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Gain Monitoring
Since launch, calibration
observations of several sources have been taken regularly with the HRC
to monitor the detector's performance. Analysis of these
observations, examining the median source PHA as a function of time,
reveals gain fatigue in both the HRC-I and HRC-S. In the HRC-I, the
decline is roughly 30% at the aimpoint and smaller (10-20%) at the
offset positions monitored (see recent memo by
Posson-Brown & Kashyap (2007) and older memo by
Posson-Brown & Donnelly (2003)). In the HRC-S, the median source PHAs show a
large decline (roughly 10%) in the initial months of the mission. In
subsequent years, there has been a decrease in gain of 2-10%, depending on the
position on the detector, giving total declines of roughly 20% at the
aimpoint, 12% along the LETG dispersion line on the central MCP, and
18% along the dispersion line on the wing MCPs. (See memo by
Posson-Brown & Donnelly (2003) for analysis at
the aimpoint, and memo by Pease &
Drake (2003) - also available as HTML - for analysis at the
aimpoint and along the dispersion lines.)
To date, the gradual lowering of the gain has not had any effect on the quantum efficiency (QE). In May 2004, members of the instrument team, operations group, and calibration team for the HRC met to discuss the possibility of addressing the gain degradation by adjusting the voltage on the detector and/or shifting the aimpoints. However, it was decided that action is not necessary at this time since the science done with the detector is not threatened by the gain fatigue. We continue to monitor the gain closely, and will take appropriate measures if science done with the detector becomes endangered.
The gain evolution affects the pulse height amplitude (PHA) values. These can be corrected using the gain correction maps to obtain location and time independent pulse invariant (PI) values. The PI values can be used with the HRC Response Matrices for calculating hardness ratios and quantiles. In addition, for the HRC-S, the PI values are used to filter HRC-S/LETG data to reduce background, as outlined in the memo LETG/HRC-S Background Filtering with CIAO and corresponding CIAO thread. The effect of the gain fatigue on PI filtering is currently under investigation, but results from the "light" filtering are still safe to use.
We have also developed a series of time-tagged gain correction maps for the HRC-I, which will be available in the CALDB (v.3.2.5) after Dec 2006. These maps correct for spatial variations in the detector gain as well as the temporal variations in the PHA due to the gain decline. These maps are discussed in the memo "HRC-I Gain Correction" (Posson-Brown & Kashyap, 2007) and are shown below. Time-dependent gain correction maps for the HRC-S are under development.
For more information, see the following calibration memos:
Evolution of PHA Response in the HRC
J Posson-Brown, H Donnelly - May 2003
Monitoring the HRC-S Gain with the LETG/HRC-S
[HTML] [PDF]
D Pease, J Drake - October 2003
Gain
Evolution in the HRC-S: Implications for Spectroscopy and
Imaging
D Pease, J Posson-Brown, C Wilton, J Drake, H Donnelly - October 2003
The HRC-I
Gain Map
C Wilton, J Posson-Brown, M Juda, V Kashyap - October 2005
and additional PDF plots:
Gain evolution at
aimpoint of HRC-S
Gain evolution at aimpoint of HRC-I
Median PHAs across
HRC-S central MCP - ArLac observations (updated Nov 06)
Median PHAs across
HRC-I - ArLac observations (updated May 08)
Rates of gain decline across
HRC-S central MCP - ArLac observations (updated Dec 06)
Rates of gain decline across
HRC-I - ArLac observations (updated Dec 06)
In standard data processing, gain maps provide multiplicative gain
correction factors as a function of location on the detector. These
gain map values are used to convert pulse height amplitude (PHA) values
to pulse invariant (PI) values, removing the spatial dependence of the
pulse channel.
We have recently developed a series of time-dependent gain maps, which
are described in detail in the memo "HRC-I Gain
Correction" (Posson-Brown & Kashyap, 2007). These maps correct for the temporal variations in PHA due to the gain decline, as
well as correcting for the spatial dependence of PHA by analyzing AR
Lac calibration observations taken yearly at the aimpoint and 20 offset
locations. We find that the spatial and temporal changes in the gain
can be treated separately. We therefore first develop spatial
correction factors by comparing the offset PHA profiles to the aimpoint
PHA profile in each year. Then, by comparing the spatially adjusted
PHA profiles from year to year, we calculated a linear time correction
factor. Note that at the aimpoint, the gain underwent a rapid
nonlinear change between Oct 1999 and Dec 2000, so care must be
exercised when using the gain correction map for this period. The gain
correction maps and their time of validity are listed below:
Gain Correction Maps
HRC-I
| filename | Date |
| hrciD1998-10-30gainN0002 | launch till 1999-Oct-04*** |
| hrciD1999-10-04gainN0003 | 1999-Oct-04 till 2000-Dec-12 |
| hrciD2000-12-12gainN0002 | 2000-Dec-12 till 2002-Jan-26 |
| hrciD2002-01-26gainN0002 | 2002-Jan-26 till 2003-Feb-22 |
| hrciD2003-02-22gainN0002 | 2003-Feb-22 till 2004-Nov-25 |
| hrciD2004-11-25gainN0002 | 2004-Nov-25 till 2005-Oct-17 |
| hrciD2005-10-17gainN0002 | 2005-Oct-17 till 2006-Sep-06 |
| hrciD2006-09-06gainN0002 | 2006-Sep-06 till present |
The image below shows all the gain correction maps for the HRC-I, with the high voltage map in the upper left corner and the most recent maps in the bottom row. They are shown with a linear scale from 0.85-3.
Last modified: 09/27/12
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