Although superconducting nanowire single-photon detectors (SNSPDs) are well studied regarding the detection of infrared/optical photons and keV-molecules, no studies on continuous X-ray photon counting by thick-film detectors have been reported so far. We fabricated a 100 nm thick niobium X-ray SNSPD (an X-SNSPD), and studied its detection capability of photons with keV-energies in continuous mode.The detector is capable to detect photons even at reduced bias currents of 0.4%, which is in sharp contrast to optical thin-film SNSPDs. No dark counts were recorded in extended measurement periods. Strikingly, the signal amplitude distribution depends significantly on the photon energy spectrum.Already more than a decade before the development of superconducting nanowire single-photon detectors (SNSPD) for the optical and near-infrared wavelength range, serious efforts had been undertaken to adapt this detection principle for X-ray photons with keVenergies. 1,2,3,4 However, these preliminary X-ray detectors struggled with latching, making it difficult to operate them as self-recovering detectors in which superconductivity recovers after photon detection events (called continuous operation mode in this letter). The need to externally reduce the bias current to a value low enough for superconductivity to recover after a detection event results in long dead times and limits the count rates.Very fast and sensitive X-ray single-photon detectors from superconducting nanowires would be very interesting for applications where very high count rates, precise timing, a good signal-to-noise ratio and response in a wide spectral range for photon counting are required.Potential applications comprise experiments with synchrotron X-ray sources, free-electron lasers and hot plasmas (as in nuclear fusion experiments), all emitting bright and pulsed X-ray radiation. In many medical imaging systems ultrafast X-ray single-photon detectors with energy resolution are desirable in order to reduce patient radiation dose. In the recently developing photon-counting X-ray computer tomography for example, long detection pulse durations can compromise the image quality by a possible overlap of succeeding photon pulses, as high photon fluxes have to be used in order to prevent motion blur. 5Recently, SNSPDs 6 and superconducting stripline detectors (SSLDs) 7,8,9,10 with superconducting film thicknesses of up to 50 nm were reported to be used in continuous mode for time-of-flight mass spectrometry (TOF-MS) of molecules with keV-energies, and ultrafast pulse recovery times and pulse rise times down to 380 ps ± 50 ps were reported. 11There is only one report on continuous X-ray photon counting with SNSPDs: Perez de Lara et al. 12 reported on the detection of 6 keV photons by a SNSPD from 5 nm thin NbN.However, there are no published studies on continuous X-ray photon detection in thick-film SNSPDs (called X-SNSPDs in this work), although they are promising candidates for ultrafast detectors in continuous mode.The absorptance of X-ray photons in thin-...
