= Fano-limited X-ray CCD energy resolution
{c}
{title2=$\Delta E_{\rm FWHM}=2\sqrt{2\ln2}\sqrt{FwE}$}
A <photon> of energy $E$ fully absorbed in <silicon> produces on average $E/w$ <electron-hole pairs>, with <variance> $F E/w$ rather than necessarily a Poisson <variance>. Thus $\sigma_E=\sqrt{FwE}$ and Gaussian FWHM is $2\sqrt{2\ln2}\,\sigma_E$. Representative <silicon> values $w\simeq3.65$ eV and $F\simeq0.12$ give about $156$ eV FWHM at $10$ keV. <read noise> in split-event <pixels>, trapping and gain errors broaden that limit; optical <pixel> size alone does not specify the <energy resolution>.<The ASCA Data Reduction Guide by NASA HEASARC>[§4.7, non-uniform CTI and Fano-limited FWHM expression]{ref}
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