Dr. Mehdi Gmar Profile

Dr. Mehdi Gmar

at CEA Saclay

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Publications (2)

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Proceedings Article | 17 March 2011 Paper

Fluence estimation by deconvolution via l1-norm minimization

J. García Hernández, D. Lazaro-Ponthus, M. Gmar, J. Barthe

Proceedings Volume 7961, 79613J (2011) https://doi.org/10.1117/12.878361

KEYWORDS: Deconvolution, Convolution, Sensors, Monte Carlo methods, Inverse problems, Radiotherapy, Electronic components, Electronic imaging, Imaging devices, Detection and tracking algorithms

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Advances in radiotherapy irradiation techniques have led to very complex treatments requiring for a more stringent control. The dosimetric properties of electronic portal imaging devices (EPID) encouraged their use for treatment verification. Two main approaches have been proposed: the forward approach, where measured portal dose images are compared to predicted dose images and the backward approach, where EPID images are used to estimate the dose delivered to the patient. Both approaches need EPID images to be converted into a fluence distribution by deconvolution. However, deconvolution is an ill-posed problem which is very sensitive to small variations on input data. This study presents the application of a deconvolution method based on l1-norm minimization; this is a method known for being very stable while working with noisy data. The algorithm was first evaluated on synthetic images with different noise levels, the results were satisfactory. Deconvolution algorithm was then applied to experimental portal images; the required EPID response kernel and energy fluence images were computed by Monte-Carlo calculation, accelerator treatment head and EPID models had already been commissioned in a previous work. The obtained fluence images were in good agreement with simulated fluence images. This deconvolution algorithm may be generalized to an inverse problem with a general operator, where image formation is not longer modeled by a convolution but by a linear operation that might be seen as a position-dependent convolution. Moreover, this procedure would be detector independent and could be used for any detector type provided its response function is known.

Proceedings Article | 30 May 2006 Paper

Detection of nuclear material by photon activation inside cargo containers

Mehdi Gmar, Eric Berthoumieux, Sébastien Boyer, Frédérick Carrel, Diane Doré, Marie-Laure Giacri, Frédéric Lainé, Bénédicte Poumarède, Danas Ridikas, Aymeric Van Lauwe

Proceedings Volume 6213, 62130F (2006) https://doi.org/10.1117/12.665571

KEYWORDS: Actinides, Sensors, Inspection, Uranium, Signal detection, Signal attenuation, Photon transport, Photodetectors, Tungsten, Data analysis

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Photons with energies above 6 MeV can be used to detect small amounts of nuclear material inside large cargo containers. The method consists in using an intense beam of high-energy photons (bremsstrahlung radiation) in order to induce reactions of photofission on actinides. The measurement of delayed neutrons and delayed gammas emitted by fission products brings specific information on localization and quantification of the nuclear material. A simultaneous measurement of both of these delayed signals can overcome some important limitations due to matrix effects like heavy shielding and/or the presence of light elements as hydrogen. We have a long experience in the field of nuclear waste package characterization by photon interrogation and we have demonstrated that presently the detection limit can be less than one gram of actinide per ton of package. Recently we tried to extend our knowledge to assess the performance of this method for the detection of special nuclear materials in sea and air freights. This paper presents our first results based on experimental measurements carried out in the SAPHIR facility, which houses a linear electron accelerator with the energy range from 15 MeV to 30 MeV. Our experiments were also modeled using the full scale Monte Carlo techniques. In addition, and in a more general frame, due to the lack of consistent data on photonuclear reactions, we have been working on the development of a new photonuclear activation file (PAF), which includes cross sections for more than 600 isotopes including photofission fragment distributions and delayed neutron tables for actinides. Therefore, this work includes also some experimental results obtained at the ELSA electron accelerator, which is more adapted for precise basic nuclear data measurements.

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