RICH with sipms Dino Tahirović Queen Mary, University of London PPRC Seminar, 30 September 2016

Outline Particle identification Ring Imaging Cherenkov (RICH) counter RICH with SiPMs Outline Particle identification Ring Imaging Cherenkov (RICH) counter Prototype module with Silicon Photomultipliers (SiPM) RICH in test beam Light concentrators performance Conclusion

Particle identification RICH with SiPMs Particle identification

Particle Identification RICH with SiPMs Particle Identification What does it mean to identify the particle? Particle is uniquely identified if its mass m and charge Z are known Charge can be determined by the sign of curvature of particle track in magnetic field The mass can be obtained from the relation for the momentum p in a magnetic field: 𝑝=𝛾 𝑚 𝛽 𝑐 The curvature of the track determines the momentum p The unknown quantity is the velocity β Different physical effects exploited to determine it Which effect? Depends on specific experiment.

Belle II Experiment An experiment dedicated to flavour physics studies RICH with SiPMs Belle II Experiment An experiment dedicated to flavour physics studies 40 times higher luminosity (8× 10 35 cm-2 s-1) than its predecessor, Belle Physics programme includes: Precision measurements (angle γ or φ3 in the unitarity triangle) Rare decays Identification of final particles is crucial for flavour studies

Belle II Spectrometer Colliding experiment RICH with SiPMs Belle II Spectrometer Colliding experiment Asymmetric energies, electrons (7 GeV) and positrons (4 GeV) Energy in the centre of mass is equal to the mass of ϒ(4S) resonance ϒ(4S) decays almost exclusively to B-mesons Branching ratios (PDG2014): 𝐵 0 𝐵 0 : 48.6 ± 0.6 % 𝐵 + 𝐵 − : 51.4 ± 0.6 % Study of b-quark

Belle II Spectrometer Particle identification in the forward region RICH with SiPMs Belle II Spectrometer Particle identification in the forward region

Belle II Inner Detector Systems RICH with SiPMs Belle II Inner Detector Systems PiXel Detector (PXD) Silicon Vertex Detector (SVD) Central Drift Chamber (CDC) Time-Of-Propagation (TOP) detector Aerogel Ring Imaging Cherenkov (RICH) Counter Immersed in magnetic field 𝐵=1.5 T

Kinematical Region of Interest RICH with SiPMs Kinematical Region of Interest Typical processes where kaon vs. pion separation is needed: Tagging process B D(*) π (“golden channel”) D(*) decaying to K, π Quark level b c s Two body B-decay (a rare process) Quark: b s or d Final K have momentum 𝑝∈ 1, 4 GeV/c Good method for PID in this kinematical region? Distribution of kaon momentum p vs. cos θ for two typical processes

Energy Loss Bethe-Bloch formula In the same material: RICH with SiPMs Energy Loss Bethe-Bloch formula In the same material: − 𝑑𝐸 𝑑𝑥 = 𝑍 2 𝑓 𝛽 Determine the velocity of the particle, 𝛽 Reproduced in Kleinknecht, p. 143 Material: 1 cm thick Ar:CH4 80:20 In the region of interest it is impractical (or impossible) to employ this method Normalized energy loss in the same material. From: J. N. Marx and D. R. Nygren, ”The Time Projection Chamber”, Physics Today 31, 46 (1978).

Time of Flight Time it takes particle to fly between two counters: RICH with SiPMs Time of Flight Time it takes particle to fly between two counters: Δ𝑡= 𝐿 1 𝛽 1 𝑐 − 𝐿 2 𝛽 2 𝑐 An impractically large detector needed Therefore, some other PID method has to be used Goal: > 3 σ Simulated pion vs. kaon separation (in σ) using energy loss and time-of-flight methods. From: K. Abe et al., Letter of Intent for KEK Super B Factory (KEK Report, 2004).

Ring Imaging Cherenkov counter RICH with SiPMs Ring Imaging Cherenkov counter

Particle Identification by Ring Imaging RICH with SiPMs Particle Identification by Ring Imaging Cherenkov radiation Cherenkov angle: cos 𝜃 𝐶 = 1 𝑛𝛽 Kinematical range: 𝑝= 1,4 GeV/c Belle II separation goal: separation better than 4σ Uncertainty on 𝜃 𝐶 should be smaller than: 𝜎< 23.5 mrad 4 =5.9 mrad Calculated Cherenkov angle as a function of momentum p for aerogel with ref. index n=1.485

Proximity Focusing RICH RICH with SiPMs Proximity Focusing RICH Aerogel radiator width d Proximity focusing, no optical system Uncertainty 𝜎 𝑡𝑟𝑎𝑐𝑘 depends on number of detected photons: 𝜎 𝑡𝑟𝑎𝑐𝑘 = 𝜎 𝑠𝑖𝑛𝑔𝑙𝑒 𝑁 𝑑𝑒𝑡𝑒𝑐𝑡𝑒𝑑 Emitted photons 𝑁∝𝑑∙ 𝑠𝑖𝑛 2 𝜃 More emitted photons  increase d Increase in 𝜎 𝑠𝑖𝑛𝑔𝑙𝑒 due to emission point uncertainty Solution: non-homogeneous aerogel produces more photons, without loss of resolution

Baseline Photon Detector in Belle II RICH with SiPMs Baseline Photon Detector in Belle II Hybrid Avalanche PhotoDiode (HAPD) Combination of a vacuum photomultiplier and a photodiode 6×6 readout channels on a chip, each with active area of 5×5 mm2 Operation principle: Photoelectron is accelerated in the electric field Hits the Avalanche PhotoDiode (APD) APD gain ~ 1000 – detectable signal Disadvantages: High working voltage Fragile Problematic operation in magnetic field Photograph of HAPD. From: Nishida et al., NIM A 787 (2015) p. 59 Schematic drawing of HAPD. From: T. Abe et al., Belle II Technical Design Report, Tech. Rep. (KEK, 2010).

Photon module with sipms RICH with SiPMs Photon module with sipms

Alternative Photon Detectors RICH with SiPMs Alternative Photon Detectors Belle II R&D included: Micro-Channel Plate (MCP) PMT HAPD Silicon PhotoMultiplier (SiPM) 𝑑𝑁/𝑑𝜆: Emitted photons in aerogel per unit wavelength, P(λ) : Probability that photon exits aerogel non-scattered Photon Detection Efficiency (PDE) of SiPM and Quantum Efficiencies (QE) for HAPD and MCP PMT

Novel Photon Detector: Silicon Photomultiplier RICH with SiPMs Novel Photon Detector: Silicon Photomultiplier Semiconductor device: thin, robust design Operating principle: Geiger-mode Avalanche PhotoDiode (G-APD) Incident photon produces electron-hole pair The pair starts an avalanche of carriers High gain, G ≈10 6 Photograph of one SiPM

Silicon Photomultiplier: Structure RICH with SiPMs Silicon Photomultiplier: Structure Biased p-n junction, 𝑈 𝑏𝑖𝑎𝑠 , above the critical point, 𝑈 𝑏𝑟𝑒𝑎𝑘𝑑𝑜𝑤𝑛 Width of depleted region: order of µm High electric field in depleted region, ~10 5 V/cm Absorption of a photon Blue photon: absorbed in p+. Electron triggers an avalanche. Red photon: absorbed in n+. Hole triggers an avalanche. Explain the drawing with the points Schematic view of one G-APD cell for p-on-n structure. From: D. Renker and E. Lorenz, JINST 4 (2009) 4004

Silicon Photomultiplier: Operation RICH with SiPMs Silicon Photomultiplier: Operation Diode volume divided into small cells (G-APDs) They can stay in supercritical state for longer time Biased diode analogous to a charged capacitor, C Signal amplitude: 𝐴= 𝐶 𝑒 𝑈 𝑜𝑣𝑒𝑟𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑈 𝑜𝑣𝑒𝑟𝑣𝑜𝑙𝑡𝑎𝑔𝑒 = 𝑈 𝑏𝑖𝑎𝑠 − 𝑈 𝑏𝑟𝑒𝑎𝑘𝑑𝑜𝑤𝑛 Diverging avalanche has to be stopped  quenching resistors into cell structure Many cells connected in parallel form one SiPM Analogue response (if 𝑁 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 ≪ 𝑁 𝑐𝑒𝑙𝑙𝑠 ) Typically, Ncells order of 1000

SiPM Characteristics Single photon sensitivity Fast response (~100 ps) RICH with SiPMs SiPM Characteristics Single photon sensitivity Fast response (~100 ps) High Photon Detection Efficiency: 𝑃𝐷𝐸=𝑄𝐸× 𝑃 𝑡𝑟𝑖𝑔𝑔𝑒𝑟 × 𝜀 𝑓𝑖𝑙𝑙 𝑄𝐸 – quantum efficiency 𝑃 𝑡𝑟𝑖𝑔𝑔𝑒𝑟 - prob. to trigger Geiger discharge 𝜀 𝑓𝑖𝑙𝑙 - active to passive area Insensitivity to high magnetic fields From: Hamamatsu Technical Data 2013

RICH with SiPMs Noise Reduction Without light signal caused by thermally excited e-h pair – dark counts Typical DCR ≈100 kHz per mm2 Optical cross-talk After-pulses We employed two methods to overcome these: Narrow detection time window – use laser trigger or scintillator signal Light guides to collect light

Histograms of integrated charge, two thresholds RICH with SiPMs Signal from SiPM Threshold near zero Laser light on SiPM Superposition of signals Dark counts have the same amplitude as the signal Charge =𝑒×𝐺×𝐴= ∫𝑉𝑑𝑡 𝑅 Integrated charge – discrete spectrum Laser emitted incident photons Dark counts Threshold at half photon amplitude Laser trigger Histograms of integrated charge, two thresholds

Sensitivity of SiPM Response to focused laser light Cell pitch 50 µm RICH with SiPMs Sensitivity of SiPM 50 µm Few cells Whole SiPM, 3600 cells Response to focused laser light Cell pitch 50 µm

Silicon Photomultiplier Arrays RICH with SiPMs Silicon Photomultiplier Arrays Hamamatsu MPPC S11834, could be easily assembled into larger area detector Whole array has 64 = 8 x 8 individual SiPMs 1 SiPM has 3600 cells connected in parallel SiPMs are spaced at 5 mm and have active area 3 x 3 mm2 40 mm

Geometrical Efficiency of Array RICH with SiPMs Geometrical Efficiency of Array 𝜖 𝑔𝑒𝑜 = 𝑎𝑐𝑡𝑖𝑣𝑒 𝑎𝑟𝑒𝑎 𝑝𝑖𝑡𝑐ℎ 2 = 3 5 2 =0.36 Light concentrators improve 𝜖 𝑔𝑒𝑜 Solid (glass) concentrators use total internal reflection Better than hollow with mirror sides (back-reflection) Simultaneously, light concentrators improve signal-to-noise ratio Ray tracing simulation

Height d vs. window width a RICH with SiPMs Shape & Dimensions Truncated pyramids Ray tracing Monte Carlo study Efficiency as function of height d and width a Simulated rays: polar angle 𝜃∈ 0, 30 ∘ 90% Efficiency Height d vs. window width a Efficiency as function of ray incident angle

RICH with SiPMs Prototype Module Module with the 4 ASD8 front-end electronic boards (HERA-B RICH) Borosilicate glass concentrators Module assembled in Al frame

Prototype rich in Test beam RICH with SiPMs Prototype rich in Test beam

RICH with SiPM in Test Beam RICH with SiPMs RICH with SiPM in Test Beam Tested at DESY synchrotron from 24th to 27th September 2013 Highly energetic electrons, momentum p=5 GeV/c (𝛽≃1) Light tight box Two aerogel tiles (n=1.048 & n=1.062) Photon detector with FE electronic Photograph of the prototype RICH counter

Signal Detection 64 SiPMs read out in binary mode RICH with SiPMs Signal Detection 64 SiPMs read out in binary mode Timing information only Hits accepted in narrow time window (red line), 6 ns Reject most of the dark counts Light concentrators double the number of detected hits  improved S/N Distribution of the time of arrival of hit w.r.t. scintillator trigger Without light concentrators With light concentrators

Cherenkov Angle from Time of Arrival RICH with SiPMs Cherenkov Angle from Time of Arrival Reconstructed Cherenkov angle 𝜃 𝐶 from hits in all 64 SiPMs Every histogram is 1 ns time window Observe the highest peak in the on-time (𝑡=0 ns) histogram

Cherenkov Angle Reconstruction RICH with SiPMs Cherenkov Angle Reconstruction Iterative procedure to include refractions at two boundaries (aerogel-to-air and air-to-light concentrator/SiPM) Plotted a circle with radius equal to the expected 𝜃 𝐶 One module covers only 9% of ring circumference 3 4 2 1 Reconstructed hits represented in Cherenkov space

Number of Detected Hits RICH with SiPMs Number of Detected Hits With concentrators: 𝑁 𝐿𝐶 =3.5 hits/track → 32 hits/ring Without concentrators: 𝑁 𝑤𝑜 =1.9 hit/track → 17 hits/ring Uncertainty: 𝜎 𝑠𝑖𝑛𝑔𝑙𝑒 = 𝑃 3 =16 mrad 𝜎 track = 16 mrad 32 =2.9 mrad Separation: 23.5 mrad 2.9 mrad ≃8σ Distribution of reconstructed Cherenkov angle Number of detected hits from the fit to: Gaussian for the signal 2nd order polynomial for the background

Number of Hits Distribution RICH with SiPMs Number of Hits Distribution Should follow Poisson distribution The mean: 𝑁 =− ln 𝑃 0 =3.2 The departure from Poisson dist. is obvious Causes: Photons emitted in brehmsstrahlung Binary mode: detected hits, not photons Distribution of number of hits per track (line) and Poisson distribution calculated from P(0) (stars)

Light concentrators performance RICH with SiPMs Light concentrators performance

All rays collected collection ratio 5 3 2 =𝟐.𝟕𝟖 RICH with SiPMs Collection Ratio Perpendicular incidence All rays collected collection ratio 5 3 2 =𝟐.𝟕𝟖 Response to laser light: Average number of detected photons N Without the light concentrators 𝑁 𝑤𝑜 With the light concentrators NLC 𝑵 𝑳𝑪 𝑵 𝒘𝒐 =2.13 Losses due to: Optical coupling and misalignment, 8% Light concentrator production (glue, air bubbles)

Optical Losses in Test Beam RICH with SiPMs Optical Losses in Test Beam PCB on which MPPC is assembled slightly deformed Imperfect optical coupling Study response to polarised light Zones of different response A, B and C easily understood (Fresnel equations) Optical loss (5±2)% Without optical grease (left); Simulation (middle); Better coupling (right) Tahirovic et al. NIM A 787 (2015) p. 203

Concentrators Cross-talk RICH with SiPMs Concentrators Cross-talk Ray-tracing simulation Registered hits in the neighbour channel without and with teflon filling Teflon filling between the pyramids

Binary Mode Loss Motivation: Number of emitted photons is small RICH with SiPMs Binary Mode Loss Motivation: Number of emitted photons is small High granularity of photon detector Probability of detection of 2 or more photons is small Simulated number of detected photons and hits with one module as function of emitted photons per track. Higher loss with more incident photons. Distribution of number of photons hitting one SiPM. The probability of multiple hits is 18%

RICH with SiPMs Conclusion SiPM as Detector for RICH

RICH with SiPMs Radiation Hardness At Belle II RICH expected integrated fluence is 10 11 neq/cm2 per year Non-ionizing radiation causes increase in dark count rate Previous studies of Hamamatsu MPPCs show that degradation starts at 109 n/cm2 Photograph of irradiated MPPC shows hot spots caused by a higher leakage current (marked red). From: Sudo et al. PoS PD09 (2009) 5 Single photon counting impossible Irradiation with 2⋅ 10 10 n/cm2 From: I. Nakamura, NIM A 610 (2009) Annealing process Single photon spectrum

Solution: Operation at Lower Temperature RICH with SiPMs Solution: Operation at Lower Temperature Dark counts are halved by reducing temperature by ~8°C Even after an irradiation with neutrons, dark counts rate would be low However, we haven’t tested whether the single photons are still detectable Dark counts rate as function of overvoltage and temperature for Hamamatsu MPPC S10931-33-050P, 3x3 mm2. From: N. Dinu et al., NIM A 787, (2015) p. 275.

8𝜎 with, 6𝜎 without light concentrators RICH with SiPMs Conclusion An excellent sensor for RICH A very high number of detected photons: 𝑁=32 hits/ring Kaon/pion separation at 4 GeV/c: 8𝜎 with, 6𝜎 without light concentrators Operation at lower temperatures would reduce dark counts even further

FIN

RICH with SiPMs B-mesons Mixing

Photon Scattering in Aerogel RICH with SiPMs Photon Scattering in Aerogel

RICH with SiPMs Processes in Aerogel Absorption and scattering lengths in aerogel. [E. Aschenauer et al., Nucl. Instr. and Meth. A 440, 338 (2000).] Transmittance of light concentrator (borosilicate glass). Effectively filtering of UV diffuse background

Proximity Focusing with Aerogel Radiator RICH with SiPMs Proximity Focusing with Aerogel Radiator Single photon uncertainty vs. aerogel thickness. [T. Matsumoto et al., Nucl. Instr. And Meth. 521, 367 (2004).] Track uncertainty vs. aerogel thickness for 3 transmission lengths Λ. [T. Iijima et al., Nucl. Instr. and Meth. A 548, 383 (2005).] Optimal thickness ~20 mm

Detected Photons with Two Aerogels RICH with SiPMs Detected Photons with Two Aerogels Expected with SiPMs: ~62 photons Lower PDE, light concentrators performance, binary loss Detected ~32 21 41

Photon Detection Efficiency RICH with SiPMs Photon Detection Efficiency Hamamatsu Tech. Data 2013 Bias voltage at test beam was ~1.5 V, Peak PDE was ~2/3 of the nominal Expected number of photons: 1.8 (without light concentrators) and 4.6 (with the light concentrators)

Biasing SiPM Correctly RICH with SiPMs Biasing SiPM Correctly By raising the bias voltage, the peak PDE increases However, the dark counts rate increases as well

Time of Arrival Distribution for Laser Scans RICH with SiPMs Time of Arrival Distribution for Laser Scans One cell Whole SiPM (3600 cells)