Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Particle identification studies with a full-size 4-GEM prototype for the ALICE TPC upgrade
Introduction
Charged-particle detectors based on Gas Electron Multipliers (GEMs) [1] have become essential components of particle and nuclear physics experiments, such as COMPASS [2], LHCb [3] and TOTEM [4], while future applications are planned for KLOE-2 [5] and CMS [6]. Up to now, however, the main applications of this kind of detectors have been high-rate tracking and in-beam detectors. The large-scale application of a Time Projection Chamber (TPC) [7] with GEM-based readout has been pioneered by the prototype developed for the FOPI experiment [8]. The prototype has been evaluated in a test beam campaign with a 1.7 GeV/c beam impinging on a carbon target. The relative resolution has been determined to 15% with about 20 samples along a trajectory of 20 cm per minimum ionizing particle [9]. This result proved the feasibility of a GEM-based TPC.
The ALICE TPC [10] is the main device for tracking and particle identification (PID) in ALICE [11]. With an overall active volume of 90 m3, it is the largest detector of its kind. The TPC employs a cylindrical field cage with a central high-voltage electrode and a gated MWPC-based (Multi-Wire Proportional Chamber) readout plane on each endplate. Charged-particle tracking and PID via the measurement of the specific energy loss is accomplished by the measurement of the ionization in 159 samples along a trajectory of 160 cm. In and central collisions a relative resolution of about 5.5% and 7% [12] is achieved, respectively. At a drift time of 100 s and an interaction rate of 50 kHz, an average event pile-up of five is expected in the active volume of the detector. Hence, the gated operation of the current TPC implies rate limitations which will not conform with the scenario of operation in during the LHC Run 3 and beyond. Therefore, the currently used gated MWPCs will be replaced to allow for continuous readout, retaining the present tracking and PID capabilities. In this mode of operation, the ion back flow (IBF) which quantifies the leakage of ions from the amplification region into the drift volume, becomes an important design parameter. The resulting accumulation of positive space charge in the active volume of the detector leads to distortions of the drift field and hence to a deterioration of the spatial resolution. The IBF must therefore be minimized to a level of 1% or less [13]. In a thorough R&D program [[12], [14], [15]], readout chambers with a 4-GEM amplification scheme have been identified as the solution fulfilling the challenging requirements of the upgrade in terms of IBF, energy resolution and operational stability. According to the baseline configuration proposed in the Technical Design Report [12], the GEM stacks contain Standard (S, pitch 140 m) and Large Pitch (LP, pitch 280 m) foils in the order S–LP–LP–S.
In this paper, we present results from a test beam campaign with a full-size prototype of a TPC Inner Readout Chamber (IROC) with a 4-GEM amplification scheme. In particular, we present its performance in terms of PID separation power, and compare the results to simulations.
Section snippets
Experimental setup
The test beam campaign at the CERN Proton Synchrotron (PS) took place in 2014 with the aim to prove that the resolution achieved by the GEM-based prototype complies both with the performance of the current MWPC-based TPC and the requirements for the upgrade. Therefore, a detector prototype with four single-mask GEM foils, in the following referred to as 4-GEM IROC, has been assembled, commissioned and tested with beams.
Track reconstruction
The front-end cards on the detector provide a digitized measurement of the amplitude on each pad at a certain time - a so-called digit. In order to reconstruct the track of an incident particle from a set of digits, several reconstruction steps are performed. The reconstruction is conducted with a dedicated framework, which partially relies on components of the AliRoot framework [22].
Data analysis
A number of selection criteria are applied to the reconstructed tracks in order to prevent biases from background and edge effects. The usage of the Cherenkov counter as reference particle identification restricts the analysis to one-track events. Only tracks with more than 32 associated clusters within a fiducial region around the center of the drift region are considered in the analysis in order to avoid any bias due to edge effects. Additional selection criteria are applied on the cluster
Simulation
In order to obtain a more solid understanding of the performance of the prototype, dedicated simulations are performed employing AliRoot [22], the framework for reconstruction, simulation and analysis in ALICE.
Straggling functions
Detector effects, such as diffusion, gain variations, and the pad response, have been shown to significantly impact the resolution [[27], [28]] and hence need to be properly described in the simulation. Therefore, a comparison of the corresponding figures of merit of simulation and test beam data is mandatory to conclusively obtain a thorough understanding of the detector response. In order to verify the performance of the simulation and to demonstrate that all physical processes involved
Summary
The increased LHC luminosity envisaged for the Run 3 and beyond implies significant upgrades of the ALICE TPC, as the gating grid of the current MWPC-based readout chambers imposes unacceptable rate limitations. The resolution is a crucial detector parameter for the particle identification via measurement of the specific energy loss. As the ALICE TPC is the main device for PID in ALICE, it is therefore of particular importance to ensure its excellent performance is retained after the
Acknowledgments
The ALICE TPC Collaboration would like to thank the LCTPC Collaboration for borrowing the readout electronics for the PS test beam and the RD51 Collaboration for useful discussions.
The ALICE TPC Collaboration would like to thank the CERN accelerator teams for the outstanding performance of the Proton Synchrotron during the test beam.
The ALICE TPC Collaboration acknowledges the following funding agencies for their support in the TPC Upgrade: Fundação de Amparo à Pesquisa do Estado de São Paulo
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