Construction and Characterization of a GEM G Bencivenni

Construction and Characterization of a GEM G. Bencivenni, LNF-INFN The lesson is divided in two main parts: 1 - construction of a GEM detector (Marco Pistilli) 2 - measurement, in current mode, of the basic parameters of a GEM detector (electron transfer functions, electric field choice. . . ) with an X-ray tube

Construction of a GEM detector 1 - description of typical detector components (including epoxy glue) for differents GEM arrangements: from small 10 x 10 cm 2 GEM to GEM used in the existing experiments (LHCb, Compass or TOTEM – I need Miranda help) 2 - discussion of the preliminary tests on GEM foils and other components (mainly HV test in N 2 - box) 3 - preparation of the components (cleaning) 4 - stretching & framing a 10 x 10 cm 2 GEM foil (following Frascati procedure) 5 - closing a 10 x 10 cm 2 detector SUPPLIED BY US: all 10 x 10 cm 2 GEM & LHCb detector components & tools NEEDS: a large table; some COMPASS or Totem detector components; N 2 bottle (w/gas regulator. . ) to flush and clean TIME required: 1 hour

Characterization of a GEM detector GOAL: show a GEM detector must be operated; the role of the transfer fields (EDrift, ETransfer, EInduction), the concept of the electron transparency and effective gain. A 10 x 10 cm 2 GEM detector is tested in current mode with an Xray-tube. SUPPLIED BY US: the detector the HV (HVGEM - type) the current-meter (1 n. A resolution) NEEDS: Xray-tube Ar/CO 2 (70/30) gas mixture TIME required: 3 hours (especially if we ask students to make the measurements themselves) DOCUMENTATION: slides explaining the measurements to be done

Spare slides

GEM: principle of operation The GEM (Gas Electron Multiplier) [F. Sauli, NIM A 386 (1997) 531] is a thin (50 μm) metal coated kapton foil, perforated by a high density of holes (70 μm diameter, pitch of 140 μm) standard photo-lithographic technology. By applying 400 -500 V between the two copper sides, an electric field as high as ~100 k. V/cm is produced into the holes which act as multiplication channels for electrons produced in the gas by a ionizing particle. Gains up to 1000 can be easily reached with a single GEM foil. Higher gains (and/or safer working conditions) are usually obtained by cascading two or three GEM foils. A Triple-GEM detector is built by inserting three GEM foils between two planar electrodes, which act as the cathode and the anode.

Electron transparency (single-GEM) Cathode Electrons: Ions Drift Field Diffusion Losses Ion trap ~50% signal only due to electron motion ~50% I-out = I-in. G. T (gain x transparency) Induction Field Anode Ion Feedback = I+drift / I-out

Electron transparency (triple-GEM) Ar/CF 4/i-C 4 H 10 = 65/28/7 GEM polarization: 375/365/355 V Gain ~ 20000

Triple-GEM operation Gain Rate Capability

LHCb: fast gas mixtures The intrinsic time spread : s(t) = 1/nvdrift , where n is the number of primary clusters per unit length and vdrift is the electron drift velocity in the ionization gap. Garfield: Magboltz + Heed simul. 9. 7 ns 4. 5 ns To achieve a fast detector response, high yield and fast gas mixtures are then necessary 5. 3 ns 4. 5 ns Ar/CO 2/CF 4 (45/15/40): q 10. 5 cm/ s @ 3. 5 k. V/cm q 5. 5 clusters/mm fast & non flammable

LHCb-GEM: full size detector performances The performances of a full size detector, in almost final configuration, have been measured at the T 11 -PS CERN facility. 2. 9 ns r. m. s. Ar/CO 2/CF 4=45/15/40 Drift = 3. 5 k. V/cm Transfer = 3. 5 k. V/cm Induction = 3. 5 k. V/cm Time resolution of two chambers in OR Efficiency measured on the last test beam