The KM3NeT Digital Optical Module: a multi-PMT approach

NEUTEL 2025 The KM3NeT Digital Optical Module: a multi-PMT approach Rea I. C. on behalf of the KM3NeT collaboration 3 Neutrino telescope map & design The extremely low neutrino cross-section and low fluxes of high-energy neutrino motivated the scientific community to instrument bodies of water and ice around the World with 3-dimensional grids of light detectors. Rea I.C. GENERAL DESIGN •Large volumes of water instrumented with arrays of detection units: vertical string-like structures coupled to electro/optical cables, equipped with optical modules. •The detection units, with a typical length of several hundreds of meters, are distributed in specific geometries, optimized for the targeted energy region. •The optical modules can host one or more PMTs, with different photocathode areas. R&D 4 The KM3NeT neutrino detector Currently under construction: at two different sites in the Mediterranean Sea •ORCA (Oscillation Research with Cosmics in the Abyss) 40 km offshore Toulon, France at 2500 m depth •ARCA (Astroparticle Research with Cosmics in the Abyss) 90 km offshore Portopalo di Capopassero, Sicily at 3500 m depth Same technology / different geometry / different physics: the main difference is the density of photosensors (different spacing) Rea I.C. 5 •KM3NeT/ARCA: high-energy neutrinos (Eν~ GeV-PeV). Main physics goal: identifying astrophysical sources of high energy neutrinos →1 km^3 of instrumented water Planned DUs: 230 (currently 51 DUs) •KM3NeT/ORCA: low-energy neutrinos (Eν ~ MeV - GeV). Main physics goal: determination of the neutrino mass ordering through neutrino oscillation studies →7 megatonnes of instrumented water Planned DUs: 115 (currently 28 DUs) The KM3NeT neutrino telescope 5 MEV GEV TEV PEV Supernova explosion Neutrino oscillations Dark matter HE neutrinos ARCA ORCA Rea I.C. The KM3NeT DU design The Detection Unit of KM3NeT (DU) is a mooring line (700 m for ARCA and 200 m for ORCA) consisting of: •1 Buoy •2 Dyneema ropes •18 DOMs •1 Anchor •Oil-filled electro-optical backbone cables with 18 optical fibers and 2 copper wires for long-distance communication and power Starting design requirements for KM3NeT detector: •instrument gigatonnes/megatonnes of water to detect corresponding neutrino fluxes •high angular resolution •operational lifetime of at least 15 years (possibly more) •70 m absorption length and 100 m scattering length at 440 nm (ARCA) → sparse instrumentation is sufficient 6Rea I.C. The KM3NeT multi-PMT design Implementation of the multi-PMT design: •Cost efficiency (& high reliability): use of off-shelf components (as much as possible) •Scalability: uniform module design •Distributed production: module assembly process distributed to multiple production sites 7Rea I.C. The Digital Optical Module (DOM) has an innovative design: - high-pressure resistant glass sphere - dense packing of 31 PMTs -calibration devices (positioning and timing) -electronics for power, readout, data acquisition and transmission - low electrical power consumption: DOM ~ 7 W •The KM3NeT multi-PMT optical module, 2022 JINST 17 P07038 The KM3NeT multi-PMT design 8Rea I.C. Main features: •the segmented photocathode area of about 1200 𝒄𝒎𝟐 in each sphere increases the sensitivity of each DOM for the incoming direction of the detected photons → broad angular coverage and good photon counting performance •the nanosecond accuracy of photon arrival time helps the reduction of the background coming from K-40 decay and bioluminescence •the impact of a PMT failure on the performance of the telescope is lowered as the module can still be operated efficiently with fewer PMTs (still 97% of detection efficiency) The KM3NeT multi-PMT design 9Rea I.C. Photodetection: the PMT •Convex bialkali photocathode, 3’’ diameter, 10-stage dynode structure: Hamamatsu R12199-02 and R14374 → performance improvement (lower TTS) •Collection ring: polished metal, 45° around the head of the PMTs, convex shape, 92% reflectance for photons in the wavelength range 375-500 nm, acceptance increased by 20-40% •Custom-designed active base: - HV generated by Cockroft –Walton circuit - analog pulse → charge amplifier → digitization by a circuit called PROMiS (PMT Read Out Mixed signal) → comparator discriminates the PMT signal vs. a tunable threshold (0.3 p.e.) The output level is kept «high» for the time that the PMT signal remains above the threshold → this Time over Threshold (ToT) is then stored and sent to shore ❑Negative HV on the photocathode. ❑An insulating coating on the outside and on the PMT bases to avoid electrical discharge between PMT and the surroundings. 10Rea I.C. •Expansion cone for the 3-inch PMTs of the KM3NeT optical modules, JINST 8 (2013) •Characterisation of the Hamamatsu photomultipliers for the KM3NeT Neutrino Telescope, JINST13 (2018) •A method to stabilise the performance of negatively fed KM3NeT photomultipliers, JINST 11 (2016) Production model: the integration Staff people from many institutes and groups in the kme3collab participate in the construction of the telescope. Then the copper wires from the penetrator are connected to the power board and the optical fiber is spliced to the add and drop filter connecter to the tranceiver. After installing the nanobeacon and the pressure gauge, the PMT support structure are equipped with PMT adn light collectio ring. The pigtail of the PMTs are conected to the octopus. A functional test of the two hemispheres is pefomerd in order to check if all components of the DOMS are ok as this is the last chance to change the faulty components. Finally yhe gel is poured, and then the DOM is closed . A final accept test is performed by placing the optical module in a dark box to test pmts. This test provides the input for the decision whether to accept or reject the optical module against a set o prefefined criteria. non conformity report is opened and the corresponding component is temporaryily removed until the problem is resolved. 3 ways to resolve: waiving the non conformity, the non conformity is corrected, the component is discarded Integration process: 1) Glueing the cooling mushroom (top) and acoustic piezo (bottom) 2) Electronics, penetrator → helium leak test, optical fiber splicing 3) Installation of PMT and light collection rings 4) Functional test 5) Gel pouring 6) DOM closure (custom made tool) 7) Acceptance test in a dark box 17 •All components are registered in a central database with a unique product identifier code, UPI. •The history of the components and the test values are tracked in the database: all the info can be recovered from the database during the detector operation and data analysis. Rea I.C. Production model: the ‘’Detector Integration Assistant’’ (DIA) Integration sheet A dedicated software, the KM3DIA - Detector Integration Assistant - , guides the integrators through the right sequence of operations, logs all relevant information and registers all DOM details in the database 18Rea I.C. Minimisation of the error Check all components Problem? Non-conformity report (NCR) Production model: the DIA Acceptance test sheet high voltage tuning… 19Rea I.C. DOM performance •Multifold coincidences in the single DOMs are shown and compared to the rates predicted by the Monte Carlo simulation. •Even with a single DOM it is possible to reject the background and identify muon-induced signals by selecting high multiplicity (≥6) coincident hits. •The general pattern in all three DOMs clearly shows a higher hit frequency on the top hemispheres of the DOMs, reflecting the fact that atmospheric muons come from above and demonstrating the directional sensitivity of the DOMs. 20 With a single optical module it is possible to distinguish the background (40K and bioluminescence) from atmospheric muons by exploiting multi-PMT coincidences. Rea I.C. The prototype detection unit of the KM3NeT detector, Eur. Phys. J. C 76 (2016) 54 DOM performance The ToT response of the PMTs detecting multiple photons has been studied in lab measurements With an analytic model is possible to obtain the gain from the ToT measurement. With the model, the PMT gain can be monitored in situ, and, if needed, it is possible to adjust the High Voltage (HV) by tuning it. 21 Lab measurement of time-over-threshold response of the Hamamatsu R12199-02 PMT to different numbers of photoelectrons To minimize the effect of aging (and hence to maximize the lifetime of the PMTs), given the typical rate of the deep-sea environment, a low nominal gain of 3 ∗ 106 has been chosen (ToT of 26.4 ns). The operational gain is obtained by tuning the HV of each PMT in situ during the telescope operation. Rea I.C. •The KM3NeT multi-PMT optical module, 2022 JINST 17 P07038 Thanks for the attention Rea I.C.