Ageing of large CsI photocathodes exposed to ionizing radiation in a gaseous RICH detector

A. Braem, G. De Cataldo, A. Di Mauro, A. Franco, A. Gallas Torreira, H. Hoedlmoser, P. Martinengo, E. Nappi, F. Piuz,

E. Schyns

Bari INFN, CERN

Attending the conference

2

RICH DETECTOR BASED ON CsI-PHOTOCATHODE

The cathode of a gaseous detector is coated with a thin layer of CsI converting photons of WL < 210nm. The single electron emitted is in turn amplified in the detector.

Usually, the coated cathode is segmented into pads allowing for a 2D-localization of the Cherenkov photons.

CsI-RICH systems under operation

DRAWBACK OF THE MWPC OPEN GEOMETRY:

THE CsI-PC IS BOMBARDED BY THE IONIC AVALANCHES

Fig. 1

Table 1

2 PCs of 78x19 cm2

0.3 m2

ISOBUTANMWPCNA44-SPS

completed

4 PCs of 64x40 cm2

1 m2

C6F14MWPCSTAR-RHIC

completed

20 PCs of 30X30 cm2

1.8 m2GEMPHENIX

In prep

3 PCs of 64x40 cm2

0.7 m2

C6F14MWPCHALL1-JLAB

running

42 PCs of 64X40 cm2

11 m2

C6F14MWPCALICE-HMPID

In assembly

16 PCs of 58x58 cm2

6 m2

C4F10MWPCCOMPASS-CERN

running

6 PCs of 0,25 m2

1.5 m2

C4F10MWPCHADES-GSI

running

AREA CsI-PC

m2RADIATORDETECTOREXPERIMENT

3

SOME HINTS ABOUT LARGE CsI-PHOTOCATHODESChemical affinity:

• CsI strongly hygroscopic, however QE (quantum efficiency) recovers from short exposure to ambient atmosphere (10-15% hygr.) (see H. Hoeldmoser’s talk at this conference)

• no reaction with gases: O2,noble gases,CO2, CH4,etc (NIM A461,2001,584)

Photocathode production• to achieve a reproducible QE performance over a large series (42 PCs), follow a strictprocedure. Main steps are:

- specific elaboration and cleaning of the PCb-pad substrate- thermal evaporation under vacuum followed by an heat treatment under vacuum- protection of the PC in a protective vessel flushed with Argon

- during any transfer or storage, keep the PC under argon flow,( H2O, O2 <10ppm)

PC QE PERFORMANCE KEPT STABLE OVER SEVERAL YEARS (NIM A515 2003 307)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

5.5 6 6.5 7 7.5 8photon energy [eV]

CsI

ph

oto

cath

od

e Q

E

PC32 (@STAR)

PC33

PC34

PC35

PC37, PC39

PC38

Fig. 2

CsI-QE differential plot for several large PCs obtained from Cherenkov events analysis.Performance has been improved from 1998 production (green curve)

4

AGEING induced at the CsI surface, driven by physisorption mechanisms such as surface photolysis or electrolysis. Ageing is observed under:

- direct exposure to high photon flux (≈10 12-14 ph/s,cm2 ,no elecrical field)(not relevant in most of RICH application)

- ion sputtering from avalanches in the wire chamber under gas gain

STATUS OF AGEING STUDIES- in the 90ies, several lab studies, mixing photon irradiation and ion sputtering, and also exposure to

air (NIM A364,1995,243; A387,1997,154; A454,2000,365)- HEP experiments at SPS test beams, NA44 and STAR using low level irradiation (NIM A502,

2003,76)- COMPASS: exposed to high level irradiation; some damaged spots observed (S. Dalla Torre, this

conference)- HADES-GSI

GOAL OF THIS WORK- study the ageing of the CsI-PC under the only ion sputtering of avalanches- using the final CsI-PCs and detector modules to be installed at the ALICE experiment- CsI-PCs never exposed to ambient atmosphere

EXPERIMENTAL METHOD- irradiate several spots of the PC with Sr90-electrons- record the accumulated charge deposited by the ion avalanches- correlate this charge with the measurement of the CsI-QE at these spots and at non irradiated reference points

5

Sr9

0

Sr9

0

C6

F1

4

Test

beam

CsI

-PC

an

nu

lar C

here

nkov

fid

uci

al zon

e

irra

dia

tion

sp

ot S

r90

Sr9

0

CsI

-PC

an

nu

lar C

here

nkov

fid

uci

al zon

e

irra

dia

tion

sp

ot

EXPERIMENTAL LAYOUT

- RICH detector equipped with a 60x48cm2 CsI-PCanode wires running horizontal (Proto-3)

- small C6F14 radiator defining a Cherenkov

fiducial zone (blue) on the CsI-PC- 3 positions for locating a collimated Sr-90 sourceirradiating spots of 32mm Ø overlapping the fiducial Cherenkov ring

Only 8 wires, overlapping the irradiated spots, are raised at full amplification voltage.The ageing will be effectivealong a stripe, 64 cm long, 35mmwide.

C6F14 radiator

2

1

34

Fig. 3 Fig. 4Proto-3

6

MEASUREMENTS during the irradiation

- Ia = anodic current of the 8 wires raised to HV

(to check possible anode wire ageing)

- I20pads = cathodic current of the 20 pads covered

by the direct irradiation cone. (Pad size: 8.0x8.4mm2)- anode wire rate and pulse height spectra- charge at each cathode pad using the analog

RO GassiplexAs seen in the fig. 6, the charge profile can be measured per pad rows (and pad columns)

Q per pad along X

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

-150 -100 -50 0 50 100 150x mm

Q a

rb u

nit

charge row 37*E-02

charge row 38*E-02

charge row 39*E-02

charge row 40*E-02

charge row 41*E-02

charge row 42*E-02

These elements provide the distribution of the ionic

charge accumulated on the CsI-cathode (mC/cm2,s)to be correlated with the expected QE variation

Sr-90 source collimator

Along the X-coord, the charge profiles extend largely out of thedirect irradiated area (blue lines), due to the multiple scattering of the electrons in the methane and the brehmstralung photons emitted from the thin mylar window

Fig. 5Fig. 6

7

x [mm]

Inorm

y [mm]

stdev Ipc( )

mean Ipc( )0.01

mean Ipc( ) 3.71

LOCAL MEASUREMENTS OF THE CsI QUANTUM EFFICIENCY (QE)

2 methods are used:

-THE VUV-SCANNER

A XY-stepping system allows to scan in vacuum a CsI-PC with a VUV-beam, Ø3-5mm, D2-lamp.

The photocurrent is recorded at every pad and normalized to a reference PM.A pad mapping of a quantity proportionnal to the QE integrated over the VUV-beam spectrum is shown in Fig. 7

- Cherenkov single photon counting (see layout in Fig. 3)Exposing the detector at a pion beam, Cherenkov rings that overlap the irradiated spotsare recorded. Further analysis provide the differential QE plot at selected location in thrCherenkov fiducial area. (see results in fig. 18, 19)

Fig. 7Fig. 8

Position 2

Position1

Position 3

8

IRRADIATIONS SEQUENCE

positionDate

start

Source

MBq

HV

V

Duration

hoursI 8wires

nA

I 20pads

nA

Dose

mC/cm2

Dose rste

mC/cm2,s

1 080604 30 2167 196 661 121 6.4 9.8E-06

2 050404 260 1980 193 563 132 6.8 9.1E-06

3 090604 260 1965 48 543 122 1.57 9.1E-06

4 151104 30 2050 101 27 0.7

Dose = (I20pad x duration[s]) / (20 pads area = 13.44cm2

VUV-SCANS SEQUENCE

position End irradiation VUV scan Days after irrad

position2 13 04 16 04 3

position 1 18 06 09 08 52

Position 2 09 08 118

Position 3 11 06 09 08 59

Position 1 09 11 142

position2 09 11 208

Position 3 09 11 149

Table 2

Table 3

2.24x10121.4x1013Part/(evt m2) inHMPID

14200100Particles/event

8x10102x1013Events

107108Runtime [s]

8x1032x105Rate [s-1]

Pb-Pbp-pALICE operation

The expected dose integrated over 10 years operation in ALICE is evaluated at

0.5 mC/cm2

FOR COMPARISON WITH ALICE EXPOSURE

9

pos 3

x [mm] pos 2

pos 1 y [mm]

pos 3

pos 1

pos 2

y [mm]

x [mm]

Inorm

anode wires

VUV-scans after irradiation at 3 positions (see table 2)

X

Y

“Inefficiency” profiles taken along pad columns (Y-coord) at different X values.The sharp edges fit the stripe of the 8 anode wires under voltage making the ageing effect well restricted to the area where ions impinge the CsI-PC

Y-scan at position 2

0

0.5

1

1.5

2

2.5

3

3.5

-170 -150 -130 -110 -90 -70Y [mm] at different X

Iref

x= -88mm

x= -72mm

x= -55,7mm

x= -39.4mm

x= -23.1mm

x= -6.8mm

x= 9.5mm

x= 26mm

x= 42mm

8 ANODE WIRES UNDER HV

Fig. 9

Fig. 10

Fig. 11

10

NOVEL BUT UNDESIRED EFFECT:

IT HAS BEEN OBSERVED THAT THE DEGRADATION OF THE QE INCREASES WITH THE TIME ELAPSED AFTER THE IRRADIATION IN ABSENCE OF GAS GAIN OR FURTHER IRRADIATION .

THIS BEHAVIOUR HAS BEEN FOUND AT THE 3 POSITIONS OF IRRADIATION WHILE NON-IRRADIATEDSPOTS WERE CHECKED TO BE STABLE

Fig. 12

0

10

20

30

40

50

60

70

80

90

0 20 40 60 80 100 120 140 160 180 200 220 240

time after irradiation [day]

dec

reas

e in

no

rmal

ized

cu

rren

t [%

]

pos2: 6.8 mC/cm2 = dose equivalent to 135 years of ALICE

pos3: 1.6 mC/cm2 = dose equivalent to 31 years of ALICE

pos1:6.4 mC/cm2 =dose equivalent to 125 years of ALICE

11

3 X-scans at position 10

0.5

1

1.5

2

2.5

3

3.5

-70 -20 30 80 130 180 230X mm at y=122.75mm

I n

orm

I pos1 1stscan

I pos1 2d scan

I pos1 3d scan

4 X-scans at position20

0.5

1

1.5

2

2.5

3

3.5

-200 -150 -100 -50 0 50 100

x [mm] at y=-121.85mm

I n

orm

Ipos2-1rst scan

Ipos2-2d scan

Ipos2-3d scan

Ipos2-4th scan

3 Y-scans in position 1

0

0.5

1

1.5

2

2.5

3

3.5

60 80 100 120 140 160 180Y [mm] at x=99.85

I n

orm

I norm1st scan

Inorm 2d scan

Inorm 3d scan

4 Y-scans at postion2

0

0.5

1

1.5

2

2.5

3

3.5

-180 -160 -140 -120 -100 -80 -60Y [mm] at X=-23.1 mm

Iref

I Norm 1st scan

I Norm 2d scan

I Norm 3d scan

I norm 4th scan

EVOLUTION OF THE INEFFICIENCY WITH THE TIME ELAPSED AFTER THE IRRADIATION

X-PROFILES ALONG X-COORD Y-PROFILES ALONG Y-COORD

POSITION1: TIME ELAPSED after irrad: 52,142,155 DAYS (table 2

POSITION 2:TIME ELAPSED after irrad: 3,118,208,220 DAYS (table 2)

Fig. 13

Fig. 14

12

f_p1_nr2 x( ) F x 0.856 1.695( ) chi2 fi0

fi1

0.404

0 1 2 3 4 5 6 7 8 9 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

measurementlognormal fit

accumulated charge [mC/cm2] within pos 1

decr

ease

in p

hoto

-cur

rent

[%

]

HOW THE CsI-QE DECREASE IS CORRELATED TO THE IONIC DOSE DEPOSITED BY THE IRRADIATION ?

AS SEEN IN FIG. 6,13 AND 14, BOTH, THE CHARGE AND CsI-QE DECREASE ARE MEASURED ALONG THE SAME PAD ROW . THEN, FIG. 15 SHOWS THE VARIATION OF THE CsI-QE DECREASE VS CHARGE ALONG THE X-COORD.THE DEPENDANCE IS NOT FOUND LINEAR. A LOGNORMAL FUNCTION IS ACTUALLY CHOSEN TO FIT THE DATA, SUGGESTING THAT THE LOSS OF PHOTOEMISSIVE SITES IS PROPORTIONAL TOTHE SITE DENSITY

Fig. 15

13

fits of the dose effect with a cumulative lognormal distribution function

0 1 2 3 4 5 6 7 8 9 100

10

20

30

40

50

60

70

80

90

100

lognormal fit 49 dinterpolated measurement 49 dlognormal fit 129 dinterpolated measurement 129 dlognormal fit 2 dinterpolated measurement 2d

integrated dose [mC/cm2]

QE

dec

reas

e [%

]

0 1 2 3 4 5 6 7 8 9 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

measurementlognormal fitcomparison to previous method

accumulated charge [mC/cm2] within pos 1

decr

ease

in p

hoto

-cur

rent

[%

]

f_p1_nr2 x( ) F x 0.856 1.695( ) chi2 fi0

fi1

0.404

Fig. 16

Fig. 17

THE DEPENDANCE OF THE CsI-QE DECREASE VS CHARGE CAN ALSO BE OBTAINED FROMTHE MEASUREMENTS MADE AT DIFFERENT DOSES AND POSITIONS (SEE FIG. 12) WHEN TAKEN AT THE SAME TIME ELAPSED AFTER IRRADIATION

AS SHOWN IN FIG. 16, THE FITTING FUNCTIONS ARE NOT THE SAME AT DIFFERENT TIMESA GOOD AGREEMENT (FIG. 17) WITH THE PREVIOUS METHOD (FIG. 15) IS ONLY OBTAINED FOR THE SAME ELAPSED TIME, OF 129 DAYS IN THIS PLOT.SUCH A BEHAVIOUR MIGHT BE CORRELATED WITH THE INCREASE OF THE AGEING EFFECT WITH TIME

expected dose for 10 y ALICE operation

(0.50 mC/cm2)

14

1

2

31

2

3

SIMULATION

1

3

2

x (pads)

y (p

ads)

Test beam with PROTO 3 in agreement with QE decrease measure by VUV scanner

The differential CsI-QE curve has to scaled down as seen in Fig. 18 to obtain a simulation close to the data (Fig. 19)

COMPARISON BETWEEN THE VUV-SCANS AND THE CHERENKOV PHOTON COUNTING

Fig. 19

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

5 5.5 6 6.5 7 7.5 8 8.5

CsI QE

ph

oto

n e

ner

gy

[eV

]

PC39

zone 1

zone 2

zone 3

Fig. 18

15

COMMENTS

THESE PRELIMINARY RESULTS ARE RAISING SEVERAL QUESTION MARKS

• WHY THE CsI-QE DECREASE EVOLVE AFTER IRRADIATION ?

• IS THE RATE OF AGEING INCREASE WITH TIME CORRELATED WITH THE INITIAL DOSE ?

• IS THE AGEING DRIVEN BY THE PARTICLE RATE OR BY THE INTEGRATED CHARGE ?

• IS IT STILL AN INFLUENCE OF RESIDUAL ADSORBED WATER ?