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Operating Hybrid Photon Detectors in the LHCb RICH counters at high occupancy. Stephan Eisenhardt, University of Edinburgh On behalf of the LHCb experiment. Introduction HPD Benefits for LHCb RICH Operation Experienced from Run 1 Photon Yields Ion Feedback Evolution & HPD Optimisation

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operating hybrid photon detectors in the lhcb rich counters at high occupancy

Operating Hybrid Photon Detectors in theLHCb RICH counters at high occupancy

Stephan Eisenhardt, University of Edinburgh

On behalf of the LHCb experiment

Introduction

HPD Benefits for LHCb RICH Operation

Experienced from Run 1

Photon Yields

Ion Feedback Evolution & HPD Optimisation

Conclusions

04.12.2013

RICH 2013, Kanagawa, 04.12.2013

lhcb rich counters

~1 cm

B

LHCb RICH Counters

b-b angular

correlation

Dipole

Magnet

RICH2

RICH1

VELO

collision

point

  • for RICH detector description and operation
  • talk byA. Papanestis
  • for RICH detector performance
  • talk by C. Matteuzzi
  • for RICH upgrade (2019)
  • talk by S. Easo

Stephan Eisenhardt

hybrid photon detectors
Hybrid Photon Detectors

Anode

Vacuum

photon detector

  • Readout:
    • encapsulated 256x32 pixel silicon sensor
    • bump-bonded to binary readout chip
    • low noise of 145 e-→ low background
    • 8-fold binary OR

 effective 3232 pixel array

    • pixel size 500mm500mm sufficient

Anode on carrier

RICH1 HPD panel

65% geometric efficiency,

incl. m-metal

  • Pixel HPDs:
    • developed in collaboration with industry

(lead partner: Photonis-DEP)

    • combines:

vacuum photon detector technology with

silicon pixel readout

    • Quartz window with S20photocathode → high QE
    • QE: increased during production: 25% → 31%
    • 20kV operating voltage (~5000 e–[eq. Si])
    • Factor 5 demagnification @ 20kV → close-packing

Stephan Eisenhardt

quantum efficiency
Quantum Efficiency

<QE> (PhotonisData): across delivery batches

  • QE right where we need it:
    • increase during production
    • single most helpful improvement to RICH performance
    • <QE @ 270nm> = 30.8% >> typical QE = 23.3%

<QE> per delivery batch

QE [%]

QE [%]

Wavelength [nm]

RMS of

batch spread

  • LHCb QA cross-check:
    • measured QE on 10% of tubes
    • confirmed Photonis data

Batch number

Stephan Eisenhardt

pixel chip threshold and noise
Pixel Chip – Threshold and Noise
  • 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)

<noise>:

145 e-

<threshold>:

1065 e-

<S/N>:

27

Stephan Eisenhardt

occupancies @ l 4x10 32 cm 2 s 1
Occupancies @ L=4x1032 cm-2s-1

02.09.2012

RICH2

~2700 photons/event

RICH1

~2400 photons/event

RICH1+2:

~500k channels

RICH1: 196 HPD

RICH2: 288 HPD

Stephan Eisenhardt

image drifts
Image Drifts

14.6 hours

30 hours

30 hours

  • Observation of image drifts for some HPD
    • especially in RICH1 with time scale 0.5-1 hour
    • while most HPDs show stable image, within 0.2 pixels
    • always the same few HPD show either:
      • continuous drifts: typically <1.5 pixels, max. <3 pixels
      • or distinct shifts
      • without periodicity or correlation to environment
    • reason not really understood, but looks like charging effect

pixel

pixel

pixel

pixel

0 5 10 15 20 25 30

0 2 4 6 8 10 12 14

0 5 10 15 20 25 30

1 pixel

1 pixel

1 pixel

19.5

19.0

18.5

18.0

17.5

17.0

16.5

16.0

1 pixel

19.0

18.5

18.0

17.5

17.0

16.5

16.0

15.5

19.0

18.5

18.0

17.5

17.0

16.5

16.0

15.5

0 400 800 1200 1600 2000

16.6

16.5

16.4

16.3

16.2

16.1

16.0

15.9

15.8

14.6 hours

time[hrs]

time[hrs]

time[hrs]

time[min]

Stephan Eisenhardt

image drifts1
Image Drifts
  • Observation of image drifts for some HPD
    • correlation in time between x- and y-movement, but not linear
  • Solution: automated monitoring of movement
    • fit image position from beam data
      • using Sobel algorithm for edge detection
    • online correction

time [min]

time [min]

14.6 hours

14.6 hours

2000

1600

1200

800

400

0

2000

1600

1200

800

400

0

15.0 15.4 15.8 16.2 16.6 17.0

15.6 16.0 16.4 16.8 17.2

y [pixel]

x [pixel]

18.0

17.6

17.2

16.8

16.4

16.0

x [pixel]

20.0

19.0

18.0

17.0

16.0

15.0

y [pixel]

photo cathode

image on anode

with edge

from Sobel fit

Stephan Eisenhardt

hpd gas atmosphere
HPD Gas Atmosphere
  • 2011: during period of increase of data rate
    • HPD saw “Beam Induced Light Events” – corona
  • 2011: during period of increase of data rate
    • HPD saw “Beam Induced Light Events”
    • spreading to other HPD
    • CO2 reported to better suppress corona than N2
    • changed atmosphere in HPD box from N2 to CO2
    • changed HV: 1816kV, at negligible efficiency loss
    • result: stable ever since

bias current of RICH1 rows vs. time

RICH1 panels

N2 CO2

corona

light from corona

in opposite panel

N2 CO2

11.5.2011 26.5.2011 5.6.2011

Stephan Eisenhardt

photon yield method
Photon Yield - Method
  • Choose the cleanest data set:

Tracks

RICH1:

ppppm+m-

event

RICH1:

typical 2012

event

  • fit shape of Cherenkov angle resolution:

Dq = qrec - qexp

  • get photon yield from area of fit to each track (using all photons, fixed shape & flat background)
  • get average track yield over sample (long run)

Stephan Eisenhardt

photon yield results
Photon Yield - Results

drop is rate dependent,

no QE degradation

  • Npe from data slightly lower than MC prediction from D*D0p+

optimised

HPD chip

settings

RICH1

C4F10

2011

2012

2010

RICH2

CF4

Stephan Eisenhardt

ion feedback monitoring
Ion Feedback – Monitoring

high IFB

low IFB

  • Process:
    • photoelectron ionises residual gas atom
    • drift of ion to photo cathode (200-300ns)
    • release of secondary photo electrons (~10-40 e-)
    • impact of ph.e. cluster on sensor (cluster size)
  • Three measurement methods:
    • 1) measure gas gain in QE setup – very sensitive
    • 2) scan delay of DAQ gate – standard lab tool
    • 3) cluster size – RICH in-situ measurement

example dark count hit maps

HPD response to 15ns LED pulses with varied delay

50ns

strobe signal

Very low IFB <<1%

hits / event

photoelectron

current vs.

bias voltage

Delay [ns]

Stephan Eisenhardt

ifb in situ monitoring
IFB – in-situ Monitoring
  • In-situ monitoring:
    • cw-laser (635nm)
    • record IFB from cluster size
    • evolution in time

cut

typical cluster size distributions

  • In physics data:
    • IFB removed by clustering
    • to first order:

no effect on PID

photon yields /event /HPD with cw-laser

RICH1 hit map: cw-laser illumination

Stephan Eisenhardt

ifb evolution
IFB – Evolution
  • Evolution: without beam
    • residual gas increases linear in time
    • typically: DIFB <0.5% / year
    • illumination anneals IFB
    • fraction of HPD evolve more quickly
  • Threshold: self-sustained IFB
    • from IFB > 5%
    • where photocathodes degrade quickly

 exchange & repair

H602003: IFB rate monitored in RICH2

Ion Feedback [%]

Ion Feedback [%]

2008

2009

2008

2009

2010

2011

2012

2010

2011

2012

Date

Date

  • Evolution: with increasing data rate
    • IFB increases stronger
    • correlated with heat (data rate)
    • the fraction of HPD evolving more quickly increased
    • a stretch for the exchange & repair

programme

IFB distribution 03/2010

Stephan Eisenhardt

ifb evolution long shutdown 1
IFB – Evolution – Long Shutdown 1
  • Evolution: during shutdown
    • some HPD show significant increase when operation conditions are not well defined
  • Strategy for LS1:
    • LV off – to keep HPDs cool
    • HV on – to allow photoelectron production
    • cw-laser on – to cause annealing
    • Si-Bias on – to monitor tubes
    • ~monthly IFB runs (needs LV on)
  • Strategy pays off:
    • 163 HPD show negative DIFB
    • 144 HPD show reduced DIFB
    • 97 HPD show continuous DIFB
    • 9 HPD show increased DIFB
    • O(50) special cases

H638003: IFB rate monitored in RICH2

Ion Feedback [%]

2008

2009

2010

2011

2012

Date

H721002: IFB rate annealed during LS1

Ion Feedback [%]

2008

2009

LS1

2013

2010

2011

2012

Date

Stephan Eisenhardt

hpd repair replacement program
HPD Repair&Replacement Program
  • Pre-Run1 Repair & Replacement program:
    • 2009: 35 HPD – catching up on old HPD which were most affected
  • 2010 prediction:
    • need to exchange O(13) HPD/year
  • Run1 Repair & Replacement program:
    • 2010: 6 HPD
    • 2011: 38 HPD
    • 2012: 39 HPD
  • 2012/13: R&D to improve stability of tube vacuum
    • see next slide
  • Long Shutdown 1 Repair & Replacement program:
    • with optimised production parameters
    • 2013: 40 HPD
    • 2014: O(40) HPD (planned)
  • Procedure:
    • remove HPD from RICH and return to Photonis
    • recuperate anode, body and Quartz window
    • build new tube
    • full Quality Assurance
    • re-introduce to RICH

RICH1 duringbuilding

HPD volumes

Stephan Eisenhardt

hpd production improvement
HPD Production Improvement
  • Standard production procedure:
    • yielded a rather large variation of DIFB
      • bulk: 0.1%/yr < DIFB < 0.2%/yr
      • tail: < 0.5%/yr (which is tolerable)
    • repair reset the clock and ‘threw dice’ again
  • Improvement:
    • introducing getters
    • optimised production recipe
    • they integrate now very well
    • dimensioned to last 10 years
  • New production procedure:
    • reliably gives very low initial IFB
    • and gives DIFB <<0.01%/yr
    • used for repair & replace in LS1

0.5%/yr

without getters

with getters

0.2%/yr

IFB vs. time: R&D sample

IFB scale: x100 zoom

near sensitivity limit

Stephan Eisenhardt

rich the lhcb pid workhorse
RICH: the LHCb PID Workhorse
  • Used by (virtually) every analysis in LHCb to do positive ID

Without

RICH

With

RICH

JHEP 10 (2012) 037

Without RICH PID, the B0 p+p- is completely dominated by B0  K+p-

Stephan Eisenhardt

conclusions
Conclusions
  • HPD benefits for LHCb RICH
    • high QE
    • low noise  low background
  • Got operational challenges under control quickly or well maintained
  • Developed reliable tools and measures to deal with IFB
    • beautiful PID properties of RICH are maintained
  • Developed now long-term fix to suppress IFB in the HPDs
    • HPD repair for Run2 (2015-18) is under way
  • RICH is the reliable PID workhorse for LHCb
    • most (student) members these days just know it from their PID selection code…

Stephan Eisenhardt

spare slides
Spare Slides
  • t

Stephan Eisenhardt

rich1 and rich2 layout

Flat mirrors

Spherical Mirrors

Support Structure

7.2 m

Central Tube

Photon Funnel + Shielding

RICH1 and RICH2 Layout

RICH1

RICH2

reversible

magnetic field

Interaction Point

Stephan Eisenhardt

lhcb operation 2010 2012
LHCb Operation 2010-2012
  • Excellent running: 2010 2011 2012
    • Beam energy 3.5TeV 3.5TeV4.0TeV
    • Luminosity [cm-2 s-1] 2x10322-4x10324x1032

successful test: 6x1032

    • Visible interactions/crossing m = 0.4m = 0.4-1.4 m = 1.6
    • Data taking efficiency >90% >91% >94%
    • High Level Trigger output to tape 3kHz 4.5kHz
    • bunch spacing 50ns 50ns50ns
    • Recorded luminosity 0.037fb-1>1.0 fb-1>2.0 fb-1

bb cross-

section +15%

design values

(25ns from 2015)

design lumi

2012

2011

LHCb lumi levelling

by beam adjustment

2010

Stephan Eisenhardt

occupancies @ l 6x10 32 cm 2 s 1
Occupancies @ L=6x1032 cm-2s-1

30.11.2012

RICH2

~3200 photons/event

RICH1

~2800 photons/event

RICH1+2:

~500k channels

RICH1: 196 HPD

RICH2: 288 HPD

Stephan Eisenhardt

magnetic distortions
Magnetic Distortions
  • Imaging in HPDs is distorted by

external magnetic fields

    • used projected test pattern with and

without field to extract corrections

    • done for both filed orientations
    • produced maps for online correction

Before

After

RICH1

Before

After

hits

RICH2

Dx =

0.18 pixel

0 mT

3 mT

0 mT

3 mT

transversal field

axial field

pixels

Stephan Eisenhardt

slide25

t

Stephan Eisenhardt