RD51 ANNUAL REPORT 2009 WG1 - Technological Aspects and Development of New Detector Structures Conveners: Serge Duarte Pinto (CERN), Paul Colas (CEA Saclay) Common projects Most activities in WG1 are meetings, where groups report their progress in certain fields and discuss results. This prevents people from redoing work and encourages the groups involved to produce results that can be checked and compared to those of others. General WG1 sessions are part of every collaboration meeting. But also outside these meetings WG1 sessions are organized, to increase the frequency of these events. An Indico section is dedicated to the activities in working group 1: http://indico.cern.ch/categorydisplay.py?categid=2114 There has been a special focus on large area detectors; two half-day meetings at CERN have been dedicated to it, and a series of EVO meetings on large area GEMs specifically. Mainly GEM and Micromegas technologies are discussed, as those are considered most promising for cost effective large area production. Most groups involved are working on slhc upgrades for forward muon systems of various experiments (ATLAS, CMS, TOTEM), for a digital hadronic calorimeter for future linear colliders, and general large area tracking applications. Another effort in this working group is to develop portable detectors. The technical challenges of portability usually do not involve the actual detector, but rather the associated instrumentation. Meetings on portability include talks about compact high voltage supplies, portable gas systems or sealed detectors (where gas is not flowed), and independent DAQ electronics with USB connection. Input for these sessions comes from groups working on medical applications, teaching, and groups with very small detectors with a pixel ASIC as readout structure. Figure 1 - Demonstration of GEM stretching and framing during the assembly training week.
In February a GEM detector assembly training was organized at CERN. The assembly of triple GEM detectors had been an obstacle for many groups. It involves stretching and framing of delicate and flexible foils, and afterwards aligned gluing of multiple framed GEMs together with a readout structure and a cathode, maintaining accurate parallel spacing. This training, initially set up as a small one-day activity, met with enthusiastic demand and grew into a large event, spanning a whole week and involving more than 50 participants from 18 institutes. The program included technical lectures, handson sessions (see figure 1) in a lab and a cleanroom, and demonstrations of Micromegas operation. Due to the success of this event a rerun is considered, which should perhaps also cover detector design. The thick GEM has been the subject of a long series of EVO meetings. It is a hole-type amplification structure similar to a GEM, but with the thin flexible substrate replaced by thicker fiberglass-reinforced epoxy board. This stiff substrate lends itself well to vacuum deposition of a CsI photoconverter, and all groups involved in these meetings use it for that purpose (all in different experiments). The manufacturing process of thickgems gives full control over the hole pitch and diameter, and the shape, size and thickness of the base material; all these parameters need to be optimized for the application. Scientific results Work on large area GEMs focused on two new techniques to overcome the existing limitations, which prohibited making foils longer or wider than ~50 cm. A single-mask technique for hole pattern transfer and a splicing method for GEM foils were developed. Both techniques were successfully implemented in a prototype of 66 66 cm 2 active area, shown in figure 2. In 2009 this prototype was tested in the RD51test beam of June (see WG7). Meanwhile development of the single-mask technique is still ongoing, and by a series of meetings on large area GEMs the new techniques were proliferated throughout the community. A particular benefit of this technique is that it is well adjusted to industrial processes and equipment, so that exporting a large job to industry will give better results at a higher production rate, and more than an order of magnitude of cost reduction. Most groups involved are working on high luminosity upgrades for forward muon systems of LHC experiments (CMS, TOTEM), R&D for a digital hadronic calorimeter (DHCAL) for future linear colliders, and general large area tracking applications.
Figure 2 - Large area GEM prototype. Micromegas detectors underwent a technical improvement with the introduction of a new fabrication method, named bulk Micromegas. Here a woven metal micromesh is laminated to the readout board between layers of photoimageable soldermask. These soldermask layers can subsequently be patterned by UV-exposure to create the pillar structure that supports the mesh. The materials involved are quite inexpensive, and the processes are industry standard, which makes it suitable for large scale production. The size of the active area one can obtain is limited only by base materials and the manufacturing machinery, the largest example built so far is 150 50 cm 2 (shown in figure 3). A technique is developed to subdivide the mesh into smaller areas in order to limit the capacitance of the structure. Industry is being surveyed for thinner and higher pitch micro-meshes, which have been shown to allow a better electron collection and a lower ion backflow. These developments are largely driven by the ATLAS forward muon upgrade effort and groups doing studies for a DHCAL or a TPC for ILC.
Figure 3 - Large area Micromegas prototype. Most of these applications, both for large area Micromegas and GEMs, foresee large scale production once the R&D phase is over. Therefore scenarios have been worked out to move part of the production process to industry. Potential industrial partners have been located and contacted. Representatives of many private firms interested in collaborating with RD51 projects have been invited to WG1 meetings, to bring them in contact with research groups. Many groups are interested in the use of resistive layers in gaseous detectors. There are two main motivations for using such layers: lateral spreading of the signals, and quenching of discharges. For spreading the signal charge the layer is applied to the readout structure, and the readout elements are either in direct contact with the layer (DC-coupled readout), or separated by an insulating layer (ACcoupled). To quench discharges the layer is applied to where a discharge takes place, for instance on the electrodes of a thickgem (as in figure 4). In Micromegas, a resistive readout serves both purposes, and orders of magnitude decrease in spark frequency and intensity have been reported. However, resistive electrodes will impact the rate capability of the detector, as is known from the experience with resistive plate chambers (RPCs). Therefore the resistive values must be well understood, and optimized for the application. Many tests are done with laminating resistive foils, screen printing resistive pastes or inks, and vapor deposition of silicon nitride.
Figure 4 - A small thickgem with resistive electrodes. Several groups reported progress in hybrid amplification structures, where the principles of different charge amplification structures are integrated in one device. A well-known example is the micro-hole and strip plate (MHSP), which works like a GEM foil with a microstrip gas pattern printed on its bottom electrode. MHSPs combine a high gain with a strong ion feedback suppression. They have special advantages in detectors working under high pressures and cryogenic conditions, where multiple GEM structures have problems with charge transfer from one GEM to the next. Recently more exotic electrode patterns were introduced, and also thick versions of these structures. Another recent development is the micro-mesh micro-pixel, which combines parallel plate amplification as in a Micromegas with amplification in the steep 1/r 2 field close to a metallic dot-shaped anode. Several groups also combine gas amplification with detection of the scintillation light coming from the avalanche or the ionization. In one case an entirely transparent microstrip gas chamber was made to this end, using indium tin oxide transparent conductors on a glass substrate. Some effort is spent on development of detectors with unconventional shapes. Cylindrical GEMs and a cylindrical Micromegas have been made and tested. These could make an ultra light barrel tracking system, without any cabling or cooling in the low-η region. GEMs were recently also made in a spherical shape, as is shown in figure 5. A spherical geometry will eliminate the parallax error in detectors where a gaseous converter is used to detect x-rays, neutrons or UV photons.
Figure 5 - The first spherical GEM foil. Working group 1 will continue its efforts to bring together people from many different groups, working for applications in various fields. With upgrade programs for slhc moving ahead, contacts with industry will be strengthened. WG1 meetings will continue to be the general platform for spreading information about new detector structures and geometries; more specific tasks or common issues will be addressed by special activities. Presentation of novel ideas and approaches is inspiring to many, and leads to crossfertilization also between different technologies and applications. We look forward to another productive year in the development of technologies.