We are developing a superconducting analog-to-digital converter (ADC) as a readout for high-resolution X-ray detectors based on a superconducting tunnel junction (STJ). The ADC has a sensitive front end which consists of a DC superconducting quantum interference device (SQUID). A signal current is digitized by this front end without using any preamplifiers. A single-flux-quantum (SFQ) pulse train whose frequency is proportional to the input current is launched by the front end, and integrated by a digital counter. The counter has a 10-bit resolution, and the integrated value is scanned and transferred to room-temperature processing modules with a frequency of 40 MHz. In this paper, the design of the ADC is described, and the preliminary results of the ADC performance test are shown. The performance of the STJ accompanied by the ADC is discussed in terms of the X-ray energy resolution.

A normal-distribution-function-shaped superconducting tunnel junction (NDF-STJ) which consists of Nb/Al-AlOx/Al/Nb has been fabricated as an X-ray detector. Current - voltage characteristics were measured at 0.4 K using three kinds of STJs, which have the dispersion parameters σ of 0.25, 0.45 and 0.75. These STJs showed very low subgap leakage current of about 5 nA. By irradiating with 5.9 keV X-rays, we obtained the spectrum of these NDF-STJs. They showed good energy resolution with small magnetic fields of below 3 mT, which is about one-tenth of those for conventional-shaped STJs.

A lumped element kinetic inductance detector (LeKID) relying on a superconducting resonator is a promising candidate for sensing high energy particles such as neutrinos, X-rays, gamma-rays, alpha particles, and the particles found in the dark matter owing to its large-format capability and high sensitivity. To develop a high energy camera, we formulated design rules based on the experimental results from niobium (Nb)-based LeKIDs at 1 K irradiated with alpha-particles of 5.49 MeV. We defined the design rules using the electromagnetic simulations for minimizing the crosstalk. The neighboring pixels were fixed at 150 µm with a frequency separation of 250 MHz from each other to reduce the crosstalk signal as low as the amplifier-limited noise level. We examined the characteristics of the Nb-based resonators, where the signal decay time was controlled in the range of 0.5-50 µs by changing the designed quality factor of the detectors. The amplifier noise was observed to restrict the performance of our device, as expected. We improved the energy resolution by reducing the filling factor of inductor lines. The best energy resolution of 26 for the alpha particle of 5.49 MeV was observed in our device.