Achievable time resolution of spectroscopic prototype DEPFET detectors

Abstract

A new generation of spectroscopic prototype DEPFET detectors provide a noise performance of 6 e−ENC rms with readout times shorter than 0.6 μs per pixel. This is enabled by the DEPFET pixels, which include a linear gate shaped transistor and the VERITAS readout ASIC, which is capable of operating the sensor in the source follower as well as in the drain current readout mode. The achievable frame rate does not depend only on the minimum processing time per pixel, but also on different possible readout schemes for the sensor array. In particular it is shown, that active DEPFET pixels can be binned together or can be operated in a window mode. The pixel binning is realized by controlling a cluster of DEPFET transistors together, in order to build one larger pixel. This results in a higher frame rate at the expense of spatial resolution and spectral performance. In the window mode solely a subset of pixels is read out while the remaining pixels are regularly cleared. The operated window provides a higher time resolution but spatial information is lost in the region outside the window. The measurements were done on 64×64 pixel detectors, which were operated either in the source follower or in drain current readout mode. In this paper we present studies and measurement results concerning the achievable time resolution of those detectors. A faster frame rate increases the fraction of misfit events—defined by signal charge collected during the readout—while decreasing the probability for pile-up events. Both effects are analyzed and compared with Monte Carlos simulations.