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For some time now, it has been apparent that the ratio of the quantum efficiency (QE) of the Back Illuminated (BI) and Front-Illuminated (FI) chips in the ACIS camera was incorrect. We resolve this discrepancy by the analysis of ground calibration data, and discuss new curves for the BI quantum efficiencies.

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.

For several years it has appeared that at energies below about 1 keV there was a significant discrepancy in the ratio the quantum efficiency of front-illuminated (FI) to back-illuminated (BI) ACIS chips. A good discussion on the grating data is given by H. Marshall.

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.

The increased charge transfer inefficiency (CTI) in the ACIS front illuminated devices causes a quantum efficiency (QE) decrease that is not accounted for in the current generation calibration files. The effect is seen as a decreasing QE with increasing distance from the readout node (Y axis). It is caused by standard flight event grades migrating to rejected grades as charge is lost to CTI during readout. The effect is large for high energy events. A ~20% loss at 5.9 keV is seen for charge clocked over the extent of a device. The loss decreases to a few percent at 1.5 keV. The loss again increases to about 50% for low energy events around the Carbon edge (0.277 keV). However, at Carbon, events are lost as their charge packets fall below the event detection threshold.

Plots of ACIS FI and BI QE. Including data tables and release notes.

Questions or Comments to:
R. Edgar

Last modified: 11/15/10

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