In situ Imaging at the NIST Neutron Imaging Facility
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In situ Imaging at the NIST Neutron Imaging Facility David L. Jacobson, Daniel S. Hussey and Eli Baltic Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899-8461, U.S.A. ABSTRACT Neutron imaging as a method to perform in situ studies of hydrogen fuel cells, hydrogen storage devices, heat pipes, and batteries has made tremendous progress in recent years. Neutrons are useful to study light elements mixed with heavy Z elements where penetration by other forms of radiation is either impossible or incapable of contrasting the light elements. Useful spatial resolution available at neutron imaging facilities is now approaching 10 micrometers. Complimentary time resolution of 30 fps or greater is also possible with a spatial resolution approaching 300 micrometers. Here we will provide an overview of the technique of neutron imaging and experimental studies with neutrons at the National Institute of Standards and Technology. Examples of in situ studies of fuel cells, hydrogen storage devices, heat pipes and batteries will be discussed. INTRODUCTION The method of neutron radiography has seen a strong resurgence in recent years due mainly to the advent of digital imaging detectors in the 1990’s. This resurgence is also due in part to the unique ability of the neutron to penetrate metal components and remain sensitive to light elements like hydrogen and lithium that are found in important alternative energy storage and conversions devices. Examples include hydrogen powered polymer electrolyte membrane fuel cells (PEMFC), hydrogen storage beds and alkaline and lithium- ion batteries. As the neutron digital imaging field has progressed, the spatial resolution has been dramatically improved from 250 μm to the current best case measured resolution of 13 μm. Neutron radiography is best implemented at a research reactor facility which can provide enough neutron intensity to allow for both dynamic imaging (greater than or equal to 30 fps) or high resolution imaging (near 10 μm). The NIST Neutron Imaging Facility (NNIF) offers users the opportunity to utilize neutrons for imaging [1,2]. A raw neutron image is called a radiograph and is a 2-dimensional image of the beam intensity that passes through an experimental device. The resulting image is simply a shadow or transmission image of the device. Regions of greater varying attenuation are described well by the Lambert-Beer relationship given by: (1) I = I 0 e − N σt where N is the atom density, σ is the neutron cross section of the atom, and t is the thickness of material that the neutron beam passes through. In some cases it is possible to quantify how much material the neutron passes through using Eq. (1). Shown in figure 1 is ı for selected elements, which shows that the interaction between neutrons and material does not depend on the atomic number, Z. This is due to the fact that the neutron interacts with the nucleus through the strong nuclear force and not with the Coulombic forces in the atom. From figure 1 it is apparent that t
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