Obtaining spectroscopic information with the ATLAS FE-I4 pixel readout chip

The ATLAS FE-I4 pixel readout chip is a 2×2cm2 state-of-the-art ASIC that is used in high-energy physics experiments and for research and development purposes. It is optimized for high hit rates and radiation hardness, but it provides only limited charge resolution. Therefore, a method generally used for the characterization of binary readout chips is presented and applied to the FE-I4 to obtain spectroscopic information with high resolution. The method relies on the ability to change the detection threshold in small steps, while measuring the particle rate, in order to record the integrated single pixel charge-spectra. The challenges resulting from the large number of channels (28,880), such as inhomogeneous threshold, low statistics per pixel and effects of charge sharing, are discussed. Charge spectra of an ATLAS FE-I4 bump bonded to a 230 μm 3D-silicon sensor from radioactive sources and electron beams are analyzed and presented. The results show that a charge resolution close to the electronic noise can be achieved proving the potential of this approach.