Gas Electron Multiplier (GEM)

INTRODUCTION

A gas electron multiplier (GEM) consists of a thin, metal-clad polymer foil, chemically pierced by a high density of holes. On application of a difference of potential between the two electrodes, electrons released by radiation in the gas on one side of the structure drift into the holes, multiply and transfer to a collection region.

The multiplier can be used as a detection unit on its own, or as a preamplifier in a multiple structure; in this case, it permits to reach large overall gains in harsh radiation environment.

 CERN produces and sells GEM foils to users inside and outside CERN for use in research and development studies. GEM foils have been developed at CERN in tight collaboration between TS/DEM and PH/GDD, with Fabio Sauli, the inventor of the GEM technology.

Example of an unframed 100x100mm GEM foil

 The use of GEM foils as Radiation Detectors is covered by two patents: Compteurs à Trous (CAT) (US 5 742 061) owned by CNRS and Gas Electron Multiplier (GEM) (WO9921211) owned by CERN.

Such use requires a license. CERN grants royalty-free licenses for Research and Development use of GEM foils. The license conditions applicable to R&D use of GEM foils are accessible.

Any commercial use of GEM foils is subject to acquiring a commercial license from CERN, on conditions to be agreed (e.g. license included in the price of each GEM sold). The license is provided by the DSU/Technology Transfer Unit at CERN. E-mail: HelpDesk-TT@cern.ch or contact Mr. Bernard Denis.

SALES

GEM modules of different sizes can be bought from the CERN Stores. Users with a CERN budget code can order directly from the Stores. Outside companies should contact Mr. Bernard Denis from the Technology Transfer service for details on how to obtain those modules. For requests different than those standard products, please contact R. De Oliveira.

Standard GEMs

SCEM number
(catalog page)

Type
(one-page description)

Engineering
drawings

Active area
[mm]

Pitch
[μm]

Outer
Hole size
[μm]

Finish

Framed
08.82.00.050.8 GEM-50x50-140-70/50-P-U

GEM 50x50

50 x 50

 140

70

Passivated

no
08.82.00.051.7 GEM-50x50-140-70/50-P-F

GEM 50x50

50 x 50

 140

70

Passivated

yes
08.82.00.100.5 GEM-100x100-140-70/50-P-U

GEM 100x100

100 x 100

 140

70

Passivated

no
08.82.00.101.4 GEM-100x100-140-70/50-P-F

GEM 100x100

100 x 100

 140

70

Passivated

yes

General features

GEM naming scheme

GEM-<width>x<length>_<pitch>_<outside_hole_size>/<inside_hole_size>_<finish>_<framed/unframed>

Parameter

Description

Value
width x length or width and length of active area in mm
Rxx Round GEM, diameter in mm
pitch distance between centres of holes in μm
outside_hole_size size of holes in copper in μm
inside_hole_size size of holes in kapton in μm
finish metal finishing P: passivated, AU: Nickel-Gold
framed framing F: framed, U: unframed

Example: "GEM 100x100-140-70/50-P-F" is a GEM with a size of 100 by 100mm, 140 μm pitch between 70 μm holes that is passivated and framed. "GEM-R10-140-70/50-P-U" is a similar GEM, but round with a 10mm diameter, and unframed.

Finish: standard Cu GEM foils are used for most of the applications - we've found that Ni/Au passivated GEMs are less robust (the maximum gain before discharge is lower than standard GEMs). Ni-Gold plated GEMs are used for UV photon detection where one of the GEM electrodes is covered with a photosensitive layer to avoid chemical interactions between the metal and photocathode. It is true that gold plated surfaces are optically better quality and don't degrade with time, but for standard applications due to robustness and price arguments the passivated ones are the best choice.

STATUS

18 April 2003

Agreement with CERN Stores about distribution (C. Saint-Jal and J-P. Lyonne).
The Stores will order 100 of each of the four proposed standard GEMs.
4 July 2003 Assignment of SCEM numbers
16 January 2004 New designs in EDMS
4 March 2005 Put in place solid system with TT for sales to outside companies
12 July 2005 Doubled stock in Stores to reduce delivery times
21 September 2006 CERN GEM considered best in quality. Lowest charge-up effects and leakage currents.
18 December 2006 CERN provides licences that cover both CAT and GEM patents, easing any commercial and legal issues.

DOCUMENTATION

CONTACTS

Licencing

Sales

Technical information