A Mo-Cu superconducting transition-edge microcalorimeter with 4.5 eV energy resolution at 6 keV

We describe a superconducting transition-edge microcalorimeter with an energy resolution of 4.5±0.1 eV full-width at half-maximum (FWHM) for Mn Kα X-rays from an 55Fe source. The thermometer consists of a photolithographically patterned Mo-Cu superconducting proximity bilayer with a 93 mK transition temperature. The device is fabricated on a Si3N4 membrane patterned as a 'flyswatter' to control the thermal conductance to the heat bath. The sensor is voltage biased with an on-chip shunt resistor, and the current is measured by a two-stage SQUID amplifier consisting of a single first-stage SQUID and a series-array second-stage SQUID