Development and Characterization of an Irradiation Site for Radiation Damage Studies of the ATLAS ITk Pixel Detector

Abstract

Silicon pixel detectors build the foundation of particle tracking in high-rate environments, delivering high spatial resolution while sustaining hit rates of up to several GHz cm^-2. In the field of modern-day high-energy particle physics, pixel detectors are operated in extreme conditions, marking the frontier of radiation levels for microelectronics. In this environment, silicon devices suffer from radiation-induced damage and subsequent performance degradation of the detector components. To allow for research, development and verification of novel tracking detectors for extreme radiation applications, irradiation sites are a crucial part of the infrastructure. <br/> In this work, a novel irradiation site for silicon detectors has been developed, characterized and commissioned at the University of Bonn. The site is located at the Bonn Isochronous Cyclotron (BIC) facility and operated with nominally 14 MeV protons with beam currents of up to 1 µA. Devices Under Test (DUTs) are situated in a thermally-insulated box at temperatures of ≤ −20 °C while being moved through the beam on a row-based meander pattern, achieving a uniform fluence distribution. A set of custom diagnostics facilitate non-destructive online beam monitoring at the site with a relative uncertainty of ≤ 2 %. The beam monitoring capabilities enable a purely beam-based, on- and offline dosimetry approach and beam-driven irradiation routine that result in highly uniform fluence profiles with spatial resolution and uncertainties of a few percent. Dedicated irradiation campaigns allow for determination of the proton hardness factor of κ = 3.75 ± 0.12, enabling one to irradiate up to NIEL fluences of 1 × 10¹⁶ neq /cm² within a few hours. Comparison of multiple irradiation sites shows good agreement of the NIEL scaling with the irradiation site at the BIC providing the largest scaling factor at the lowest relative uncertainty. <br/> Using the established irradiation infrastructure, radiation hardness studies of an ITkPix pixel detector assembly are performed. After proton irradiation up to the anticipated ATLAS ITk end of life levels for the sensor and readout chip, respectively, the assembly is functional and can be operated within specifications in a lab setting. Subsequent test beam measurements show a hit detection efficiency of up to 91 % with the indication of possibly achieving higher efficiencies by increasing the sensor bias voltage. In conclusion, an irradiation site has been implemented at the BIC, allowing for irradiations up to extreme fluence levels with low uncertainty, using protons with high NIEL scaling. Radiation damage studies of an ITkPix pixel detector assembly show its suitability for operation up to the radiation levels anticipated for the ATLAS ITk.