Development of Dielectric X-Ray Microcalorimeter

For future X-ray astronomy, a microcalorimeter array that has both mega-pixel imaging capability and eV-level high energy-resolution is desirable. In order to realize it, thermometers with negligible self-heating, multiplexing readout and close packing are essential components. We propose a novel detector concept, a dielectric microcalorimeter (DMC) and present current design studies. The DMC uses dielectric pixels as thermometers which form LC resonators in GHz band. The signals from many pixels can be easily multiplexed in a similar way to kinetic inductance detectors. The dielectric pixels are easy to be integrated into large and dense arrays. However, dielectrics with temperature-dependent permittivity below 2 K were not known. We evaluated quantum ferroelectric strontium titanate (STO) used in a capacitive thermometer higher than 2 K as a suitable dielectric for the DMC pixels. We fabricated STO capacitors and measured their capacitance from 2 K down to 80 mK. As a result, we found that their capacitive thermometer sensitivity at 100 mK are dlogC/dlogT similar to 10(-3), and they can be sensitive enough to detect X-ray with a resonator. We report the concepts, the measurement details, expected response to X-ray irradiation and our designs of the DMC.