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Uppal Kalan, India

Belabbas I.,University of Abderrahmane Mira de Bejaia | Chen J.,University of Caen Lower Normandy | Nouet G.,CIMAP
Physica Status Solidi (C) Current Topics in Solid State Physics | Year: 2016

We have carried out computer atomistic simulations, based on Density Functional Theory, to investigate energetics and core structure of the undissociated basal screw dislocation in wurtzite GaN. Our calculations enabled us to establish the energetic hierarchy of two stable core configurations. The shuffle core configuration, which is formed by fully coordinated atoms, was demonstrated to be more energetically favorable than the glide configuration, which exhibits sp2-like hybridization. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Miro S.,CEA Saclay Nuclear Research Center | Costantini J.M.,CEA Saclay Nuclear Research Center | Bardeau J.F.,University of Maine, France | Chateigner D.,CNRS Crystallography and Material Science Laboratory | And 2 more authors.
Journal of Raman Spectroscopy | Year: 2011

Raman spectroscopy was used to study the radiation damage of fluorapatite single crystals and sinters. Krypton and iodine ion irradiations were performed at high energies (∼1 MeV amu-1) for fluences ranging between 1 × 1011 and 5 × 1013 cm-2. Evolution of the symmetric stretching mode of the PO4 3- tetrahedral building blocks (strongest Raman mode observed at 965 cm-1) versus ion fluence was investigated. After irradiation, this peak decreases in intensity and a second broader peak appears at lower wavenumber. The well-resolved peak has been assigned to the crystalline phase, and the broader one to the amorphous phase. The integrated intensity ratios of these two peaks versus fluence are in good agreement with the damage fractions determined by X-ray diffraction (XRD). Fits of the amorphous fraction versus fluence show that the amorphization mechanisms is dominated by a single-impact process for iodine ions and by a double-impact process for krypton ions in the case of single crystals and sinters. For both irradiations, complete amorphization could not be obtained. The amorphous fraction saturates at a maximum value of 88% for sinters and 72% for single crystals. This is attributed to a recrystallization effect which is more important in single crystals than in sinters. For both types of samples, the crystalline peak shifts slightly to a lower wavenumber with fluence, and then shifts back to its initial value for an amorphous fraction larger than 60%. This feature is attributed to a stress relaxation, as shown in the XRD data, which is accompanied by a decrease of the crystalline peak full-width at half-maximum. © 2011 John Wiley & Sons, Ltd. Source

Belabbas I.,University of Abderrahmane Mira de Bejaia | Chen J.,University of Caen Lower Normandy | Nouet G.,CIMAP
Computational Materials Science | Year: 2014

We have carried out a complete and consistent study on core configurations of the three types of threading dislocations in wurtzite GaN. Their atomic and local electronic structures were analyzed and systematically compared, at the level of Density Functional Theory. Screw and mixed threading dislocations were found to introduce both deep and shallow gap states, while most of core configurations of the edge dislocation introduce only shallow states. We demonstrated that the existence of an extended one-dimensional metallization, associated with unoccupied gap states, is a specific feature of threading screw dislocations. The extended metallization along with the high dispersion of the energy gap states are at the origin of the experimentally observed high electric conduction along threading screw dislocations in GaN. The presence of nitrogen vacancies in the core of screw dislocations is predicted to enhance their electric conduction. © 2014 Elsevier B.V. All rights reserved. Source

Miro S.,CEA Saclay Nuclear Research Center | Costantini J.M.,CEA Saclay Nuclear Research Center | Haussy J.,CEA DAM Ile-de-France | Chateigner D.,CNRS Crystallography and Material Science Laboratory | Balanzat E.,CIMAP
Journal of Nuclear Materials | Year: 2012

We have studied the damage induced in fluorapatite (Ca 10(PO 4) 6F 2) sinters after 70-MeV Kr, 120 MeV I and 163-MeV Au ion irradiations at room temperature. On the basis of X-ray powder diffraction data we conclude that fluorapatite is not completely amorphized due to ion-induced recrystallization. This recrystallization of the amorphous phase is greatly enhanced for Au ions with a high electronic stopping power. We also have used the 3He(d, p) 4He nuclear reaction to study the migration of implanted 3-MeV 3He ions after swift heavy ion irradiations. The proton yield curves versus deuteron energy for irradiated samples exhibit two bumps for high fluences. These excitation curves are deconvoluted by using a computer code based on a two-diffusion equation model of helium atoms in two accumulation zones. Optimizations of the model parameters give access to the diffusion coefficients and helium depth profiles in the two zones. This yields two broad peaks in the helium depth profiles, the first one is near the end-of-range region and the second one is shifted at about half way between the surface and the first peak. This shift is interpreted as a Radiation-Enhanced Diffusion (RED) effect which is found to increase with fluence for Kr ions, and with electronic stopping power from Kr to Au ions. © 2012 Elsevier B.V. All rights reserved. Source

Huber B.A.,CIMAP | Malot C.,CIMAP | Domaracka A.,CIMAP | Solov'yov A.V.,FIAS
Journal of Physics: Conference Series | Year: 2012

The 1st Nano-IBCT Conference entitled "Radiation Damage in Biomolecular Systems: Nanoscale Insights into Ion Beam Cancer Therapy" was held in Caen, France, in October 2011. The Meeting was organised in the framework of the COST Action MP1002 (Nano-IBCT) which was launched in December 2010 (http://fias.uni-frankfurt.de/nano-ibct). This action aims to promote the understanding of mechanisms and processes underlying the radiation damage of biomolecular systems at the molecular and nanoscopic level and to use the findings to improve the strategy of Ion Beam Cancer Therapy. In the hope of achieving this, participants from different disciplines were invited to represent the fields of physics, biology, medicine and chemistry, and also included those from industry and the operators of hadron therapy centres. Ion beam therapy offers the possibility of excellent dose localization for treatment of malignant tumours, minimizing radiation damage in normal healthy tissue, while maximizing cell killing within the tumour. Several ion beam cancer therapy clinical centres are now operating in Europe and elsewhere. However, the full potential of such therapy can only be exploited by better understanding the physical, chemical and biological mechanisms that lead to cell death under ion irradiation. Considering a range of spatio-temporal scales, the proposed action therefore aims to combine the unique experimental and theoretical expertise available within Europe to acquire greater insight at the nanoscopic and molecular level into radiation damage induced by ion impact. Success in this endeavour will be both an important scientific breakthrough and give great impetus to the practical improvement of this innovative therapeutic technique. Ion therapy potentially provides an important advance in cancer therapy and the COST action MP1002 will be very significant in ensuring Europe's leadership in this field, providing the scientific background, required data and mechanistic insight which are indispensable for the optimization of this new therapy. The conference gathered 115 participants originating from 28 countries and addressed a large number of highly relevant aspects concerning ion propagation in biological matter, the production of secondary particles along the ion tracks as electrons, holes and radicals, and their propagation in the biomolecular medium. In particular, the attack of DNA molecules and proteins by electrons and free radicals, the relative importance of direct and indirect damage processes as well as the role of the environment were discussed. Not only were fundamental mechanisms and processes elucidated, but radiobiological scale effects, multi-scale approaches and recent advances in the theoretical description of the underlying complex phenomena were also presented. Aspects linked to the energy deposition (LET), the characteristics of the Bragg peak and new techniques of dosimetry and radiolysis were highlighted. Furthermore, methods for increasing the therapy efficiency by using radio sensitizers and the state-of-the-art of defining precise patient treatment plans, identifying the clinical benefits of this type of therapy, were also addressed. We would like to thank all participants for the lively exchange of ideas and results, thus making this conference a very fruitful event. Furthermore, we appreciate the financial support of the sponsors of this conference, in particular of the COST Action MP1002 financed by ESF. We would also like to express our thanks to all authors of these proceedings, as well as to the reviewers for their time, efforts and recommendations made during the preparation of this volume. Finally, many thanks to U G Huber for a careful proof-read of this manuscript. We look forward to the 2nd Nano-IBCT Conference, which will be held in spring 2013. Caen, 15 March 2012 Bernd A Huber, Christiane Malot, Alicja Domaracka and Andrey V Solov'yov The Editors The PDF also contains details of the Conference Committees and Sponsors and a list of participants. Source

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