The penumbral imaging technique has proven to be ideally suited for neutron imaging. The French CEA has successfully installed a neutron imaging system at the LLE (Rochester-New York) in June 2000. Images of the 14MeV fusion neutrons produced in the target have been recorded in the range 1012 to 1014 with a two-point resolution of 45 micrometers. The detector used was a 15cm diameter circular array composed of plastic scintillator elements. For several of the CEA experiments, bubble detectors developed for General Atomics simultaneously recorded neutron images. The SIRINC (Simulation and Reconstruction Imaging Neutron Code) code has been used to unfold neutron images obtained both with the segmented scintillator detector and with the bubble detector. We first describe the experimental setup and detector designs, then compare the sensitivity, quantity of information, and signal to noise ratio for those two detectors. Then raw and unfolded images are presented. The spatial resolution obtained for the unfolded images are estimated and compared for the two detectors types.
KEYWORDS: Spatial resolution, Sensors, Image resolution, Filtering (signal processing), Signal to noise ratio, Electronic filtering, Point spread functions, Digital filtering, Image fusion, National Ignition Facility
In Inertial Confinement Fusion (ICF) experiments, radiation from the compressed core is increasingly reabsorbed. For the largest experiments, the only radiation to escape is the 14 MeV fusion neutrons in which we must turn to learn of the physical processes taking place. The most important parameters are the shape and the size of the compressed core and this involves imaging the neutrons produced by the fusion reactions. The penumbral technique is ideally suited to neutron imaging and the feasibility of this technique has been demonstrated at the Lawrence Livermore National Laboratory in the United States. At the Phebus laser facility in France, this method has been used in image compressed ICF cores with diameters of 150 micrometers yielding approximately 109 neutrons, and the overall spatial resolution obtained in the reconstructed source was approximately 100 micrometers . On the Laser Megajoule project which is the equivalent of the National Ignition Facility in the United States, the spatial resolution required to diagnose high-convergence targets is 10 micrometers . We wish first to obtain a spatial resolution of 30 micrometers to image source with a diameter <EQ 100 micrometers at a neutron yield in the range of 1011 - 1014 neutrons/4(pi) . A collaborative experimental program with the Laboratory for Laser Energetics at the University of Rochester in this perspective is planned. At the same time, there is a research program in collaboration with Laval University (Quebec) concerning coded aperture designs and the associated reconstruction techniques. In this article we first review the basic requirements of such imagery and the concept of the penumbral imaging technique. Then we concentrate on the parameters that condition the spatial resolution and the description of our imaging system. Finally, we survey the reconstruction techniques used followed by results and comparative evaluation of those methods.
In Inertial Confinement Factor (ICF) experiments, radiation from compressed core is increasingly reabsorbed. For the largest experiments, the only radiation to escape is the 14 MeV fusion neutrons to which we must turn to learn of the physical processes taking place. The most important parameters are the shape and the size of the compressed core and this involves imaging the neutrons produced by the fusion reactions. The penumbral technique is ideally suited to neutron imaging and the feasibility of this technique has been demonstrated at the Lawrence Livermore National Laboratory in the United States. At the Phebus laser facility in France, this method has been used to image compressed ICF cores with diameters of 150 micrometers yielding approximately 109 neutrons, and the overall spatial resolution obtained in the reconstructed source was approximately 100 micrometers . On the Laser Megajoule project which is the equivalent of the National Ignition Facility in the United States, the spatial resolution required to diagnose high-convergence targets is 10 micrometers . We wish first to obtain a spatial resolution of 30 micrometers to image source with a diameter <EQ 100 micrometers at a neutron yield in the range of 1011 - 1014 neutrons. A collaborative experimental program with the Laboratory for Laser Energetics at the University of Rochester in this perspective is planned. At the same time, there is a research program in collaboration with Laval University concerning coded aperture designs and the associated reconstruction techniques. In this article we first review the basic requirements of such imagery and the concept of the penumbral imaging technique. Then we concentrate on the aperture design criteria and on the quantity of information necessary to achieve high spatial resolution. Finally, we survey the reconstruction techniques used followed by results and comparative evaluation of those methods.
Images of neutron distributions provided by penumbral imaging are degraded by highly signal-dependent noise, so it is difficult to separate object information from noise and to solve the inverse problem. The Richardson-Lucy restoration algorithm is a well-known method to deconvolve images given the point-spread function of the aperture, but it is very sensitive to noise fluctuations. We present a way to denoise images by a multiresolution method, and then apply a modified version of the Richardson-Lucy algorithm twice to deconvolve and to denoise simultaneously neutron images.
In inertial confinement fusion experiments, images of neutron distributions provided by a large aperture imaging system are degraded by multiplicative and signal-dependent noise. In this case, it is difficult to separate the object's fluctuations from those of the noise. Wiener filtering is a well-known method which takes noise into account. Unfortunately, the Wiener filter is a low-pass filter which blurs the contours. The propose of this paper is to introduce two new approaches which restore most of the object's fluctuations. The first approach is a Wiener adaptive filtering method that consists of locally adjusting the value of the cutoff frequency. This adjustment is based on the basic properties of the human visual system where the visibility of noise can be reduced considerably in the neighborhood of a 'strong' contour. The contour detection is realized by means of a special class of quadratic Volterra filters. These filters are approximately equivalent to the product of a local mean estimator with a high pass filter. The second approach consists in applying a multiresolution decomposition of the raw image followed by a wavelet thresholding that changes for each resolution layer. Results and comparative evaluation of these two methods will also be presented.
Penumbral images of neutron distributions form laser fusion experiments provided by large aperture imaging systems are degraded by signal-dependent noise. Most image processing algorithms assume that the signal and the noise are stationary. The purpose of this paper is to introduce a new approach using a locally adaptive non-stationary filter. This method modifies the stationary Wiener filter approach by trading off noise removal against resolution. We shall describe the method and show some results.
Neutron source images of a compressed DT target burn region were obtained in 1994 on the Phebus laser facility at CEL-V, by using a penumbral imaging technique. The symmetric biconical aperture generates a signal-dependent noise which degrades the spatial resolution of the system. The recorded image must be corrected for both instrumental effects and noise. Then by deconvolving from the point spread function of the aperture, we obtain the neutron source distribution. The information structure contained in the raw image and the reconstruction techniques used are explained. Practical and theoretical problems that need to be resolved, including the amount of information in the raw data, will be discussed. Also results and comparative evaluation of those methods are presented.
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