  For X-ray imaging spectroscopy, high spatial resolution over a large field of
view is often as important as high energy resolution, but current X-ray
detectors do not provide both in the same device. Thermal Kinetic Inductance
Detectors (TKIDs) are being developed as they offer a feasible way to combine
the energy resolution of transition edge sensors with pixel counts approaching
CCDs and thus promise significant improvements for many X-ray spectroscopy
applications. TKIDs are a variation of Microwave Kinetic Inductance Detectors
(MKIDs) and share their multiplexibility: working MKID arrays with 2024 pixels
have recently been demonstrated and much bigger arrays are under development.
In this work, we present our first working TKID prototypes which are able to
achieve an energy resolution of 75 eV at 5.9 keV, even though their general
design still has to be optimized. We further describe TKID fabrication,
characterization, multiplexing and working principle and demonstrate the
necessity of a data fitting algorithm in order to extract photon energies. With
further design optimizations we expect to be able to improve our TKID energy
resolution to less than 10 eV at 5.9 keV.
