ACIS Pileup: Analysis of XRCF Phase H Data

Results of analysis of XRCF ACIS pileup tests H-IAI-CR-1.001, 1.003 and 1.005 are presented. In this series of tests with monotically increasing Al Kα source flux (ranging from ∼ 0.3 to ∼ 3.0 ACIS counts/frame in the absence of pileup) incident on a frontside-illuminated CCD, various symptoms of pileup — image degradation, grade migration, count rate non-linearity, and event pulse height confusion — are observed and quantified. As the fraction of events consisting of two or more photons increases (from 3% to 26% over this suite of tests) the image core broadens slightly while the proportion of events occupying multiple pixels increases markedly. A large fraction of events (∼ 23%) are “lost” from the “standard” ASCA grade set (02346) at the highest flux. We tabulate the rates of singleand multiple-photon events, and the event pileup fraction, as functions of the ACIS count rate in the absence of pileup (as inferred from beam normalization detector data). Non-linearity of the single-photon count rate is evident, as is the loss of photons both to “non-standard” grades and (possibly) to off-line event amplitudes.