ACIS Detector Efficiency

Monitoring and analysis of ACIS detector efficiency by the instrument team.

Updated monthly.

Detector efficiency at 6 keV

The Fe-55 calibration source has a half-life of 2.73 years. The plots below show the ratio of the measured counting rate of the Mn-Kα line to a model with the predicted decay rate of Fe-55 for all ten ACIS CCDs (bottom 100 rows only to minimize the effect of CTI change). The data are all completely consistent with this decay rate implying no change in the quantum efficiency at 6 keV.

Detector efficiency at 700 eV and 1.5 keV

ACIS is known to be suffering from contamination which reduces the low energy detection efficiency. The decrease in efficiency due to contaminant can be monitored by measuring the ratio of the Mn-L to Mn-Kα lines and the Al-K to Mn-Kα lines. As shown above, except for the radioactive decay of the source, there has been no change in the Mn-Kα detection efficiency since launch. Any change in this ratio is then due to increasing contamination. For the highest statistics, only data from the S3 CCD is shown below.

The first plot shows the transmission and the second the optical depth at 700 eV as a function of time.

Transmission and optical depth data

The same for 1.5 keV

Transmission and optical depth data

Ratio of optical depths at 1.5 keV to 700 eV. The dotted line is the mean of the ratio.

• 03/29/07 - Temperature filter upper limit for contamination monitoring changed from -119.7 to -119 which increases the number of data points for 2005 and beyond. Data bin size doubled to decrease statistical scatter.
• 04/03/07 - Added Al-K contamination monitoring and Al-K/L-complex optical depth ratio. Temperature filter upper limit for 6 keV QE monitoring changed from -119.7 to -119.
• 04/09/07 - Changed Mn-K decay monitoring so that S1 and S3 use the bottom 100 rows only, like the FI devices. Previously used full CCD.