Lab tests of Thick GEMs (THGEM) 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 70 µm 55 µm 5 µm 50 µm GEM hole cross sectionAvalanche simulation Electrons Ions 60 % 40 %

Some THGEM pictures R3R3 P1P1 W2W2 P 1 : D=0.8 mm Pitch=2 mm Rim=0.04 mm Thick=1mm R 3 : D=0.2 mm Pitch=0.5 mm Rim=0.01 mm Thick=0.2mm W 2 : D=0.3 mm Pitch=0.7 mm Rim=0.1 mm Thick=0.4mm R 3 section

Sources & Parameters of the THGEMs used THGEM Diameter (mm) Pitch (mm) Rim (mm) Thick (mm) W1W W2W *P *P R3R R4R Sources Photons energy Average number of primary electrons in Ar/CO 2 (70/30) Rates available 55 Fe 5.87 KeV210 W/O collimation up to 300 Hz X-Ray (Cu) 8.8 KeV, 8.9 KeV 320 With collimation (1mm of diameter) up to 120 KHz *Except for the P i geometry we always used 30/70 CO 2 /Ar gas mixture !!

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

Cu X-Ray setup

Electronics Setup and acquisition THGEM in the chamber Power Supply CAEN 471A Gas system with mass flow meter mixing (30%CO 2 70% Ar) 142 A ORTEC Preamplifier G 472 ORTEC Amplifier Digital Oscilloscope ADC (LRS ch) + DAQ (CAEN controller C111) FAN I/O Le Croy 428F DISCRIMINATOR Le Croy 821 SCALER CAEN N 145 Delay

@ out of the ADC range Gain variations larger than a factor of 2 ! W2W2 d=0.3mm Pitch=0.7mm Rim=0.1mm One of the first trial: scan in 40 Hz with the THGEM characterized by the “Weizmann geometry”

Our best result so far with R 3 ( d=0.2mm pitch=0.5mm rim=0.01 thick=0.2mm ) 15% 55 Fe Source Uncollimated Rate =260 Hz Long time scan (~ 4 days) <10% X-Ray Source Collimated Rate =6.6 KHz Short time scan (< 1 days) R3R3

RATE=460 Hz R= primary e- in 55 Fe primary e - in X-Ray R= = 0.66 R= ADC ch. peak position with 55 Fe ADC ch. peak position with X-Ray R= = Comparison between different sources R3R3

Rate capability R3R3 Rate effect on signal amplitude: ~ 20%, varying the rate by 3 orders of magnitude! Also, from current measurement  gain ~ 700

Cut spectrum due to the threshold on the discriminator giving the trigger signal ‘ Gain  const e –V/t t ~ 50V R3R3 Gain Estimation for different signal amplitudes

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 chemically Before chemical polishing After chemical polishing Residuals after mechanical drilling 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 CsI 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….