Particle identification studies with a full-size 4-GEM prototype for the ALICE TPC upgrade

Abstract

A large Time Projection Chamber is the main device for tracking and charged-particle identification in the ALICE experiment at the CERN LHC. After the second long shutdown in 2019/20, the LHC will deliver Pb beams colliding at an interaction rate of about 50 kHz, which is about a factor of 50 above the present readout rate of the TPC. This will result in a significant improvement on the sensitivity to rare probes that are considered key observables to characterize the QCD matter created in such collisions. In order to make full use of this luminosity, the currently used gated Multi-Wire Proportional Chambers will be replaced. The upgrade relies on continuously operated readout detectors employing Gas Electron Multiplier technology to retain the performance in terms of particle identification via the measurement of the specific energy loss by ionization d$E$/d$x$. A full-size readout chamber prototype was assembled in 2014 featuring a stack of four GEM foils as an amplification stage. The performance of the prototype was evaluated in a test beam campaign at the CERN PS. The d$E$/d$x$ resolution complies with both the performance of the currently operated MWPC-based readout chambers and the challenging requirements of the ALICE TPC upgrade program. Detailed simulations of the readout system are able to reproduce the data.

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 ${\pi }^{-}$ beam impinging on a carbon target. The relative $\mathrm{d}E∕\mathrm{d}x$ resolution has been determined to $\sim$15% with about 20 samples along a trajectory of $\sim$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 $\sim$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 $\mathrm{d}E∕\mathrm{d}x$ is accomplished by the measurement of the ionization in 159 samples along a trajectory of $\sim$160  cm. In $\mathrm{pp}$ and central $\text{Pb–Pb}$ collisions a relative $\mathrm{d}E∕\mathrm{d}x$ resolution of about 5.5% and 7% [12] is achieved, respectively. At a drift time of 100 $\mathrm{\mu }$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 $\text{Pb–Pb}$ 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 $\mathrm{\mu }$m) and Large Pitch (LP, pitch 280 $\mathrm{\mu }$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.