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

Double-THGEM photon-imaging detector UV 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

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)

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

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

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

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=40-120 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: 40-120 microns: Single-THGEM: gain ~ 5,000 Double-THGEM: gain ~20,000

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

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

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.

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

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

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

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!

Photoelectron extraction: effective QE

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

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

Repeated: extraction from CsI pc: Ar+5%CH4 This work CsI E=Voltage/2cm Extraction efficiency: Ar/5%CH4: 67%@550V/cm  Same in both works Breskin et al. NIM A483 (2002) 670

Need to add a “quencher” Repeated: extraction from CsI pc: Ne This work scintillation Extraction efficiency: Depends on Ne purity! After turbo: ~50% @ 300V/cm Flushed: ~60% @ 380V/cm “Static”: ~70% @ 420V/cm Extr. Efficiency In ultra-pure Ne (with getter): ~40% @ 900V/cm Scintillation limit! Need to add a “quencher” Coelho et al NIMA 581(2007)190

Extraction from the CsI pc in: Ne+CH4 This work Ne/5%CH4 vacuum Ne/5%CH4: ~53% @ 400V/cm Ne/23%CH4: ~69% @ 550V/cm NO SCINTILLATION! This work Ne/23%CH4 Similar to Ar/5%CH4 vacuum

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

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 105 -106 TRIESTE: Ar/30%CO2, with 0 rim: 2-THGEMs 6x104

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?

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