Superconducting detectors have superior advantages in achieving high sensitivity, fast response, and high energy resolution. In the past decades, they have been successfully applied to a superconducting nanowire single photon detector (SNSPD), a transition edge sensor (TES), and a microwave kinetic conductance detector (MKID). We first proposed the idea of a superconducting sensor called a current-biased kinetic inductance detector (CB-KID) .The CB-KID has a significant difference from others because it utilizes a voltage signal coming from a sudden change in a kinetic inductance of a hot spot in a superconducting meanderline under feeding a modest DC bias current and its low-loss propagation along a superconducting stripline. We need a netron conversion 10B layer to detect charge-neutral neutrons. A delay-line technique makes it possible to conduct the neutron transmission imaging in two dimensions only with the four-terminal CB-KID device. The detector has been systematically investigated to understand its characteristics and to optimize the operating conditions for improving the spatial resolution, the temporal resolution, and the detection efficiency [2,3]. The practical usefullness tests were done with various samples in view of good linearity between the Gd-islands sizes evaluated not only by neutron images but also by SEM images over the wide range of sample sizes , a demonstration of the narrow-area Bragg-edge neutron transmission , the Bragg-dip analyses for observing mosaic structure in a SmSn mixed metal , and the confirmation of spatial resolution down to 16 mm .
In this study, we used a delay-line-CB-KID measurement system to obtain a microstructure of Wood’s metal alloy (Bi 50%, Pb 25%, Sb 12.5%, Cd 12.5%, melting temperature 75.2°C) under beam power of 812 kW at beamline BL10 of J-PARC (MLF). The Wood’s metal is composed of four phases, of which one is a Cd-rich needle-like phase of an average width of 25 µm and length of 5 mm. Since Cd is a strong neutron absorber, it is suitable for observing the fine mosaic structure contrastingly by neutron transmission imaging. We modified our cryostat system to be able to mount a room-temperature sample for conducting the experiments under pulsed neutrons. After neutron-transmission imaging with the Wood’s metal at its initial state, we melted the Wood’s metal sample and sodified it again by slow cooling during the same beam time. We were successful in observing an impressive change in morphorogy of the phases by neturon-transmission imaging.
Since CB-KID is a cryogenic detector, the sample to be investigated has been located near the CB-KID sensor at an operating temeperature. Neutron imaging by CB-KID with cryogenic-temperature samples is not so convenient for users to exchange a sample and to conduct the open beam normalization of imaging in a limited beam time. Room-temperature sample imaging achieved in the present study would be very useful to apply our CB-KID system to versatile different samples of interest in the future.
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).
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Keywords: Superconducting neutron detector, CB-KID, Neutron, Wood's metal