Post on 05-Jan-2016
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)