Super-Kamiokande Detector
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Super-Kamiokande Detector Mauro Mezzetto INFN – Sezione di Padova XXV Giornate di Studio sui Rivelatori, Cogne, 23 febbraio 2016 Few initial remarks • I’m not a member of SK collaboration. I’m a member of the T2K collaboration, which is a «user» of SK. • SK had been designed in the early 90’s, so it’s not at the leading edge of technology • However its performances are outstanding • … and the physics results are «more than outstanding» • I took material from published papers and from three lectures: – T. Kajita lessons at the Nufact ‘05 School, Capri, 2005 – Y. Koshio «Overview of the Calibration System in SK», 2012 – S. Yamada «Commissioning of the new electronics and online system for the Super-Kamiokande Experiment”, 2009 PremiNobel Nobel Prizes alla to Neutrinofisica dei Physicsneutrini 1995: for the detection of the neutrino Fred Reines 1988: for the neutrino beam method and the demonstration of the doublet structure of the leptons through the discovery of the muon neutrino Lederman Stainberger Schwartz 2002: for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos Ray Davis Koshiba 2015: for the discovery of neutrino oscillations, which shows that neutrinos have mass Kajita Mc Donald To my knowledge: SK is the only hep experiment to be awarded twice Breakthrough Prize 2015 Atsuto Suzuki Yifang Wang Yoichiro Suzuki Kamland Daya Bay Super-Kamiokande Takaaki Kajita K. Nishikawa Art McDonald Kam-Biu Luk Super-Kamiokande K2K & T2K SNO Daya Bay The origins: KamiokaNDe • Kamiokande had been designed (early 80’s) mainly to search for proton decay. • Proton decay is the main signature of Grand Unified Theories (GUT): barionic number conservation is an accidental low in the standard model (not ralated to any gauge symmetry) and should be violated in higher unification schemes 29 • At the time the estimated decay time was τp=10 years, needed order of 1kton material (1 kton of water contains 3 x 1032 protons), KamiokaNDe was 3 kt. • Atmospheric neutrinos came into game because they are a natural background for proton decay. • Soon after the commissioning (1985) the collaboration worked hard to decrease the detection threshold in order to be sensitive to solar neutrinos. • And few weeks after the low threshold run it detected neutrinos from the SN1987A supernova explosion • The great successes of KamiokaNDe and the need of more statistics to achieve the necessary sensitivity convinced the collaboration to propose a 10 times bigger detector: Super-Kamiokande, which started data taking on 01/04/1996 Super-Kamiokande 11,146×(50cm φ PMT) : Inner detector 40% photo-cathode coverage Number of observed Ch. photons ～ 6 /MeV (excluding scattered or reflected photons) 1,885×(20cm φ PMT) : Outer detector (Less photo-cathode coverage: Only needed to identify muons) 2m active detector region + 0.6m layer (no photon detection) 50,000 ton water Cherenkov detector γ (and neutron) shield (Fid. Mass is 22,500 tons) Physics in Super-Kamiokande Solar neutrinos: energy spectrum down to 4 MeV. Requires extremely good energy scale, maximum PMT coverage with sensitivity to the single photoelectron, good timing. Atmospheric neutrinos: energy spectrum from 100 MeV to several GeV. Electron events are measured by calorimetry, muon events by range (but SK contains muons up to 1.4 GeV) Beam neutrinos: energy spectrum from 200 Mev to 3 GeV. Require good PID performances (e- vs π0 and e- vs µ-). Requires good direction reconstruction (see later) SuperNovae neutrinos: in a galactic SN explosion order of 105 events are expected in 10s. Strong constraint to electronics and DAQ. Proton decay: requires zero background. Extreme performances in PID and energy reconstruction. Relic SN Neutrinos Indirect Dark Matter searches. Achievements Solar neutrinos: first experiment to detect energy spectrum and direction of solar ν. Nobel Prize 2002 and Breaktrhrough Prize 2015 Atmospheric neutrinos: discovery of neutrino oscillations: Nobel Prize 2015 and Breaktrhrough Prize 2015 SuperNovae neutrinos: detection of the neutrinos emitted by SN1987A. A crucial test of SN Models. Nobel Prize 2002 and Breaktrhrough Prize 2015 Beam neutrinos: First confirmation of atmospheric neutrinos with a long baseline experiment: K2K. First direct detection of oscillation appearance and first measurement of θ13 by T2K. Both awarded with the Breaktrhrough Prize 2015 Proton Decay: the «GUT Killer»: all the simplest (and most elegant) GUT models killed by the SK limits. Susy severely matched. Indirect DM searches: via DM annihilation in the Sun producing neutrinos. The best limit for spin dependent interaction models. A Sun picture taken from deep underground … Detecting Cherenkov photons Number of Ch. photons with λ=300- 600 nm emitted by a relativistic particle per cm = 340. Need an efficient detection of the electron photons. Large PMTs Photomultiplier tube (PMT) 50cm φ ν (Super-K) 20cm φ 1 (SNO) cosθ = nβ n (refractive index)=1.34 in water θ=42deg. for β=1 T. Kajita lectures at Nufact 05 school, Capri. Detecting Cherenkov photons and event reconstruction lectron PMT ν Color: timing Size: pulse height Time: vertex position Multiple direction Coulomb scattering Pulse height (number of pe’s): energy T. Kajita lectures at Nufact 05 school, Capri. Photomultipliers (Hamamatsu R3600) NIM A329 (1993) 299-313, NIM A501 (2003) 418-462 HV system by CAEN Quantum Efficiency Transit Time Single p.e. SNO: the water Cherenkov exp that shared with SK the Nobel Prize 2015 Various types of atmospheric neutrino events (1) ・Both CC ν and νµ (+NC) FC (fully contained) e ・Need particle identification to separate νe and νµ ν ・12,000events Outer detector (no signal) Single Cherenkov Single ring electron-like Cherenkov event ring muon- like event Color: timing Size: pulse height Various types of atmospheric neutrino events (2) Signal in the PC (partially contained) outer detector ν ・97% CC νµ ・900 events T. Kajita lectures at Nufact 05 school, Capri. Various types of atmospheric neutrino events (3) ・ almost pure CC ν Upward going µ muon ・1800 throught muons ・400 stopping muons ν Upward Upward stopping through- muon going muon Particle identification electron-like muon-like event event e: electromagnetic shower, µ: propagate almost straightly, multiple Coulomb scattering loose energy by ionization loss Difference in the event pattern 2 p.e.(obs'd) − p.e. (expected) Particle ID χ 2 = e or µ ∑ σ θ <70deg p.e. Particle ID results Cosmic ray μ e from μ decay ε=99% Beam Neutrino energy reconstruction Direction of neutrino = known Quasi-elastic µ- For quasi-elastic events: ν + n → µ + p µ (E , p ) θµ µ µ Eµ, θµ Eν ν p 2 mN Eµ − mµ 2 Eν = mN − Eµ + pµ cosθ µ Background (non-quasi-elastic int.) - νµ + n → µ + p + π µ θ (Eµ, pµ) Quasi- µ ν Elastic π p Non-QE Below threshold T. Kajita lectures at Nufact 05 school, Capri. Eν (reconstructed) – Eν (true) T2K experimet Detect νµ−νe oscillations to measure θ13 and δCP • Beam neutrino from the J- Parc 30 GeV syncrotron • SK at 295 km, off-axis with respect to the neutrino direction • A sophisticated close detectors system (280 m from the target) to measure the neutrino beam before oscillations Data taking started in 2009 Long shutdown for the 2011 tsunami Taking data First experiment to detect νµ−νe oscillations and measure θ13 Water purification system The key feature to detect solar and SN neutrinos Air Purification Calibration Nucl.Instrum.Meth. A737 (2014) 253-272 Requirements • Number of photo-elections and arrival timing for each PMT • Understand water quality in detail • Systematic errors of reconstruction: energy, position, direction, efficiency • Long term monitoring of PMT gain, water condition, energy scale etc. Pre - Calibration Aim: have all the PMT’s tuned to the same overall gain (QE x HV gain) • 400 PMT’s had been prepared with precise gain measurement before SK start • Installed uniformely in the detector • HV of the other PMT’s adjusted thanks to the Xenon flush lamp Initial calibration Number of photo-electrons Electronics calibration • Relation channel vs pC HV adjustment • Used ~ p.e. light for each PMT • Adjusted QE x HV gain Gain measurement QE measurement • At the single p.e. level • Absolute/Relative PMT gain Charge with p.e. for data Charge with p.e. for MC • Fine tuning for PMT correction efficiency HV adjustment • Light source (Xenon lamp + Scintillator ball) at the center of the detector • Adjust number of photo-electorns in all PMTs to reference PMT (light non uniform in all the PMTs due the cylindrical shape of SK, light reflections and water transparency) • QE x HV gain is then equalized Gain adjustment • Relative gain of each PMT measured by the ratio of low/high intensity of laser light (30 levels of intensity). Correction of observed photo-electons for each PMT • Absolute gain averaged for the whole detector determined by the single p.e. peal by Nickel calibration • Problem: electronics pedestal drift in time Detector calibration by an electron LINAC Precise calibration of absolute energy scale, energy resolution, and angular resolution using electron LINAC. ENDCAP 0.1mm thick Ti window • Beam energy: 5 ~ 16 MeV/c • Beam energy spread: < 0.5 % Nucl.Instrum.Meth. A421 (1999) 113-129 16N calibration With a Deuterium-Tritium (DT) generator Data taken at various positions in the detector. Uniform direction complementary to LINAC calibration. 16 16 DT generator D+T→He+n n+ O→p+ N(τ (14.2 MeV) 1/2 = 7.13 s) 16N decay (precisely known): 106 n/pulse 6.129MeVγ + 4.29MeVβ (66.2%), 10.419 MeV β(28.0%), etc. ~1%