Spectroscopic Performance of DEPFET active Pixel Sensor Prototypes suitable for the high count rate Athena WFI Detector

The focal plane of the WFI of Athena consists of two sensors. One features a large field of view of 40' x 40' and one is forseen to be used for bright point like sources. Both parts base on DEPFET active pixel sensors. To fulfil the count rate requirement for the smaller sensor of less than 1% pile-up for a one Crab source it has to have a sufficient high frame rate. Since therefore the readout becomes a large fraction of the total photon integration time, the probability of measurements with incomplete signals increases. A shutter would solve the problem of these so called misfits but is not in agreement with the required high throughput of more than 80%. The Infinipix design has implemented a storage in addition to separate the collection and the readout of the charges without discarding them. Its working principle was successfully shown by Bahr et al.(1) on single pixel level. For the further development three layout variants were tested on a 32 x 32 pixel array scale. The measurements of the spectroscopic performance show very promising results even for the intended readout speed for the Athena WFI of 2.5 mu s per sensor row. Although, there are still layout and technology improvements necessary to ensure the reliability needed for space missions. In this paper we present the measurement results on the comparison of the three prototype layout variants.