Focal Plane Temperature

In addition to being affected by the temperature of the readout electronics, the gain is also dependent on the temperature of the focal plane (FP). Part of the the MIT CSR sub-assembly calibration efforts included operating each flight chip at off-nominal temperatures to study the influence of this effect on gain. Table 4.66 lists the mean gains and gain variation dependence on chip temperature, as calculated at MIT CSR . Each chip was operated at ^-110 C$\hbox{$^\circ$ }$, ^-120 C$\hbox{$^\circ$ }$ and ^-130 C$\hbox{$^\circ$ }$ and illuminated with an ⁵⁵Fe source. Gains were determined by fitting a gaussian to the Mn K$\alpha$ line and assuming an identically linear (i.e. no offset) relationship between the centroid and 5.895 keV. The differences between the MIT CSR mean gains and the LLTV/XRCF mean gains (Table 4.65) result from not using the flight electronics at MIT CSR.

Before calculating the gain dependence on DEA temperature, we must account for the fact that the FP was operated at slightly different temperatures during the LLTV and XRCF calibrations. Using the (dln(G)/dT_chip) calculated from the MIT measurements, the expected increase in gains from operating the FP at <^-117.5> C$\hbox{$^\circ$ }$ instead of ^-110 C$\hbox{$^\circ$ }$ is on order of 0.002 ADU/eV. This amount has no affect on the derivation of the gain dependence on DEA temperature. For comparison, .002 ADU/eV corresponds to roughly twice the thickness of a line in in Figure 4.97. If we neglect the very small effect caused by different FP operating temperatures, the DEA temperature dependent variations are determined by the linear fit term divided by the mean gain. These values are also listed in Table 4.66
 
 

Table 4.66: Gain dependence on chip temperature and DEA temperature

 

FP Location	MIT CSR		LLTV/XRCF
	Mean Gain at chip=-120 C$\hbox{$^\circ$ }$ (eV/ADU) (HIREFS)	Gain Temp Variation (dln(G)/dTchip) x10-5 (C-1)	Gain Temp Variation (dln(G)/dTDEA) x10-4 (C-1)
I0	3.990	< 6	2.0
I1	3.972	< 6	3.8
I2	3.865	-10	3.7
I3	4.250	< 6	4.1
S0	4.059	54	10
S1(BI)	5.134	< 6	2.7
S2	4.042	< 6	3.7
S3(BI)	5.109	< 6	4.4
S4	4.375	-8	4.4
S5	4.316	< 6	3.0
mean			4.2