1. Status of simulation studies of IBF for GEMs
Taku Gunji
Center for Nuclear Studies
University of Tokyo
Thanks a lot for much discussion with our ALICE GEM-TPC upgrade colleagues

2. Motivation
In gaseous avalanche detectors, back drifting ions generated by avalanches to cathode can limit the detector performance and lifetime.
This is the issue for TPC and gaseous photomultipliers under high rate operations.
GEM is very attractive to suppress IBF.
Multi-GEM layers
Exotic GEMs (Flower GEM, MHSP, COBRA GEMs)
This work is to do the systematic studies of IBF and to search for the optimal configurations for IBF.
<0.25% IBF at gain = 2000 (without gaiting grid for the ALICE GEM-TPC upgrade)
with keeping high efficiency and good energy resolution

3. Simulation status (in 2012)
First simulation studies and comparison with the measurements.
No good agreement. Our reference was (partially) biased by the space-charge effect (rate dependence on IBF).
Investigation of space-charge effect to the IBF
Qualitative agreement. More studies are needed.

4. Update since then
(No update on dynamical space-charge simulation for IBF studies…)
Systematic studies to search for the optimal solutions of IBF.
Play with multi-GEM layers and the fields under Ar and Ne based gas mixture
3 or 4 GEM layers
Effect of hole alignment between different layers
Play with different types of GEMs and exotic GEMs
Conical GEMs, Flower GEM, COBRA GEMs

5. 3 GEMs (Et1 and Et2 scan) e
High Et
Low Et
IBF with 3 GEMs: Et1 and Et2 scan in Ar/CO2(70/30)
VGEM1=260, VGEM2=360, VGEM3=460. GEM1&2 mis-aligned.
% with high Et1 (6kV/cm) and low Et2 (0.5kV/cm)
Ions from GEM1 and GEM2 are main contributors.

6. 3 GEMs (VGEM2 and VGEM3 scan)
IBF with 3 GEMs: VGEM2 and VGEM3 scan in Ar/CO2(70/30)
Et1=6kV/cm, Et2=0.5kV/cm, GEM1&2 mis-aligned.
% as VGEM2 << VGEM3

7. Conical GEMs 0.2-0.3% of IBF can be possible (for Ar/CO2)
Further decrease of IBF by different pitch/hole or shape?
GEMs with conical hole shape
Conical GEM with upper hole size < lower hole size is preferable? (in terms of extraction of electrons and IBF)

8. IBF with Conical GEMs
IBF with 3 GEMs under Et1=6kV/cm and Et2=0.5kV/cm
Use conical GEMs for GEM1 and GEM3.
x2-3 improvement by conical GEMs (40/70um for GEM1 and 40-70/70um for GEM3)

9. 3 GEMs in Ne/CO2 IBF with 3 GEMs under Ne/CO2 (90/10)(= ALICE-TPC)
High Et
Low Et
IBF with 3 GEMs under Ne/CO2 (90/10)(= ALICE-TPC)
Multiplication under Et=6kV/cm (M=7 in 2mm gap)
1-2% of IBF under Et1=4kV/cm and Et2=0.5kV/cm
Adding N2 (5-10%) to achieve higher Et

10. 3 GEMs in Ne/CO2/N2 IBF with 3 GEMs under Ne/CO2/N2(90/5/10)
Multiplication under Et=6kV/cm (M=1.3 in 2mm gap)
0.6-2% of IBF under Et1=4kV/cm and Et2=0.5kV/cm

11. 4 GEMs in Ne/CO2 IBF with 4 GEMs. x2-x4 improvement of IBF
High Et
Low Et
High Et
IBF with 4 GEMs. x2-x4 improvement of IBF
0.5-1% of IBF with 4 GEM configurations under high Et3 and low Et2.

12. 4 GEMs in Ne/CO2/N2 IBF with 4 GEMs. x2-x4 improvement of IBF
0.3-1% of IBF with 4 GEM configurations under high Et3 and low Et2.

13. Effect of Hole alignment
IBF with 3 GEMs. Ne/CO2 (90/10)
IBF vs. hole distance between GEM1 and GEM2
Strong alignment dependence (Et1>2-4kV/cm)
No effect of the alignment under Et=1kV/cm
aligned
mis-aligned
Et1=1kV/cm
Et1=2kV/cm
Et1=4kV/cm

14. Next Steps for IBF with std. GEMs
IBF with high Et depends on the hole alignment.
Need to randomize the alignment (for the real case)
to be able to compare the measurements and to search for the optimal solutions under realistic conditions.
In this case, alignment depends on the hole position and need to do simulations by generating the seeds over many places…
More effective way compared to this procedure?
Ne/CO2=(90/10)
Real data
Gain=
from Munich

15. Another types of GEMs Flower-GEM: Mis-aligned holes between GEM1-GEM2
Absorb ions at GEM1 Bottom under high Et
Promising results by F. Tessarotto (INFN-Trieste)
x10 decrease of IBF from Et=0.8kV/cm  2.4kV/cm
(Mini-week on June, 2012)
F. Tessarotto (INFN-Trieste)

16. Another types of GEMs Thick-COBRA-GEM:
Additional pattern on the electrode
Effectively blocking ions
with VAC. x10 improvement
of IBF/layer.
J.F.C.A Velosa et al., NIMA 639, 2011,
J.F.C.A Velosa et al., NIMA 639, 2011,
A. Lyashenko et al. NIMA 598 (2009) 116

17. R&D of Thick COBRA GEM in CNS-Tokyo
Thick double COBRA GEM by CNS-Tokyo and SciEnergy
t=0.2mm/0.4mm, pitch=0.5mm/1mm
First measurements on IBF and
comparison with quick simulations
Ar/CF4(95/5)
Simulation
(geometry is not
identical)
100/50um
Suppression of IBF x7/x4
depending on drift space.
Detailed studies with
simulations will be done.

18. IBF of thick COBRA vs. Ed/Drift space
IBF at fixed VAC strongly depends on Ed and drift space
Changing VAC leads:
Potential difference above GEM(A) and COBRA(C) gets larger
Decrease absolute potential around GEM
Ed decreases much if Ed is small or drift space is small. (Ed*d is small)!!
If Ed*d is much larger than VAC, IBF is not so much improved.
drift space
edge of the GEM

19. IBF with exotic GEMs IBF simulations with Flower GEM and COBRA GEMs.
0.2% of IBF with Flower GEM (2,3 GEMs) and 3 COBRA GEMs
mis-align
1 STD GEM + 1 cobra GEM
align
mis-align of
cobra-GEM2

20. Summary and Outlook
Search for the optimal solutions for IBF with GEMs in simulations.
Improvement of IBF by x2-x4 with 4 GEMs and high Et1/Et3 and x2-x3 with conical GEM.
However, hole alignment is an issue under the operation of higher Et1 (and Et3).
Unlikely to adjust the hole alignment for standard GEMs (large GEMs)….
Rely on the random alignment? Uniformity? Combination of different GEMs with hole size and pitch?
Go back to tune under Et=1kV/cm (no alignment effect..)? Extraction?
Still room for the thoughts…. Comparison with the measurements.
0.2% of IBF is possible with exotic GEMs (Flower GEM, Thick COBRA GEM)
For thick(-COBRA)-GEM, less difficulty in mis-alignment (framed, robust).
IBF seems to be ok. Need to verify another issues (collection of electrons, energy resolution, stability and charge-up)

21. Backup slides

22. Triple THGEM configurationD
7.6 mm 1
700 V
2.5 mm 2
1575 V
2.5 mm 3
1575 V
2.0 mm
400 V A
Flipping the central THGEM provides the maximally misaligned configuration
Flipping the central M2.4 provides the maximally misaligned configuration
tests performed in aligned and misaligned configuration
all parameters have been varied, all currents + signal amplitudes measured
RD51 miniweek, WG1, CERN, /06/2012, IBF studies with THGEMs Fulvio TESSAROTTO

23. IBF for misaligned THGEMs
Numbers are IBF%
RD51 miniweek, WG1, CERN, /06/2012, IBF studies with THGEMs Fulvio TESSAROTTO