Recent Results From Beam Tests of the ALPIDE Pixel Chip for the Upgrade of the ALICE Inner Tracker

Abstract

The planned upgrade of the A Large Ion Collider Experiment (ALICE) inner tracking system (ITS) aims at improving the ALICE performance in terms of spatial resolution and data rate readout. The new ITS will be a low-material-budget, high-granularity detector. It comprises seven concentric layers of monolithic active pixel sensors (MAPS), called ALPIDE, which are based on the TowerJazz 180-nm CMOS technology. In this article, we report on detection performance and radiation hardness studies of ALPIDE sensors. We show that these sensors keep having high detection efficiency for minimum ionizing particles (MIPs) (>99%) and low fake-hit rate while being radiation hard to some ${ 1.7} \times { 10}^{ 13}\,\,1$ MeV $\text{n}_{\text {eq}}$ cm−2 of nonionizing energy loss (NIEL) and 2.7 Mrad of total ionization dose (TID), which exceeds the expected radiation load during the detector lifetime ten times. ALPIDE sensors thus completely fulfill the design requirements. In addition, we show that measured cluster shape frequencies obtained at different operating points (thresholds, bias voltages) are well described with the state-of-the-art ALICE Monte Carlo simulation.