Neutron Imaging Detector with Scintillation Light Magnification and Amplification

Abstract

Neutron imaging is an ideal method to study water transport phenomena in proton exchange membrane fuel cells (PEMFCs). Neutrons readily penetrate aluminum, carbon and platinum, yet have a high sensitivity to hydrogenous materials, including water. In addition, unlike x-rays, neutrons are a non-destructive probe and do not alter the performance of PEMFCs due to irradiation. As such, there have been numerous studies that revealed the heat and mass transport phenomena in the flow fields and gas diffusion layers [1], freeze operation [2], impacts of carbon corrosion [3], and the impact of porosity in thick non-precious metal catalyst layers [4]. The primary limitation of neutron imaging is the achievable spatial and temporal resolutions. Current state of the art spatial resolution is about 20 µm, with a 20 minute image acquisition time. The spatial resolution is primarily limited by the range of the charged particles that are the result of the neutron capture reaction and are used to detect the neutron. This range is of order 5 µm for many common detector schemes. The temporal resolution is limited by the brightness of neutron sources, which are in general many orders of magnitude less bright than synchrotron x-ray sources; thus to have reasonable time resolution conventional neutron imaging requires the use of beam defining apertures that are of order 1 mm for high resolution imaging of PEMFCs, so that one cannot make use of geometric image magnification. We are pursuing three efforts to improve the achievable image spatial resolution and time resolution. The simplest approach to improving the spatial resolution is to place a narrow slit in front of the test section and scan it across the active area. The challenge of this method is to create a narrow, well-defined, completely opaque slit with opening area ~1 µm. KAERI and Pusan National University have devised a Gadox powder filling method that enables creating such slits, enabling imaging with 1 µm resolution [5]. By amplifying the scintillation light and using a high speed camera, it is possible to record individual neutron scintillation events and using a centroid algorithm determine the position of the neutron with high precision (less than 10 µm). The measured through-plane water content of a PEMFC is shown in figure 1 for the present state of both methods. The final effort is the development of a neutron microscope objective based on reflective neutron optics [6]. With a lens, one no longer requires the use of small beam defining apertures and thus the image temporal resolution can increase by at least a factor of 100. We will report on the progress of the first optics to reach 20 µm resolution and discuss the possibility of achieving 1 µm spatial resolution through neutron image magnification. References T.A Trabold et al, “Use of neutron imaging for proton exchange membrane fuel cell (PEMFC) performance analysis and design”, Handbook of fuel cells v. 5, 2009. R Mukundan et al, “Performance of PEM fuel cells at sub-freezing temperatures”, ECS Transactions 11 (1), 543-552, 2007.JD Fairweather et al, “Effects of cathode corrosion on through-plane water transport in proton exchange membrane fuel cells”, JECS 160 (9), F980-F993, 2013.D Spernjak et al, “Water Management in PEM Fuel Cells with Non-Precious Metal Catalyst Electrodes”, 228th ECS Meeting abstracts, p. 1540, 2015.J. Kim et al, “Fabrication and characterization of the source grating for visibility improvement of neutron phase imaging with gratings”, RSI 84(6):063705, 2013.D. Liu et al “Demonstration of achromatic cold-neutron microscope utilizing axisymmetric focusing mirrors”, Applied Physics Letters 102 (18), 183508, 2013. Figure 1 Comparison of the image quality of the current state of the art (a), the slit imaging method with 4 µm resolution (b), and the reconstructed image from centroiding (c). Figure 1