CsI-TGEM vs. CsI-MWPC photodetector

Some part of this work was performed in collaboration between CERN and Breskin

group

There are two options for planar photodetectors:

TGEMs/RETGEMs(see P. Martinengo talk)

MWPC

(Currently used in ALICE and COMPASS RICH)

or

Some considerations to be taken into account:

I) Cherenkov light detection efficiency II) Discharges at low ratesIII) Discharges at high rate (additional gain drop withrate, cathode exciataion effectIV) Gases

I) Cherenkov light detection efficiency

QTGEM=Qvac εextr(E,λ)

QE in gas QE in vacuum

Fractionof photoelectronsextracted from PC(depends on E and gas)

Npe=QTGEM (λ) I(λ)fpe

fpe~ exp(-Ath/A0

For MWPC:

Εextr measurements

Such curves were measures by many authors, see for example

the lates publicationJ. Escada et al., JINST 4:P11025,2009

QTGEM=Qvac εextr(E,λ) Seffεcoll (A)

QE in gas QE in vacuum

Active area

Fractionof photoelectronsextracted from PC(depends on E and gas)

Fractionof photoelectronscollected into the TGEMholes- depends ongas gain

Npe=QTGEM (λ) I(λ)fpe

fpe~ exp(-Ath/A0

For a single TGEM/RETGEM

QTGEM=Qvac εextr(E,λ) Seffεcoll (A) εtransf1.. εtransf1

QE in gas QE in vacuum

Active area

Fractionof photoelectronsextracted from PC(depends on E and gas)

Fractionof photoelectronscollected into the TGEMholes- depends ongas gain

For cascaded TGEMs/RETGEMs

=1 =1

In avalanche mode

0.0 0.5 1.0 1.5 2.0 2.5 3.00.4

0.5

0.6

0.7

0.8

0.9

1.0Atm. pressureGas flow mode

Ext

ract

ion e

ffic

iency

EDrift

(kV/cm)

CH4 CF4 Ne/10% CF4 Ne/5% CF4 Ne/10% CH4 Ne/5% CH4 Ar/5% CH4

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.30

1

2

3

4

5

Ele

ctric

File

d (k

V/c

m)

Distance from center between holes (mm)

VTHGEM

= 500 V

VTHGEM

= 700 V

VTHGEM

= 900 V

Electric filed on the top TGEM electrode

Εextr:estimations in the case

of TGEM/RETGEM

C. Azevedo , et al., 2010 JINST 5 P01002

εcoll (A) measurements in Breskin group(single photoelectron mode)

C. Azevedo , et al., JINST 5 P01002, 2010

εcoll (A) measurements in our group at CERN(pulsed mode)

ΔIsat

ΔIback= ΔIsat εcoll A

εcoll (A)= ΔIback/ AΔIsat

Pulsed D2 lamp

Drift mesh

Back plane

TGEMCsI

Avalanche

εcoll (A): results of indirect measurements in laboratory

QTGEM≈65%QMWPC

Beam

Cherenkov light

40mm

3mm

3mm

4.5mm

Drift gap 10mm

R/O pads 8x8 mm2

Front end electronics (Gassiplex + ALICE HMPID R/O + DATE + AMORE)

CsI layerDrift mesh

Ne/CH4 90/10

Indirect QTGEM measurements on the beam(Summer2010)

4 mm CaF2 window

~20o ~37o

Time after coating [h]

Normalized photocurrent

CsI quality control

HMPIDlevel

Analysis of the beam test data shows that for the given geometrical layout the QE of TGEM (after geometrical corrections) is compatible to one of the HMPID (which confirms the scan data!)

Monte Carlo simulations well reproduce the experimental data

QTGEM≈70-80%QMWPC

Ne/CH4 Ne/CF4

2

3

4

5

1

Pad plain

RETGEMs

CsI

Drift mesh

Wasmanufactured

New,exists

Old,exist

Wasmodified

Old,exist

Proto-4(schematic side view)

~70

3

3

3

~60

Rc= ~135~30

11

30

Direct QTGEM measurements (November 2011)

C6F14 radiator

135

The top view of the frame #3 (from the electronics side)

Feethroughts RETGEM supporting flame

New holes

Cherenkovring

TGEMs

4

3

1

2

5

6

Data are still under analysis, however qualitatively QETGEM≈ 50%QMWPC

Raether limit:Amaxn0=Qmax=106-107 electrons,

where n0 is the number of primary electrons created in

the drift region of the detector

(Qmax depends on the detector geometry and the gas composition/track density)

II.1) Discharges in TGEMs/RETGEMs Low rate

General curve

High rates

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

1.0E+01 1.0E+03 1.0E+05 1.0E+07

Counting rate density (counts/s/mm2)

To

tal c

ha

rge

in

av

ala

nc

he

(e

lec

tro

ns

)

1

2

3

45

6

7

Forbidden region(by breakdown)

General limit for all micropattern gaseous detectors:(a very similar curves were measured for MWPC as well,

however the physics behind is different)

See also : P. Fonte et al, NIM A419,1998, 405;Yu. Ivaniouchenkov et al, NIM A422,1999, 300;P. Fonte et al., Nuclear Science, IEEE Transactions 46, Issue 3, Part 1, June 1999 Page(s):321 - 325P. Fonte et al ICFA Insrum Bull., Summer 1998 issue, SLAC-JOURNAL, ICFA-16

For physicsbehind this effectsee V. Peskov et al., arXiv:0911.0463 ,2009

V. Peskov et al., JINST 5 P11004, 2010

Results obtained wit bare (not coated with CsI) TGEMS

100 102 104 106 108104

106

108

T

ota

l a

vala

nch

e c

ha

rge

[e]

counting rate [Hz/mm2]

Single

Double

Triple Sparks region

Rate effect for CsI coated TGEMs

Alice region

II.2 Discharges in MWPC

The maximum gain is determined by a feedback loop

Discharges in thin wire detectors

Primary avalanche

Small gain

VUV photons

Larger gain

Secondary photoelectrons

Secondary avalanches

Aγ=1(Aγph=1 or Aγ+=1)

Geiger mode in quenched gases

Geiger discharge is not damaging. One can observed signals~1V directly on 1MΩ input of the scope (no amplifier is needed)

cfront=106-107 cm/s

This discharge is not destructive because there is no any

conductive bridge between the anode and the cathode

Space charge effect on gas amplification. In this figure taken from [A.H. Walenta, Physica Scripta 23, 1981, 354 ] G/G0 is the gas gain

relative to zero counting rate, Q is a total charge in a single avalanche and n is particle rate/wire length.

Rate effect in MWPC

V. Peskov et al., JINST 5 P11004, 2010

Discharges in thick wire detectors

Anode wire(grounded via amplifier)

Cylindrical cathode

Avalanche-V

Transition to streamer occurs whenAn0≥Qmax=108electros

Self-quenched streamer

Strimers give huge amplitudes but the are not harmful as well

Streamers cannot propagateto the cathode because theelectric field drops as 1/r

Streamer

Signal’s amplitude in proportional and streamer modes

Avalanche Streamer

…so in practice:

in bare MWPC steamers (and streamers triggered discharges) may appear in “weak”

regions (not well stretched wire, dust and cetera) or regions of dielectric surfaces

The maximum achievable gain is determined by afeedback loop: Am=1/γ,where γ is a probability of creation secondary electrons(as a results there are no sparks in presence of Ru or Fe source)

Typical results obtained with CsI-MWPC:

Voltage (V)

Gain

III.3. Cathode excitation effect

1.00E-07

1.00E-05

1.00E-03

1.00E-01 0 100 200 300 400 500 600

Wavelength (nm)

QE

Quantum efficiency vs. wavelength of metal (rhombus) and CsI (triangles) cathodes measured in as ingle-wire counter before a corona discharge (solid symbols) and immediately after the corona discharge was interrupted

0.001

0.01

0.1

1

0 10 20 30 40 50

Time (min)

Q a

t 546 n

m (

arb

un

its)

Changes in QE vs. time for Cu (rhombus) and CsI (triangles) photocathodes

V. Peskov et al., arXiv:0911.0463 , 2009

Cathode excitation effect in CsI single wire-counters

CsI pc

Metal pc

CsI pc

Metal pc

0 50 100 150 200

0

100

200

300

400

Cou

ntin

g ra

te [

Hz]

Time [min.]

Ne+10%CH4

gain raised to 105 after intense X-ray irradiation

After-pulsesvisible-photon pulses

CsI-coated Triple-THGEM:the cathode excitation effect

0 50 100 150 2000

200

400

600

800

1000

coun

ting

rate

[Hz]

Time [min.]

MWPC with CsI

visible-photon pulsesafter pulses

Conclusion: triple TGEM less suffering from the cathode excitation effect than MWPC

Continuous discharge is possible due to thecathode excitation effect

Recent measurements with CsI-MWPC and with CsI-TGEM

IV.Gases

0 1000 2000 300010-3

10-1

101

103

105

107

Gai

n

THGEM HV [V]

Ne/10%CF4

CF4

current mode UV light

CsI-MWPC can efficiently operate only in CH4. This arise safety concernsIn contrast TGEMs/RETGEMs can operate in many gases. This open possibility to use windowless design when GEM or TGEM/RETGEM operates in the same gas as uses for the Cherenkov radiator

Conclusions:

●CsI MWPC reached their operational limit in gain ( 5x104 ) and in QE (80-90% of Qvac)● Discharges are possible in MWPC due to the design features, defects and cathode excitation effect● In contrast TGEMS/RETGEMs have several advantages and unexploited yet potentials:higher gain, possibility to increase Aeff and thus QE, wider choice of gases, possibility to exploit windowless designs,less troubles from the cathode excitation effect… and more● Thus it looks that TGEM/TRETGEMs is an attractive option for some gaseous RICH detectors and for this reason is still under consideration and studies for the ALICE RICH upgrade

Spare

TGEM+MWOC option (sugested by Hungarin team)

Feature #4Cathode excitation effect

(closely related to the rate effect physics-see

P. Fonte et al.,IEEE Nuc.Sci46,1999,321)