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

2. 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

3. Particle identification
RICH with SiPMs
Particle identification

4. 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.

5. 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× 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

6. 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

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

8. 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

9. 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

10. 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).

11. 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).

12. Ring Imaging Cherenkov counter
RICH with SiPMs
Ring Imaging Cherenkov counter

13. 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

14. 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

15. 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).

16. Photon module with sipms
RICH with SiPMs
Photon module with sipms

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. Geometrical Efficiency of Array
RICH with SiPMs
Geometrical Efficiency of Array
𝜖 𝑔𝑒𝑜 = 𝑎𝑐𝑡𝑖𝑣𝑒 𝑎𝑟𝑒𝑎 𝑝𝑖𝑡𝑐ℎ 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

27. 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

28. 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

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

30. 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

31. 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

32. 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

33. 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

34. 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 =2.9 mrad
Separation: 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

35. 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)

36. Light concentrators performance
RICH with SiPMs
Light concentrators performance

37. All rays collected collection ratio 5 3 2 =𝟐.𝟕𝟖
RICH with SiPMs
Collection Ratio
Perpendicular incidence
All rays collected collection ratio =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)

38. 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

39. 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

40. 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%

41. RICH with SiPMs
Conclusion
SiPM as Detector for RICH

42. RICH with SiPMs
Radiation Hardness
At Belle II RICH expected integrated fluence is 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⋅ n/cm2
From: I. Nakamura, NIM A 610 (2009)
Annealing process
Single photon spectrum

43. 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 S P, 3x3 mm2.
From: N. Dinu et al., NIM A 787, (2015) p. 275.

44. 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

45. FIN

46. RICH with SiPMs
B-mesons Mixing

47. Photon Scattering in Aerogel
RICH with SiPMs
Photon Scattering in Aerogel

48. 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

49. 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

50. 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

51. 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)

52. 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

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