Several useful properties of superconducting devices are applicable to fulfilling high-sensitivity superconducting detectors [1]. We developed a superconducting detector, named as a current-biased kinetic-inductance-detector (CB-KID), which particularly targets applications in neutron beams [2].

CB-KID consists of two orthogonal X and Y superconducting Nb meanderlines and a Nb ground plane. This superconducting stripline structure guarantees long-distance electromagnetic wave propagation of a signal. In addition, a ¹⁰B neutron conversion layer is deposited on top of the CB-KID to utilize a ⁷Li particle and a ⁴He particle emitted toward opposite directions accompanying with the nuclear reaction between incident neutrons and ¹⁰B atoms. When one of them excites temporary quasiparticles locally at a hotspot on the X and Y meanderlines, a transient change of the kinetic inductance under a small DC-bias current produces a pair of voltage pulses which is in proportional to a time derivative of the kinetic inductance and DC-bias current, and propagates toward both ends of the meanderlines as electromagnetic waves. We record signal arrival timestamps with a temporal resolution of 1 ns by using a high-speed time-to-digital converter (TDC) of the Kalliope-DC circuit. Since a signal-arrival-time difference at both ends can be used to identify the hotspot position, thus a neutron transmission image is determined in two-dimensions with high spatial resolution from signal arrival timestamps of a signal quartet in two meanderlines by a delay-line method. We call the combination of CB-KID and delay-line method as a delay-line CB-KID system.

A pulsed neutron source is characterized to generate a wide energy range of neutrons simultaneously at a regular repetition intervals (25 Hz at J-PARC). Generated neutrons fly through a beamline of a certain length L and are separated by energy according to the time of flight. Thanks to high temporal resolution and multi-hit tolerance of delay-line CB-KID, energy-resolved neutron-transmission measurements and imaging are conveniently achievable. For single crystal samples, of which the neutron scattering cross section has a sufficiently large fraction in the total cross section, the wavelength dependence of the neutron transmission spectrum tends to show dip structures associated with Bragg diffractions. The wavelength of the Bragg dip is determined by the crystalline orientation of the single crystal and the index of reflection.

CaF₂ has a CaF₂-type face-centered cubic structure and is often used as an optical window. CaF₂ window has various properties depending on how they are cut. The CaF₂ window (15 mm 2 mm^t) used for the present study was cut in a random-crystalline orientation. We measured neutron transmission images and transmission spectra for a CaF₂ single crystal using pulsed neutrons with a source size D = 100 mm at BL10 NOBORU of the Material and Life science experimental Facility (MLF), J-PARC [3]. The sample was placed at 20 mm upstream side from the detector at room temperature in air. The detector was kept at 5 K during the neutron irradiation experiments under beam power of 711 kW and the collimator ratio of L/D = 140.

We observed dip structures in the neutron transmission spectrum of the CaF₂ single crystal sample. The Miller indices were successfully assigned for the observed Bragg dips, and thus we were able to determine the crystal orientation of the CaF₂-window sample. We also identified dip structures due to nuclear resonance absorption of Ca and F nucleus in the energy range of epithermal neutrons. The combination of delay-line CB-KID with ¹⁰B converter and pulsed neutron source is applicable to Bragg-dip analysis and identification of constituent nuclei by the nuclear resonance absorption in a non-destructive manner.

This work is partially supported by Grant-in-Aid for Scientific Research (A) (No. JP21H04666) and Grant-in-Aid for Early-Career Scientists (No. JP21K14566) 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 project program (No. 2021P0501).

[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 transmission spectroscopy, CB-KID