Operating Hybrid Photon Detectors in the LHCb RICH counters at high occupancy

Operating Hybrid Photon Detectors in theLHCb RICH counters at high occupancy

Introduction HPD Benefits for LHCb RICH Operation Experienced from Run 1 Photon Yields Ion Feedback Evolution & HPD Optimisation Conclusions

RICH 2013, Kanagawa, 04.12.2013

Stephan Eisenhardt, University of EdinburghOn behalf of the LHCb experiment

04.12.2013

2Stephan EisenhardtRICH 2013, Shonan, 04.12.2013

LHCb RICH Counters

RICH2

RICH1

VELO

DipoleMagnet

b-b angular correlation

collisionpoint

~1 cm

B

for RICH detector description and operation talk by A. Papanestisfor RICH detector performance talk by C. Matteuzzifor RICH upgrade (2019) talk by S. Easo


Hybrid Photon Detectors

Anode

Vacuumphoton detector

Pixel HPDs: – developed in collaboration with industry

(lead partner: Photonis-DEP)– combines:

vacuum photon detector technology withsilicon pixel readout

– Quartz window with S20 photocathode → high QE– QE: increased during production: 25% → 31%– 20kV operating voltage (~5000 e– [eq. Si])– Factor 5 demagnification @ 20kV → close-packing

Anode on carrier RICH1 HPD panel65% geometric efficiency,

incl. m-metal

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

effective 3232 pixel array– pixel size 500mm500mm sufficient

4Stephan Eisenhardt

Quantum Efficiency 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> @ 270 nm (per batch)

19

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0 2 4 6 8 10 12 14 16 18 20 22 24batch no.

aver

age Q

E [%

] .

<QE> per batch

running <QE> (batch 0-25)

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

Q

E [%

]

Wavelength [nm] RMS ofbatch spread

<QE> per delivery batch

QE

[%]

Batch number

<QE> (Photonis Data): across delivery batches

RICH 2013, Shonan, 04.12.2013

5Stephan Eisenhardt

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)

electonic noise of pixel chip

0

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16050 65 80 95 11

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noise [e-]

HPD

.

global threshold setting

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HPD

.

signal-over-noise of pixel chip

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S/N

HPD

.

<threshold>:1065 e-

<noise>:145 e-

<S/N>:27

RICH 2013, Shonan, 04.12.2013


Occupancies @ L=4x1032 cm-2 s-1

02.09.2012 RICH1~2400 photons/event

RICH2~2700 photons/event

RICH1+2:~500k channels

RICH1: 196 HPD

RICH2: 288 HPD

7Stephan Eisenhardt

16.6

16.5

16.4

16.3

16.2

16.1

16.0

15.9

15.8

pixe

l

time[min]0 400 800 1200 1600 2000

1 pixel

14.6 hours

19.018.518.017.517.016.516.015.5

pixe

l

0 5 10 15 20 25 30 time[hrs]

1 pixel

30 hours19.519.018.518.017.517.016.516.0

pixe

l

0 5 10 15 20 25 30 time[hrs]

1 pixel30 hours

19.018.518.017.517.016.516.015.5

pixe

l

1 pixel

14.6 hours

0 2 4 6 8 10 12 14 time[hrs]

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

Image Drifts

RICH 2013, Shonan, 04.12.2013

8Stephan Eisenhardt

14.6

hou

rs

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[min

]

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17.6

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16.8

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16.0 15.6 16.0 16.4 16.8 17.2

time

[min

]

x [pixel]y [pixel]

14.6

hou

rs

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

RICH 2013, Shonan, 04.12.2013

Image Drifts

photo cathodeimage on anodewith edgefrom Sobel fit

9Stephan Eisenhardt

2011: during period of increase of data rate– HPD saw “Beam Induced Light Events” – corona

RICH 2013, Shonan, 04.12.2013

HPD Gas Atmosphere 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: 1816kV, at negligible efficiency loss– result: stable ever since

RICH1 panelscorona

light from coronain opposite panel

bias current of RICH1 rows vs. time

N2 CO2

N2 CO2

11.5.2011 26.5.2011 5.6.2011


Photon Yield - Method

RICH1:typical 2012event

RICH1:ppppm+m-

event

Trac

ks

– Choose the cleanest data set:

– 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)

11Stephan Eisenhardt

RICH1C4F10

RICH2CF4

2010 2011 2012

optimisedHPD chipsettings

RICH 2013, Shonan, 04.12.2013

Photon Yield - Results

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

Photon Yield 2011 Npe from data : ppppm+m- Npe from MC : D*D0p+

calculated true

RICH1 : Aerogel 4.3 ± 0.9 8.0 ± 0.6 6.8 ± 0.3

RICH1 : C4F10 24.5 ± 0.3 28.3 ± 0.6 29.5 ± 0.5

RICH2 : CF4 17.6 ± 0.2 22.7 ± 0.6 23.3 ± 0.5

drop is rate dependent,no QE degradation

12Stephan Eisenhardt

photoelectroncurrent vs.bias voltage

RICH 2013, Shonan, 04.12.2013

Ion Feedback – Monitoring

Strobe Scan H524004

0.0

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Delay [ns]

Hits

Per

Eve

nt

Raw hitsClustersPoisson estimateIon Feedback x 100

Very low IFB <<1%

h

its /

even

t

Delay [ns]

HPD response to 15ns LED pulses with varied delay

50nsstrobe signal

example dark count hit maps

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


IFB – in-situ Monitoring

RICH1 hit map: cw-laser illuminationphoton yields /event /HPD with cw-laser

typical cluster size distributions

cut

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

In physics data:– IFB removed by clustering

– to first order: no effect on PID


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

IFB distribution 03/2010

201220112010

20092008

Ion

Feed

back

[%]

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

201220112010

20092008

Ion

Feed

back

[%]

Date

H602003: IFB rate monitored in RICH2


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

201220112010

20092008

Ion

Feed

back

[%]

Date

H638003: IFB rate monitored in RICH2

201220112010

20092008Io

n Fe

edba

ck [%

]

Date

2013

H721002: IFB rate annealed during LS1

LS1


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 during building

HPD volumes


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

IFB vs. time: R&D sample

0.5%/yr

0.2%/yr

without getters

with getters

near sensitivity limit

IFB scale: x100 zoom


RICH: the LHCb PID Workhorse Used by (virtually) every analysis in LHCb to do positive ID

JHEP 10 (2012) 037 017.0009.0262.0)KB()B(

0

0

ppp

BrBr

WithoutRICH

WithRICH

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


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…


Spare Slides t


RICH1 and RICH2 LayoutFlat mirrors

Spherical Mirrors

Photon Funnel + Shielding

Central Tube

Support Structure

7.2 mRICH2RICH1

Interaction Pointreversiblemagnetic field


LHCb Operation 2010-2012 Excellent running: 2010 2011 2012

– Beam energy 3.5TeV 3.5TeV 4.0TeV– Luminosity [cm-2 s-1] 2x1032 2-4x1032 4x1032

successful test: 6x1032

– Visible interactions/crossing m = 0.4 m = 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 50ns 50ns– Recorded luminosity 0.037fb-1 >1.0 fb-1 >2.0 fb-1

LHCb lumi levellingby beam adjustment

design values

design lumi

bb cross-section +15%

(25ns from 2015)

2012

2011

2010


Occupancies @ L=6x1032 cm-2 s-1

30.11.2012 RICH1~2800 photons/event

RICH2~3200 photons/event

RICH1+2:~500k channels

RICH1: 196 HPD

RICH2: 288 HPD


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

RICH1BeforeAfter

RICH2 BeforeAfter

Dx =0.18 pixel

axial field transversal field

pixels

hits

0 mT 3 mT

0 mT 3 mT


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Operating Hybrid Photon Detectors in the LHCb RICH counters at high occupancy