Charge calibration of MALTA2, a radiation hard depleted monolithic active pixel sensor

Abstract

MALTA2 is a depleted monolithic active pixel sensor (DMAPS) designed for tracking at high rates and typically low detection threshold of $\sim 150{\mathrm{e}}^{-}$. A precise knowledge of the threshold is crucial to understanding the charge collection in the pixel and specifying the environment for sensor application. A simple procedure is developed to calibrate the threshold to unit electrons making use of a dedicated charge injection circuit and an Fe-55 source with dominant charge deposition of $1600{\mathrm{e}}^{-}$. The injection voltage is determined which corresponds to the injection under Fe-55 exposure and is the basis for charge calibration. The charge injection circuit incorporates a capacitance with design value of ${\mathrm{C}}_{\mathrm{inj}}=$ 230 aF. Experimentally, the average capacitance value for non-irradiated samples is found to be ${\mathrm{C}}_{\mathrm{inj},\exp }=$ 257 aF. The 12 % divergence motivates the need for the presented precise calibration procedure, which is proposed to be performed for each MALTA2 sensor.