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Production and Tests of Hybrid Photon Detectors for the LHCb RICH Detectors. Stephan Eisenhardt, University of Edinburgh On behalf of the LHCb experiment. Introduction Hybrid Photon Detectors Production Test results Conclusions. LHCb. HPD. RICH 2007, Trieste, 17.10.2007. RICH2. RICH1.

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Production and tests of hybrid photon detectors for the lhcb rich detectors

Production and Tests of Hybrid Photon Detectors for the LHCb RICH Detectors

Stephan Eisenhardt, University of Edinburgh

On behalf of the LHCb experiment


Hybrid Photon Detectors


Test results




RICH 2007, Trieste, 17.10.2007



Rich photondetector requirements

RICH1 RICH Detectors





RICH Photondetector Requirements

single event full LHCb simulation used in performance studies

photodetector area: 3.3 m2

single photon sensitivity: 200 - 600 nm

quantum efficiency: >20%

good granularity: 2.5 x 2.5 mm2

active area fraction: 65%

# of electronic channels: 500k

LHCb DAQ rate: 40MHz

rad. tolerant: 3kRad/year

 answer: 484 Hybrid Photon Detectors

Stephan Eisenhardt

Hybrid photon detector hpd
Hybrid Photon Detector (HPD) RICH Detectors

  • Photon detector:

    • Quartz window, S20 photocathode

      • Typical  QE dE > 0.7eV

    • Cross-focussing optics (tetrode structure):

      • De-magnification by ~5

      • Active diameter 75mm

         484 tubes for overall RICH system

    • 20 kV operating voltage (~5000 e– [eq. Si])

  • Anode:

    • 25632 pixel Si-sensor array (“Alice mode”)

      small pixels  low noise

    • bump-bonded to binary readout chip

    • assembly encapsulated in vacuum tube

    • “LHCb readout mode”: 8-fold binary OR

       effective 3232 pixel array

    • pixel size 500mm500mm sufficient



photon detector

Stephan Eisenhardt

Hpd manufacture anode

20 RICH Detectorsm

Detector chip (Canberra)

Assembly probing

High T bump-bonding (VTT)

Readout chip (IBM)

Wafer probing

Packaging (HCM)

Anode testing

Visual inspection and plating control

Ceramic carrier (Kyocera)

Brazing (DEP) and gold-plating (CERN)

HPD manufacture – Anode


  • 7 companies/institutes

  • 6 countries

  • coordinated by LHCb

tests by LHCb

Stephan Eisenhardt

Hpd manufacture tube @dep
HPD manufacture – Tube (@DEP) RICH Detectors

HPD tube production (DEP)

Vacuum [email protected] 300°C

Photo-cathode deposition and vacuum sealing

HPD cabling and potting

Tube body assembly

Final HPD

testing by LHCb

Anode incoming inspection and testing

Anode testing

QE measurement and anode testing

Stephan Eisenhardt

Photon detector test facilities

Dark box RICH Detectors


flat & pointing

light source

Electronics & Power supplies


Photon Detector Test Facilities

  • Photon Detector Test Facilities (PDTF): (Edinburgh & Glasgow)

    • 2 test stations per site

    • design test rate: 1 HPD / day / site

    • standard preparation and automated test programme per HPD: ~6hrs

    • extended tests: on ~10% of HPDs

      • Quantum Efficiency (Edinburgh)

      • Backpulse Signal (Glasgow)

  • HPD Storage:

    • under He-free atmosphere: N2 gas flow (0.2 l/min)

PDTF station

Stephan Eisenhardt

Pdtf tests
PDTF – Tests RICH Detectors

  • Comprehensive test of every function and parameter of the HPD:

Electron Optics /

Tube Volume



HV Stability

Field Distortions

Ion Feed Back

Vacuum Quality


Dark Count

Response to light

Quantum Efficiency

HPD Body


Quartz window

Pin Grid Array

Sensor position

Readout Chip



DAC linearity

Readout modes

Dead Channels

Noisy Channels

Pixel masking



Silicon Sensor

IV Curve



Efficiency (Backpulse)

Stephan Eisenhardt

Pdtf automation of tests

Bias Voltage Scan RICH Detectors

HV Scan

Strobe Scan

HV ramp up

PDTF Taskflow

Temp monitor

Threshold Scan

Dark Count – Alice mode

LED light – Alice mode

LED monitor

HV monitor

Bias V monitor


Dark Count - LHCb mode

IV Scan

LED light - LHCb mode

Power ON

PDTF – Automation of Tests


  • parameter setting

  • data taking

  • logging

  • data analysis

  • report generation

    human control:

  • parameter choice

  • online displays

  • offline reports

Stephan Eisenhardt

Testing programme summary
Testing Programme – Summary RICH Detectors


pass: 547 ~98%

fail: 12 ~ 2%

Stephan Eisenhardt

Mechanical tests
Mechanical Tests RICH Detectors

  • 555 HPD passed

  • 2 HPD failed on first test

    • leaned by ~0.4mm

       tubes repaired to pass as well

  • t

point of first possible contact


mechanical test jig

HPD : 83.0mm



Teflon tape: 0.1mm

Jig : 83.4mm

gap: 0.1mm

any contact = failure

Stephan Eisenhardt

Pixel chip threshold and noise
Pixel Chip – Threshold and Noise RICH Detectors

  • excellent signal over noise: specification <measured>

    • average signal charge @ 20kV: C = 5000 e-

    • average threshold: T = < 2000 e- 1065 e-

    • average electronic noise: N = < 250 e- 145 e-

    • signal over noise: S/N = (C-T)/N > 12 27

      (min, max) = (21,33)


145 e-


1065 e-



Stephan Eisenhardt

Anode channel yields
Anode – Channel Yields RICH Detectors

  • development of Flip-Chip bump-bonding

    for O(104) channels:

  • excellent yields for response of individual pixels:

    • in “Alice mode” 8192 pixels / HPD

    • spec: > 95% working

      (< 400 pixels dead)

    • all HPD within spec

    • noisy pixels: negligible

dead pixels / HPD

20 m

noisy pixels / HPD

Alice mode:

fine resolution

Stephan Eisenhardt

Anode leakage current
Anode – Leakage Current RICH Detectors

Leakage Current @ 80V bias

  • goal: typical value of: LC ~ 1mA

  • achieved for all bare chips when unpowered

  • in powered HPD: 1W heat dissipation

    • anode heat up by ~12-15 °C

    • increase in leakage current: ~*2 for 6 °C


IV scans for sample of HPDs

Leakage Current [nA]

  • found two classes:

    • low current @ 80V (<1mA):

      quadratic behaviour up to 90V bias

    • medium current @ 80V (~1mA…3mA):

      turn up point between: 40…60V

Bias Current [nA]

Bias Voltage [V]

Stephan Eisenhardt

Imaging RICH Detectors

pulsed LED run

(200k events, ~3 npe/event)

fit for sensor position

displacement in y [mm]

cylindrical reflection:

reflection on Al coating

circles: LHCb pixel Ø

displacement in x [mm]

fit for image diameter

sensor displacement:

due to positioning error

>1mm (2 LHCb pixel):

signal loss possible

in magnetic field


linear demagnification:

<D> = 5.45

Stephan Eisenhardt

linear demagnification

Photoelectron response
Photoelectron Response RICH Detectors

  • HV scan: look for photon yield

    • onset of response

    • onset of charge sharing between pixels

    • slope due to increasing efficiency

      for back-scattered e-

      (only partial energy deposit)

  • all accepted HPD pass

pixel hit rate

cluster hit rate

photoelectrons / event

HV [kV]

Stephan Eisenhardt

Anode response bias voltage scan
Anode Response – Bias Voltage Scan RICH Detectors

  • Bias voltage scan: look for photon yield

    • onset of response

    • bias of full depletion

    • plateau of over-depletion >50V

  • all accepted HPD pass

pixel hit rate

cluster hit rate

photoelectrons / event



Anode Bias [V]

Stephan Eisenhardt

Dark count response
Dark Count Response RICH Detectors

  • Dark Count settling after first HV ramp up:

    • observation of signals without light source

    • typical decay:

      factor 2 in 30min after initial ramp-up

    • time constants vary

  • all accepted HPD pass

pixel hit rate

dark counts / event

cluster hit rate

Time [min]

Stephan Eisenhardt

Dark count

H516018: 10.0 kHz/cm RICH Detectors2

H516009: 7.3 kHz/cm2

high red sensitivity

increased IFB prob.

Dark Count

  • all accepted HPD have a very low

    dark count < 20kHz/cm2

    • DC = 5 kHz/cm2 :

      1% probability for 1 hit / HPD / event

    • 497 HPD with DC < 5 kHz/cm2

settled Dark Count from high statistics run

  • in the range 5…20 kHz/cm2:

    • two types:

      • high red sensitivity

      • increased IFB probability

    • perfectly fine to be used in RICH


Dark Count [kHz/cm2]

Stephan Eisenhardt

Ion feed back
Ion Feed Back RICH Detectors

  • due to e- ionising residual gas atoms

     ion produces bunch of photoelectrons at photocathode

     cluster of hits with 200-300ns delay

  • we find: very low IFB  very good tube vacuum at fabrication

HPD response to 15ns LED pulses with varied delay

Ion Feed Back from delayed cluster signals

<IFB> = 0.04%


max. 1%

Very low IFB <<1%

hits / event


Delay [ns]

50ns strobe signal

Ion Feed Back [%]

Stephan Eisenhardt

Quantum efficiency dep data
Quantum Efficiency – DEP Data RICH Detectors

<QE> (DEP Data): across delivery batches

  • Excellent sensitivity:

    • increase due to process tuning at DEP

    • single most helpful improvement to RICH performance

    • <QE @ 270nm> = 30.8%

      >> typical QE = 23.3%

<QE> per delivery batch

QE [%]

RMS of

batch spread

QE [%]

Wavelength [nm]

  • more tuning improvements:

    • fill of sensitivity dip between UV and visible

    • reduction of red sensitivity @ 800nm

      • anti-correlated to blue sensitivity

      • cause of thermal e--emission (dark count)

Batch number

Stephan Eisenhardt

Qe lhcb verification
QE – LHCb Verification RICH Detectors

  • PDTF measurement:

    • 7 wavelengths, 10nm bandpass filter

    • error: 2%

    • 76 HPD measured

  • PDTF QE measurements typically

    matches DEP values within 3%

4 tests across QE range


all tests: PDTF vs. DEP

wavelength [nm]

  • PDTF measurements confirm

    shape of spectra & absolute values

  •  full trust in DEP measurements


Stephan Eisenhardt


Conclusions RICH Detectors

  • Production & testing of >550 HPDs has finished

  • Rigorous test programme with: ~98% of HPDs accepted

    pass: 547 HPD

    fail: 12 HPD

  • HPDs meet requirements of LHCb RICH detectors

  • Very good results for vacuum quality and Dark Count

  • Excellent results on Quantum Efficiency and S/N

  • DEP Quantum Efficiency results confirmed by PDTF

  • Commissioning is underway

    with 288 HPD installed in RICH2

Stephan Eisenhardt

Backup slides
Backup Slides RICH Detectors

Stephan Eisenhardt

Classification system
Classification System RICH Detectors

  • guideline for usability in RICH:

    • 161x class A+: exceed specifications significantly

    • 282x class A: clear pass in all aspects

    • 60x class B : may fail specs, but recommended for usage

      • HPDs with slightly increased dark count

    • 42x class E: flagged with an issue, still usable in RICH

      • HPDs with increased LC or 1…5% dead pixels

    • 12x class F : clear fail  reject

  • 12 failed HPDs:

    • 9x replaced with good HPD

    • 3x accepted as failure within LHCb responsibility

  • misc:

    • 4x repaired, retested and accepted as good

    • 2x anode problem, but usable,

      under study, not classified

pass: 545+2 of 559

~ 98%

fail: 12 of 559

~ 2%

Stephan Eisenhardt

Qe pdtf test setup

Filter RICH Detectors





Halogen Lamp

Dark Box


photocurrent: <160nA

image Ø:

~ 50mm

Fused Silica



Shutter , 10nm BP filter , IR or VIS block , ND filter



IPD [pA]


QE – PDTF Test Setup

  • measurement of the photocurrent, referenced with calibrated photodiode

  • differing DEP parameters:

    • Bias: 900V

    • Ø: 10-15mm

    • large photo currents

default: 100V

cross-check: 22V (just below He ionisation threshold)

Stephan Eisenhardt

Qe effect of degrading vacuum
QE – Effect of degrading Vacuum RICH Detectors

  • degraded vacuum causes:

    • increase of Ion Feed Back

    • increase of charge per photoelectron

    • increase of measured photo current

    • fake increase in determined QE

  • cure:

    • measure photo current below

      He ionisation threshold

    • PDTF : IV curves 0…500V

    • PDTF : bias = 100V, 22V

    • DEP : bias = 900V

case of extreme

vacuum degradation

for good illustration


Stephan Eisenhardt

Photoelectron efficiency backpulse
Photoelectron Efficiency – Backpulse RICH Detectors

  • comparison of binary to analog event yield with constant light source

    • binary: through readout chip  npe

    • analog: measurement of the charge pulse on the bias line  Poisson <m>

       capacity of whole chip: noise*104 wrt. single pixel

  • Poisson fit to analog spectrum

  • Results: efficiency = npe / <m>

    strobe length

    efficiency 25ns 50ns

    PDTF 2007 88% 94%

    (production HPDs)

    CERN 2004 84% 92%

    (prototype HPDs)

    error estimation pending

analog ph.el. spectrum

photo electrons

 data

 fit


an almost

perfect match





fit yields Poisson <m>




ADC counts

Stephan Eisenhardt