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

MALTA2 is a depleted monolithic active pixel sensor (DMAPS) designed for tracking at high rates and typically low detection threshold of $200e^{-}$. Threshold calibration is crucial to understanding the charge collection in the pixel. 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 $1611e^{-}$. The injection voltage corresponding to the signal under exposure of Fe-55 is determined and serves as the basis for charge calibration. The charge injection circuit incorporates a capacitance with design value of $C_{\mathrm{inj}}=230aF$. Experimentally, the average capacitance value for non-irradiated samples is found to be $C_{\mathrm{inj,exp}}=255\pm 35aF$ and varies between sensors by 14% in terms of standard deviation. The deviation from the design value as well as among sensors motivates the need for the presented calibration procedure, which is proposed to be performed for each MALTA2 sensor and reduces the uncertainty down to 3% depending on the sensor.