Spectrum

Figure 4.114 shows the spectrum of the ExtCalSrc from S2. The data consists of a 9.2 ks exposure of HST data. After accounting for the different gains of each quadrant, the data from each output was summed together to increase the signal. The most obvious features are the easily discernible K$\alpha$ and K$\beta$ lines needed for accurate gain measurements. A number of other, well-understood features are present and are discussed below.

  

Figure 4.114: ExtCalSrc spectrum (0-7 keV) on S2 from a 4 $\times$ 9.2 ks integration

The lowest energy complex is a blend of Mn L$\alpha$, Mn L$\beta$, Fe L$\alpha$, and Fe L$\beta$ lines. The CCD resolution does not allow differentiation between the Mn and Fe lines ($\Delta$E $\approx$ 60 eV apart) and they appear as a single line. See Section 4.11.2.1 for a detailed discussion on the characteristics of the L complex. The next feature is the Al K$\alpha$emission line with its very well defined low-energy shoulder. Right above that is the Si K$\alpha$ fluorescence line. These photons are produced when an incident photon of higher energy interacts inside the CCD and produces a Si K$\alpha$ photon. If the photon travels far enough from the interaction sight in the proper direction, it can be detected. Regardless on whether the Si K$\alpha$ photon is detected, if it travels sufficiently far from the initial absorption site, the charge deposited at that site will equal the energy of the incident photon - 1740 eV (Si K$\alpha$ energy). These feature are the escape features of an emission line. Both the Mn and Ti K$\alpha$ escape features are clearly present and nearly gaussian. Ti K$\beta$ escape is also present, but with fewer events. Mn K$\beta$ escape is also present, but is hidden by the Ti K$\beta$ line. The well-defined, but irregularly shaped feature around 2.2 keV is the Au M complex, a number of closely spaced Au M lines. The lines are produced by fluorescence of the gold coating on the outer surface of the radioactive sources. Finally, we have indicated where the pile-up line of Al K$\alpha$ would occur. There is a hint of counts above the background at the appropriate energy, but low statistics prevents a firm detection. Certainly, pile-up is always present at some level, but the incidence for the ExtCalSrc is small and will only decrease as the ⁵⁵Fe decays. For a discussion of the features present above 7 keV, we again refer the reader to detailed discussion in Section 4.11.2.1. Table 4.81 lists the lines and energies of all the labeled features in Figure 4.114.

 

Table 4.81: ExtCalSrc lines in the 0-7 keV band

Line	Energy (keV)	Line	Energy (keV)
Mn L $\alpha _{1,2}$	0.636	<Ti K $\alpha _{1,2}$> esc	2.768
Mn L$\beta_{1}$	0.647	2 $\times$ <Al K $\alpha _{1,2}$>	2.974
Fe L $\alpha _{1,2}$	0.704	Ti K$\beta_{1}$ esc	3.191
Fe L$\beta_{1}$	0.717	<Mn K $\alpha _{1,2}$> esc	4.155
<Al K $\alpha _{1,2}$>	1.487	<Ti K $\alpha _{1,2}$>	4.508
<Si K $\alpha _{1,2}$>	1.740	Mn K$\beta_{1}$ esc	4.750
Au M $\alpha _{1,2}$	2.112	Ti K$\beta_{1}$	4.931
Au M$\beta$	2.20	<Mn K $\alpha _{1,2}$>	5.895
Au M$\gamma$	2.410	Mn K$\beta_{1}$	6.490