Quantification of Sub-Pixel Dynamics in High-Speed Neutron Imaging

Abstract

The high penetration depth of neutrons through many metals and other common materials makes neutron imaging an attractive method for non-destructively probing the internal structure and dynamics of objects or systems that may not be accessible by conventional means, such as X-ray or optical imaging. While neutron imaging has been demonstrated to achieve a spatial resolution below 10 μm and temporal resolution below 10 μs, the relatively low flux of neutron sources and the limitations of existing neutron detectors have, until now, dictated that these cannot be achieved simultaneously, which substantially restricts the applicability of neutron imaging to many fields of research that could otherwise benefit from its unique capabilities. In this work, we present an attenuation modeling approach to the quantification of sub-pixel dynamics in cyclic ensemble neutron image sequences of an automotive gasoline direct injector at a 5 μs time scale with a spatial noise floor in the order of 5 μm.

Measurement of needle and armature dynamics in a gasoline direct injector by high-speed neutron imaging

“…This image is a map of how the path length through phase A changed, scaled by the attenuation coefficient difference between the phases A and B. This image can then be analyzed according to the approach described in our previous work, 16 in which the geometry and attenuation coefficients of the internal components of the injector are extracted from a neutron CT scan and are then used to create a model of how the path length seen by each pixel on the detector would change if the geometry were displaced by a given amount. This model can then be fit to the log-normalized image described in equation ( 5) to estimate the displacement that best fits the data.…”

“…In this case, we operated the injector in synchronization with the MCP detector at an injection frequency of 50 Hz to make ensembles consisting of ∼5–6 × 10 6 injection events for each movie. As described in previous work, 16 ensemble image sequences were acquired in a series of shutters with time bin sizes ranging from 5.12 μs to 20.48 μs. The raw data, which is in the form of neutron counts per pixel per time bin, were overlap corrected, 19 re-binned to 20.48 μs (equivalent to 48,828 frames/s), and normalized to count rate to account for the variation in number of successfully acquired shutters.…”

“…Two-dimensional path length models were developed for each following the approach described in our previous work. 16 The models are illustrated in Figure 5, with the armature displaced horizontally by 0.25 px and vertically by 0.5 px, and the valve ball displaced horizontally by 0.1 px and vertically by 0.5 px. The distance maps of the reference and displaced geometry are visually indistinguishable, but in the distance difference map, the displacement is readily apparent.…”

“…The values measured within the solid components ( Σ A ) are effectively constant, whereas the values for the fuel surrounding the components ( Σ B ) will vary with path length due to beam hardening, as discussed in our previous work. 16 There it was shown that selecting a representative value of Σ B based on the average path length produces good results, but there is uncertainty in the fit due to uncertainty in the absolute value of Σ B , which is affected not only by beam hardening by also by incoherent scattering, which will vary with sample-to-detector distance. Here we have included upper and lower bounds on Σ B based on the path length range in each component to provide an estimate of the uncertainty in the fit.…”

“…In recent work, we demonstrated that high-speed neutron imaging could resolve oscillatory motions of an injector needle on the order of 5 μm at 5 μs time scale. 16 However, the field of view in that effort was restricted to only the very end of the needle near the valve ball. In this work, we extend the field of view to cover the entire injector, offering a complete picture of the internal mechanical dynamics while simultaneously imaging the internal and external fluid dynamics.…”