The latest developments in a technique dating back to Ernest Rutherford.
In 1908, Rutherford was the first to use a gas-filled wire counter to study natural radioactivity. To celebrate 100 years of gas counters, and in particular to look ahead to new developments in gas-based detectors, some 100 physicists gathered at Nikhef, Amsterdam, on 16–18 April. They were on a mission: to work towards the foundation of the RD51 collaboration, devoted to further research and development of micropattern gas detectors (MPGDs).
Fabio Sauli from the TERA Foundation and CERN reviewed how, in 100 years, gas detectors developed from Geiger counters to multiwire proportional chambers, drift chambers and time-projection chambers (TPCs) – detectors that are now widely used in high-energy and nuclear physics experiments. The need for gas detectors that could operate at high counting rates led to the development of micro-strip gas chambers. However, they proved difficult to operate in challenging conditions and were prone to aging and sparking. Nevertheless, the gas-detector community stood up to the challenge. The invention of MPGDs, such as the micromesh gaseous structure chamber (the MicroMegas) and gas-electron multiplier (GEM) detector, appears to have solved these problems.
Progress in MPGDs
These detectors have small avalanche gaps and therefore a rapid signal development, implemented in slightly different ways. In MicroMegas detectors the electron multiplication takes place in the narrow gap between a thin cathode mesh with holes and the anode. GEMs, on the other hand, have an insulating polymer foil with thin metal coatings on both sides, and the multiplication takes place in the holes in the foil. Such MPGDs are already in use in difficult environments, such as in the COMPASS experiment at CERN, and various ideas exist to develop MPGDs further into robust, economic, fast and, potentially, large-area tracking detectors with a low material budget (one-fifth to one-tenth of that in typical silicon detectors).
The workshop heard about progress towards various further improvements for MPGDs. Ioannis Giomataris of DAPNIA-Saclay presented new developments in MicroMegas detectors, such as bulk and large-area construction, and also spoke about various applications. Recent advances in thick GEM detectors formed the focus of the talk by Amos Breskin of the Weizmann Institute, while CERN’s Serge Duarte looked at how to make large GEMs. In a slightly different vein, Vladimir Peskov from CERN described work on resistive-electrode thick GEMS, which are designed to give higher gain without sparking.
With recent developments in silicon wafer processing technology it is now possible to grow the thin cathode grid of a MicroMegas detector right on top of a silicon pixel chip (figure 1). Such a set-up (known as “Ingrid”) integrates detector and read-out electronics optimally in one structure, as Victor Blanco Carballo from Twente University and Lucie de Nooij from Nikhef demonstrated (figure 2). Sparks in the narrow gap between the cathode and the anode can destroy the pixel chip, but Nicolas Wyrsch of the Institute of Microtechnology, Neuchatel, showed that with a layer of amorphous silicon on the pixel chip, the detector can withstand sparking.
There are numerous applications of MPGDs, a few of which were discussed during the workshop. In R&D studies, thick GEMs are used for the detection of single photons in Cherenkov imaging counters. At Jefferson Lab, a new multipurpose spectrometer is being developed, where GEMs could be used in particle tracking at high rates. GEMs are also being developed for digital hadron calorimetry in experiments proposed for the International Linear Collider (ILC) – a very high granularity can be achieved with small cells that are either “on” or “off”. Groups working on experiments for the ILC have in addition designed large TPCs with MPGD read-out, and both GEMs and MicroMegas are being considered for this role.
In other developments, MicroMegas detectors could read out a TPC for the Tokai-to-Kamioka experiment in Japan, or be used as muon detectors at high counting rates, such as in the upgrade of the ATLAS detector at CERN for the upgraded LHC, the Super-LHC (SLHC). A gas-pixel transition-radiation tracker based on MicroMegas is under study, and MicroMegas detectors are excellent technology choices for experiments that aim to detect rare events, such as searches for weakly interacting massive particles and solar axions, and studies of neutrinoless double beta-decay. MPGDs also have applications in astronomy and medicine as X-ray imaging detectors, and in neutron detection.
The workshop also discussed future read-out chips. The TimePix chip is derived from the Medipix2 chip, but with a time measurement for each pixel, which is an important asset for gas detectors. Michael Campbell from CERN talked about the Medipix3 chip, which is now under development, and Jan Timmermans of Nikhef discussed the requirements of TimePix-2, a successor of TimePix, and how this chip could be a general purpose read-out chip.
The RD51 collaboration
In a workshop at CERN in September 2007, participants realized that future progress in MPGDs would be best served by tighter collaboration. This led to the formation of a protocollaboration, working towards an R&D proposal: “Development of micropattern gas detectors technologies.” Now some 50 institutes in Europe, the US and Asia have declared an interest, and a proposal for this collaboration, RD51, was submitted to the LHC committee on 2 July, following the workshop at Nikhef where Leszek Ropelewski from CERN was elected spokesperson, and Maxim Titov of CEA-Saclay was elected co-spokesperson.
The objectives of RD51 are to form a technology-oriented collaboration; to share common investments and infrastructure, such as test beams, radiation facilities and production lines; to develop common standards; to optimize the communication and sharing of knowledge; and to collaborate with industrial partners. The collaboration intends to perform technological studies for the optimization and industrialization of each manufacturing technology, and to develop radiation-hard devices that can operate beyond the limits of present devices (e.g. for detector upgrades for the SLHC). In addition, RD51 will work towards the integration of detector-simulation software, such as Garfield and Magboltz, with Geant4. It will also study the synthesis of MPGD front-end electronics into a number of read-out approaches, optimize read-out integration with detectors, and develop large-area MPGDs with CMOS read-out.
• Slides from the workshop are available online at Indico: see indico.cern.ch/conferenceDisplay.py?confId=25069
The next RD51 workshop will take place in Paris on 13–15 October 2008. For further details, visit http://indico.cern.ch/conferenceDisplay.py?confId=35172