The absolute quantum efficiency (QE) of ACIS chips S2 and S3 is derived at several energies from flat field data taken at the X-Ray Calibration Facility (XRCF). The data were taken in May 1997. Absolute quantum efficienty is derived by comparing count rates of the ACIS chips (derived from high-speed tap data) and a Flow Proportional Counter (FPC) in the target line. The FPC in question (fpc_hn) was absolutely calibrated at the synchrotron at BESSY.
We find that the Back-Illuminated chips S1 and S3 had their QE underestimated in released (version N0003) CALDB products, and discuss a corrected product. These curves have been released in Summer 2004 as version number N0005 of the ACIS QE files.
The sense and size of this correction is approximately what is needed for consistency with grating measurements (Marshall et al. 2003) and for some of the ACIS imaging mode observations of clusters of galaxies.
This memo has nothing to do with contamination of ACIS, but rather addresses the QE of ACIS at the time of launch.
However, astrophysical sources cannot resolve the issue of which (if either) of the QE curves is correct, since we have no a priori knowledge of the spectra or intensities of celestial x-ray sources.
We have therefore undertaken a re-analysis of some of the flat field data taken at the X-Ray Calibration Facility (XRCF) in the spring of 1997. In this experiment, an electron impact point source illuminated the flight ACIS camera and several calibrated proportional counters.
In this memo, we compute the absolute quantum efficiency of the ACIS S2 (FI) and S3 (BI) chips at two low energies (Oxygen K-alpha, 0.525 keV and Copper L-alpha, 0.929 keV) based on these ground calibration data. We find that the released QE curves in the CALDB are essentially correct for S2, but are underestimated for S3 by ~9%.
In the next few months we will analyze XRCF data at the iron L and K lines (approximately 0.7 and 6.0 keV), and attempt to construct a better, physically motivated, QE curve for the S3 chip.
Our best estimate for release of a CALDB product is summer of 2004.
For further information, please consult our papers in the 2003 Chandra Calibration Workshop.
Last modified: 11/15/10
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