High Resolution Neutron Imaging Developments for PEMFC, AEMFC, and Electrolyzers
Authors: Hussey Daniel S , LaManna Jacob M , Baltic Elias , Jacobson David L 
Published: February 2020

In 2017, we reported the first neutron imaging measurements of the through-plane water content in an operating PEMFCs with a spatial resolution of ~2 µm [1]. The proof of concept detector worked stably, the system is far from being a routine user instrument for a number of reasons. 1) In order to avoid overlapping neutron capture events, images must be acquired with a short exposure time, about 3 ms. The sustained frame rate of the proof of concept system was about 30 fps. This resulted in a live time fraction of about 0.09, that is, a 4h acquisition to capture 400,000 frames had the counting statistics of a 20 min image, or about 5 neutrons per pixel. 2) Even using a single stage image intensifier to amplify the light signal from the neutron capture event, camera read noise was still a limiting factor in image quality, requiring the use of a cooled camera. 3) Event reconstruction was performed in software after the data set acquisition, and the initial frame processing speed was about 2 fps, so that a data set of about 400,000 frames required about 55 h to obtain the final image. We have acquired a new, low-noise camera with x3 higher sustained frame rate which will increase the live time of the measurement. As well, we expect to take receipt of an image intensifier in summer 2019 with a 10x boost that in the future will enable the use of a less cooled camera that has 1000 fps capability, which would result in a near 100% live time, or improved event reconstruction with the currently available suite of cameras. Further, the analysis code has been converted to run on a GPU, realizing a factor 4 improvement in frame processing time to about 8 fps (14 h to process 400,000 frames). It is expected that this reconstruction time can be further improved as the GPU utilization is not yet fully engaged.
Another approach to improving the neutron spatial resolution is to employ a Wolter optic, which converts a pinhole optics-based neutron imaging beam line into a conventional microscope. The optic is composed of nested shells of electroformed nickel neutron mirrors, so that the image is free of chromatic aberrations. We have updated our optical design [2] to include a condensing optic [3]. Based on design calculations, we expect the updated microscope to have a spatial resolution of better than 3 µm, and a time resolution that is a factor 1000 higher than what is achievable at NIST’s thermal neutron imaging facility at BT2. With such intensity gains, the measurement time to reach similar water thickness uncertainty as currently achievably in 20 minutes will be about 10 s.
References
[1] D. S. Hussey, J. M. LaManna, E. Baltic, and D. L. Jacobson, “Neutron imaging detector with 2 μm spatial resolution based on event reconstruction of neutron capture in gadolinium oxysulfide scintillators,” Nucl. Instruments Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip., vol. 866, 2017.
[2] H. Wu, B. Khaykovich, X. Wang, and D. S. Hussey, “Wolter Mirrors for Neutron Imaging,” in Physics Procedia, 2017, vol. 88.
[3] P. Jorba et al., “High-resolution neutron depolarization microscopy of the ferromagnetic transitions in Ni3Al and HgCr2Se4 under pressure,” J. Magn. Magn. Mater., vol. 475, pp. 176–183, Apr. 2019.