Optimum temporal and contrast considerations in extremely high speed motion neutron radiography Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/wp988p30j

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  • A facility developed using the OSU TRIGA pulsing capability and high speed motion photography has been used successfully for more than a decade to produce dynamic neutron radiographs of solid propellant combustion, two phase flow, and liquid streamflow. The existing imaging components have undergone little modification since the experiment's inception and include a LIF-ZnS neutron imaging scintillator, an image intensifier, and a high speed framing camera. Previous research has shown motion of large objects traveling at 100ft/s may be resolved at a framing rate of 10,000frame/s sampling each 40 microseconds. Few experiments, however have challenged system response beyond half this framing rate. Upon demand to record increasingly shorter events with greater time resolution during the TRIGA 8ms pulse width, the camera was improved to allow exposure times as short as 2 microseconds. The existing system, however, demonstrates a minimum practical exposure time of 3 microseconds under the best flux and processing conditions for ordinance radiography. The OSU high speed neutron radiography facility exhibits temporal and spacial resolution limitations as well as contrast restrictions when attempting to record extremely high speed events. An attempt was made to define present limitations in order to recognize areas of improvement. Transfer and modulation efficiency of each component (i.e. the reactor, collimator, object, neutron scintillator, intensifier, camera optics, and film) was analyzed and image blur due to object motion accounted. Treated analytically, system modulation, exposure, and noise contribution to the film image showed adequate agreement to existing measurements. This analysis demonstrated the most serious exposure loss is due to camera optics itself, static resolution is limited predominantly by the scintillator, and phosphor persistance at exposure times below 50 microseconds seems the most significant cause of motion blur. Quantum noise should deteriorate image quality if neutron exposure falls below 10⁶n/cm² but observation is exposure limited at this level. To improve exposure reouires replacement of the rotating prism camera for more efficient optical transfer; however, use of a faster film would increase system response on the short term. Scintillator resolution improvement would appear more practical once other components of the system have improved efficiency. Finally, increased flux levels and scintillator-photocathode efficiency would improve gain and especially noise conditions but is difficult to accomplish without compromise in resolution.
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