Abstract
We are developing high energy resolution X-ray microcalorimeters based on iridium and gold (Ir/Au) phase transition thermometers. Here we analyze the signal behavior of a 200×200 µm2 Ir/Au transition edge sensor (TES) with iridium and gold having the thicknesses of 100 and 25 nm, respectively. The energy resolution of this device was 15.4 eV full width at half maximum (FWHM) at 5.9 keV X-ray energy. For operation at bias resistances lower than 140 mΩ, we observed two distinct decay components, a slow component followed by a very fast component, which is thought to be due to the variation of current distribution inside the TES. However, taking into consideration the nonlinearity associated with the slow component, a very fast signal response makes the operation of this device promising at low-resistance bias points.