L’irrésistible ascension des détecteurs MPGD

L’invention des détecteurs gazeux à microstructures (MPGD) a ouvert la voie à une nouvelle génération de détecteurs, ayant une très haute définition spatiale et adaptés aux cadences élevées. Leurs autres atouts sont des zones sensibles étendues, une grande stabilité opérationnelle et une résistance aux radiations améliorée. Beaucoup d’équipes dans le monde développent des dispositifs de ce type pour de futures expériences, non seulement dans les accélérateurs de particules, mais aussi pour l’astrophysique des particules, la physique nucléaire, l’imagerie médicale, la science des matériaux et les inspections de sécurité. Ces applications diverses étaient l’objet de la première Conférence internationale sur les MPGD, organisée à l’Académie orthodoxe de Crète, à Kolympari (Grèce).

The invention of micropattern gaseous detectors (MPGDs), in particular the gas electron multiplier (GEM) by Fabio Sauli and the micromesh gaseous structure (Micromegas) by Ioannis Giomataris, has triggered a range of active research and development on a new generation of gaseous detectors (CERN Courier June 2006 p37). These technologies, together with other new micropattern detector schemes that have arisen from these initial ideas, are now enabling the development of detectors with unprecedented spatial resolution and high-rate capability, which also have large sensitive areas and exhibit operational stability and increased radiation hardness. Many groups worldwide are developing MPGD devices for future experiments, not only at particle accelerators but also in nuclear and astroparticle physics, as well as for applications such as medical imaging, material science and security inspection.

This range of activity was the subject of the first international conference on MPGDs, which was organized at the Orthodox Academy of Crete, in Kolymbari, Greece, on 12–15 June 2009. The RD51 collaboration, which was established to advance the technological development and application of MPGDs, actively participated in the conference and held its collaboration meeting immediately afterwards on 16–17 June. The Orthodox Academy conference centre offered an ideal environment for the detailed examination of MPGD issues, together with the exchange of ideas and lively discussions that took place in both meetings. Crete is after all where, according to the myths of Daedalus and Talos, technology emerged during the Minoan civilization.

From COMPASS to the ILC

The history of MPGDs is much shorter, but nevertheless it is already rich in results and prospects. In 1999 COMPASS at CERN became the first high-energy physics experiment to use large-area Micromegas and GEM detectors in high-rate hadron beams. Micromegas produced with the new "microbulk" technology have backgrounds of a few 10–7 counts/s/keV/cm2. They might allow big improvements in the research potential of experiments that are searching for rare events (such as CAST, MIMAC and NEXT). Three time-projection chambers (TPCs) developed for the Tokai to Kamioka (T2K) project are using large pixellized Micromegas made using bulk technology to read out data from some 80,000 channels. This promising neutrino oscillation experiment reported impressive technological progress and results. Meanwhile, GEMs are about to be used in the TOTEM experiment at the LHC (CERN Courier September 2009 p19).

Review talks on future accelerators and upgrades, in particular the sLHC and the International Linear Collider (ILC) projects, covered the physics potential and set the requirements for detectors. MPGDs are in a favourable position thanks to their excellent properties. Research and development has already begun on a pixellized tracker (namely GridPix, or the Gas On Slimmed Silicon Pixels [GOSSIP] detector) for the upgrades of the LHC experiments, aiming for a spatial precision of around 20 μm. MPGDs are also good candidates for upgrading end-cap muon detection (with a precision of around 25 μm). Detectors with large surface areas pose a serious problem, however, owing to the huge number of read-out channels. A modified MPGD with controlled charge dispersion on a resistive anode-film laminated above the read-out plane would allow wide pads (about 2.7 mm), thus reducing significantly the number of channels.

GEMs and variations of Micromegas are being designed for digital hadron calorimetry and for TPCs and their read-out electronics at the ILC. The spatial resolution, which is not affected by a magnetic field, is reaching a record 50 μm for this application. The ion feedback suppression offered by the MPGDs is particularly important for operation at high rates. The new development of an integrated Micromegas (INGRID) on top of silicon micropixel anodes offers a novel and challenging read-out solution, and is under study both for a TPC at the ILC and for a vertex detector for ATLAS. Recent results using a triple-GEM structure combined with either Medipix or Timepix read-out electronics were also presented at the conference.

Multiple applications

Moving away from applications in particle physics, the strip-resistive-electrode thick GEM (S-RETGEM) could be used as flame/smoke detectors for the detection of forest fires at distances up to 1 km, compared with a range of about 200 m for commercially available UV-flame detectors. A detector structure inspired by the Micromegas concept, the Parallel Ionization Multiplier (PIM-MPGD), is being developed in collaboration with medical researchers for use in radio-pharmaceutical β-imaging, with a spatial resolution of 30 μm.

X-ray polarimetry for astrophysical applications now has a powerful tool, with intense development work on GEMs and thick GEMs (THGEMs) based on the pure noble gases xenon, argon, and neon. Interesting developments on GEMs and micropixel (μPIC) detectors operating as large-area VUV gas photomultiplier tubes were also presented at the conference. THGEMs are being assessed for applications in ring-imaging Cherenkov detectors and are also being used in a novel nuclear-imaging technique (3γ imaging) for medical purposes.

The construction of MPGDs is now moving away from planar geometry, but not without difficulties. Cylindrical Micromegas, as used in the CLAS12 detector at Jefferson Lab, and the triple-GEM structure developed for the KLOE experiment at Frascati, do not lose their performance compared with planar ones. Spherical GEMs are also being tested to fight parallax effects that pose limitations in many applications.

Rui De Oliveira of CERN presented the excellent research, development and innovation taking place at CERN in close collaboration with the GEM and Micromegas groups. He presented new photolithography and etching techniques that aim to improve several aspects of the performance of MPGDs, e.g. in robustness, homogeneity, sparking and electronics integration. MPGDs are now being manufactured with areas larger than around 0.5 m2, but further developments are needed for detectors for the sLHC and ILC. Industry has quickly become involved in MPGDs; several companies in Europe, Japan and the US are already manufacturing MPGD elements.

The conference proved the ideal occasion for discussions about the common aspects of all of the variations of MPGDs: field mapping, simulations, gases, electronics etc. All of the groups involved, and the two communities, GEM and Micromegas, came together in a fruitful collaboration. In addition, they were able to sample some of the beauty of Crete, present and past, with two special lectures, one on the history of Crete and the city of Chania, and one on Cretan flora. Participants also enjoyed walking excursions in the gorge of Samaria or visiting the archaeological site of Knossos. The conference dinner featured local delicacies, traditional Greek and Cretan music as well as dancing.

• For more information on the conference and to see the presentations, visit http://candia.inp.demokritos.gr/mpgd2009. The contributions will be submitted for publication in the open-access journal JINST, http://jinst.sissa.it.