Superconducting detectors are one of the most important applications of superconductivity, and various detectors have been developed [1]. We have been developing a unique superconducting neutron detector, called current-biased kinetic-inductance-detector (CB-KID) [2].

CB-KID comprises 1) Nb ground plane, 2) SiO₂ insulating layer, 3) Y superconducting Nb meanderline, 4) SiO₂ insulating layer, 5) X superconducting Nb meanderline, 6) SiO₂ insulating layer, and 7) ¹⁰B neutron conversion layer. The nuclear reaction between incident neutrons and ¹⁰B atoms at the neutron conversion layer emits ⁷Li ions and a-particles with opposite momentum. When one of them excites quasiparticles temporary in the X and Y meanderlines at a local hotspot, pairs of voltage pulses are generated under DC-bias currents, and propagate toward both ends of the meanderlines as electromagnetic waves. Signal arrival time difference between both end is proportional to the local hotspot position, thus the signal quartet detected at both end of X and Y meanderlines arrows us to determine the hotspot position in two-dimensions with high spatial resolution. It is so-called delay-line method. To obtain high spatial resolution with CB-KID using the delay-line method (delay-line CB-KID), we employed Kalliope-DC readout circuits as a high-speed time-to-digital converter with a temporal resolution of 1 ns.

High temporal resolution of delay-line CB-KID brings high neutron energy resolution in combination with pulsed neutron source, in which wide energy range neutrons are simultaneously generated at regular time intervals. Generated neutrons fly through a beamline of a certain length and are separated by energy according to the time of flight. Thanks to multi-hit tolerance of delay-line CB-KID, energy resolved neutron transmission measurements and imaging are available in the same experiment without changing the experimental setup.

We measured neutron transmission images and transmission measurements for SmSn₃ single crystals in Sn ingot with pulsed neutrons at BL10 NOBORU of the Material and Life science experimental Facility (MLF), J-PARC [3]. Neutron irradiation experiments were performed with the collimator ratio of 190 and beam power of 920 kW. The detector temperature was kept at 7.9 K.

We visualized distribution of Sm atoms in the sample thanks to a huge neutron absorption cross section of Sm with the neutron energy range of thermal neutrons. We also confirmed the appearance of nuclear resonance dips of Sm at the energy range of epithermal neutrons. It demonstrates that the combination of delay-line CB-KID with ¹⁰B converter and pulsed neutron source is applicable for wide energy range neutron imaging and spectroscopy in a single experiment without changing the setup.

This work is partially supported by Grant-in-Aid for Scientific Research (A) (Nos. 16H02450, 21K14566, and JP21H04666) from JSPS. The neutron irradiation experiments at the Materials and Life Science Experimental Facility (MLF) of the J- PARC were conducted under the support of MLF program (No. 2020P0201).

[1] K. D. Irwin, Appl. Phys. Lett. 66, 1998 (1995); P. K. Day et al., Nature 425, 817 (2004); J. Zmuidzinas, Annu. Rev. Condens. Matter Phys. 4, 169 (2012).
[2] T. Ishida et al., J Low Temp Phys 176, 216 (2014); H. Shishido et al., Appl. Phys. Lett. 107, 242601 (2015); H. Shishido et al., Phys. Rev. Appl. 10, 0440440 (2018); Y. Iizawa et al., Supercond. Sci. Technol. 42, 125009 (2019).
[3] K. Oikawa et al., Nucl. Instrum. Methods Phys. Res. A 589, 310 (2008).

Keywords: Superconducting detector, Neutron imaging, CB-KID