Gamma-ray spectrometers using superconducting transition edge sensors with external active feedback bias

We are developing X-ray and gamma-ray spectrometers with high absorption efficiency and high energy-resolution for X-ray and gamma-ray spectroscopy. They are microcalorimeters consisting of a bulk Sn absorber coupled to a Mo/Cu multilayer superconducting transition edge sensor (TES). We have operated these microcalorimeters with an external active feedback bias to linearize the detector response, improve the count rate performance, and extend the detection energy range. We measured an energy resolution of 120 eV FWHM for 60 keV incident gamma-rays with no degradation of resolution from active bias. We present X-ray and gamma-ray results and operation of this detector design in both bias modes.