Development of new circuit with CSA for X-ray astronomical SOI pixel detector - improving energy resolution

We have been developing a monolithic active pixel sensor with the silicon-on-insulator (SOI) CMOS technology for use in future X-ray astronomical satellite mission. Our objective is to replace the X-ray Charge Coupled Device (CCD), which is the standard detector in the field, by offering high coincidence time resolution (~ 50 ns), superior hit-position readout time (~ 10 μs), and wider bandpass (0.5 - 40 keV) in addition to having comparable performances in imaging spectroscopy. In order to realize this detector, we have developed prototype detectors, called “XRPIX” series. XRPIX contains comparator circuit in each pixel to detect an X-ray photon injection; it offers intra-pixel hit trigger (timing) and two-dimensional hit-pattern (position) outputs. Therefore, XRPIX is capable of direct access to selected pixels to read out the signal amplitude. In our previous study, we evaluated its basic performance and obtained the X-ray spectra by this system. The next step is improvement in spectroscopic performance. Then, we designed a new prototype, called “XRPIX3”, which has charge sensitive amplifier (CSA) in each pixel. The pixel circuit with CSA works good. It is 3.4 times higher gain as compared with normal pixel circuit. Furthermore, XRPIX3 resolved Mn-K_α and Mn-K_β successfully for the first time in our series. The readout noise is 33 e- rms and the energy resolution is about 300 eV FWHM at 5.9 keV. In this paper, we report on the design and test results of this new device.