Charge reconstruction from binary hit data on irradiated MALTA2 Czochralski sensors

MALTA2 is a depleted monolithic active pixel sensor (DMAPS) designed for tracking at high rates and is produced in the modified Tower 180 nm CMOS imaging technology. The pixel pitch is 36.4 µm and the sensing layer consists of either high resistivity epitaxial or Czochralski silicon. The implementation of a small collection electrode features a small pixel capacitance and offers low noise. Typically a detection threshold of around 200 e⁻ is used. It is calibrated making use of a dedicated charge injection circuit on chip and an Fe-55 source. In this contribution, MALTA2 sensors are characterized in terms of hit detection efficiency inside the pixel at fine threshold steps. A comparison is made for samples before and after irradiation at different substrate voltages and with different doping concentration of the internal n-layer. Data was taken at CERN SPS test beam campaigns in 2023 and 2024, using the MALTA beam telescope consisting of six MALTA tracking planes with 4 µm spatial and 2 ns timing resolution. A reconstruction of the most probable signal amplitude from binary hit data is performed. A two-dimensional map of the collected charge is obtained with sub-pixel resolution. The presented method provides an in-beam alternative to grazing angle studies or Edge-TCT for determining a charge collection profile.