Spatially-averaged Relative Quantum Efficiency Measurements

The results of the relative quantum efficiency measurements described above in Section 4.7.1 are presented in Table 4.52. The mean relative efficiencies given in the table are the centroids obtained from Gaussian fits to histograms of the superpixel ratios in each measurement. These values have been corrected for pileup and are our best estimates for the spatially averaged relative efficiency between the corresponding flight and reference detectors. Listed in parentheses are measurements of the spatial variation (RMS) in efficiency across each chip, obtained by computing the standard deviation of the superpixel ratios. (Note that the RMS variations in efficiency are heavily influenced the framestore cover shielding at the edge of each device. An alternative measure of spatial variation, which is more representative of the majority of the area of each device, is presented and discussed in Figures 4.77-4.86 in section 4.7.2.2.) The name of the reference device used during each measurement is listed in the table, along with the focal plane position of each flight device. Note the QE ratio listed for w140c4r at 525 eV is referenced to w134c4r, a backside chip. No measurement relative to w103c4 was made at this energy because of time constraints. Also, the QE ratio for w134c4r relative to w203c2 at 677 eV was derived from the corresponding ratio for w140c4r relative to w134c4r. This was necessary because reference device w203c2 was misaligned during calibration of w134c4r at this energy.
 
 

Table 4.52: Spatially Averaged Relative Detection Efficiency at CSR

 

Flight	Position	Mean (RMS) Relative Efficiency vs. Energy (keV)							Reference
Device		0.525	0.677	1.740	2.015	4.509	5.894	8.040	Device
		O	F	Si	P	Ti	Fe55	Cu
w203c4r	I0	0.972	0.992	0.996	0.982	0.994	0.977	0.933	w190c3
		(0.040)	(0.041)	(0.046)	(0.046)	(0.046)	(0.045)	(0.043)
w193c2	I1	0.885	0.768	0.980	0.951	1.005	1.034	1.097	w103c4
		(0.045)	(0.040)	(0.052)	(0.049)	(0.054)	(0.055)	(0.060)
w158c4r	I2	0.914	0.804	0.986	0.968	1.007	1.029	1.096	w103c4
		(0.040)	(0.037)	(0.047)	(0.045)	(0.047)	(0.048)	(0.055)
w215c2r	I3	0.999	1.015	0.997	0.989	0.996	0.980	0.929	w190c3
		(0.041)	(0.040)	(0.051)	(0.050)	(0.052)	(0.053)	(0.046)
w168c4r	S0	1.064	1.089	1.004	1.014	0.997	0.972	0.916	w190c3
		(0.042)	(0.044)	(0.050)	(0.050)	(0.050)	(0.048)	(0.047)
w182c4r	S2	1.009	0.962	1.003	1.002	1.014	1.067	1.210	w103c4
		(0.042)	(0.042)	(0.053)	(0.052)	(0.053)	(0.055)	(0.064)
w457c4	S4	1.020	1.070	1.005	1.020	1.000	0.999	1.000	w190c3
		(0.030)	(0.043)	(0.031)	(0.032)	(0.031)	(0.031)	(0.031)
w201c3r	S5	1.004	1.025	1.001	0.991	1.000	1.000	1.005	w190c3
		(0.032)	(0.034)	(0.038)	(0.037)	(0.039)	(0.039)	(0.040)
w140c4r	S1	1.030	1.078						w134c4r
		(0.037)	(0.038)
			2.507	1.035	1.442	0.934	0.763	0.631	w103c4
			(0.114)	(0.050)	(0.082)	(0.080)	(0.054)	(0.041)
w134c4r	S3	3.457	2.955	1.108	1.570	1.014	0.882	0.743	w203c2
		(0.113)	(0.184)	(0.061)	(0.087)	(0.061)	(0.054)	(0.051)
w190c3	Ref.	0.890	0.767	0.986	0.949	1.010	1.058	1.164	w103c4
		(0.020)	(0.017)	(0.018)	(0.014)	(0.014)	(0.016)	(0.051)
w203c2	Ref.	0.872	0.787	0.979	0.950	1.004	1.009	1.041	w103c4
		(0.016)	(0.016)	(0.015)	(0.015)	(0.015)	(0.016)	(0.021)

The correction factors used to correct the count rates at CSR for pileup are presented in Table 4.53. The values listed in parenthesis are the pileup correction factors applied to the count rates from the reference devices.
 
 

Table 4.53: Pileup Correction Factors Applied to Count Rates from CSR

 

Flight	Position	Flight (Reference) Pileup Correction vs. Energy (keV)							Reference
Device		0.525	0.677	1.740	2.015	4.509	5.894	8.040	Device
		O	F	Si	P	Ti	Fe55	Cu
w203c4r	I0	1.053	1.074	1.075	1.097	1.063	1.068	1.169	w190c3
		(1.055)	(1.075)	(1.076)	(1.100)	(1.064)	(1.070)	(1.185)
w193c2	I1	1.061	1.031	1.033	1.033	1.026	1.064	1.067	w103c4
		(1.071)	(1.041)	(1.076)	(1.079)	(1.060)	(1.146)	(1.142)
w158c4r	I2	1.068	1.032	1.022	1.033	1.027	1.066	1.066	w103c4
		(1.076)	(1.041)	(1.050)	(1.078)	(1.061)	(1.151)	(1.141)
w215c2r	I3	1.051	1.075	1.073	1.095	1.066	1.068	1.166	w190c3
		(1.051)	(1.075)	(1.074)	(1.097)	(1.067)	(1.070)	(1.182)
w168c4r	S0	1.068	1.074	1.073	1.097	1.067	1.070	1.166	w190c3
		(1.064)	(1.068)	(1.074)	(1.096)	(1.068)	(1.073)	(1.184)
w182c4r	S2	1.042	1.039	1.033	1.034	1.026	1.065	1.073	w103c4
		(1.042)	(1.041)	(1.075)	(1.078)	(1.059)	(1.143)	(1.140)
w457c4	S4	1.067	1.074	1.078	1.098	1.066	1.073	1.200	w190c3
		(1.066)	(1.069)	(1.078)	(1.096)	(1.066)	(1.074)	(1.201)
w201c3r	S5	1.065	1.070	1.078	1.096	1.065	1.072	1.184	w190c3
		(1.065)	(1.069)	(1.079)	(1.098)	(1.066)	(1.073)	(1.184)
w140c4r	S1	1.073	1.037						w134c4r
		(1.072)	(1.034)
			1.037	1.030	1.048	1.032	1.049	1.018	w103c4
			(1.040)	(1.076)	(1.078)	(1.061)	(1.140)	(1.144)
w134c4r	S3	1.063	1.033	1.068	1.096	1.077	1.057	1.057	w203c2
		(1.049)	(1.031)	(1.070)	(1.062)	(1.060)	(1.062)	(1.176)
w190c3	Ref.	1.062	1.070	1.067	1.091	1.052	1.078	1.087	w103c4
		(1.070)	(1.093)	(1.069)	(1.097)	(1.119)	(1.172)	(1.166)
w203c2	Ref.	1.035	1.031	1.034	1.032	1.026	1.056	1.070	w103c4
		(1.041)	(1.040)	(1.078)	(1.076)	(1.059)	(1.127)	(1.158)

In Table 4.54 we have used the values given in Table 4.52 to compute QE ratios referenced to w190c3 for each flight device. The results are plotted in Figure 4.76. Most of the relative efficiency points in the upper panel of the figure lie within 5% of unity, and all but two of the points lie within 10%, indicating that at each energy the absolute efficiencies of the eight frontside devices are nearly identical. In the lower panel we see the backside devices are about three times more efficient than the frontside devices at low energies (< 1 keV), but are less efficient than the frontside devices at high energies (> 3-4 keV).
 
 

Table 4.54: Spatially Averaged Relative Detection Efficiency at CSR--Referenced to w190c3

 

Flight	Position	Mean (RMS) Relative Efficiency vs. Energy (keV)
Device		0.525	0.677	1.740	2.015	4.509	5.894	8.040
		O	F	Si	P	Ti	Fe55	Cu
w203c4r	I0	0.972	0.992	0.996	0.982	0.994	0.977	0.933
		(0.040)	(0.041)	(0.046)	(0.046)	(0.046)	(0.045)	(0.043)
w193c2	I1	0.994	1.002	0.994	1.002	0.996	0.977	0.942
		(0.053)	(0.054)	(0.056)	(0.053)	(0.055)	(0.054)	(0.076)
w158c4r	I2	1.027	1.049	1.000	1.020	0.997	0.972	0.941
		(0.048)	(0.050)	(0.050)	(0.050)	(0.049)	(0.049)	(0.073)
w215c2r	I3	0.999	1.015	0.997	0.989	0.996	0.980	0.929
		(0.041)	(0.040)	(0.051)	(0.050)	(0.052)	(0.053)	(0.046)
w168c4r	S0	1.064	1.089	1.004	1.014	0.997	0.972	0.916
		(0.042)	(0.044)	(0.050)	(0.050)	(0.050)	(0.048)	(0.047)
w182c4r	S2	1.133	1.255	1.017	1.056	1.005	1.008	1.039
		(0.052)	(0.058)	(0.057)	(0.056)	(0.055)	(0.055)	(0.083)
w457c4	S4	1.020	1.070	1.005	1.020	1.000	0.999	1.000
		(0.030)	(0.043)	(0.031)	(0.032)	(0.031)	(0.031)	(0.031)
w201c3r	S5	1.004	1.025	1.001	0.991	1.000	1.000	1.005
		(0.032)	(0.034)	(0.038)	(0.037)	(0.039)	(0.039)	(0.040)
w140c4r	S1	3.487	3.269	1.050	1.519	0.925	0.721	0.542
		(0.190)	(0.154)	(0.054)	(0.088)	(0.080)	(0.053)	(0.047)
w134c4r	S3	3.386	3.032	1.100	1.572	1.008	0.841	0.664
		(0.139)	(0.203)	(0.065)	(0.093)	(0.064)	(0.055)	(0.062)

 

Figure 4.76:  Relative detection efficiencies of front-illuminated (upper panel) and back-illuminated (lower panel) ACIS flight CCDs as a function of energy - referenced to w190c3.

We have computed the mean relative efficiency and the maximum absolute deviation from the mean for each CCD type as a function of energy (see Table 4.55). The largest absolute deviations from the mean ($\sim$10-20%) in the frontside devices are at 525 eV, 677 eV, and 8 keV. The deviations at 525 eV and 677 eV are due to the high relative efficiency of w182c4r at these energies, which in turn is caused by that detector's relatively thin oxide layer. The reason for the large deviations at 8 keV is still unknown.

From the RMS variations listed in Table 4.54 it can be seen that spatial variations in efficiency across the frontside devices are low ($\sim$3-6%). About two-thirds of the RMS variation is the result of shadowing by the framestore covers. Thus, we find the detection efficiencies of the frontside devices are highly uniform within each chip and between the chips.

The spatial variations in efficiency across the backside devices are generally larger ($\sim$5-8%) than in the frontside devices, but the maximum absolute deviations from the mean of the backside devices are of comparable amplitude to those of the frontside devices. Hence the spatially averaged quantum efficiencies of the two backside devices are in reasonably good agreement with each other.
 
 

Table 4.55: Mean Relative Detection Efficiencies for FI and BI CCDs at CSR--Referenced to w190c3

 

Device	Mean (Max. Abs. Dev.) Relative Efficiency vs. Energy (keV)
Type	0.525	0.677	1.740	2.015	4.509	5.894	8.040
	O	F	Si	P	Ti	Fe55	Cu
FI	1.027	1.062	1.002	1.009	0.998	0.986	0.963
	(0.106)	(0.193)	(0.015)	(0.046)	(0.006)	(0.022)	(0.076)
BI	3.436	3.151	1.075	1.546	0.967	0.781	0.603
	(0.050)	(0.119)	(0.025)	(0.027)	(0.041)	(0.060)	(0.061)