L. Ropelewski

Discharge studies and prevention in the gas electron multiplier (GEM)

The gas electron multiplier (GEM) used as single proportional counter or in a cascade of two or more elements, permits to attain high gains and to perform detection and localization of ionizing tracks at very high radiation rates. As in other micro-pattern detectors, however, the occasional occurrence of heavily ionizing trails may trigger a local breakdown, with possible harmful consequences on the device itself and on the readout electronics. This paper describes a systematic investigation of the discharge mechanisms in single and multiple GEM structures, and suggests various strategies to reduce both the energy and the probability of the discharges.

Further developments and beam tests of the gas electron multiplier (GEM)

Abstract We describe the development and operation of the Gas Electron Multiplier (GEM), a thin insulating foil metal-clad on both sides and perforated by a regular pattern of small holes. The mesh can be incorporated into the gas volume of an active detector to provide a first amplification channel for electrons, or used as stand alone. We report on the basic properties of GEMs manufactured with different geometries and operated in several gas mixtures as well as on their long-term stability after accumulation of charge equivalent to several years of operation in high-luminosity experiments. Optimized GEMs reach gains close to 10 000 at safe operating voltages, permitting the detection of ionizing tracks, without other amplifying elements, on a simple Printed Circuit Board (PCB), opening new possibilities for detector design.

Progress on large area GEMs

Abstract The Gas Electron Multiplier (GEM) manufacturing technique has recently evolved to allow the production of large area GEMs. A novel approach based on single mask photolithography eliminates the mask alignment issue, which limits the dimensions in the traditional double mask process. Moreover, a splicing technique overcomes the limited width of the raw material. Stretching and handling issues in large area GEMs have also been addressed. Using the new improvements it was possible to build a prototype triple-GEM detector of ∼ 2000 cm 2 active area, aimed at an application for the TOTEM T1 upgrade. Further refinements of the single mask technique allow great control over the shape of the GEM holes and the size of the rims, which can be tuned as needed. In this framework, simulation studies can help to understand the GEM behavior depending on the hole shape.

Evidence for non-exponential elastic proton-proton differential cross-section at low |t| and √ s = 8 TeV by TOTEM

Abstract The TOTEM experiment has made a precise measurement of the elastic proton–proton differential cross-section at the centre-of-mass energy s = 8 TeV based on a high-statistics data sample obtained with the β ⁎ = 90 m optics. Both the statistical and systematic uncertainties remain below 1%, except for the t-independent contribution from the overall normalisation. This unprecedented precision allows to exclude a purely exponential differential cross-section in the range of four-momentum transfer squared 0.027 | t | 0.2 GeV 2 with a significance greater than 7 σ . Two extended parametrisations, with quadratic and cubic polynomials in the exponent, are shown to be well compatible with the data. Using them for the differential cross-section extrapolation to t = 0 , and further applying the optical theorem, yields total cross-section estimates of ( 101.5 ± 2.1 ) mb and ( 101.9 ± 2.1 ) mb , respectively, in agreement with previous TOTEM measurements.

Performance of GEM detectors in high intensity particle beams

Abstract We describe extensive tests of Double Gas Electron Multiplier (GEM) and Triple GEM detectors, including large size prototypes for the COMPASS experiment, exposed to high intensity muon, proton and pion beams at the Paul Scherrer Institute and at CERN. The measurements aim at detecting problems possibly appearing under these harsh operating conditions, the main concern being the occurrence of discharges induced by beam particles. Results for the dependence of the probability for induced discharges on the experimental environment are presented and discussed. Implications for the application of GEM detectors in experiments at high luminosity colliders are illustrated.

The Q2 dependence of the structure function ratio F2Sn/F2C and the difference RSn - RC in deep inelastic muon scattering

The Q(2) dependence of the structure function ratio F-2(Sn)/F-2(C) for 0.01 < x < 0.75 and 1 < Q(2) < 140 GeV2 is reported. For x < 0.1 the size of shadowing decreases linearly with In Q(2) and the maximum rate is about 0.04 at x = 0.01. The rate decreases with x and is compatible with zero for x greater than or equal to 0.1. The difference R(Sn) - R(C), where R is the ratio of longitudinally to transversely polarised virtual photon absorption cross sections, is also given. No dependence on x is seen and the average value is 0.040 +/- 0.021 (stat.) +/- 0.026 (syst.) at a mean Q(2) of 10 GeV2.

Electron collection and ion feedback in GEM-based detectors

We report the results of systematic experimental investigations on electron transmission and ion feedback in a single gas electron multiplier (GEM) detector for the operating region close to unity gain. Critical factors for obtaining good collection are the transverse diffusion coefficient and the hole diameter. A unit gain GEM can be used for instance as first element in a cascade to improve collection efficiency and ion feedback suppression in time projection chambers.

Characterization of GEM detectors for application in the CMS muon detection system

The muon detection system of the Compact Muon Solenoid experiment at the CERN Large Hadron Collider is based on different technologies for muon tracking and triggering. In particular, the muon system in the endcap disks of the detector consists of Resistive Plate Chambers for triggering and Cathode Strip Chambers for tracking. At present, the endcap muon system is only partially instrumented with the very forward detector region remaining uncovered. In view of a possible future extension of the muon endcap system, we report on a feasibility study on the use of Micro-Pattern Gas Detectors, in particular Gas Electron Multipliers, for both muon triggering and tracking. Results on the construction and characterization of small triple-Gas Electron Multiplier prototype detectors are presented.

Construction and performance of large-area triple-GEM prototypes for future upgrades of the CMS forward muon system

At present, part of the forward RPC muon system of the CMS detector at the CERN LHC remains uninstrumented in the high-η region. An international collaboration is investigating the possibility of covering the 1.6 &#60; |η| &#60; 2.4 region of the muon endcaps with large-area triple-GEM detectors. Given their good spatial resolution, high rate capability, and radiation hardness, these micro-pattern gas detectors are an appealing option for simultaneously enhancing muon tracking and triggering capabilities in a future upgrade of the CMS detector. A general overview of this feasibility study will be presented. The design and construction of small (10×10 cm2) and full-size trapezoidal (1 × 0.5 m2) triple-GEM prototypes will be described. During detector assembly, different techniques for stretching the GEM foils were tested. Results from measurements with x-rays and from test beam campaigns at the CERN SPS will be shown for the small and large prototypes. Preliminary simulation studies on the expected muon reconstruction and trigger performances of this proposed upgraded muon system will be reported.

Performance of the Totem Detectors at the LHC

The TOTEM Experiment is designed to measure the total proton–proton cross-section with the luminosity-independent method and to study elastic and diffractive pp scattering at the LHC. To achieve optimum forward coverage for charged particles emitted by the pp collisions in the interaction point IP5, two tracking telescopes, T1 and T2, are installed on each side of the IP in the pseudorapidity region 3.1≤|η|≤6.5, and special movable beam-pipe insertions — called Roman Pots (RP) — are placed at distances of ±147 m and ±220 m from IP5. This article describes in detail the working of the TOTEM detector to produce physics results in the first three years of operation and data taking at the LHC.