All the development described in these pages have been made with detectors built at CERN making use of GEM foils manufactured by CERN's printed circuit workshop. The GDD group has usually a small stock of small and medium-size GEMs, with an active area of 27x27 to 100x100 mm2, made on a 50 µm kapton support with 5 µm copper on each side. The hole pattern is as described in the GENERAL section, with 70 µm holes at 140 µm pitch. Samples can be obtained at a nominal cost (mail to Fabio Sauli). The foils can be provided raw or framed; size and geometry of the GEMs as well as a short description of an assembled detector are described in the attached pdf document.
For larger quantities, different shapes or geometry, contact directly the CERN EP-DT-EF section leader, Rui De Oliveira (Rui.de.Oliveira@cern.ch). See also:
Other groups have manufactured GEMs or obtained them from industry.
All GEMs are electrically tested at CERN before mounting or shipping:
- at the printed circuit workshop with an ohmmeter: accepted if the resistivity in air between the two sides exceeds 2 GOhm;
- in our clean room in dry air (< 30% humidity): accepted if the leakage current is below ~5 nA at 500 V (R>100 GOhm). Sometimes this value is reached only gradually (the voltage is increased in steps, the current decreasing with time at each step). Residual dust is often "burned away" with sudden micro-discharges during the voltage ramp-up. In rare cases we have observe high current or shorts (see below); these GEMs are retreated or rejected.
The presence of a few "blanks" or "whites" (local absence of the metal and underlying kapton), due to unavoidable inhomogenety in the material, has not consequences in the general operating behavior. Care should be taken however in handling and mounting not to offend these regions.
In the operating gas (e.g. Ar-CO2) the leakage current is below 1 nA at 500 V for standard 10x10 cm2foils.
If necessary because of exposure to dust or excessive current, the following procedure is recommended: in a clean room the GEM is washed in ethyl alcohol and suspended until dry. A second de-ionized water bath may follow. Gentle blowing with nitrogen helps. In some cases (persistent short presumably due to metallic fragments detaching from the holes) we recovered the GEM by immersion in a mild copper etchant, in conditions providing removal of part (less than one half) of the metal thickness. Standard washing procedures as used for printed circuit should follow.