The non-equilibrium response of a high-resolution Ti/Au X-ray microcalorimeter

We have studied the response of a high resolution Ti/Au transition edge sensor (TES) microcalorimeter with an energy resolution of 6.2±0.7 eV at 1.5 keV and 7.8±0.9 eV at 6.4 keV. We find that the sensitivity α≡R/T dR/dT of the TES varies significantly along the superconducting-to-normal transition that the bias point traverses during a pulse. Furthermore, α is reduced significantly during a pulse compared to its equilibrium value calculated from the DC characteristics. This leads to an overestimate of the expected energy resolution when basing the prediction on the equilibrium sensitivity.