High speed motion neutron radiography Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/vd66w355t

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  • A system to perform neutron radiographic analysis of dynamic events taking place over the order of several milliseconds has been developed at Oregon State University. The design consists of a TRIGA reactor capable of pulsing to 3000 MW peak power, a neutron beam collimator with an L/D of about 30:1, a lithium fluoride-zinc sulphide (LiF-ZnS) neutron scintillator screen, an image intensifier and a 16 mm high speed movie camera. The TRIGA reactor pulse and beam port collimator combine to yield a peak thermal neutron flux of about 4.2 x 10 ¹¹ n/cm ² -sec at the object with a pulse width (FWHM) of 9 milliseconds. During this pulse duration sequential neutron radiographs are taken by the imaging system which detail any movement occurring in the object. The imaging system operates by converting the neutron signal to light, amplifying the light signal and recording the information on 16 mm Rapid Arts Recording film. LiF-ZnS neutron scintillator screens of 2/1 weight ratio of ZnS to LiF and 0.13 mm thickness constructed at OSU with sodium silicate binder are found to be very successful. The construction of the scintillators plus both an analytical model and experimental study of their performance is described. These screens have good light yield, good resolution and sufficiently rapid light decay characteristics to allow individual framing at over 5000 frames/second. The image intensifier is a two stage, 40 mm diameter tube. An 5-20 photocathode and a P-11 output phosphor are employed in the intensifier for their rapid decay characteristics and spectral response. The tube has a fiber optics input faceplate onto which the scintillator screen is pressure held for radiography. Actual imaging through the intensifier is superior to the conventional scintillator screen in contact with film method, apparently due to the non acceptance of wide angle light by the fine fiber optics. The high speed movie camera focuses on the intensifier output screen. Modulation transfer function analysis of the system indicates that the scintillator screen and the collimator L/D ratio are the two weakest areas of image quality in the system. The intensifier and high speed camera exhibit much better quality. Image quality can also be subject to statistical limitations due to the very short exposure times (about 80 psec at 5000 frames/second) per frame in the imaging system. Experimental studies indicate that the image quality becomes statistically limited for general detail below about 10⁷ n/cm ² fluence incident on the object. Subject to the object thickness and level of detail desired, the OSU system may be operated at up to 10,000 frames/second without experiencing this problem. For fine detail though it is necessary to enhance the object contrast by appropriate doping with a high contrast material such as gadolinium. This high speed motion neutron radiography technique may be applied to the interrogation of rapid dynamic events that have imaging characteristics suitable for neutron radiography inspection. High speed motion neutron radiographs have been taken of the ballistic cycle of both live and blank 7.62 mm munition rounds and of two phase flow as represented by air bubbles passing through water.
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