Résumé

Conférence sur les détecteurs gazeux microstructurés

Les détecteurs gazeux microstructurés ont ouvert une ère nouvelle dans la technologie des détecteurs et servent de référence pour le développement des détecteurs gazeux au-delà du LHC. Ils pourraient finalement permettre toute une série de nouveaux concepts de détecteurs de rayonnement, non seulement pour la science fondamentale mais aussi pour l’imagerie médicale, les inspections de sécurité et l’industrie. La deuxième édition de cette conférence, MPGD2011, s’est tenue à l’Université de Kobe. Elle a été l’occasion de faire un état des lieux de la recherche dans ce domaine très dynamique.

Micropattern gaseous detectors (MPGDs) have opened a new era in state-of-the-art technologies and are the benchmark for gas-detector developments beyond the LHC. They could eventually enable a plethora of new radiation-detector concepts in fundamental science, medical imaging, security inspection and industry. Given the ever-growing interest in this rapidly developing field, an international conference series on MPGD detectors was founded in 2009 to provide a scientific forum to review the current highlights, new results and concepts, applications and future trends, with the first conference organized in Crete (CERN Courier December 2009 p23). The second in the series, MPGD2011, took place in Kobe on 29 August – 1 September. With it being two years since the previous meeting, there were many new developments to discuss at MPGD2011.

The conference was held at the Maiko Villa Kobe hotel, which is located near the Akashi Strait Bridge. Connecting the Japanese mainland with Awaji island, this is the world’s largest suspension bridge. It was clearly visible from the venue and symbolically emphasized the connection and synergy of the worldwide communities. Half of the 120 participants were from overseas, visiting from 16 countries. Attendance was clearly unaffected by the Great East Japan Earthquake on 11 March 2011, which was in contrast to many other international conferences and events in Japan in 2011 that were cancelled owing to low participation from foreign countries following the disaster.

Japan is the most advanced of any country in terms of having a successful partnership between academia and industry in the development of particle-physics detectors. MPGD developments have been an active field in the country since the early 1990s, shortly after the invention of the micro-strip gas chamber (MSGC). However, in the Asian region and especially in Japan, most MPGD R&D has been carried out independently from other countries. Elsewhere, worldwide interest in the technological development and the use of the novel MPGD technologies led to the establishment of the international research collaboration RD51 at CERN in 2008. By 2011, 80 institutes from 25 countries had joined the collaboration. Only one institute from Japan – Kobe University – has so far joined RD51, although there is an annual domestic MPGD workshop with some 80 participants and around 30 presentations. Holding the international MPGD conference in Japan, followed by a meeting of the RD51 collaboration on 2–3 September, was highly important from the perspective of improving communication and enhancing the synergy between the worldwide MPGD communities.

MPGDs are a relatively novel kind of particle detector, based on gaseous multiplication using micro-pattern electrodes instead of thin wires in a multiwire proportional chamber (MWPC). By using a pitch size of a few hundred micrometres, which is an order-of-magnitude improvement in granularity over wire chambers, these detectors offer an intrinsic high rate-capability (> 106 Hz/mm), excellent spatial resolution (around 30–50 μm) and single-photoelectron time resolution in the nanosecond range. The MSGC, a concept invented by Anton Oed in 1988, was the first of the microstructure gas detectors. Further advances in photolithography techniques gave rise to more powerful devices, in particular, the gas-electron multiplier (GEM) of Fabio Sauli in 1997, and the micromesh gaseous structure (Micromegas) of Ioannis Giomataris and colleagues in 1996. Both of these devices exhibit improved operational stability and increased radiation hardness. During their evolution, many types of MPGDs have arisen from the initial ideas, such as the thick GEM (THGEM), the resistive thick GEM (RETGEM), the microhole and strip plate (MHSP) and the micropixel gas chamber (μ-PIC).

Today, a large number of groups worldwide are developing MPGDs for:

• future experiments at particle accelerators (upgrades of muon detectors at the LHC and novel MPGD-based devices for time-projection chambers (TPCs) and digital hadron calorimetry at a future linear collider);

• experiments in nuclear and hadron physics (KLOE2 at DAΦNE, the Panda and CMB experiments at the Facility for Antiproton and Ion Research, STAR at the Relativistic Heavy Ion Collider, SBS at Jefferson Lab and many others);

• experiments in astroparticle physics and neutrino physics;

• and industrial applications such as medical imaging, material science and security inspection.

This report cannot summarize all of the interesting developments in the MPGD field but it illustrates the richness with a few conference highlights and their implications.

During the three days of MPGD2011, results were presented in 39 plenary talks – including three review talks – and some 30 posters. Five industrial companies linked closely to MPGD technologies also exhibited their products.

Marcel Demarteau of Argonne National Laboratory discussed the paramount importance of the interplay between future physics challenges and the development of advanced detector concepts, with instrumentation being the enabler of science, both pure and applied. The greatest payoffs will come from fundamentally reinventing mainstream technologies under a new paradigm of integration of electronics and detectors, as well as integration of functionality. As an example, several conference talks discussed recent progress in the development of integrated Micromegas (InGrid) directly on top of a CMOS micropixel anode (the Timepix chip), which offers a novel and fully integrated read-out solution. These detectors will be used in searching for solar axions in the CAST experiment at CERN and are also are under study for a TPC at the International Linear Collider and for a pixellized tracker (the "gas on slimmed silicon pixels" or GOSSIP detector) for the upgrades of the LHC experiments.

A key point that must be solved to advance with MPGDs is the industrialization of the production and manufacturing of large-size detectors. Rui de Oliveira of CERN discussed the current status of the new facility for large-size MPGD production at CERN, which will be able to produce 2 m × 0.6 m GEMs, 1.5 m × 0.6 m Micromegas and 0.8 m × 0.4 m THGEMs. He also presented recent developments and improvements of fabrication techniques – single-mask GEMs and resistive "bulk Micromegas". GEMs and Micromegas prototypes have been produced in the CERN workshop with a size of nearly 1 m2 for the ATLAS and CMS muon upgrades for the future high-luminosity LHC (the HL-LHC project, Designs on higher luminosity). Large-area cylindrical GEMs are currently being manufactured for the KLOE2 inner tracker.

Moving away from applications in particle physics, large-area MPGDs are being developed for muon tomography to detect nuclear contraband and for tomographic densitometry of the Earth. Industry has also become interested in manufacturing MPGD structures; technology-transfer activities and collaboration have been actively pursued during the past year with several companies in Europe, Japan, Korea and the US.

One of the highlights of MPGD2011 was the recent trend in the development of MPGDs with resistive electrodes. This technique is an attractive way to quench discharges, thus improving the robustness of the detector against sparks. There were more than 10 presentations devoted to resistive MPGDs. The resistive bulk Micromegas for the ATLAS muon upgrade (MAMMA) employs a 2D read-out board utilizing resistive strips on top of the insulator, covering copper strips (figure 2). This industrial-assembly process allows regular production of large, robust and inexpensive detector modules. The design has achieved stable operation in the presence of heavily ionizing particles and neutron background, similar to the conditions expected in the ATLAS cavern in the HL-LHC upgrade. There were also other presentations describing basic developments of the GEM, THGEM and μ-PIC using resistive materials.

Alexey Buzulutskov of the Budker Institute of the Nuclear Physics, Novosibirsk, reviewed recent advances in cryogenic avalanche detectors, operated at low temperatures (from a few tens of degrees kelvin down to a few degrees). Recent progress in the operation of cascaded MPGDs at cryogenic temperatures could pave the road toward their potential application in: the next-generation neutrino physics and proton-decay experiments; liquid argon TPCs for direct dark-matter searches; positron-emission tomography (PET); and a noble-liquid Compton telescope combined with a micro-PET camera.

The MPGD2011 conference also featured a physics presentation announcing the observation of electron-neutrino appearance events using the beam from the Japan Proton Accelerator Research Complex to the Super-Kamiokande detector (CERN Courier September 2011 p6). The results were mainly based on the three large-volume TPCs, instrumented with bulk Micromegas detectors and read out via some 80,000 channels. This is a good example of the interplay between physics and technology. Last, but not least, interesting results of gaseous photomultipliers with caesium-iodide and bialkali photocathodes coupled to GEM, THGEM and Micromegas structures, were reported at the conference. A sealed prototype of an MPGD sensitive to visible light has been produced by Hamamatsu.

• For more information about the conference and for the presentations, see http://ppwww.phys.sci.kobe-u.ac.jp/~upic/mpgd2011. The contributions will be submitted for publication in the open-access journal JINST, http://jinst.sissa.it.