Several types of superconducting detectors have successfully been utilized in various significant fields such as cosmology, dark matter, quantum communications, etc. due to their high performance [1,2,3,4,5]. The imaging by the superconducting detector was supposed to be limited at 20,000 pixels at most due to the heat inflow from room temperature to cryogenic detector through the readout wires. However, a paper claiming a 400,000-pixel camera with a delay-line technique was published in 2023 with a lot of attention [2]. A current-biased kinetic inductance detector CB-KID is a superconducting delay-line detector proposed by us to observe a local stimulus [1,5]. It uses a versatile 50-ohm matching impedance in signal transportation in the inside of the detector. A superior merit of CB-KID was demonstrated by realizing a neutron imager, i.e., it simultaneously realized a delay-line neutron transmission imaging system (an imaging function named as Role-I) and a time-of-flight energy dispersive spectroscopy system (a function named as Role-II) [1,6]. This must be useful in material sciences. CB-KID consists of orthogonal XY superconducting meanderlines and a neutron conversion layer, with a meander period of 1.5 μm (waveguide width 900 nm), a waveguide length of 151 m, and a sensing area of 15 mm × 15 mm. Initially, a signal arriving time to an end electrode (X or Y) was measured by a Kalliope-I readout circuit equipped with a 1ns time-to-digital converter TDC and the position was allocated to a certain segment as the smallest unit by the delay-line method. A higher spatial resolution requires a higher-temporal resolution. Since the propagation velocity along the waveguide of the signal is about 20% of the speed of light, and a position resolution remained at a level of 7 - 15 times of the meanderline period 1.5μm [1,6]. A rising time of the signal to a threshold value is slightly delayed from a nuclear-event-occurrence time. A time over threshold (ToT) acquisition of our Kalliope-I readout circuit has a dead zone, i.e., ToT is discarded when it is less than 8ns. Therefore, it could not be used for delay-time correction of the event time. ToT is a lasting time, where a signal amplitude stays over the TDC threshold. We developed a new 30ps operating Kalliope-II circuit to aim for a resolution limit down to 1.5μm. The Kalliope-II readout circuit utilized a high-resolution TDC (HR-TDC) and a front-end main circuit for continuous readout DAQ (AMANEQ) [7]. High-temporal 30ps resolution measurement of ToT enabled us to conduct a delay-time correction rather satisfactory. This significantly improved the positional resolution (Role-I). Thanks to delay-time correction, a typical data acquisition time of a high-resolution image with the Kalliope-II system effectually became faster than the Kalliope-I system. A Role-II function is also realized with HR-TDC. By referring to operating principle of CB-KID, one notices that the measurement object can be expanded not only to neutrons but also to other sorts of stimuli. The pixel size of the Kalliope-II imager becomes as small as 1.5 μm × 1.5 μm when it is determined by 30ps operating readout circuit. It is impressive to note that this corresponds to an untrodden 100,000,000-pixel camera of the 15 mm × 15 mm sensitive area as a Role-I CB-KID [1,6]. This is in marked contrast with performances achieved by other superconducting detectors [2,3,4,5]. The time-dependent spectroscopy with CB-KID becomes possible to find wide applications in other fields too as a Role-II CB-KID.
Acknowledgements: This research was supported by Grant-in-Aid for Scientific Research (A) (JP16H02450, JP21H04666), Grant-in-Aid for Early-Career Scientists (JP21K13566, JP23K13690) from Japan Society for the Promotion of Science, and J-PARC Project Research (2024P0501). We thank for the support provided by R. Honda, who developed the HR-TDC and AMANEQ circuits, in constructing the measurement system.
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2. B. G. Oripov et al., Large-scale nanowire camera with a single-photon sensitivity, Nature 622(2023) 730-734.
3. C. Nones et al., High-impedance NbSi TES sensors for studying the cosmic microwave background radiation, Astron. Astrophys. 548 (2012) A17.
4. J. Chiles et al., New constraints on dark photon dark matter with superconducting nanowire detectors in an optical haloscope, Phys. Rev. Lett. 128 (2022) 231802.
5. F. Grünenfelder _et a_l., Fast single-photon detectors and real-time key distillation enable high secret-key-rate quantum key distribution systems, Nat. Photonics 17(2023) 422–426.
6. T. Ishida et al., Neutron Transmission CB‑KID Imager Using Samples Placed at RoomTemperature, J. Low Temp. Phys. 214 (2024) 152-157.
7. R. Honda, M. Ikeno, M. Shoji, T. Takahashi, 2021 Autumn Meeting of the Physical Society of Japan, 16pV1-10 (in Japanese).
Keywords: pulsed neutrons, transmission imaging, high spatial resolution, readout circuit.