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calpnalgo
July 1, 2008
Abstract
Library of dedicated calibration algorithms for EPIC PN
1 Instruments/Modes
Instrument Mode
EPIC PN IMAGING, TIMING, BURST
2 Use
pipeline processing n.a.
interactive analysis n.a.
3 Description
3.1 General
This package is only called via the CAL and has no interface to the user.
3.2 CTI correction
In CCDs part of the charge released by an absorbed photon is lost during transfer to the readout node.
This loss depends on several parameters, in particular on the position where the photon was detected, its
energy, the temperature of the CCD, and the saturation of traps by charges preceding along the readout
direction. In order to determine how these parameters aect the charge loss of the pn-CCD cameras,
more than three billion events were recorded in extensive sets of calibration measurements from February
1998 to January 1999.
For deriving the CTI, each CCD column was split into macro pixels, which were determined by adaptive
binning to contain a suфcient number of rst singles around the line feature. The charge losses were then
obtained, column by column, by cross-correlating a spectral template accumulated from all rst singles
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of a column with spectra from the macro pixels of the same column, to yield a rst estimate of the energy
shifts. This value was then used to improve the template and the cross-correlation was repeated until
convergence was achieved.
In order to obtain a general CTI correction code, a model based on the capture and emission process
of electrons in deep level traps was developed. This model, described in detail in [?], considers the
dependence of the CTI on energy and temperature, and takes the eect of precursors and the presence
of noisy pixels into account. It was calibrated with the spatially resolved charge losses obtained with the
technique described above, and makes it possible to calibrate not only the spectral response of each pixel,
but also the mutual in uence of pixels along the same CCD readout column.
3.3 Gain correction
In the pn-CCD, 768 individual ampliers allow fast readout, but imply dierent gains for the dierent
readout channels which all have to be determined and corrected for. The gains of the 768 readout nodes
were derived by comparing the position of CTI corrected calibration lines (in adu) with their known
energy. Since all gain values were found to be close to 5 adu, they were normalized to yield the constant
conversion factor 1 adu = 5.00 eV at the nominal temperature, independent of energy, for E > 3 keV.
More details on the CTI and gain correction can be found in [?].
3.4 Response matrix
The model which is used to describe the energy response of the pn-CCD is the so-called partial event
model (see [?]). The model accounts for the eects of incomplete charge collection close to the detector
surface. An eфciency function CCE is dened that re ects the portion of charge collected if a photon
is absorbed in a certain depth. This function is folded with the probability of absorption in that depth
which is given by the absorption law. The resulting analytical function for the spectral shape is then
folded with the Gaussian noise distribution given by the Fano noise and the system noise (electronic and
transfer noise).
Dierent processes are responsible for dierent spectral features that are visible in the detected channel
distribution of infalling monochromatic X-rays. Absorption in the oxide causes detection of very few
electrons, resulting in a at shelf that extends down to the detector noise peak. If the photon is absorbed
in the silicon, but close to the silicon surface, only a part of the generated electrons will reach the readout
node, causing a shoulder at the low-energy side of the main peak. The variation of this portion with
absorption depth is described by the CCE. If the absorption takes place deep in the detector all generated
electrons will be detected. The relative strength of the at shelf, shoulder and main peak is dependent
on energy.
Monochromatic synchrotron radiation at 15 dierent energies ranging from 0.150 keV to 15 keV was used
to calibrate the energy response of the pn-CCD detector. The resulting channel spectra were t with the
partial event model and the model parameters derived as function of energy. The detector response matrix
is then lled using the partial event model with the parameters derived from the ground calibrations.
4 Errors
This section documents warnings and errors generated by this task (if any). Note that warnings and
errors can also be generated in the SAS infrastructure libraries, in which case they would not be docu-
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mented here. Refer to the index of all errors and warnings available in the HTML version of the SAS
documentation.
none issued by this package
5 Input Files
6 Output Files
7 Algorithm
subroutine calpnalgo
...
end subroutine calpnalgo
8 Comments
9 Future developments
References
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