura, C. Vignoli, A. Zhang, A. Zani
PPrepared for submission to JINST
Design and implementation of the new scintillation lightdetection system of ICARUS T600
B. Ali-Mohammadzadeh, a M. Babicz, b , c W. Badgett, d L. Bagby, d V. Bellini, a R. Benocci, e M. Bonesini, e A. Braggiotti, f , g S. Centro, f A. Chatterjee, h , d A.G. Cocco, i M. Diwan, l A. Falcone, e C. Farnese, f A. Fava, d D. Gibin, f A. Guglielmi, f W. Ketchum, d U. Kose, c A. Menegolli, m G. Meng, f C. Montanari, m , d M. Nessi, c F. Pietropaolo, c , f A. Rappoldi, m G.L. Raselli, m , M. Rossella, m C. Rubbia, c , n , o P. Sala, p , c A. Scaramelli, m F. Sergiampietri, c , q M. Spanu, l , D. Torretta, d M. Torti, e F. Tortorici, a F. Varanini, f S. Ventura, f C. Vignoli, o A. Zhang l , and A. Zani p for the ICARUS Collaboration a Department of Physics and Astronomy, University of Catania and INFN, Catania, Italy b Institute of Nuclear Physics PAN, Kraków, Poland c CERN, Geneve, Switzerland d Fermi National Laboratory, Batavia IL, USA e Department of Physics, University of Milano Bicocca and INFN, Milano, Italy f Department of Physics and Astronomy, University of Padova and INFN, Padova, Italy g CNR Padova, Padova, Italy h Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh PA, USA i Department of Physics, University of Napoli and INFN, Napoli, Italy l Brookhaven National Laboratory, Brookhaven NY, USA m Department of Physics, University of Pavia and INFN, Pavia, Italy n Gran Sasso Science Institute, L’Aquila, Italy o INFN, Laboratori Nazionali del Gran Sasso, Assergi, Italy p Department of Physics and INFN, University of Milano, Milano, Italy q INAF Torino, Torino, Italy
E-mail: [email protected] Corresponding author. now at Department of Physics, University of Milano Bicocca and INFN, Milano, Italy now at Department of Physics and Astronomy, Stony Brook University, Stony Brook NY, USA a r X i v : . [ phy s i c s . i n s - d e t ] S e p bstract: ICARUS T600 is the far detector of the Short Baseline Neutrino program at Fermilab(USA), which foresees three Liquid Argon Time Projection Chambers along the Booster NeutrinoBeam line to search for LSND-like sterile neutrino signal. The T600 detector underwent a significantoverhauling process at CERN, introducing new technological developments while maintaining thealready achieved performances. The realization of a new liquid argon scintillation light detectionsystem is a primary task of the detector overhaul. As the detector will be subject to a huge flux ofcosmic rays, the light detection system should allow the 3D reconstruction of events contributingto the identification of neutrino interactions in the beam spill gate. The design and implementationof the new scintillation light detection system of ICARUS T600 is described.Keywords: Photon detectors for UV, visible and IR photons (vacuum) (photomultipliers, HPDs,others); Noble liquid detectors (scintillation, ionization, double-phase); Scintillators, scintillationand light emission processes (solid, gas and liquid scintillators); Time projection Chambers (TPC) ontents The ICARUS T600 detector is the largest Liquid Argon Time Projection Chamber (LAr-TPC) everoperated on a neutrino beam for oscillation studies. It took data from 2010 to 2013 in the INFNGran Sasso Laboratory (Italy), both with atmospheric neutrinos and with the CERN Neutrinos toGran Sasso (CNGS) beam. After an intense refurbishing operation, carried out at CERN in theframework of the Neutrino Platform activities (WA104/NP01), the entire apparatus was moved toFermilab (IL, USA), where it will operate as far detector of the Short Baseline Neutrino (SBN)program [1]: three liquid argon detectors, placed along the Booster Neutrino Beam (BNB) line andoperating at shallow depth, will investigate the possible presence of sterile neutrino states.The realization of a new light detection system, sensitive to the photons produced by the LArscintillation, is a fundamental feature for the T600 operation at shallow depth. A threshold of100 MeV of deposited energy, a time resolution of the order of ≈ The ICARUS T600 detector is made of two identical cryostats, filled with about 760 t of ultra-pureliquid argon [2]. Each cryostat houses two TPCs with 1.5 m maximum drift path, sharing a commoncentral cathode made of punched stainless-steel panels. The cathode plane and field cage electrodes,composed by stainless-steel tubes, generate an ideally uniform electric field E =
500 V/cm.Charged particles interacting in liquid argon produce both scintillation light and ionizationelectrons. Electrons are drifted by the electric field to the anode, made of three parallel wire planes.A total of 53248 wires are deployed, with 3 mm pitch, oriented on each plane at a different angle(0 ◦ , ± ◦ ) with respect to the horizontal direction. By appropriate voltage biasing, the first twowire planes record signals in a non-destructive way, while the ionization charge is collected andmeasured on the last plane. The electronics was designed to allow continuous read-out, digitizationand independent waveform recording of signals from each wire of the TPC, with 400 ns samplingtime and 12-bit dynamic range [3]. The information of the ionization track occurrence time,combined with the electron drift velocity ( v ≈ . µ s at E =
500 V/cm) provides the eventcoordinate in the drift direction. The composition of the three views from the TPC wires yields thetrack projection on the anode plane. This information allows obtaining a full 3D reconstruction ofthe tracks, with a spatial resolution of about 1 mm [4].The precise information of the event occurrence time is given by the LAr scintillation lightwhich permits the generation of a light-based trigger signal and a preliminary identification ofevent topology for fast selection purposes [5]. The light information is a fundamental feature forthe identification of signals related to the neutrino beam induced events. This requires a highperformance light detection system as described in the following sections. Scintillation light emission in LAr is due to the radiative decay of excimer molecules Ar ∗ producedby ionizing particles, releasing monochromatic VUV photons ( λ ≈
128 nm) in transitions from thelowest excited molecular state to the dissociative ground state. The emitted light is characterizedby a fast ( τ ≈ τ ≈ . µ s) decay components. Their relative intensity depends on dE / dx , ranging from 1:3 for minimum ionizing particles, up to 3:1 for alpha particles. This isotropiclight signal propagates with negligible attenuation throughout each TPC volume. Indeed, LAr isfully transparent to its own scintillation light, with measured attenuation length in excess of severaltens of meters and Rayleigh-scattering length of about 1 m [6]. Because of their short wavelength– 2 –he scintillation photons are absorbed by all detector materials without reflection, leaving time andamplitude information unaffected during the photon path to the light detectors. A scintillation light detection system based on 74 ETL9357FLA (8 (cid:48)(cid:48) diameter) PMTs mountedbehind the wire chambers was adopted in the T600 detector for the LNGS run [7]. The sand-blastedglass window of each device was coated with about 200 µ g/cm of Tetraphenyl Butadiene (TPB),to convert the VUV photons to visible light. ICARUS at Fermilab will take data at shallow depth,facing more challenging experimental conditions than at LNGS. The light detection system willcomplement the 3D track reconstruction performed with the use of the TPC wires, thus contributingto identify neutrino interactions occurring in the BNB spill gate structure and rejecting the expected ≈
10 kHz cosmic background. This new environment requires a number of improvements, namelythe adoption of a PMT model with better performances, an improvement of the sensitivity downto 100 MeV, a time resolution O (1 ns) and an increase of the light detection granularity. This lastrequirement is needed to localize the track associated with every light pulse along the ≈
20 m of thelongitudinal detection direction, with accuracy better than 1 m, namely shorter than the expectedaverage spacing between cosmic muons in each TPC image. In this way, the light detection systemwould be able to unambiguously provide the absolute timing for each track, and to identify, amongthe several tracks in the LAr TPC image, the event in coincidence with the neutrino beam spill.The adoption of large area PMTs coated with TPB was considered the best solution for the lightdetection system upgrade. The use of alternative devices, such as SiPM detectors, was considerednot mature enough for applications in large volume LAr-TPC because of their small sensitive surface.
Dedicated Monte Carlo simulations were realized to design and optimize the light detection systemfor the refurbishing of the T600 detector.Initially, the focus was put on the geometrical properties of the propagation of the VUVscintillation light in ICARUS, with some simplifications on the features of the topology of theconsidered class of events [8]:1. electromagnetic (e.m.) showers, mimicking Neutral Current (NC) and ν e Charged Current(CC) interactions from BNB;2. single crossing cosmic muons, which represent the most abundant source of background;3. muons generated from ν µ CC interactions.Fine details of physical events, such as e.m. showers shape or particle multiple scattering, werefound to be less important than the spatial resolution achieved with 8 (cid:48)(cid:48) diameter devices spacedby O (1 m). Muons were schematized as straight lines, while e.m events as clusters of points with1 MeV deposited energy each. Showers energy spanned from 100 MeV to 1 GeV, to cover allthe expected energy range in the SBN configuration. Muons generated from ν µ CC interactionswere a superposition of the other two event topologies. From each point along the simulatedtrack, the proper number of photons was generated isotropically; due to the short wavelength,– 3 –Ar scintillation light is absorbed by all the detector material, so no reflection was assumed. ARayleigh scattering length of 90 cm was considered. A 5% overall Quantum Efficiency (QE) wasconservatively assumed for the PMTs, which includes wavelength shifting conversion efficiency andgeometrical factor: about 50% of the light is loss during conversion. This QE value was adoptedfollowing the working hypothesis of using ETL9357FLA PMTs coated with TPB by means of aspraying technique, as realized for the LNGS run [7]. An error of ± ±
10% uncertainty on the number of collected photons were assumed, to take into account the PMTresponse uncertainties according to experimental measurements results [8, 9].Different PMT positioning layouts with 8 (cid:48)(cid:48) and 5 (cid:48)(cid:48) diameter PMTs were considered to studythe performance both for cosmic muons and for e.m. showers in the T600 detector. Configurationswith different numbers of PMTs were also considered, starting from 27 devices (8 (cid:48)(cid:48)
PMTs) upto 210 devices (5 (cid:48)(cid:48)
PMTs) per TPC. The pattern of each layout was constrained by the existingmechanical structure of the T600: the requirement was not to change it, exploiting the free spacealready available in this structure.For what concerns the event position reconstruction, simulations were carried out to evaluatethe capabilities of the different configurations to localize the e.m. showers, mainly along the 18 mlength on the beam direction (z axis). The error on the event position reconstruction was calculatedas the difference between the actual geometrical center of the event and the one derived from theaverage on the PMT coordinates, weighted on the light collected by each PMT. The best resultswere obtained by the set of geometries with the highest numbers of PMTs, as shown in figure 1 for a)
27 (8 (cid:48)(cid:48) ) PMTs, b)
90 (8 (cid:48)(cid:48) ) PMTs, and c)
210 (5 (cid:48)(cid:48) ) PMTs. Anyway the difference among them isnot significant and performance improvements can be obtained just by refining the reconstructionalgorithm. For example, just considering in the average on the PMT position only those devices witha signal above a threshold of 10 phe, as shown in figure 1 d ) , the 90 (8 (cid:48)(cid:48) ) PMTs configuration showsa localization capability which is better than the one obtained with the 210 (5 (cid:48)(cid:48) ) PMTs configuration.A more detailed study of the PMT system performance was then carried out within LArsoft,which is a framework supporting a shared base of physics software across LAr-TPC experiments.In particular, the ICARUS T600 detector description and the particle generation and propagation inthe ICARUS volume are determined within Geant4 software which is implemented in LArsoft. TheICARUS T600 inner detectors main components (wires, PMTs, cathode, field cage) are faithfullyreproduced. Both single BNB ν µ and ν e interactions and cosmic ray samples were generatedinside the active volume of one of the two modules of ICARUS. LArsoft framework covers basiccomponents of the real scintillation detector system and includes all relevant physical processes.With the complete simulation, individual physical factors that can affect the performance of thedetector system, such as detector geometry, surface finishing, decay time and scintillation yield ofscintillator as well as the actual response of PMTs, presented in Sections 4.1 and 4.2, and front-endelectronics, are taken into account [10].The impact of the layout with 90 (8 (cid:48)(cid:48) ) PMTs for each TPC in terms of neutrino vertex localiza-tion, as obtained from LArsoft simulation of BNB ν e CC and ν µ CC events, is illustrated in figure 2:provided the timing information from all PMTs is available, an accuracy of less than 15 cm and aprecision of about 30 cm (70 cm) for ν e CC ( ν µ CC) was obtained by estimating the neutrino vertexposition from the light barycenter of the first three hit PMTs.Finally, this analysis opens the possibility to directly match the event position as determined by– 4 – igure 1 . Evaluation of the precision on the localization of the actual interaction position along the beamdirection z for e.m. showers as determined with the custom simulation. The considered layouts are: a) (cid:48)(cid:48) ) PMTs; b)
90 (8 (cid:48)(cid:48) ) PMTs; c)
210 (5 (cid:48)(cid:48) ) PMTs. Figure d) shows the result obtained with the layout with 90(8 (cid:48)(cid:48) ) PMTs by considering only those devices with a signal above a threshold of 10 phe. The FWHM and thepercentage of events localized with an error greater than 10, 20, 30 60 and 90 cm are indicated in each figure. Figure 2 . LArsoft evaluation of the precision on the localization of the actual neutrino vertex position, alongthe beam direction z, for ν e ( Left ) and ν µ ( Right ) for the layout with 90 (8 (cid:48)(cid:48) ) PMTs: a precision of about 30cm (70 cm) for ν e CC ( ν µ CC) is obtained. the analysis of the charge with light information coming from PMTs. This could help in developinga quick first level event tagging using the light signals. As an example of the localization capability,two neutrino events reconstructed from the TPC wires are shown superimposed with the map ofPMTs in in figure 3.The described MC simulations led to select the 90 (8 (cid:48)(cid:48) ) PMTs layout for installation on the– 5 – igure 3 . Examples of neutrino events simulated with LArsoft and reconstructed from the TPC wiressuperimposed with the color map of PMTs having a light signal exceeding 10 photo-electrons. Top: ν µ CC,1.2 GeV deposited energy. Bottom: ν e CC 0.9 GeV deposited energy.
ICARUS T600 TPCs. This layout implies the use of 360 PMTs with 8 (cid:48)(cid:48) diameter, correspondingto a coverage of 5% of the wire plane surface. The estimation of the number of photo-electronscollected per MeV of deposited energy in a single TPC gives an average of about 15 phe/MeV(9 phe/MeV for events close to the cathode). The possibility to adopt the scintillation light fortriggering and timing purposes with events down to 100 MeV is then assured in the whole TPCvolume.
The realized PMT layout, shown in figure 4, features 360 total Hamamatsu R5912-MOD PMTsdeployed in groups of 90 devices behind each wire chambers. Since the PMT glass windows is nottransparent to the scintillation light produced in liquid argon, each unit was coated with a properwavelength shifter re-emitting in the visible. The PMTs were installed using dedicated mechanicalsupports. A laser calibration system permits the timing calibration of the single units.In this section the ICARUS T600 scintillation light detection system is described, presentingthe main characteristics of its different components.
In order to identify the most suitable model to the requirements of the light detection system ofICARUS T600, a test campaign was carried out on different PMT samples manufactured by differentproducers, such as Hamamatsu and ETL [11–14]. All the PMTs taken into consideration feature an– 6 – igure 4 . Scheme of the adopted geometry with 90 PMTs behind each wire plane and picture of the actualconfiguration. (cid:48)(cid:48) hemispherical glass window with bialkali photocathode on platinum undercoating, to guaranteehigh performance at cryogenic temperatures. The evaluation of their conformity was based on thefollowing considerations:- The scintillation light detection system should guarantee a good sensitivity to ionizing interactionsin LAr down to an energy deposition of 100 MeV. To this end the quantum efficiency and itsuniformity over the sensitive surface of PMTs have a strong impact on the global efficiency of thesystem. The effective values of these parameters resulting from actual measurements on PMTprototypes are considered distinguishing features for the model selection.- The dynamics of the scintillation light detection system should permit the recording of thescintillation light fast component pulses and, at the same time, of single photons arriving from theslow component de-excitation. In addition it has to cope with the expected wide variation of lightintensity which depends on the deposited energy and on the geometry of interactions inside theLAr volume. Taking into account a standard electronics for PMT signal recording without pulseamplification, a gain G ≈ at cryogenic temperature is necessary to detect single photons. Inaddition the PMT dynamics should permit the generation of anode pulses without remarkablesaturation up to hundreds of photoelectrons. – 7 – igure 5 . The PMT Hamamatsu R5912-MOD. - The light collection system should be able to provide unambiguously the absolute timing of eachinteraction and identify, among the several tracks in the LAr-TPC image, the event in coincidencewith the neutrino beam spill. In order to achieve O (ns) timing resolution, fast PMT pulses areneeded. Moreover good stability of the transit time as a function of temperature and appliedvoltage, low time spread and a good uniformity over the PMT sensitive windows surface arerequired.- Since a large number of PMTs is used, the presence of a high dark count rate can affect thedetector performance inducing stochastic coincidences at trigger level. From each PMT a single-photo-electron physical background rate of tens of kHz is expected in LAr. This rate consists ofresidual photons produced from the decay of Ar or other radioactive contaminates. Thereforean intrinsic dark count rate of a few kHz at cryogenic temperatures is judged acceptable. On theother hand the absence of bursts, sparkling, lightening effects or other noise generating pulsesabove the single photon level is considered mandatory.- ICARUS T600 will operate in absence of external magnetic fields, except for the earth’s intrinsicfield. Therefore the main PMT performances should not be degraded by external magnetic fieldsof the order of gauss at different axial orientations.- The adopted devices should withstand low temperatures and the relative high pressure as expectedin LAr immersion. All the considered PMT models were subjected to a series of thermal shocksto highlight possible mechanical or cracking problems.Best results were obtained with the Hamamatsu R5912-MOD device (see figure 5) which wastherefore selected for installation. The main features and the characteristics resulting from the testsare summarized in table 1.For the upgrade of the ICARUS T600 scintillation light detection system, 400 HamamatsuR5912-MOD PMTs were procured in 2016. All the samples were tested at room temperature and60 of them were also characterized at cryogenic temperatures, in liquid argon bath during a testcampaign carried out at CERN in 2016 and 2017 [15]. All the 400 PMTs were rated compliant withthe requirements for installation in the T600. – 8 – able 1 . Main acceptance requirements for the Hamamatsu R5912-MOD
Spectral Response 300 ÷
650 nmWindow Material Borosilicate glass (sand blasted)Photocathode Bialkali with Pt under-layerMax suppy voltage (anode-cathode) 2000 VPhotocathode Q.E. at 420nm ≥ ±
5% of mean valueNumber of dynodes 10Typical Gain 1 × at 1500 VNominal anode pulse rise time ∗ ≤ ∗ ∗ − Max. transit time variation 2.5 ns (center-border)Transit time spread (RMS) 0.7 nsPulse linearity variation ∗∗ ≤
10% up to 150 phe ≤
50% up to 300 phe ∗ Values for G = × ∗∗ Values for G = . × at 87 K [14]Each device was equipped with a proper base voltage divider directly welded on the PMT flyingleads. The base circuits, entirely passive, were manufactured with SMD resistors and capacitorsable to withstand the LAr temperature. The reference voltage distribution ratio is the standardrecommend by Hamamatsu. A particular care was devoted to the choice of damping resistors toimprove the PMT time response. Detailed design and specifications are presented in reference [15]. The glass window of this PMT model is not transparent to the scintillation light produced by liquidargon. The sensitivity to vacuum-ultra-violet (VUV) photons was achieved by depositing a layer ofa proper wavelength shifter on the PMT windows.1,1,4,4-Tetraphenyl-1,3-butadiene, or TPB, is an organic fluorescent chemical compound gen-erally used as wavelength shifter, thanks to its extremely high efficiency to convert ultra-violetphotons into visible light. To obtain effective TPB layers on a large number of PMTs, ensuring atthe same time a high repeatability and reliability of the operation, a dedicated thermal evaporatorwas instrumented and a specific evaporation procedure was defined [16].The thermal evaporator consists of a vacuum chamber, 68 cm high and 64 cm diameter, closedat both sides by means of two large flanged plates . The PMT to be coated is fastened to a specificrotating support looking downwards and inclined of 40 ◦ angle with respect to the vertical direction,as shown in figure 6. The rotating structure, fixed below the chamber top plate, is connected to anexternal motor by a ferrofluid-based feedthrough allowing a rotation speeds of 10 turns/min. The The thermal evaporator and the optical test system cited in this paper were funded by the italian INFN (âĂIJIstitutoNazionale di Fisica NucleareâĂİ) and MIUR (âĂIJMinistero dellâĂŹIstruzione, dellâĂŹUniversitÃă e della RicercaâĂİ)within the PRIN (âĂIJProgetto di Rilevante Interesse NazionaleâĂİ) program. – 9 – igure 6 . Picture of the instrumented evaporator with a PMT fastened on the rotating support. vacuum chamber houses a temperature controlled
Knudsen cell , placed on the bottom plate at adistance of about 14 cm below the PMT surface. For each deposition the cell crucible was filledwith about 0.8 g of TPB and left to evaporate at a temperature of 220 ◦ for about 10 min, yieldinga uniform TPB coating of about 220 µ g/cm on the PMT sensitive surface. This density valuewas proven to guarantee a high conversion efficiency and the absence of adhesion instabilities onsand-blasted glass at cryogenic temperatures after immersion in LAr [17].The effectiveness of this technique from the point of view of deposition uniformity and lightconversion efficiency was validated by simulations and experimental tests before being acceptedfor the TPB coating of the 360 Hamamatsu R5912 of ICARUS T600 light detection system. Thetreatment of a total of 365 PMTs was carried out at CERN Technology Department in around 120working days. The distribution of the resulting TPB coatings is shown in figure 7.The effective value of the quantum efficiency at λ =
128 nm and its uniformity as a function ofthe position on the photocathode window was measured by means of an optical test system on 10PMT samples. The quantum efficiency was evaluated by comparing the currents given by the PMTunder VUV illumination and by a reference calibrated photodiode . Values are distributed in the0 . ÷ .
15 range, with an average value of 0.12, while for each PMT the uniformity results to be Light at λ =
128 nm is generated by a 30 W deuterium lamp (McPherson 632) and selected by a monochromator(McPherson 234/302). The reference is a NIST calibrated photodiode. – 10 – igure 7 . Distribution of the resulting TPB coating densities. Each sample is related to a PMT evaporationrun of the series production. In addition to the needed 360 PMTs, 5 more samples were coated as spare units.
Figure 8 . (Left) Distribution of quantum efficiency resulting from the measurement of 10 PMT samplesafter the TPB coating by evaporation. (Right) Example of measurement of quantum efficiency variation as afunction of the radial distance from the center. within ± Each of the two T600 LAr cryostats features a mechanical structure that sustains the differentinternal detector subsystems and the control instrumentation. The three wire-planes of each TPCare held by a sustaining/tensioning frame positioned onto the longitudinal side walls of the cryostat.The stainless-steel supporting structure has dimensions of 19.6 m in length, 3.6 m in width and3.9 m in height, subdivided in 9 sectors, 2 m long each. PMTs are located in the 30 cm space behindthe wire planes, 10 unit for each frame sector, as shown in figure 9.The PMTs are mounted onto the mechanical frames by means of PEEK TM holders in form ofslabs with dimensions of 350 ×
250 mm , 10 mm thick, held up by stainless-steel bars 3 m long, as– 11 – igure 9 . Drawing of the PMT positioning behind the wire planes. Units of measurement are millimiters.Three frame sectors are displayed. shown in 9. The support system allows the PMT positioning behind the collection planes ≈ A negative power supply is adopted and two independent coaxial cables are used to provide eachPMT with high voltage and to read out the anode signals. Signal cables are RG316/U, 7 m longwith a BNC connector on one end. High voltage cables are HTC-50-1-1, 7 m long, with a SHVconnector on one end. The non-terminated ends of the cables are directly soldered on the PMTbases.For each group of 10 PMTs mounted in the same frame sector, two bundles of 10 signal cablesand 10 power supply cables are deployed along the mechanical structure up to the frame top, asoutlined in figure 12. Each bundle is then driven through a stainless steel chimney (20 cm diameter,1 m long), vertically mounted on the detector roof. The top edge of each chimney hosts a set offeedthrough flanges for the interconnection of various elements of the detector (PMT signal andpower supply, wire signals and biasing, optical fibers and sensors). The PMT flanges are from– 12 – igure 10 . Picture (left) and CAD drawing (right) of the PMT support.
Figure 11 . Picture of the PMT supporting system. – 13 – igure 12 . Outline of the PMT cables deployment along the mechanical structure and their distribution onthe top chimneys.
Table 2 . Flange main characteristics
Flange type DN100CF DN100CFNumber of feedthroughs 10 10Connection type (Int/Ext) BNC/BNC SHV/SHVImpedance 50 Ohm 50 OhmShield type Grounded GroundedVoltage 1000 V 6000 VMin. temperature − ◦ C − ◦ CVacuum UHV UHVAllectra Ltd., each hosting 10 SHV-SHV or 10 BNC-BNC feedthrough connectors mounted onstainless-steel DN100CF high vacuum flanges. A photo showing the assembly of the PMT flangesis shown in figure 13. The main characteristics of the flanges are listed in table 2.The electrical connection between PMT flanges and electronics, located in a building alcoveadjacent to the detector, consists of 360 signal cables (RG316/U with BNC-MCX termination) and360 high voltage cables (RG58/U with SHV-SHV termination) deployed on cable-trays. In orderto guarantee uniformity among the different channels, all the cables are 37 m in length. The actualtotal cable length from PMT base to electronic channel input is 44 m. A detailed study on the effectsof the use of these extremely long cables shows a reduction of the high frequency components ofthe PMT signal resulting in an increase of the rise-time to 8 ns [18]. These effects are includedin Monte Carlo simulations described in Section 3.3, to evaluate possible effects on the systemperformance. – 14 – igure 13 . Pictures showing the assembly of the PMT flanges. Signal cables and BNC flanges are shown inthe top pictures, while HV cables and SHV flanges are presented in the bottom pictures.
PMT electronics is designed to allow continuous read-out, digitization and independent waveformrecording of signals coming from the 360 PMTs of the light detection system. This operationis performed by 24 V1730B digitizers. Each module consists of a 16-channel 14-bit 500-MSa/sFLASH ADC. The 2 Vpp input dynamic range well fits the PMT response in terms of linearity andsaturation (see table 1). In each board 15 channels are used for the acquisition of PMT signals,while a channel is left for possible future implementations. During the acquisition, data stream ofeach channel is continuously written every 2 ns in a circular memory buffer of 5kSa, correspondingto 10 µ s , allowing the recording of both components of the LAr scintillation light, i.e. photonsfrom fast and slow decays of exited excimers to ground state, as described in Section 3.1. Whena boards receives an external trigger request, the active buffers are frozen, writing operations aremoved to the next available buffers and stored data are available for download via optical links .The amplitude of the prompt signals from the fast component of the scintillation light, withoutany pulse integration, is exploited for trigger purposes. To this aim, V1730B boards generate apattern of digital pulses (200 ns, LVDS logic standard) mapping the PMT signals exceeding digitallyprogrammed thresholds, set to few photoelectrons [19]. The ICARUS Trigger System, which will The total memory available for each channel is 5.12 MSa, divisible into a maximum of 1024 buffers. Data read out is based on the CAEN proprietary CONET2 (Chain162 able Optical NETwork) protocol allowing upto 80 MB/s data transfer. – 15 –e described in dedicated papers, generates a first level PMT trigger pulse whenever the coincidenceof the discriminated PMT signals satisfy a defined multiplicity inside a neutrino beam gate window.For generation and distribution of high voltages, the same power supply system designed byICARUS for the LNGS run is adopted. For each cryostat, housing 180 PMTs, a primary -2000 V isgenerated by a BERTAN 210-02R. The primary voltage is finely regulated and distributed to 180PMTs by four CAEN A1932AN boards, 48 channels each, housed in a CAEN SY1527 crate. Thelinear technology employed in this system results in extremely low output ripple, as demonstratedduring the ICARUS data taking at LNGS. This is a fundamental feature required for the lightdetection system to prevent the induction of PMT noise onto the wire planes. In order to performa study on the performance of the ICARUS PMT electronics and other detector subsystems beforethe final detector operation at Fermilab, a LAr test facility was instrumented at CERN [20].The apparatus consists of a 1.5 m cylindrical cryostat filled with LAr and instrumented with 10Hamamatsu R5912-MOD PMTs, 6 of them coated with TPB. Scintillation light data were takenby exposing the system to cosmic rays. Figure 14 shows an example of PMT signal taken withthis facility, demonstrating the capability of detecting both components of scintillation light anddemonstrating the required performance of the electronics. To identify interactions associated to the neutrino beam and reject the expected huge cosmicbackground, the occurrence time of each event should be reconstructed with a resolution better than1 ns. This can be achieved by means of a precise determination of the time delay of the responseof each PMT, that may drift in time for temperature excursions, power supply variations or otherreasons. The monitoring of the timing values during data-taking can be accomplished with cosmicrays or by delivering a fast calibration pulse to each individual channel.A fast-laser based calibration system has been developed for the time calibration and monitoringof each PMT channel. Its layout is outlined in figure 15, while additional information on theemployed components can be found in reference [21]. Fast light pulses (60 ps FWHM, 120 mWpeak power, emission at 405 nm) are generated by a laser diode (Hamamatsu PLP10) settled in thebuilding electronics alcove. Light pulses feed, through 50 µ m patch cables and an optical switch(Agiltron Inc.), 36 optical flanges mounted in the same 36 chimneys used for the PMT signal cables,on the opposite site of the BNC flanges. The main characteristics of the optical flanges are shownin table 3. Inside each chimney, a 1 ×
10 optical splitter (Lightel Technologies Inc.) delivers theinput laser signal to 10 (50 µ m, 7 m long) injection fibers deployed along the mechanical frames,to convey the calibration signal to each PMT, as shown in figure 16.A specially shaped stainless steel pipe (2.5 mm diameter, 20 cm long), fixed inside the PMTsustaining structure, drives the end section of each optical fiber and allows the light focusing on thedevice windows, as shown in figure 17.The laser pulses should be delivered to the PMT photocathodes with minimal attenuation andwithout deterioration of the original timing characteristics. Extensive tests were performed onthe different components at both room and cryogenic temperatures, to ensure that selected itemscomply with these requirements [21]. Figure 18 shows the distributions of time delay and fraction This work was carried out in the framework of the CERN Neutrino Platform WA104/NP01 activities. – 16 – igure 14 . Example of PMT signal (absolute value) recorded with the CERN 10-PMT facility in linear ( Up )and logarithmic ( Down ) scale. The presence of both the slow and the fast components of the scintillationlight can be noticed.
Figure 15 . Diagram of the laser calibration system. – 17 – able 3 . Optical flange main characteristics
Manufacturer VACOM GmbHFlange type DN40CFConnection type (Int/Ext) FC/FCinside fiber MM 50 µ m core; NA 0.2Min. temperature − ◦ CMax. temperature 75 ◦ CVacuum UHV
Figure 16 . Left picture: DN200CF side of one nibble with a 1 ×
10 splitter (on the right) and a 10-channelpatch panel to connect the internal patches to the outputs of the splitter (on the left). Right picture: frontview of the DN200CF to DN40CF nibble with a mounted FC/FC optical feedthrough.
Figure 17 . Picture showing the stainless steel pipe which drives the end section of the calibration opticalfiber toward the PMT window. A blue-light laser spot on the PMT surface can be noticed. of transmitted light, obtained from measurements at room temperature on the initial sample of410 injection patches, using the laboratory setup of reference [22]. The delay dispersion over the7 m cable length is within 90 ps, with an average value of about 45.59 ns, while the dispersionon the transmission is around 8% with an average value of ≈ bath) shows that transmission and delay are similar to the ones measuredat room temperature.The final sample of 360 internal injection patches was then selected, requiring fibers with the– 18 – igure 18 . a) distribution of 7 m injection patches time delay; b) distribution of transmission for the samesample. highest transmission and similar delays.The numbers here presented for the laser system only concern the components within thecryostats, which are relevant for the purpose of this paper, while the main characteristics of theexternal system are presented elsewhere [21]. The installation of the various components is stillin progress and did not allow a precise calibration of the entire system on a channel-by-channelbasis. Anyway, the different test results indicate that the expected performances of the lasercalibration system, such as an intrinsic time calibration resolution of about 100 ps on the singlePMT channel [22], well fit the calibration requests for PMT time equalization at the level of 1 ns [1].The complete description of the laser system performances will be the subject of a forthcomingpaper. The full light detection system was tested at Fermilab after installation, in order to check thefunctioning of all the PMT channels, evaluate their performance before cooling down, evaluate theeffectiveness of the internal optical-fiber light-distribution system. To this purpose, a subset ofthe final electronic chain was used. This allowed gaining experience on how to program the newelectronics and how to synchronize it with other detector subsystems, such as the trigger and theTPC electronics. Test data were recorded with triggers generated by means of a pulse generatorwith and without the combination of laser light pulses through the optical-fiber light-distributionsystem. An example of PMT signal shape recorded in combination with a laser light pulse and a10 µ s random trigger acquisition are shown in figure 19.A first quick check was carried out to highlight possible problems related to the detector transferto Fermilab. We found 351 working PMTs, a PMT sparking when applied voltage exceeded 957 V,two dead PMTs, and 6 PMTs with some issue when illuminated with the laser source. These PMTsare are undergoing further investigation.The PMT signal analysis was mainly focused on the gain calibration and dark count rate. Thegain calibration of the working PMTs was carried out by acquiring PMT waveforms at a minimumof 3 voltage points. The gain was evaluated by fitting the charge distribution with the analytical– 19 – igure 19 . Left : example of actual PMT signal shape recorded in combination with a laser pulse.
Right : a10 µ s off-light recording showing two random single-electron pulses. Figure 20 . Left : distribution of voltages to attain a gain G = for a set of 351 PMTs. Right : distributionof voltage variation (percentage) with respect the calibration performed at CERN [15]. The results areconsistent with a standard deviation of about 5%. expression described in [23]. In figure 20, the distribution of the applied voltage needed to attaina gain of G = for the 351 working PMTs is shown. Results are consistent with a standarddeviation of about 5% with respect the calibration performed at CERN [15].The dark rate of PMTs was evaluated at Fermilab by counting the number of random singlephotoelectron pulses using light off data and dividing the total number by the effective acquisitionlive time. An overall average rate of about 1.6 kHz was found. This value is consistent with theaverage rate of 2.1 kHz previously measured at room temperature before the installation of thePMTs, taking into account that a different measurement technique was adopted at CERN [15]. The new scintillation light detection system for the ICARUS T600 LAr-TPC, realized for its operationat Fermilab in the context of the SBN program, includes the use of 360 large area PMTs mountedbehind the wire planes and a fast-laser calibration system. The high performance of this detectionsystem in terms of sensitivity, granularity and time resolution, will allow ICARUS to cope with thelarge cosmic ray background by identifying the events associated with the neutrino beam.– 20 –o this purpose the system was extensively simulated, the components were precisely charac-terized and the installation procedures and techniques were carefully defined.Preliminary tests carried out after transport and installation of the apparatus at Fermilab verifiedthe performances of the light detection system required for the identification of signals related toneutrino beam induced events.
Acknowledgment
This work was funded by INFN in the framework of the CERN WA104/NP01 program finalizedto the overhauling of ICARUS detector and was supported by the Fermi National AcceleratorLaboratory under US Department of Energy contract No. DE-AC02-07CH11359. The researchactivities were also subsidized by the EU Horizon 2020 Research and Innovation Programme underthe Marie Sklodowska-Curie Grant Agreement No. 822185.
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