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 10x10cm2 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 N2 - box) 3 - preparation of the components (cleaning) 4 - stretching & framing a 10x10 cm2 GEM foil (following Frascati procedure) 5 - closing a 10x10cm2 detector SUPPLIED BY US: all 10x10 cm2 GEM & LHCb detector components & tools NEEDS: a large table; some COMPASS or Totem detector components; N2 bottle (w/gas regulator.. ) to flush and clean TIME required: 1 hour

Characterization of a GEM detector GOAL: show how a GEM detector must be operated; the role of the transfer fields (E Drift, E Transfer, E Induction ), the concept of the electron transparency and effective gain. A 10x10cm2 GEM detector is tested in current mode with an Xray-tube. SUPPLIED BY US: the detector the HV (HVGEM - type) the current-meter (1nA resolution) NEEDS: Xray-tube Ar/CO2 (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 A386 (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 V between the two copper sides, an electric field as high as ~100 kV/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.

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

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

Gain Rate Capability Triple-GEM operation

LHCb: fast gas mixtures The intrinsic time spread :  (t) = 1/nv drift, where n is the number of primary clusters per unit length and v drift is the electron drift velocity in the ionization gap. To achieve a fast detector response, high yield and fast gas mixtures are then necessary Garfield: Magboltz + Heed simul. Ar/CO 2 /CF 4 (45/15/40):  10.5 cm/  3.5 kV/cm  5.5 clusters/mm fast & non flammable 9.7ns5.3ns 4.5 ns

2.9ns r.m.s. Time resolution of two chambers in OR Ar/CO 2 /CF 4 =45/15/40 Drift = 3.5 kV/cm Transfer = 3.5 kV/cm Induction = 3.5 kV/cm The performances of a full size detector, in almost final configuration, have been measured at the T11-PS CERN facility. LHCb-GEM: full size detector performances Efficiency measured on the last test beam