Crystalline Orientation of CaF2 window determined by Neutron Transmission Imaging using a Delay Line Current-Biased Kinetic-Inductance Detector

Nov. 30 11:20-11:35

*Hiroaki Shishido1,2, The Dang Vu3,4, Kazuya Aizawa4,8, Kenji M. Kojima3,5, Tomio Koyama3, Kenichi Oikawa4, Masahide Harada4, Takayuki Oku4, Kazuhiko Soyama4, Shigeyuki Miyajima6, Mutsuo Hidaka7, Soh Y. Suzuki8, Manobu M. Tanaka9, Shuichi Kawamata3, Takekazu Ishida3
Department of Physics and Electronics, Graduate School of Engineering, Osaka Metropolitan University1
The Center for Research and Innovation in Electronic Functional Materials, Osaka Metropolitan University2
Division of Quantum and Radiation Engineering, Osaka Metropolitan University3
Materials and Life Science Division, J-PARC Center, Japan Atomic Energy Agency4
Centre for Molecular and Materials Science, TRIUMF and Stewart Blusson Quantum Matter Institute, University of British Columbia5
Advanced ICT Research Institute, National Institute of Information and Communications Technology6
Advanced Industrial Science and Technology7
Computing Research Center, Applied Research Laboratory, High Energy Accelerator Research Organization (KEK)8
Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK)9

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 mmt) 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