by Piotr Gasik & Laura Fabbietti . Published: 27 May 2015

The largest detector ever built based on the GEM technology was assembled in the past few weeks at the Physik-Department of Technische Universität München (TUM) and CERN during a “School of ROC”. This ambitious enterprise was started and coordinated by Dr. Piotr Gasik and Dr. Robert Muenzer, both very active scientists within the ALICE TPC upgrade project at the TUM group.

Piotr had this idea to bring together to TUM and CERN over a period of six weeks 40 physicists, engineers and technicians from eleven institutes worldwide to build the world’s largest prototype of this detector technology” reports Prof. Laura Fabbietti, who leads the TUM group involved in the ALICE TPC upgrade. “This ambitious project should serve at the same time as a training camp for all the participants, to learn how to build a IROC/OROC, and also to build the first prototype of the outer readout chamber (OROC) for the Time Projection Chamber of the ALICE experiment at CERN. And it did work out amazingly well! People learned, had fun and at the end the OROC baby was successfully tested”.

This represents an important milestone for the ALICE TPC upgrade, which is standing at its incipit. In the coming 2.5 years, 72 new readout chambers will be assembled and tested at the participating institutes in Europe and in the USA under the coordination of Dr. Piotr Gasik, who is responsible for the building and quality assurance of the chambers.

After the next big upgrade at the Large Hadron Collider (LHC) at CERN, these new ROCs will be installed in ALICE  replacing the present multiwire proportional chambers in the TPC.

The world’s largest Time Projection Chamber (TPC) is the main component of the experiment ALICE at CERN, which records the tracks and the speed of fast charged particles. These particles are formed when lead ions collide with almost speed of light. Only at these emerging extreme temperatures and particle densities, the strong nuclear force can be overcome so that the protons and neutrons of the lead nuclei “melt” to a plasma of “free” quarks and gluons for a short period of time. After diverging and cooling, several thousand particles such as electrons, photons or pions emerge.

However, the present MWPC (Multi Wire Proportional Chamber) readout of the TPC will no longer meet the increasing readout rate requirements: from 2019 on, the LHC will deliver ion beams with lead ion bundles colliding at a rate of 50 kHz, which is about a factor of 100 above the present readout rate of the TPC. In order to study the properties of the formed quark gluon plasma, it is crucial to characterize all of the resulting particles created in such a collision. Therefore, the increased luminosity can only be exploited by significantly increasing the readout rate of the TPC. It is foreseen to replace the existing MWPC-based readout chambers by Gas Electron Multiplier (GEM) detectors to overcome the rate limitations imposed by the present gated readout scheme.

In order to meet the challenging requirements of the upcoming detector upgrade, an extensive research and development programme lead by Prof. Harald Appelshaeuser from the Frankfurt University is underway. A first milestone has now been reached in April 2015: the prototype of the Outer Readout Chamber (OROC) has been assembled. With an active GEM detector area of 0.68 square meters, this is the largest GEM detector ever built. The prototype for the OROC was commissioned in the framework of the workshop “School of ROC”. It took place from 8 March to 17 April 2015, conducted in cooperation with the Central Technology Laboratory (ZTL) under the direction of Dr. Roman Gernhäuser (TUM).


40 physicists, engineers and technicians from eleven institutes from Germany, Switzerland, Finland, Romania, Hungary, USA and Brazil met at the Physics Department of the TUM and at CERN. The participants tested and mounted twelve GEM foils for the readout chamber, coordinated by Dr. Piotr Gasik, who is also in charge of the building of all the ROCs within the upgrade project. The foils were then assembled at CERN in four layers to form a three-segmented prototype of an external readout chamber. First tests were successful.

The overall active readout detector area sums up to 34 square meters, for which a total of 137 square meters of GEM foil will be needed, with nearly a total of 4 billion holes of micrometer size etched into it. With the experience gained at the workshop, the participants will be mounting and testing the detector segments within the coming months. The new GEM detector is planned to replace the present technology after the LHC upgrade in 2019.

This sets the starting point of a new adventure, as Dr. Werner Riegler, technical coordinator of the ALICE experiment, commented about the new OROC results: “For most of detector builders and developers the oscilloscope picture represents the nice epilogue of an enterprise, for us it is just the beginning”.