Next: Preliminary Assessment of Wavelength Up: Absolute Calibration of Reference Previous: Methodology
When invoked, the unity command normalizes all redistribution function to unity at each energy. This is equivalent to assuming the branching ratios are not a function of energy. Although this is generally incorrect, by limiting the analysis to a specific energy range (.3 - 4 keV) and to a particular subset of events (ASCA grades 0,2,3,4,6), the assumption is valid. The threshold values are matched to those used in the extraction of events from the CCD output. Although the same CCD/electronics combination was used for the BESSY calibration, the energy scale and noise conditions can vary slightly between the two measurements. The gain, offset, and broadening options account for the differences in the operating conditions and modifies the RM accordingly.
|unity||u||N/A||assumes unity branching ratios|
|split threshold||p||ADU||standard MIT values; depends on CCD electronics|
|event threshold||v||ADU||standard MIT values; depends on CCD electronics|
|grid spacing||E||keV||defines the energy grid that rspgen uses|
|gain||g||N/A||scales the ECD data|
|offset||o||ADU||offsets the ECD data|
|broadening||b||ADU||decreases or increases the ECD in quadrature|
|tailing||t||N/A||scales the low energy redistribution|
The purpose of the BESSY experiments is to obtain an absolute calibration for the reference detectors. This goal is achieved by multiplying a model of the CCD quantum efficiency with the RM, convolving the model response with the input spectrum, and comparing the results to the data. The parameters of the CCD QE model are adjusted to the find the best fit to the data. The current CCD model is the Slab and Stop Model synthesized by K. Gendreau [Gendreau1995]. It approximates the gate structure as piecewise-uniform layers of Si, SiO2, and Si3N4 and the channel stop as layers of Si and SiO2 with finite width. The final two CCD parameters are the width of the pixel and the depletion depth. The model assumes that the gate and channel stop are dead, and only those photons interacting in the depleted silicon will be detected.
Fitting the BESSY data with this model does not constrain all the parameters. The channel stops occupy a small area compared to the gate structure. This reduces the role of the channel stop parameters on the overall fit and leads to degeneracy in these three best-fit parameters. Rather than determining these dimensions from BESSY data, we have measured the channel stops by other methods and frozen the parameters at these values. For the energy range of the White Light calibrations (E < 4 keV), the quantum efficiency is mainly determined by the gate parameters and is insensitive to the depletion depth. Therefore, this parameter is also frozen at a value determined by another method (see Section 4.6.2 for details). Table 4.15 gives the parameters names, describes it and states whether or not it is fixed.
|Dead||Si gate thickness||free|
|SiOx||SiO2 gate thickness||free|
|SiNit||Si3N4 gate thickness||free|
|Depl||Depletion depth||frozen-determined by branching ratio method|
|CSWidth||Channel Stop width||frozen-determined by SEM measurements|
|CSSiOx||SiO2 stop thickness||frozen-determined by SEM measurements|
|CSSi||Si P+ thickness||frozen-determined by mesh experiments|
|Pix||Pixel width||frozen at 24|
|3l: see Section 4.6.2|
|3l: see Section 4.5|
A total of eleven devices were characterized at BESSY during six separate 48 hour shifts. Table 4.16 shows when each CCD was calibrated, how many data sets were taken, and the number of CCD positions illuminated. A typical measurement consisted of acquiring 2000 frames. Integration times ranged from .83 - 1.53 seconds, depending on which readout electronics were used. Storage ring currents ranged from 2 to 50 electrons, but typically the ring current was adjusted to either 10 or 20 electrons (again, this depended on readout electronics) in order to provide 350 counts/frame/quadrant. For a typical measurement, this yields on order of 3 x 106 counts in the 0.3-4.0 keV band over the illuminated part of the CCD.
Reduction of the data begins by extracting events from the raw data and saving the location, pulse-height value, and frame number of each event in an event list. The storage ring current is monitored by the BESSY facility, and this data stream is searched for the loss of electrons from the storage ring during our measurements. If such a loss occurred, the event list is temporal filtered. Pileup effects are very dependent on the initial flux rate, and limiting the data to a single ring current allows the most accurate correction. Finally, the event list is further filtered by event grade selection and a XSPEC compatible PHA file is produced. At this time, the data from an entire quadrant is averaged together in a 5mm x 6mm spatial bin.
|Chip||Date||Data Sets||Chip Positions||Electronics|
|w34c3||Mar 95||7||4||acis 3|
|w34c3||Apr 95||17||4||acis 3|
|w34c3||May 95||6||5||acis 6|
|w34c3||Aug 95||3||1||acis 3|
|w103c1||Mar 95||7||4||acis 3|
|w102c3||May 95||6||5||acis 6|
|w103c2||May 95||5||5||acis 6|
|w103c4||May 95||6||5||acis 6|
|w103c4||Aug 95||5||5||acis 3|
|w147c3 (BI)||Aug 95||15||5||acis 3|
|w148c4 (BI)||Aug 95||6||5||acis 3|
|4l: Wavelength Shifter Measurement|
|4l: ACIS-2C device|
The rspgen-generated RM, an ``atable'' model of the incident synchrotron radiation spectrum, the CCD model, and the PHA file are loaded into XSPEC. The atable model (/usr/acis/atable/bessy_new.mod) has two parameters, an overall normalization factor and another parameter that exists for historical reasons but is no longer used. The data is rebinned, the energy range limited to 0.3-4.0 keV and the model parameters are left free or frozen according to Table 4.15. If a pileup correction is needed, the A. Rasmussen mtable correction is also loaded into XSPEC. It has one phenomenological parameter that corresponds to the incident flux rate. XSPEC then fits for gate thicknesses, the source normalization and the pileup parameter.
Figure 4.47, Figure 4.48, and Figure 4.49
show the best fit models with the data for individual quadrants of
detectors w190c3, w190c1, and w103c4. The RMs used for the fit can be found in:
Pileup corrections have only been applied to the data for devices w190c1 and w190c3. Table 4.17 shows the best-fit parameters, the RMS error, and the normalization accuracy for each reference detector as well as listing the values of the frozen parameters.
CCD Parameters (in )
1.000 +/- .003
0.994 +/- .003
0.941 +/- .003
10l: typical 90 % confidence error is +/- 0.008 m
10l: typical 90 % confidence error is +/- 0.011 m
10l: w103c4 has no pileup correction applied
As a whole, the model fits for detectors w190c1 and w190c3 are quite good. In both cases the data:model ratio shows two deviation from unity: a narrow feature around 1.8 keV and a systematic underestimation of the flux above 2.2 keV. One explanation for the narrow feature could be an underestimation of the Si fluorescence. Analysis of the response function data indicates that the fluorescence yields in the current response matrices are too low (see Section 4.3.2). The excess above 2.2 keV results from the good, but not exact, correction for pileup. Future work includes applying a Jones model correction to the data for all three reference detectors (see Section 4.4). The best-fit normalizations are within 1 % of the calculated value, but could change with an improved pileup algorithm and inclusion of more accurate fluorescence data. Another measure of the goodness of fit for these two devices is the comparison of the derived gate thicknesses. w190c1 and w190c3 came from the same wafer, and hence, underwent the exact same fabrication process. The differences between the derived thickness for the Si, SiO2, and Si3N4 layers are well within the errors.
The fit for reference detector w103c4 is noticeably worse than the other two devices. The RMS error is higher, and the best-fit normalization is nearly 6% too low. Above 2 keV, the model underestimates the data by almost 10%. The deviation from unity illustrates the importance of correcting the data for pileup. At the same time, however, the best-fit values for the three gate layers are reliable. Studies with devices w190c1 and w190c3 indicate that neglecting pileup influences the RMS error and normalization but has only small effects on the best-fit gate thicknesses. This behavior is consistent with the low level pileup model discussed in Section 4.4. To first order, pileup shifts events out of the acceptable grades (this explains the low best-fit normalization) and deposits some small fraction of these events' charge into non-physical, higher energy events (this accounts for the excess of counts above 2 keV).
Extensive measurements at MIT CSR have shown that the QE of front-illuminated CCDs have little spatial variation over bin sizes of .77 mm2. Because the intensity and shape of the BESSY spectrum changes over the illumination pattern, similar measures of fine spatial uniformity are non-trivial. Our current spectral fitting procedure is to integrate all the data into 5mm x 6mm spatial bins. Table 4.18 shows the average counting rate (cts/sec/e- ring current) normalized to the y001 position for each quadrant of w103c4 at five different CCD positions. Data is from the August 95 shift. The counting rates for y768 are expected to be higher than those for any other quadrants since the beam current was lower for this measurement and pileup effects should be smaller. Statistical errors for the data sets are less than .002. Finally, on average there is one bad column per quadrant that does not extend the length of the CCD. This analysis does not account for the reduction of the active area caused by the bad column, and this introduces an uncertainty on order of .004 to the ratios.
|CCD Position||Ring Current||Quad A||Quad B||Quad C||Quad D|
A final check of the BESSY data is a comparison of the relative quantum efficiency measurements made at MIT CSR. Reference standards w190c3 and w103c4 were calibrated with respect to one another at discrete energies using the HEXS (see Section 4.7.1 for details). The model fitting to the BESSY data yields absolute efficiencies for both devices. Dividing these continuous curves by one another provides an independent check of both the MIT CSR measurements and the quality of the CCD model. The upper panel of Figure 4.50 shows the absolute quantum efficiencies determined from the synchrotron data. The higher efficiency of w103c4 is easily understood as it has a thinner gate oxide layer than w190c3. The bottom panel of Figure 4.50 shows the discrete relative measurements made at MIT CSR vs. the continuous BESSY-derived relative quantum efficiencies. The errors associated with the MIT CSR values are systematic and currently estimated at 2 %.