Alex Malins

We previously proposed a method to detect neutrons by using a current-biased kinetic inductance detector (CB-KID), where neutrons are converted into charged particles using a ¹⁰ B conversion layer. The charged particles are detected based on local changes in kinetic inductance of X and Y superconducting meanderlines under a modest DC bias current. The system uses a delay-line method to locate the positions of neutron- ¹⁰ B reactions by acquiring the four arrival timestamps of signals that propagate from hot spots created by a passing charged particle to the end electrodes of the meanderlines. Unlike conventional multi-pixel imaging systems, the CB-KID system performs high spatial resolution imaging over a 15 mm × 15 mm sensitive area using only four channel readouts. Given the large sensitive area, it is important to check the spatial homogeneity and linearity of detected neutron positions when imaging with CB-KID. To this end we imaged a pattern of ¹⁰ B dot absorbers with a precise dot pitch to investigate the spatial homogeneity of the detector. We confirmed the spatial homogeneity of detected dot positions based on the distribution of measured dot pitches across the sensitive area of the detector. We demonstrate potential applications of the system by taking a clear transmission image of tiny metallic screws and nuts and a ladybug. The image was useful for characterizing the ladybug noninvasively. Detection efficiencies were low when the detector was operated at 4 K, so we plan to explore raising the operating temperature towards the critical temperature of the detector as a means to improve counting rates.

We previously succeeded in constructing and demonstrating the capability of a neutron imaging system based on a superconducting current-biased kinetic inductance detector (CB-KID). In the present work, we systematically studied the characteristics of the superconducting neutron detector to improve the spatial resolution and detection efficiency. We found that the number of neutron detection events with CB-KID remarkably increased when the detector temperature increased from 4 K to the critical temperature T _c. We observed systematic changes of neutron signals as a function of the detector temperature from 4 K to T _c. We evaluated the detection efficiency of the CB-KID detector, and compared with PHITS Monte Carlo simulations, which modeled the sequential physical processes for the ¹⁰B(n,α)⁷Li reaction, the transport dynamics, and the energy deposition by particles including neutrons, ⁴He particles, ⁷Li particles, photons, and electrons.

Dissolved 137Cs discharge represents approximately 30% of the total 137Cs discharge from the forested upstream catchment of the Ohta River in Fukushima, Japan [1]. It is thought that a major source of the dissolved 137Cs entering the river water may be leaching from forest litter [1]. A watershed simulation based on the distribution coefficient (Kd) that modelled water, sediment, and particulate and dissolved 137Cs transport could not reproduce the seasonal variability of the base flow dissolved 137Cs concentrations, nor the peaks in concentration that occurred during storms [2].We developed a combined watershed-biogeochemistry model for simulating dissolved and particulate 137Cs discharge from forest catchments to describe the two phenomenon as mentioned above. A compartment model for the forest ecosystem was appended to the General-purpose Terrestrial fluid-Flow Simulator (GETFLOWS) watershed code. The compartment model included compartments for undecomposed and decomposed litter, with transfer from the former into the latter depending on temperature. A pathway for dissolved 137Cs input to forest streams was linked from the decomposed litter compartment.The results from a simulation with the new simulation model reproduced the seasonal variability of dissolved 137Cs concentrations and the peaks occurring during storms. Therefore the new modelling results add weight to the theory that leaching from decomposed litter can input dissolved 137Cs concentrations in river water in Fukushima Prefecture. The developed model is expected to be useful for further explorations into factors affecting dissolved 137Cs input to river water in forested catchments. [1]Tsuji et al., 2016. J. Geophys. Res. Biogeosci. 121, 2588-2599.[2]Sakuma et al., 2018. J. Environ. Radioact. 184-185, 53-62.