1. THGEM: Introduction to discussion on UV-detector parameters for RICH
Amos Breskin
Weizmann
Discussion topics:
THGEM: hole-layout geometry?
THGEM : rim or no rim?
THGEM gain: single or double?
Gas?
Fields?
Readout?
Stability of gain?
Rates?  CsI aging?
RTHGEM?
RD51 meeting, Paris, October 2008

2. Double-THGEM photon-imaging detectorUV
Single-photons:
Robust
Single-photon sensitivity
Effective single-photon detection
8ns RMS time resolution
Sub-mm position resolution
>MHz/mm2 rate capability
Cryogenic operation: OK
EDrift
EHole
ETran
EHole
In view of possible applications of THGEM-based imaging detectors we have studied the localization properties of a 2D 10x10 cm2 detector made of two THGEMs in cascade, with a simple 2D readout electrode. The coupling of the THGEMs and the readout are matched via a resistive anode, made of sprayed Carbon on G10 substrate. The detector was operated at atmospheric pressure in Ar/CH4 and its performance was studied with soft keV X-rays. To obtain the best spatial resolution, without modulations in the position response of the periodic RO structure, the geometrical size of the induced signal from the multiplier, typically less than a mm, had to spread out matching the 2mm pitch of the RO structure. The resulting spatial position of the event is defined by the center of gravity of the avalanche
EInd
Induced-signal width matched to readout-pixel size.
One readout solution:
Resistive anode

3. TO RIM OR NOT TO RIM…? Higher gain but: Lower gain but:
Higher charge-up
Studied extensively
Lower gain but:
Low charge-up
Higher effective CsI surface
Need further studies
(e.g. e-collection into holes)

4. geometrical parameters of THGEMs studied at Weizmann
Thickness
t [mm]
Drilled hole
diameter
d [mm]
Etched Cu
[mm]
Pitch
a [mm]
Ref PC
area
[%]
Low (L) or Atm (A)
pressure 1
1.6
1 (no etching) 7 98 L* 2 4 94 3
1.2 92
4, 6
1.5 42
L*+A 5
3.2
0.4
0.5
0.7 56 A 8
0.8 9
0.3 54
1 atm 10
1.0 77 11
2.2
Standard GEM
0.05
0.055
.07
.14
83%
92%
UV: Gain did not vary much with geometry
Active CsI area  larger with no rim

5. THGEM - Gain vs rim size: Ar/5%CH4
pitch = 1 mm; diameter = 0.5 mm;
rim=40; 60; 80; 100; 120 mm
Double-THGEM
6 keV x-rays
104

6. GAIN vs RIM size: TRIESTE results: Ar/30%CO2
104

7. GAIN vs RIM-size in pure Ne
VERY NEW!
GAIN vs RIM-size in pure Ne
104
pitch = 1 mm
diameter = 0.5 mm
thickness = 0.4 mm
rim= microns
9 keV x-rays
pitch = 1 mm; diameter = 0.5 mm; thickness=0.4mm
rim=40; 60; 80; 100; 120 mm
SIMILAR GAIN LIMIT WITH X-RAYS for RIM: microns:
Single-THGEM: gain ~ 5,000
Double-THGEM: gain ~20,000

8. GAIN single- & double-THGEM: UV (recall)
Etrans = 3 kV/cm
104
t=0.4,
d=0.3,
etched d=0.5,
a=0.7,
area 54%
Etrans = 1 kV/cm
2-THGEM: 100 higher gain & lower HV. Etrans affects total gain

9. SINGLE-THGEM: EFFECT OF HOLE-PITCH: UV
(recall)
104
t=0.4,
d=0.3,
etched d=0.5,
a=1.0,
area=77%
t=0.4,
d=0.3,
etched d=0.5,
a=0.7,
area 54%
Single-THGEM: Varying the hole pitch from 0.7 to 1 mm: minor effect on gain

10. e- collection efficiency into holes (recall)
Ref PC
t=0.4,
d=0.3,
etched d=0.5,
a=0.7,
area 54%
DEPENDS ON DIFFUSION  GAS & FIELDS
Large hole smaller diffusion effects  full collection at very low gains compared to standard GEM.

11. e- collection efficiency vs hole-pitch (recall)
d=0.3,
etched d=0.5,
a=1.0,
area=77%
t=0.4,
d=0.3,
etched d=0.5,
a=0.7,
area 54% e
Ref PC
Larger pitch need higher DVTHGEM & higher gain

12. e- extraction efficiency from holes vs E trans (recall)
d=0.3,
etched d=0.5,
a=0.7,
area 54%
VARIES WITH GAS & THGEM PARAMETERS

13. MAX GAIN  hole diameter ~ thickness
GAIN vs HOLE DIAMETER/THICKNESS
104
6keV x-rays
MAX GAIN  hole diameter ~ thickness

14. Reversed Drift-field Focusing is done by hole dipole field.
Relative e
Ref. PC
Edrift E
E=0
MIP
Full transfer efficiency was measured for drift field value equal to 0 and the electron focusing is solely due to the strong dipole electric field established within the THGEM holes. Setting Edrift at slightly reversed (negative) value will reduce the detector’s sensitivity to ionizing background, as all ionizing electrons produce by the background will drift away from the multiplier
Focusing is done by hole dipole field.
Maximum efficiency at Edrift =0 (like in GEM).
Slightly reversed Edrift (50-100V/cm)
good photoelectron collection & low sensitivity to MIPS (~5-10%) !
Attention: gas and field dependent!

15. Photoelectron extraction: effective QE

16. FIELD AT THE THGEM CsI SURFACE
0.4mm thick
0.3mm holes
0.7mm pitch
>3kV/cm
Electric field on photocathode surface
created by the hole dipole field
DVTHGEM=2200V
DVTHGEM=1200V
DVTHGEM=800V e
Ref PC
We may consider a specific possible application of the THGEM for a photon detector with a reflective CsI PC deposited on THGEM top surface; it can be applied for instance for RICH. Here we require a high field on the PC surface in order to have an high QE, a good electron focusing into the holes for high detection efficiency and of course a low sensitivity for ionizing background radiation.
The figure on the right shows a simulation of the electric field on the top surface of the THGEM along the line interconnecting two adjacent hole centres, for various voltage difference value applied across the THGEM. As it is shown, for a value above 800 Volt the field exceeds 3 kV/cm all over the surface. Under this relatively high electric field, in a multiplier layout with a ref PC, the photoelectron backscattering in the gas is low and this guarantees an efficient extraction from the Ref PC into the gas.
High field on the PC surface (high effective QE)
Also at low THGEM voltages (e.g. in Ne mixtures!)
Attention: varies with hole-pitch & hole-voltage
C. Shalem et al. NIM A558 (2006) 468

17. BACKSCATTERING ON GAS: EFFECTIVE QE
No data for Ar/CO2 & Ne/CH4
scintillation
simulation Ne
Noble gases
Breskin et al. NIM A483 (2002) 670
Coelho et al NIMA 581(2007)190

18. Repeated: extraction from CsI pc: Ar+5%CH4
This work
CsI
E=Voltage/2cm
Extraction efficiency:
Ar/5%CH4:
 Same in both works
Breskin et al. NIM A483 (2002) 670

19. Need to add a “quencher” Repeated: extraction from CsI pc: Ne
This work
scintillation
Extraction efficiency:
Depends on Ne purity!
After turbo: 300V/cm
Flushed: 380V/cm
“Static”: ~70% @ 420V/cm
Extr. Efficiency
In ultra-pure Ne
(with getter):
900V/cm
Scintillation limit!
Need to add a
“quencher”
Coelho et al NIMA 581(2007)190

20. Extraction from the CsI pc in: Ne+CH4
This work
Ne/5%CH4
vacuum
Ne/5%CH4: 400V/cm
Ne/23%CH4: 550V/cm
NO SCINTILLATION!
This work
Ne/23%CH4
Similar to Ar/5%CH4
vacuum

21. Gain stability Discussed yesterday
Depend on gas, THGEM-parameters, gain, rate. To determine in “real conditions” with CsI/UV.
Seems better with no rim 6x104
To check with Ne-mixtures and high-rate UV

22. Gain comparison for UV:
WIS: Ar/5%CH4 & Ar/30%CO2, with 0.1mm rim:
1-THGEM 105 & 2-THGEMs 107
Ne, Ne/CH4 1-THGEM
TRIESTE: Ar/30%CO2, with 0 rim:
2-THGEMs 6x104

23. Summary - considerations for a RICH w CsI
1. Gain  Single- vs double-THGEMs
Lower voltage per THGEM
Larger dynamic range – less sensitive to heavily ionizing BGND
Optimize transfer field! (gas-dependent)
2. Gas
Diffusion: affects e- collection eff. into the holes
Vhole: affects effective-QE (photoelectron extraction from CsI)
3. Photon detection efficiency  Effective-QE + e-collection + gain
Effective-QE Hole layout & field at CsI surface
Extraction fields should be calculated vs hole-layout & rim-size & gas
e- collection efficiency should be measured in pulse-counting mode
with no-rim THGEMs in the selected gas (not simple but method known)
4. Gain-stability  rim vs no-rim
5. Induction field  defined by readout type
6. Drift field above CsI slightly reversed to reduce MIPs sensitivity
7. RTHGEM? If in Ne: low HV, stable operation…
8. Max gain in LARGE AREA THGEM (defects limit)
9. RICH tests: how? Who? When?

24. Ne or Ne/CH4 But still to verify: Attractive low voltages
Similar e-extraction efficiency to Ar/CH4
High gain for x rays & UV  dynamic range
But still to verify:
Calculate fields on PC surface to estimate e- extraction eff. in real conditions
large diffusion  e- collection efficiency into holes should be measured.
 optimize transfer field
 Maybe need to increase hole size (loss eff. surface)
 Max gain in rimless THGEM
Raether limit 
verify if lower sensitivity to heavily ionizing BGND