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
Quantum efficiency of ground irradiated device as a function of row
and energyPostscript version
- C. Grant, 03/07/00
A measurement of the CTI effect on the quantum efficiency of
the FI chipsPostscript version
- A. Vikhlinin, 03/03/00
to the QE page
to the ACIS Calibration page