Development of Superconducting Nanostrip X-Ray Detector for High-Resolution Resonant Inelastic Soft X-Ray Scattering (RIXS)

To achieve a resolving power greater than 100 000, we are developing a superconducting nanostrip single photon detector (SSPD) for an ultrahigh-resolution RIXS spectrometer in the Taiwan Photon Source. To achieve the target resolving power, the SSPD, combined with a microstripline (MSL) structure, which acts as a delay line, should have a line pitch of 0.5 μm. This study uses numerical simulations to design SSPD. It consists of a meander of a 50-nm-thick Nb nanostrip and a 45-nm-thick SiO2 underlayer. The width of the line and the gap of the meander are 400 and 100 nm, respectively. The estimated velocity of the transmitted pulses due to X-ray absorption in the Nb nanostrip of the above SSPD is ~5% of the speed of light in free space. A detection area of 16 mm × 5 mm is sufficient to allow a time difference of ~1 ns between neighboring nanostrips to be distinguishable using conventional time to digital converters (TDCs). We fabricated a trial SSPD imager with dimensions of 4 mm × 0.1215 mm, and measured signal pulse characteristics for ions simulating soft X-ray photons. During an experiment, the maximum time difference of 7.5 ns between output pulses from both ends of the nanostrip successfully exhibited the proper imaging operation.