CTI-Induced Quantum Efficiency Loss in ACIS Front Illuminated Devices

April 5, 2000

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.

Memoranda discussing this effect are posted below. The CXC is calibrating the effect, and will implement a software correction as soon as possible. In the mean time, please be advised of the effect as you analyze your data.

Mitigating charge transfer inefficiency in the Chandra X-ray Observatory's ACIS instrument
- L. Townsley, P. Broos, G. Garmire, J. Nousek, 02/24/00
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    Quantum efficiency of ground irradiated device as a function of row and energy
    - C. Grant, 03/07/00

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    A measurement of the CTI effect on the quantum efficiency of the FI chips
    - A. Vikhlinin, 03/03/00

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