Abstract
Gas detector development is one of the pillars of the research in fundamental physics. Since several years, a new concept of detectors, called Micro Pattern Gas Detector (MPGD), allowed to overcome several problems related to other types of commonly used detectors, like drift chamber and micro strips detectors, reducing the rate of discharges and providing better radiation tolerance. Among the most used MPGDs are the Gas Electron Multipliers (GEMs). Invented by Sauli in 1997, nowadays GEMs have become an important reality for particle detectors in high energy physics. Commonly deployed as fast timing detectors and triggers, their fast response, high rate capability and high radiation hardness make them also suitable as tracking detectors. The readout scheme is one of the most important features in tracking technology. Analog readout based on the calculation of the center of gravity technique allows to overcome the limit imposed by digital pads, whose spatial resolution is limited by the pitch dimensions. However, the presence of high external magnetic fields can distort the electronic cloud and affect the performance. The development of the micro-TPC reconstruction method brings GEM detectors into a new prospective, improving significantly the spatial resolutionin presence of high magnetic fields. This innovative technique allows to reconstruct the 3-dimensional particle position, as Time Projection Chamber, but within a drift gap of a few millimeters. In these report, the charge centroid and micro-TPC methods are described in details. We discuss the results of several test beams performed with planar chambers in magnetic field. These results are one of the first developments of micro-TPC technique for GEM detectors, which allows to reach unprecedented performance in a high magnetic field of 1 T.
Highlights
The Gas Electron Multiplier (GEM) is a newly developed gaseous electron multiplier invented in 1997 by Fabio Sauli [1]
The drift gap can be considered as a tiny time projection chamber: the time information related to each fired strip allows to perform a local track reconstruction in the few-mm drift gap
The (xi, zi) points are fitted with a linear function z = ax + b of the beam test measurements is to study the performance as a function of several parameters, such as high voltage, field setting, gas mixture, drift gap, etc., in order to set the optimal working point needed to achieve the best efficiency and spatial resolution
Summary
The Gas Electron Multiplier (GEM) is a newly developed gaseous electron multiplier invented in 1997 by Fabio Sauli [1]. It consists of a thin insulating polymeric foil (∼ 50 μm of kapton) metal coated on both sides (3−5 μm of copper), and pierced by a high density regular matrix of holes with a typical diameter of about 50/70 μm [2] realized with a photolithographic method. Better performance is obtained by means of three GEM (triple-GEM) foils between the cathode and the anode instead of one: gain values up to 105 can be achieved working at lower voltages while reducing the discharge probability. All the results reported here are related to the triple-GEM configuration
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