Ion-irradation-induced damage in nuclear materials: Case study of a-SiO2 and MgO

Tesis doctoral inédita cotutelada por la Université Paris-Sud (Université Paris-Saclay), Centre de Sciences Nucléaires et de Sciences de la Matière (CSNSM) y la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Aplicada. Fecha de lectura: 21-06-201

Microdosimetry in low energy proton beam at therapeutic-equivalent fluence rate with silicon 3D-cylindrical microdetectors

In this work we show the first microdosimetry measurements on a low energy proton beam with therapeutic-equivalent fluence rates by using the second generation of 3D-cylindrical microdetectors. The sensors belong to an improved version of a novel silicon-based 3D-microdetector design with electrodes etched inside silicon, which were manufactured at the National Microelectronics Centre (IMB-CNM, CSIC) in Spain. A new microtechnology has been employed using quasi-toroid electrodes of 25μm diameter and a depth of 20μm within the silicon bulk, resulting in a well-defined cylindrical radiation sensitive volume. These detectors were tested at the 18 MeV proton beamline of the cyclotron at the National Accelerator Centre (CNA, Spain). They were assembled into an in-house low-noise readout electronics to assess their performance at a therapeutic-equivalent fluence rate. Microdosimetry spectra of lineal energy were recorded at several proton energies starting from 18 MeV by adding 50μm thick tungsten foils gradually at the exit-window of the cyclotron external beamline, which corresponds to different depths along the Bragg curve. The experimentalyF¯values in silicon cover from (5.7 ± 0.9) to (8.5 ± 0.4) keV μm-1in the entrance to (27.4 ± 2.3) keV μm-1in the distal edge. Pulse height energy spectra were crosschecked with Monte Carlo simulations and an excellent agreement was obtained. This work demonstrates the capability of the second generation 3D-microdetectors to assess accurate microdosimetric distributions at fluence rates as high as those used in clinical centers in proton therapy

Endommagement induit par irradiation ionique dans des matériaux pour le nucléaire : étude de cas du a-SiO₂ et du MgO

One of the most important challenges in Physics today is the development of a clean, sustainable, and efficient energy source that can satisfy the needs of the actual and future society producing the minimum impact on the environment. For this purpose, a huge international research effort is being devoted to the study of new systems of energy production; in particular, Generation IV fission reactors and nuclear fusion reactors are being developed. The materials used in these reactors will be subjected to high levels of radiation, making necessary the study of their behavior under irradiation to achieve a successful development of these new technologies. In this thesis two materials have been studied: amorphous silica (a-SiO₂) and magnesium oxide (MgO). Both materials are insulating oxides with applications in the nuclear energy industry. High-energy ion irradiations have been carried out at different accelerator facilities to induce the irradiation damage in these two materials; then, the mechanisms of damage have been characterized using principally Ion Beam Analysis (IBA) techniques. One of the challenges of this thesis was to develop the Ion Beam Induced Luminescence or ionoluminescence (which is not a widely known IBA technique) and to apply it to the study of the mechanisms of irradiation damage in materials, proving the power of this technique. For this purpose, the ionoluminescence of three different types of silica (containing different amounts of OH groups) has been studied in detail and used to describe the creation and evolution of point defects under irradiation. In the case of MgO, the damage produced under 1.2 MeV Au⁺ irradiation has been characterized using Rutherford backscattering spectrometry in channeling configuration and X-ray diffraction. Finally, the ionoluminescence of MgO under different irradiation conditions has also been studied.The results obtained in this thesis help to understand the irradiation-damage processes in materials, which is essential for the development of new nuclear energy sources.Un des plus grands défis de la Physique aujourd’hui est de créer une source d’énergie propre, durable et efficace qui puisse satisfaire les besoins de la société actuelle et future avec le minimum d’impact sur l’environnement. Dans ce cadre, un grand effort de recherche internationale est dévoué à l’étude de nouveaux systèmes de production d’énergie ; réacteurs de fission de Génération IV et réacteurs de fusion nucléaire sont en particulier en train d’être développés. Les matériaux utilisés dans ces réacteurs seront soumis à des hauts niveaux de radiation, ce qui rend nécessaire l’étude de leur comportement sous irradiation pour permette le succès du développement de ces nouvelles technologies. Dans cette thèse, deux matériaux ont été étudiés : la silice amorphe (a-SiO₂) et l’oxyde de magnésium (MgO). Ces deux matériaux sont des oxydes isolants avec des applications dans l’industrie de l’énergie nucléaire. Des irradiations avec des ions de haute énergie ont été réalisées sur différentes plateformes d’accélérateurs d’ions pour induire l’endommagement de ces deux matériaux par irradiation ; ensuite, les mécanismes d’endommagement ont été caractérisés en utilisant, principalement, des techniques d’analyse par faisceau d’ions (techniques IBA).Un des objectifs de cette thèse était de développer la technique d’ionoluminescence (qui est une technique IBA très peu connue) et de l’appliquer à l’étude des mécanismes d’endommagement par irradiation des matériaux, démontrant alors le potentiel de cette technique. L’ionoluminescence de trois types différents de silice (avec des différentes teneurs en OH) a ainsi été étudiée en détail et utilisée pour décrire la création et l’évolution des défauts ponctuels sous irradiation. Dans le cas de MgO, l’endommagement produit par irradiation avec des ions Au⁺ à 1.2 MeV a été caractérisé en utilisant la technique de spectrométrie de rétrodiffusion Rutherford en configuration de canalisation et la diffraction des rayons X. Finalement, l’ionoluminescence de MgO sous différentes conditions d’irradiation a aussi été étudiée. Les résultats obtenus dans cette thèse aident à comprendre les processus d’endommagement par irradiation dans les matériaux, ce qui est indispensable pour le développement de nouvelles sources d’énergie nucléaire

Damage accumulation in MgO irradiated with MeV Au ions at elevated temperatures

International audienceThe damage accumulation process in MgO single crystals under medium-energy heavy ion irradiation (1.2 MeV Au) at fluences up to 4 x 1014 cm(-2) has been studied at three different temperatures: 573, 773, and 1073 K. Disorder depth profiles have been determined through the use of the Rutherford back-scattering spectrometry in channeling configuration (RBS/C). The analysis of the RBS/C data reveals two steps in the MgO damage process, irrespective of the temperature. However, we find that for increasing irradiation temperature, the damage level decreases and the fluence at which the second step takes place increases. A shift of the damage peak at increasing fluence is observed for the three temperatures, although the position of the peak depends on the temperature. These results can be explained by an enhanced defect mobility which facilitates defect migration and may favor defect annealing. X-ray diffraction reciprocal space maps confirm the results obtained with the RBS/C technique. (C) 2016 Elsevier B.V. All rights reserved

Multi-arrays of 3D cylindrical microdetectors for beam characterization and microdosimetry in proton therapy

International audienceThe present work shows the performance of two new large microdosimetry multi-array systems having two different configurations, namely, pixel and strip configurations. They cover radiation sensitive areas of 1.9 cm × 0.1 cm and 5.1 cm × 0.1 cm, respectively. The microdosimetry systems are based on arrays of 3D cylindrical silicon microdetectors. The 3D electrodes are etched inside the silicon and have a 25 μm diameter and a 20 μm depth. Each of these unit cells is completely isolated from the others and has a well defined 3D micrometric radiation sensitive volume. The pixel-type device consists of 25 × 5 independent silicon-based detectors (500 in total), each one connected to a readout channel, collecting information in 2D in the transverse planes to the particle beam direction. The distance between the individual detectors (pitch) is 200 μm in the horizontal axis and 250 μm in the vertical one. In the case of the strip-type system, we have 512 “columns” (or strips) of 10 detectors per column. Each strip is connected to a readout channel, giving us information in one dimension, but with better statistics than a single pixel. In this system, both the horizontal and vertical pitches are 100 μm.Both systems have been tested under proton beam irradiations at different energies between 6 and 24 MeV to obtain the corresponding microdosimetry quantities along the Bragg peak and distal edge. The measurements were performed at the Accélérateur Linéaire et Tandem à Orsay (ALTO, France). The microdosimetry quantities were successfully obtained with spatial resolutions of 100–250 μm. Experimental results were compared to Monte Carlo simulations and an overall good agreement was found. Both microdetector systems showed a good microdosimetry performance under clinical-equivalent fluence rates along distances of several centimeters. This work demonstrates that the two new systems having different configurations can be clinically used as microdosimeters for measuring the lineal energy distributions in the context of proton therapy treatments. Additionally, they could be also used for beam monitoring.</jats:p

Microdosimetry performance of the first multi-arrays of 3D-cylindrical microdetectors

International audienceThe present work reports on the microdosimetry measurements performed with the two first multi-arrays of microdosimeters with the highest radiation sensitive surface covered so far. The sensors are based on new silicon-based radiation detectors with a novel 3D cylindrical architecture. Each system consists of arrays of independent microdetectors covering 2 mm$\times$2 mm and 0.4 mm$\times$12 cm radiation sensitive areas, the sensor distributions are arranged in layouts of 11$\times$11 microdetectors and 3$\times$3 multi-arrays, respectively. We have performed proton irradiations at several energies to compare the microdosimetry performance of the two systems, which have different spatial resolution and detection surface. The unitcell of both arrays is a 3D cylindrical diode with a 25 $\mu$m diameter and a 20 $\mu$m depth that results in a welldefined and isolated radiation sensitive micro-volume etched inside a silicon wafer. Measurements were carried out at the Accélérateur Linéaire et Tandem à Orsay (ALTO) facility by irradiating the two detection systems with monoenergetic proton beams from 6 to 20 MeV at clinical-equivalent fluence rates. The microdosimetry quantities were obtained with a spatial resolution of 200 $\mu$m and 600 $\mu$m for the 11$\times$11 system and for the 3$\times$3 multi-array system, respectively. Experimental results were compared with Monte Carlo simulations and an overall good agreement was found. The good performance of both microdetector arrays demonstrates that this architecture and both configurations can be used clinically as microdosimeters for measuring the lineal energy distributions and, thus, for RBE optimization of hadron therapy treatments. Likewise, the results have shown that the devices can be also employed as a multipurpose device for beam monitoring in particle accelerators

First experimental measurements of 2D microdosimetry maps in proton therapy

International audienceBackground Empirical data in proton therapy indicate that relative biological effectiveness (RBE) is not constant, and it is directly related to the linear energy transfer (LET). The experimental assessment of LET with high resolution would be a powerful tool for minimizing the LET hot spots in intensity-modulated proton therapy, RBE- or LET-guided evaluation and optimization to achieve biologically optimized proton plans, verifying the theoretical predictions of variable proton RBE models, and so on. This could impact clinical outcomes by reducing toxicities in organs at risk. Purpose The present work shows the first 2D LET maps obtained at a proton therapy facility using the double scattering delivery mode in clinical conditions by means of new silicon 3D-cylindrical microdetectors. Methods The device consists of a matrix of 121 independent silicon-based detectors that have 3D-cylindrical electrodes of 25-mu m diameter and 20-mu m depth, resulting each one of them in a well-defined micrometric radiation sensitive volume etched inside the silicon. They have been specifically designed for a hadron therapy, improving the performance of current silicon-based microdosimeters. Microdosimetry spectra were obtained at different positions of the Bragg curve by using a water-equivalent phantom along an 89-MeV pristine proton beam generated in the Y1 proton passive scattering beamline of the Orsay Proton Therapy Centre (Institut Curie, France). Results Microdosimetry 2D-maps showing the variation of the lineal energy with depth in the three dimensions were obtained in situ during irradiation at clinical fluence rates (similar to 10(8) s(-1) cm(-2)) for the first time with a spatial resolution of 200 mu m, the highest achieved in the transverse plane so far. The experimental results were cross-checked with Monte Carlo simulations and a good agreement between the spectra shapes was found. The experimental frequency-mean lineal energy values in silicon were 1.858 +/- 0.019 keV mu m(-1) at the entrance, 2.61 +/- 0.03 keV mu m(-1) at the proximal distance, 4.97 +/- 0.05 keV mu m(-1) close to the Bragg peak, and 8.6 +/- 0.1 keV mu m(-1) at the distal edge. They are in good agreement with the expected trends in the literature in clinical proton beams. Conclusions We present the first 2D microdosimetry maps obtained in situ during irradiation at clinical fluence rates in proton therapy. Our results show that the arrays of 3D-cylindrical microdetectors are a reliable microdosimeter to evaluate LET maps not only in the longitudinal axis of the beam, but also in the transverse plane allowing for LET characterization in three dimensions. This work is a proof of principle showing the capacity of our system to deliver LET 2D maps. This kind of experimental data is needed to validate variable proton RBE models and to optimize LET-guided plans

Performances of the Two First Single Spoke Prototypes for the MYRRHA Project

International audienceThe MYRRHA project aims at the construction of an accelerator driven system (ADS) at MOL (Belgium) for irradiation and transmutation experiment purposes. The facility will feature a superconducting LINAC able to produce a proton flux of 2.4 MW (600 MeV - 4 mA). The first section of the superconducting LINAC will be composed of 352 MHz (β = 0.37) Single Spoke Resonators (SSR) housed in short cryomodules operating at 2K. After a brief presentation of the cryomodule design, this paper will aim at presenting the RF performances of the SSR tested in vertical cryostat in the framework of European MYRTE project (MYRRHA Research and Transmutation Endeavour) and at comparing experimental results (Lorentz forces, pressure sensitivity, multipacting barriers…) to simulated values

Characterization of the Charge Collection Efficiency in Silicon 3-D-Detectors for Microdosimetry

International audienceNew silicon 3-D-microdetectors have been developed to perform microdosimetry measurements for applications in hadron therapy. In this work, the charge collection efficiency (CCE) of an improved second generation of microdetectors having two different thicknesses (10 and 20 μm ) and a diameter of 25 μm has been studied by means of the ion beam induced charge (IBIC) technique. New methods to study the active volume and the CCE of microdosimeters are proposed and verified here. The results show that, for this new generation of microdetectors, the CCE is 100% for radial distances up to 10.2 μm from the center of the device, and it rapidly decays between 10.2 μm and the detector edge. The characterization of the CCE conducted here will allow us to completely explain the energy spectra obtained during the microdosimetry studies performed in clinical centers

Microdosimetry in low energy proton beam at therapeutic-equivalent fluence rate with silicon 3D-cylindrical microdetectors

International audienc