Lab tests of Thick GEMs THGEM S Dalla

Lab tests of Thick GEMs (THGEM) S. Dalla Torre, Elena Rocco, L. Ropelewski, F. Tessarotto May – August 2007

Outline: Geometry of the THGEM tested; Sources & Setup; First discouraging results; Different geometry and encouraging results; The rim effect; Conclusions.

GEM Principle Ions 5 µm 50 µm 40 % 60 % 55 µm 70 µm GEM hole cross section Electrons Avalanche simulation

Some THGEM pictures W 2: D=0. 3 mm Pitch=0. 7 mm Rim=0. 1 mm Thick=0. 4 mm R 3 W 2 P 1: D=0. 8 mm Pitch=2 mm Rim=0. 04 mm Thick=1 mm P 1 R 3: D=0. 2 mm Pitch=0. 5 mm Rim=0. 01 mm Thick=0. 2 mm R 3 section

Sources & Parameters of the THGEMs used THGE M Diameter (mm) Pitch (mm) Rim (mm) Thick (mm) Sources W 1 0. 3 0. 8 0. 1 0. 4 W 2 0. 3 0. 7 0. 1 0. 4 *P 1 0. 8 2 0. 04 1 *P 2 0. 8 2 0 1 R 3 0. 2 0. 5 0. 01 0. 2 R 4 0. 3 0. 7 0 0. 4 55 Fe X-Ray (Cu) Photons energy Average number of primary electrons in Ar/CO 2 (70/30) Rates available 5. 87 Ke. V 210 W/O collimation up to 300 Hz 320 With collimation (1 mm of diameter) up to 120 KHz 8. 8 Ke. V, 8. 9 Ke. V *Except for the Pi geometry we always used 30/70 CO 2/Ar gas mixture !!

Structure of the chamber used for testing GAS INLET Non segmented anode (copper foil); Inlet and outlet (on the cover) for the gas; Flux gas of 5 l/h. Section view of the structure inside the chamber DRIFT d_drift THGEM DRIFT d_ind ANODE IMPORTANT: Before installing bath, backing in the oven of the THGEM to avoid leakage current. THGEM

Cu X-Ray setup

Electronics Setup and acquisition Power Supply FAN I/O Le Croy 428 F CAEN 471 A THGEM in the chamber 142 A ORTEC Preamplifier Gas system with mass flow meter mixing (30%CO 2 70% Ar) G 472 ORTEC Amplifier DISCRIMINATOR Le SCALER CAEN Croy 821 N 145 Delay ADC (LRS 2259 12 ch) + DAQ (CAEN controller C 111) Digital Oscilloscope

One of the first trial: scan in time @ 40 Hz d=0. 3 mm with the THGEM characterized by the “Weizmann geometry” Rim=0. 1 mm Pitch=0. 7 mm Gain variations larger than a factor of 2 ! @ out of the ADC range W 2

Our best result so far with R 3 (d=0. 2 mm pitch=0. 5 mm rim=0. 01 thick=0. 2 mm) 55 Fe Source Uncollimated Rate =260 Hz R 3 X-Ray Source Collimated Rate =6. 6 KHz <10% 15% Long time scan (~ 4 days) Short time scan (< 1 days)

Comparison between different sources R 3 primary e- in 55 Fe R= primary e- in X-Ray R= 210 = 320 0. 66 ADC ch. peak position with 55 Fe R=ADC ch. peak position with X-Ray 417. 8 R= RATE=460 Hz 563. 5 = 0. 74

Rate capability R 3 Rate effect on signal amplitude: ~ 20%, varying the rate by 3 orders of magnitude! Also, from current measurement gain ~ 700

Gain Estimation for different signal amplitudes R 3 ‘ Cut spectrum due to the threshold on the discriminator giving the trigger signal Gain const e –V/t t ~ 50 V

Rim effect Is this dramatic gain increase with time a rim effect? (Recall that the increase is much smaller with 10 micron rim). Try thicker THGEM, larger holes w/o rim … We come back to the Weizmann geometry (d=0. 3 mm, pith=0. 7 mm, thick=0. 4), but w/o rim

NO gain increase with time ! but again stability only at moderate gains (~ 700) next step: chemical polishing to remove sharp edges and asperities due to copper drilling.

Time stability- THGEM polished Residuals after mechanical drilling chemically Before chemical polishing After chemical polishing THEN WE CREATE A SMALL RIM!!!

Rate capability – THGEM polished chemically

Conclusions: In the very near future we are : Characterizing a DOUBLE THGEM configuration; Measuring the first THGEM coated with the Cs. I in the test beam; The work is in progress (and promising!), but there’s still a long way to go: Geometry role; Technological production; Different gasses….