U. Werthenbach

Simulation of electrostatic properties and gas amplification in microstrip gas chambers and comparison with measurements

Abstract The electrostatic field in MSGCs is computed using the charge simulation method. Bulk and surface currents are taken into account as well as dielectric properties. The gas amplification and the electric field are estimated as a function of the geometric parameters of the structure, of the applied voltages, and of the conductivity of the substrate. The simulations are compared with measurements using glass substrates of 50, 100 and 500 μm thickness. They show that for a pad readout at the back plane thin substrates and large gaps between anode and cathode strips are favorable. The asymmetry of the signals induced on the cathode strips should allow to improve the position resolution of MSGCs.

Studies of aging and HV break down problems during development and operation of MSGC and GEM detectors for the inner tracking system of HERA-B

Abstract The results of 5 years of development of the inner tracking system of the HERA-B experiment and first experience from the data taking period of the year 2000 are reported. The system contains 184 chambers, covering a sensitive area of about (20×20) cm 2 each. The detector is based on microstrip gas counters (MSGCs) with diamond-like coated (DLC) glass wafers and gas electron multipliers (GEMs). The main problems in the development phase were gas discharges in intense hadron beams and aging in a high radiation dose environment. The observation of gas discharges which damage the electrode structure of the MSGC led to the addition of the GEM as a first amplification step. Spurious sparking at the GEM cannot be avoided completely. It does not affect the GEM itself but can produce secondary damage of the MSGC if the electric field between the GEM and the MSGC is above a threshold depending on the operation conditions. We observed that aging does not only depend on the dose but also on the spot size of the irradiated area. Ar–DME mixtures had to be abandoned whereas a mixture of 70% Ar and 30% CO 2 showed no serious aging effects up to about 40 mC /cm deposited charge on the anodes. X-ray measurements indicate that the DLC of the MSGC is deteriorated by the gas amplification process. As a consequence, long-term gain variations are expected. The Inner Tracker has successfully participated in the data taking at HERA-B during summer 2000.

First steps towards a foil microstrip chamber

In this paper we present developments towards a foil microstrip detector. Gas amplification has been observed on 10 μm wide strips obtained by photolithography on 50 μm thick polyimid foils. Compared to plates, foils have the advantage of flexibility and can be used for cylindrical structures with r-φ and φ-z readout, they allow for arbitrarilly shaped pickup electrodes on the back side, and they are of low cost. We emphasize the importance of surface currents for a reliable performance of foil chambers and generally for any kind of device where gas amplification is produced on electrodes in contact with an insulating medium.

Gas gain and signal length measurements with a triple-GEM at different pressures of Ar-, Kr- and Xe-based gas mixtures☆

Abstract We investigate the gas gain behaviour of a triple-GEM configuration in gas mixtures of argon, krypton and xenon with 10% and 30% of carbon dioxide at pressures between 1 and 3 bar . Since the signal widths affect the dead time behaviour of the detector we present signal length measurements to evaluate the use of the triple-GEM in time-resolved X-ray imaging.

Carbon coated gas electron multipliers

Abstract Gas electron multipliers (GEMs) have been overcoated with a high resistivity 1014– 10 15 Ω / □ amorphous carbon layer. The coating avoids charging up of the holes and provides a constant gain immediately after switching on independent of the rate. The gain uniformity across the GEM is improved. Coating opens the possibility to produce thick GEMs of very high gain.

Sparks in MSGCs

Abstract We have measured the spark rate as a function of voltages and geometric parameters and investigated the damage to Au, Al, Cr, W and Rh electrodes. Measurements of the spark signals compare well with simulations. We propose a new device, the Macro Gap Chamber.

AN OPTICAL READOUT FOR A FIBER TRACKER

The performance of 16 and 64 channel photomultipliers coupled to scintillating fibres has been tested. The devices are sensitive to single photoelectrons, show little gain losses for magnetic fields up to 100 Gauss and have moderate optical cross-talk. The maximum channel to channel gain variations reach a factor two for the 16 channel version and a factor of four for the 64 channel PM. The measurements and simulations indicate that the photomultipliers are well suited for the light detection in fibre trackers.

Study of thin substrates for microstrip gas chambers

Abstract Microstrips have been produced on glass and plastic supports of various thickness. Flexible, 100 μm thick glass plates were successfully operated. Glass foils are less vulnerable to sparks than plastic foils, are less sensitive to intensity changes and respond faster to voltage changes but have similar advantages. A structure on a 7 μm thin polyimide foil showed gas amplification with slightly reduced energy resolution as compared to the same structure on a thick foil. Its time stability during irradiation was unsatisfactory.

On image reconstruction with the two-dimensional interpolating resistive readout structure of the Virtual-Pixel detector ☆

The two-dimensional interpolating readout concept of the Virtual-Pixel detector (ViP detector) goes along with an enormous reduction of electronic channels compared to pure pixel devices. However, the special concept of the readout structure demands for adequate position reconstruction methods. Theoretical considerations reflecting the choice and the combination of linear algorithms with emphasis on correct position reconstruction and optimised spatial resolution are presented. A subsequent two-dimensional coordinate transformation further improves the image response of the system. Measured images show how far the theoretical predictions of the simulations can be verified and to what extent they can be used to improve the position reconstruction.

Investigation of the performance of an optimised MicroCAT, a GEM and their combination by simulations and current measurements ☆

A Micro compteur a Trous (MicroCAT) structure which is used for avalanche charge multiplication in gas filled radiation detectors has been optimised with respect to maximum electron transparency and minimum ion feedback. We report on the charge transfer behaviour and the achievable gas gain of this device. A three-dimensional electron and ion transfer simulation is compared to results derived from electric current measurements. Similarly, we present studies of the charge transfer behaviour of a Gas Electron Multiplier (GEM) by current measurements and simulations. Finally, we investigate the combination of the MicroCAT and the GEM by measurements with respect to the performance at different voltage settings, gas mixtures and gas pressures.