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To investigate the linear energy transfer (LET) dependence of the response of a PTW-60019 Freiburg microDiamond detector, its response was compared to the response of a plane-parallel Markus chamber in a 62 MeV/n mono-energetic carbon ion beam. Results obtained with two different experimental setups are in agreement. As recommended by IAEA TRS-398, the response of the Markus chamber was corrected for temperature, pressure, polarity effects and ion recombination. No correction was applied to the response of the microDiamond detector. The ratio of the response of the Markus chamber to the response of the microDiamond is close to unity in the plateau region. In the Bragg peak region, a significant increase of the ratio is observed, which increases to 1.2 in the distal edge region. Results indicate a correlation between the under-response of the microDiamond detector and high LET values. The combined relative standard uncertainty of the results is estimated to be 2.38% in the plateau region and 12% in the distal edge region. These values are dominated by the uncertainty of alignment in the non-uniform beam and the uncertainty of range determination.

This study was initiated following conclusions from earlier experimental work, performed in a low-energy carbon ion beam, indicating a significant LET dependence of the response of a PTW-60019 microDiamond detector. The purpose of this paper is to present a comparison between the response of the same PTW-60019 microDiamond detector and an IBA Roos-type ionization chamber as a function of depth in a 62MeV proton beam. Even though proton beams are considered as low linear energy transfer (LET) beams, the LET value increases slightly in the Bragg peak region. Contrary to the observations made in the carbon ion beam, in the 62MeV proton beam good agreement is found between both detectors in both the plateau and the distal edge region. No significant LET dependent response of the PTW-60019 microDiamond detector is observed consistent with other findings for proton beams in the literature, despite this particular detector exhibiting a substantial LET dependence in a carbon ion beam.

Purpose: In this work, ion recombination is studied as a function of energy and depth in carbon ion beams. Methods: Measurements were performed in three different passively scattered carbon ion beams with energies of 62 MeV/n, 135 MeV/n, and 290 MeV/n using various types of plane-parallel ionization chambers. Experimental results were compared with two analytical models for initial recombination. One model is generally used for photon beams and the other model, developed by Jaffe, takes into account the ionization density along the ion track. An investigation was carried out to ascertain the effect on the ion recombination correction with varying ionization chamber orientation with respect to the direction of the ion tracks. The variation of the ion recombination correction factors as a function of depth was studied for a Markus ionization chamber in the 62 MeV/n nonmodulated carbon ion beam. This variation can be related to the depth distribution of linear energy transfer. Results: Results show that the theory for photon beams is not applicable to carbon ion beams. On the other hand, by optimizing the value of the ionization density and the initial mean-square radius, good agreement is found between Jaffes theory and the experimental results. As predicted by Jaffes theory, the results confirm that ion recombination corrections strongly decrease with an increasing angle between the ion tracks and the electric field lines. For the Markus ionization chamber, the variation of the ion recombination correction factor with depth was modeled adequately by a sigmoid function, which is approximately constant in the plateau and strongly increasing in the Bragg peak region to values of up to 1.06. Except in the distal edge region, all experimental results are accurately described by Jaffes theory. Conclusions: Experimental results confirm that ion recombination in the investigated carbon ion beams is dominated by initial recombination. Ion recombination corrections are found t

Due to the development and increased use of hadrontherapy, reference dosimetry of charged particle beam, and in particular the absorbed dose to water measurement using ionization chambers, has to be improved to decrease the uncertainty of the treatment and improve its consistency. Ion recombination is one of the corrections that need to be applied to an ionization chamber measurement that is potentially much larger in a carbon ion beam as compared to conventional photon or electron beams. This work aims at measuring the ion recombination correction (ks) for parallel-plate ionization chambers in a carbon ion beam from a slow cycle synchrotron.

Developments in hadron therapy require efforts to improve the accuracy of the dose delivered to a target volume. Here, the determination of the absorbed dose under reference conditions was analysed. Based on the International Atomic Energy Agency TRS-398 code of practice, for hadron beams, the combined standard uncertainty on absorbed dose to water under reference conditions, derived from ionisation chambers, is too large. This uncertainty is dominated by the beam quality correction factors, kQ,Q0, mainly due to the mean energy to produce one ion pair in air, wair. A method to reduce this uncertainty is to carry out primary dosimetry, using calorimetry. A kQ,Q0 -value can be derived from a direct comparison between calorimetry and ionometry. Here, this comparison is performed using a graphite calorimeter in an 80-MeVA-1 carbon ion beam. Assuming recommended TRS-398 values of water-to-graphite stopping power ratio and the perturbation factor for an ionisation chamber, preliminary results indicate a wair-value of 35.5 ± 0.9 J C-1.

Purpose/Objective To validate experimentally a GATE/GEANT4-based(G4) Monte Carlo (MC) model in heterogeneous media for dedicated pencil beam scanning in proton therapy. Comparisons between measurements and MC simulations using G4 and PENELOPE-proton are presented. A comparison against analytical modeling from commercial TPS is also investigated. This work evaluates the impact of heterogeneities on range prediction, beam shape and depth dose changes. Materials and Methods The MC model for pencil beam based on G4 has been validated in water and PMMA phantoms (Grevillot et al Phys. Med. Biol.(2011)) reproducing pristine Bragg peaks for a series of individual energies (from 100 to 226.7 MeV) with 0.7 mm range and 0.2 mm spot size accuracy. The same optical model was implemented in PENELOPE-proton. In order to validate the beam model in heterogeneous media, phantoms made of stacked slabs with different densities and known compositions were used. Two experimental test cases including solid water (SW), lung (LN-300) and bone (SB3) tissue-equivalent material were investigated. Depth-dose distributions for a monoenergetic single spot and 10x10cm² composite fields were measured using Gafchromic EBT3 films and the ionization chamber (IC) PPC05 in all configurations. To measure accurately the Bragg peak position, a stack of films of 2x2cm² was inserted in the last centimeter of the proton range. Results Figure 1 shows results for one heterogeneous configuration. All doses-to-medium were converted to dose to water using stopping power ratios. Dose distributions were arbitrarily normalized in the middle of the second SW region. Bragg peak positions are reproduced by MC simulations within 1mm in both configurations (Table 1). IC measurements, G4 (binary-cascade) and PENELOPE-proton simulations are within 2%/2 mm. Point-to-point mean difference of 1.2% is observed between G4 (precompound) and measurement in the first 15 cm of the phantom and increased to 7.2% after bone insert unti

Purpose: To evaluate the uncertainties and characteristics of radiochromic film-based dosimetry system using the EBT3 model Gafchromic ® film in therapy photon, electron and proton beams. Material and methods: EBT3 films were read using an EPSON Expression 10000XL/PRO scanner. They were irradiated in five beams, an Elekta SL25 6 MV and 18 MV photon beam, an IBA 100 MeV 5 × 5 cm 2 proton beam delivered by pencil-beam scanning, a 60 MeV fixed proton beam and an Elekta SL25 6 MeV electron beam. Reference dosimetry was performed using a FC65-G chamber (Elekta beam), a PPC05 (IBA beam) and both Markus 1916 and PPC40 Roos ion-chambers (60 MeV proton beam). Calibration curves of the radiochromic film dosimetry system were acquired and compared within a dose range of 0.4-10 Gy. An uncertainty budget was estimated on films irradiated by Elekta SL25 by measuring intra-film and inter-film reproducibility and uniformity; scanner uniformity and reproducibility; room light and film reading delay influences. Results: The global uncertainty on acquired optical densities was within 0.55% and could be reduced to 0.1% by placing films consistently at the center of the scanner. For all beam types, the calibration curves are within uncertainties of measured dose and optical densities. The total uncertainties on calibration curve due to film reading and fitting were within 1.5% for photon and proton beams. For electrons, the uncertainty was within 2% for dose superior to 0.8 Gy. Conclusions: The low combined uncertainty observed and low beam and energy-dependence make EBT3 suitable for dosimetry in various applications. © 2012 Associazione Italiana di Fisica Medica.

Introduction The main advantage of ion beam therapy over conventional therapy is an improved dose conformation and a significant lower dose to healthy tissues and organs at risk [1]. To take advantage of hadrons properties, treatment has to be delivered as it is planned with optimal accuracy. Pencil beam scanning (PBS) delivery technique is nowadays the most advanced technique for hadron beam therapy and needs proper quality assurances processes. For many years, Monte Carlo (MC) codes have been used to benchmark treatment planning systems (TPS) in conventional radiotherapy. Using a general purpose MC code, like GEANT4, it is possible to evaluate modalities such as those using combinations of different particles [2]. A pencil beam scanning model for proton treatment plan simulation using GEANT4/GATE platform has been validated in a water phantom for IBA PBS dedicated system [3]. In this work, we present preliminary results on validation in heterogeneous phantom using GEANT4/GATE model. Moreover simulations with a new MC code, called PENELOPE-proton (F. Salvat), were performed. Materials and methods 1. Measurements in heterogeneous phantom The pencil beam MC model was validated in a water phantom, reproducing pristine Bragg peaks for all tested energies (from 100 to 226.7 MeV) with accurcay of order of 0.7mm (range) and 0.2 mm (spot size). A spread-out Bragg peak with 10 cm modulation was reproduced with 0.8 mm range accuracy and a maximum dose difference of less than 2% point-to-point [3]. Measurements and simulation will be performed in similar beam delivery configurations in a heterogeneous phantom made of slabs collection with different densities and compositions (Fig 1.). Reference pristine Bragg peaks will be measured using plane parallel chamber in both configurations. Dose will be normalized to the number of protons via Faraday-cup measurements. Shifts for measurements are scheduled in January the Essen proton therapy center, Germany, with the IBA PBS dedicate

On the basis of a standard one-sided lubrication-type model, an analysis is carried out pertaining to a small vicinity of a contact line of a volatile nonpolar perfectly wetting macroscopic liquid sample surrounded with its pure vapor and attached to a smooth uniformly superheated solid surface. The behavior of the liquid film is governed by the effects of evaporation, capillarity, and the disjoining pressure. The kinetic resistance to evaporation, as well as the dependence of the local saturation temperature on the local liquid pressure are accounted for. Within the localized approach pursued, a steady configuration of the film on a flat substrate is studied such that at one end (say, to the left) it asymptotically attains an adsorbed microfilm in equilibrium with the vapor, while to the right it gets on to a constant slope (contact angle of the "microstructure"). For moving contact lines in the situations like drop spreading or bubble growth in the boiling process, this microstructure is relevant in the quasi-steady sense, provided that the displacement velocity is not too large. The paper focuses on a numerically based parametric study expressing the contact angle and evaporation flux characteristics as functions of the system parameters. Asymptotic expansions at both ends of the film are elaborated in some detail and relied on in the numerics. Asymptotic results from the literature involving certain limiting cases of the system parameters are critically examined. At last, the Marangoni and the vapor-recoil effects are additionally incorporated and their possible importance is assessed., info:eu-repo/semantics/published

info:eu-repo/semantics/published

Les recherches que nous présentons dans ce manuscrit s’inscrivent dans le cadre de l’analyse des phénomènes de transport fondamentaux impliqués lors du processus d’évaporation d’un film liquide mince. Outre les mécanismes macroscopiques (résistance thermique du solide, capillarité, thermocapillarité, …) qui influencent le comportement de tels films, des développements fondamentaux et expérimentaux ont mis en évidence le rôle significatif d’effets microscopiques, comme les forces de van der Waals [11,96,117]. L’objectif de cette thèse est double. Il s’agit tout d’abord de caractériser les phénomènes locaux qui influencent le processus d’évaporation et ensuite, d’étendre notre étude à une échelle globale “macroscopique”. Ce manuscrit est divisé en deux parties qui correspondent à ces deux objectifs. L’étude décrite dans la première partie propose une contribution originale à la modélisation de l’évaporation des films minces, y compris au voisinage des lignes de contact. De manière générale, nous cherchons à mettre en évidence l’influence de phénomènes qui se déroulent aux petites échelles sur le transfert thermique d’un film mince déposé sur une paroi plane et chauffée. Dans le cadre de l’hypothèse de lubrification, deux modèles sont dès lors développés. Le premier modèle décrit l’évaporation d’un film liquide mince dans sa vapeur pure tandis que le second modèle porte sur l’évaporation d’un film liquide mince dans un gaz inerte. Les diverses recherches menées sont principalement orientées vers la quantification, d’une part, des angles de contact apparents générés par l’évaporation, malgré le caractère parfaitement mouillant du couple liquide-solide utilisé et, d’autre part, des flux de chaleur et de matière interfaciaux. Une particularité du premier modèle est qu’il généralise divers modèles existants [15,25,86,117] en regroupant un ensemble de phénomènes spécifiques et complexes tels que le saut de température à l’interface liquide-vapeur, la résistance thermi, Doctorat en Sciences de l'ingénieur, info:eu-repo/semantics/nonPublished

This paper describes a multi-scale model for evaluating the radial heat transfer within a grooved heat pipe evaporator. The model is composed of two parts, macroscopic and microscopic, which cannot be decoupled from each other. In the macroscopic part, we solve the heat conduction problem in the solid and in the liquid phases, thanks to a finite-element method allowing high flexibility in the definition of the groove geometry. In order to avoid the classical singularity problem at the contact line (where the liquid-vapor interface meets the groove wall), in addition to taking the solid thermal conductivity into account, we do not impose the saturation temperature but a mixed condition along the interface. We show in particular that the interface temperature equals the saturation temperature (at given vapor pressure), except in the microscopic region where it increases and reaches the solid temperature. In this microscopic zone, a classical lubrication-type theory allows to determine the apparent contact angle, taking into account the influence of small-scale effects, such as the variation of the saturation temperature with the disjoining pressure and with the meniscus curvature. In particular, analytical relationships and correlations are presented for the apparent contact angle, which allow an efficient coupling between macroscopic and microscopic scales. In this paper, attention is devoted to the numerical treatment of both regions, their coupling, and the influence of the macroscopic heat flux and local small-scale effects on the distribution of temperature in the groove., info:eu-repo/semantics/published

Some classical lubrication-type models are examined further and applied to the study of thin liquid film evaporation and condensation on heated or cooled horizontal and flat homogeneous substrates. The focus is on particular solutions in the form of sessile droplets surrounded by an adsorbed (or precursor) film. Using a combination of analytical results and numerical simulations, particular attention is devoted to the apparent contact angles and their evolution when evaporation or condensation takes place. Different forms of the disjoining pressure isotherm are considered, allowing both partial and complete wetting situations to be studied. The influence of an inert component in the gas phase, in addition to vapor, is considered through a slightly modified form of the mathematical expression for the phase change rate. The role of thermocapillary flows in the vicinity of the contact lines receives particular attention, while the vapor recoil is showed to be negligible in general., info:eu-repo/semantics/published

Lubrication-type theories have turned out to be useful in a wide range of scales, being applied to study the dynamics of liquid films of thicknesses ranging between the millimeter and the nanometer. As they moreover allow a deep physical understanding, and are in some cases amenable to analytical treatment, they provide a powerful tool for studying the transition between regimes affected (or even dominated) by inertia, and regimes dominated by contact forces with a solid, for instance. In this paper, recent results of the authors are presented, focusing on thin liquid films flowing down inclines, ultra-thin films of complex fluids featuring density variations, and evaporating contact lines., info:eu-repo/semantics/published

The quantitative determination of overall heat transfer in the vicinity of contact lines is crucial for many heat transfer devices such as heat pipes and boilers. In this context, a long-wave evolution equation describing the evaporation of an ultra-thin film is solved, focusing on nonlinear solutions in the form of contact lines connecting a constant slope region to an adsorbed precursor film. First, the latter film is found to be stable to hydrodynamic disturbances, via linear stability analysis. Then, the main characteristics of evaporating contact lines are analyzed, with particular attention to the sharp peak of the heat flux occurring in the transition region, which results in a microscopic (though non-negligible) contribution to the overall heat transfer. The latter is then quantified as a function of the thermal conductivity of the solid, the mass transfer resistance, the interface curvature and the Van der Waals forces, including their influence on the saturation temperature. For small superheat, a useful scaling behavior is found for the apparent contact angle and for the heat flux characteristics., info:eu-repo/semantics/published

Despite their crucial importance in the fields of microfluidics and in heat transfer technologies, contact lines are not yet completely understood from the modeling point of view, in particular when involving heat and fluid flows coupled by means of evaporation/condensation processes. In this presentation, such non-equilibrium contact lines are analyzed theoretically, using a lubrication-type equation incorporating relevant micro-scale effects, i.e. kinetic resistance to phase change, disjoining pressure and influence of interfacial curvature. In addition, the effect of an inert gas is also addressed. It is shown that steady contact line shapes can be accurately described by an analytical solution based on matched asymptotic expansions in a certain meaningful limit. For moving contact lines, the dependence of the apparent contact angle upon the contact line velocity is also investigated, and compared to numerical simulations of the evolution equation, for droplet-like solutions. Finally, the relevance of our results for the macroscopic modeling of heat transfer processes such as boiling, is also discussed., info:eu-repo/semantics/published

A new lubrication-type model applying to evaporating ultra-thin films is presented. This evolution equation generalizes earlier ones found in the literature by incorporating the effects of finite thermal conductivity of the vapor and of temperature discontinuity across the evaporating/condensing interface. The latter effect is accounted for through a set of generalized interfacial conditions derived from irreversible thermodynamics. The model is first applied to determine the conditions under which a film of given thickness is stable to hydrodynamic disturbances. Then, steady nonlinear solutions in the form of contact lines connecting a constant-slope region with an adsorbed film are found, and particular attention is devoted to the peak of the mass flux at the contact line, as well as to the apparent contact angle. The influence of various effects on these quantities is also considered., info:eu-repo/semantics/published