Maria Velaora
email: maria.velaora.auditeur@lecnam.net2francois.guena@paris-lavillette.archi.fr3mor@arsisarc.gr

François Guena

Konstantinos Moraitis

Integrating Virtual Reality Interface into architectural education Virtual Urban Environment Design and Real-Time Simulated Behaviors

Keywords: interface, interactive, behavior, scale, virtual reality

This paper proposes an educational approach for learning architecture based on prototype digital reality experiences tailored for students' needs. We use a combination of tools and methods from different disciplines, such architecturology and game design. The hypothesis is tested on architecture students from the course 'Architectural Design 9: Urban Design' at the National Technical University of Athens, fall semester, 2018-2019.We propose using the demo-application 'Behavior Interactive Interface' (BII) for Unity 3D with preset behaviors of virtual actors, as a demo-tool in order to for evaluate the effectiveness of urban design proposals in serving self-improvement of the architectural designing skills of students. The integration of such a method facilitates cognitive interactions through real-time immersive visualization during the process of learning. We expect to provide students all the necessary new assets of reality's experimentation for their future professional life in a compact theoretical and practical educational package.

INTRODUCTION

Immersive and virtual reality environments present nowadays an augmenting interest for researchers, designers, psychologists, programmers, neuroscientists, artists and architects as a result of the established visual production and communication of ideas in modern culture. This essay targets architecture's students and the way they perceive the concept of scale and its transformations in virtual immersive and multiuser environments. To be able to provide methodologies to students, we apply to the epistemological field of phenomenology in architecture, perception and game design. The merge of these disciplines can provide dynamic 3D representations of architectural models in real-time multiuser dynamic systems. The context of this article constitutes the basis of an ongoing PhD research carried out in collaboration between the School of Architecture of Paris La Villette (ENSAPLV) and the Architecture Department of the National Technical Uni-versity of Athens (NTUA).In the context for this doctoral research we present a pilot-framework of educational workflow in architecture. As a starting point we launch a questionnaire and has been answered by 97 students and young professionals of architecture, with medium age 26 years old. 79.2% replied the affirmatively to the question: "Sometimes during the design, I encountered difficulties in conceiving the space on different architectural scales (1:2000, 1:500, etc.) and make the transition between them». This result [Table 1] indicates that more than half of the architecture students are in need of new methodological approaches concerning teaching architectural work and representation.

Scientific Anchoring

A thorough research and communication with other disciplines as game design, neuroscience and art, is necessary to be done from the architects in education, so as to place the implication of interaction with different scales during the architectural conception. Our didactic proposal expects to elicit the emerging cultural cognitions, biases and perspective illusions and let the student be expressed during the architectural conception at the same time that our proposal provides students a tailor-made tool of analysis for conception's assistance that allows a rigorous and relativist regard at the architectural proposal.The resultant of the two, perceptual illusion and psychological illusion evokes an illusion of reality in the mind, although subjectively varying for each person in strength and experiences. (Panofsky)ln a sense, perspective transforms psychophysiological space into mathematical space. It negates the differences between front and back, between right and left, between bodies and intervening space ("empty" space), so that the sum of all the parts of space and all its contents are absorbed into a single "quantum continuum. " (Panofsky). We opt to use techniques to facilitate the communication of architects' ideas. L'échelle n'existe pas...il existe des échelles, (Boudon)

Toward Interaction

There is still a huge demand by the students of architecture to integrate in the studies of the university and develop educational methods about approaching architectural interactive design that include the experimentation of creating an ephemeral interactive immersive experience in real-time. (table )We are going to evaluate our experiment by a "mise en scene" of importing simulated behaviors in the virtual environment.

Phenomenological object of Architecture

The object of architecture as 3D model of architecture (physical or digital) is different than the field of tools that architects use traditionally to teach and learn architecture (drawings and maquettes). Philip Boudon and his team differentiated in the past the object of speech and the field of speech [START_REF] Lecourtois | From Architecturology to Architecturological research[END_REF]. In our case the object of that speech is defined as the static 3D object virtual environment able to receive the presence of the user/s as different from the physical in terms of scale, perception and understanding. Understanding the designing transition from one scale to another is made with difficulty by students and professionals in architecture, especially in terms of understanding the sizes of territories and parts of the city. The theoretical model proposed as Applied Architecturology constitutes a solid ground that allows observation and evaluation of the object of architecture independently of the different styles of architecture [START_REF] Lecourtois | From Architecturology to Architecturological research[END_REF]. The scale of global coexists in permanence with the scale of local, in different levels of perception. Although cognitively this is possible, the representation of a map in 1:1 scale is considered impossible (...). Applying the concept of scale is needed for the cognitive process of a "reduction of" or "augment to" the perception of space.

Phenomenological object of Perception

Urban territories as parts of the city are constantly exposed, represented and transformed by international digital media, video games; thus, these urban territories are often interpreted by means of communication.

Social networks accompanied with the daily use of the mobile phone and perpetual connection on the Internet, creates the sense of direct access to information. This asset of "accessibility" is an asset that increases city's performance. The cognitive interaction between the user and the application, similarly the interaction between observer and environment leads to cognitive maps (Lynch).

The immediate access to city maps has influenced the perception of urban cartography, giving the notion of distance a new virtual "materiality". The virtual interactive environment is built to explore a virtual identity, behavior or flow in a cognitive map.

"Architecture is considered as a cognitive activity of thought of future space" [START_REF] Lecourtois | From Architecturology to Architecturological research[END_REF] and with the digital revolution of the last decade, the perception of future space is changing according to added cultural phenomena and forming reality assets for further exploration.

Phenomenology and Game Design

Lived space than logical space -teletopyGame design and the industry of video games (Sim City, GTA, Minecraft) augment the sense of presence in virtual environments. Games propose realities and world building and give dimensions to them. There is no single one virtual reality (Steur,1993) as there is no "one" reality. "Architecture is considered as space of thought rather than the space itself" [START_REF] Lecourtois | From Architecturology to Architecturological research[END_REF]; computer games can be considered places of action (interaction) rather than places themselves. The field of game design permits interactions into the 3d virtual environment and thus allows the emergence of new aspects of educational approaches for architects, since [...] in experimental forms of urban planning it is hard to oversee all the actual situations that may occur in advance". [START_REF] Tan | Play the City[END_REF]) -(Mark Schmeink, Real World play the City Game examples)The large-scale methodological approach of traditional teaching has little integrated methods and notions that include interactive digital design. Does a "gamification" of architectural conception reveal content about cognitive operations? How do young architects conceive and build reality?Several displays and tools can assist during the architectural education, a know-how accessible to the students permits the user to observe and evaluate their models.

Hypothesis

By applying the concept of scale, we use visual interactive representations, to evaluate by means of immersion, the efficiency of urban design proposals and to verify the implementation of complexes' buildings into the urban tissue. Transforming the virtual environment into a rapidly visitable and habitable place, would be a way to augment learning capacity and self-evaluation for students aposteriori of the activity of conception.The methodology of integration is based on principles from game design field and architectural education.

METHODOLOGY

The traditional educational method of conception of the architectural work involves the creation of small 2-4 student groups working in collaboration.

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In the form of atelier, students exercise architectural design, while teachers, in an open space, discuss about the urban design of the proposed theme, in our case to implement complexes of housing buildings into the Athens city's tissue. As an educational method remains an open process, since students of the group can even participate during the correction of other groups. Dialogue and participation are the key foundation of the teaching method, as it includes presence and interactivity of both teachers and students.The experiment was conducted for this research during the teaching work for the course Architectural Design 9: Urban Design under the subject "Social Housing" at NTUA, with the help of the students during 9th fall semester in architecture, 2018-2019. The hypothesis is tested, throughout the prototyping of an experimental application for Android devices, in particular tablets. We launch a demoapplication to be able visualize in immersive mode the dimensions of the city, to give measures to values and do observations in a dynamic virtual environment.

Case study in architecture

The exercise of the semester consists in the creation of 150-200 houses, for 600-700 persons by designing and integrating new neighborhoods within the urban fabric. In an area of 14,000 square meters, in Philadelphia, Attica, the student groups analyze the urban fabric and propose an intervention strategy based on land use, heights of surrounding buildings, roads' wideness and the natural and artificial landmarks. The study starts with scales of 1:2000 and 1:1000 with either drawings or physical models. The theme of social housing is a complex architectural issue. It transits through many different architectural scales. The scales in which the exercise is analyzed concerned the investigation at the urban level (1:2000 and 1:1000), the volumetric integration of the building in the urban fabric in 1:500, the architectural and landscape design in 1:200 and the scale of the residence to 1:100, overall the exercise runs through design reading and understanding to 4-5 scales or more (spectrum of scales from 1:2000 -1:100). The theme given by the professors, simulated in the virtual environment can be easily explorable, a 1.4-hectare land (or 14.000 m2), surface big enough to be considered as a surface of "playing the city" and create interactions. It is here that Applied Architecturologic Scales for example geometric scale, scale of the model, scale of visibility, optical scale, scale of the city can refer to the "mise en scene" of cognitive activity operations that we implement into an interactive game engine.The theme "social housing" impose the interrogation and application of several scales. Every scale has its level of abstraction as well its level of detailing. Moreover, it is collaborative work, the goal is to reconstruct the tissue of the city by the implementation of houses, respecting several cultural and functional rules. It can be described as a cognitive activity of high-level complexity.The three factors of collaboration, participation and immersion during the conception leads to an 3d object implementation in Unity 3d. These three factors are significant during playing and learning cognitive procedures. As trivia anecdotical information, I refer to the three students that participated to our demo, during the exercise of conceiving urban space, when admitted that they were playing Monopoly during conceiving their proposal.The first implementation contains all the cognitive operations to be evaluated.

Framework description

The framework of the experiment is to build an educational digital example based on 3d environment of the students and to implement to it a meaningful interaction. For this process, we the software Unity 3D as game engine for the development of our demo. The scientific team of Paris 8 University developed the open source plug-in Behavior Interactive Interface (BII) for virtual actors. The Behavior Interactive Interface (BII) build by Unity 3D, is an open source software that is developed inside the framework of the European Project Virtual Artistic Laboratory: @postasis with Paris 8 University, (2018-2020).

For our demo we use the beta version of this interface.This interface is made with a procedural logic approach of designing and setting models in action. The ideation of this plug-in was made by two architects and one artist inside the framework of the European project @postasis and the development of the beta version by architect Calin Segal.

Characteristics of the Interface

The user can change different views, Top, Axonometric, First Person Controller and Observer. Then, the user of the interface BII can set direction (random, forward, on map) to the actors (3d objects), as well as functions (spawn, destroy, follow, increase, etc.) by controlling conditions of proximity, collision and counter time.

Define scale for experiment

Favorable scale for urban implementation 1:500 concerning the level of abstraction. This is the level of abstraction for the buildings' implementation for the exercise. Though the level of abstraction about the ground floor (landscape) is 1:200.At the 1:500 scale stage, students evaluate their measure and integrate 150-200 residences and public uses in the study plot. Guided by the movement of pedestrian-car, private, public and intermediate space, the students place their buildings in the study plot. Emphasis is placed on the accesses of the movements, the difference between the axis and the canvas, the scale of the road and the relationship between the road width and the building, the natural lighting of the apartments, the good orientation, the possibility of volatility of the construction, also the materials that the students choose to propose and use. Determine an objective scale of presence. Here the scale takes temporal notion, that needs dimensions of presence to exist. Time and distance perception changes into the application of virtual environment.

Preparation of implementation interface. The three-dimensional model of the background is given by the teaching team in Sketch Up File and Rhino and set the virtual environment. Unity 3D software, plugin BII Implement low polygon environment * Define the total volume of the space as a Navigation Mesh Define your lightening conditions Test: merge scales as cognitive operation for understanding dimensions, and their correspondence to real dimensions.

Define Navigation Mesh

The second part of integrated methodology refers to the definition of the terrain for "gameplay", in informatic language the Navigation Mesh, as a rigid surface where actors can navigate and interact. Our surface to explore must be one single mesh, as one 3d object. We will use the "bake" button into BII to make the demos surface our navigation mesh. In our case, this mesh refers to the 14 he real territory of Attica concerning the architectural exercise of implementation. Tools of delocalization can help this set up background environment for the demo.The navigation mesh provides presence measurement techniques as perception of reality is given to students through mediated by both automatic and control processes. (Pillai). The Navigation Mesh represents the terrain of interaction and it is a part of the 3d virtual environment where virtual actors can interact. It is where we take measures of virtual presence, interaction and time passed inside.

Set Metrics

Questions arise regarding perception of time and distance into the virtual environment.Set a point of reference so as to measure distance in the virtual environment.We are working in an abstract way to representations that correspond the scale 1:500, thought the experience provide a first-person navigation as well, an axonometric and a top view. True reality exists independently of perception (Pilai).

Set Conditions to virtual actors. The Navigation

Mesh is the terrain for setting specific conditions to virtual objects (relations and interactions between objects). These variable conditions can be controlled based on proximity, collision, and counter. (distance, limit, time) of 3d objects.The user defines a constant movement to actors (directional, random, on path, targeted object, avoid, collide, etc.) (common conditions)Finally, by creating actions (functions) by setting movement and conditions. Define interactions and relations by proposed actions. Functions: Spawn, Increase Scale, Follow, Destroy, increase speed, Decrease speed, etc.) Figure 7 Set Metrics as a tool of reference.

Demo and virtual environment proposed

Pertinence: persistent of the cubic geometrical layout , rigiditity, resistance at deformation Dimension Maintain a cubic-metric mesure

Observations and feedback

Reference Economise space, create centralityFirst Person Perspective Compressed perspective Exploration in 1:10 scale Manipulation Graphic design

The 3d model or the virtual environment endures the transformation of perspective. From a first Draft -eCAADe RIS 2019 | 7 point of view, the creation of an application that concerns a specific view by giving size to this territory.

Demo and Spawn actors' function.

This demo proposed can be seen as a architecturologic model, not a model in design neither a model for designing. It is posed as a knowledge model on activity of design [START_REF] Lecourtois | From Architecturology to Architecturological research[END_REF], as tool for evaluation.This demo creates a reality of place to instant visualization with the notion of "inhabiting the unbuilt and the far". The simulation of vivid representations of reality; as actors, is facilitating the apprehension on a cognitive level. The function (or concept) of applying scale is scientific and it is related to the perception of scale we have for the reality and the world as known terrain. "All of these concepts help to describe the operating mechanisms of architectural conception in general, that is to say detached from reality. ", (Lecourtois, 2011)Spawn Function, Counter Time passed 12, distance 10We experiment with an interface so as to introduce dynamic objects in the static architectural virtual environments of the students and make observations about the movement and the "mise en scene" of the building. Interaction with dynamic objects that can simulate actors into the virtual environment as it permits the researchers and students to observe actual situations that may occur at the urban planning. This process of combination to pass from the virtualized to the virtual space, can be a highly immersive cognitive process as the student of architecture is becoming part creation of a reality. The association of architecture with game design principles could facilitate the comprehension of no-tions related to scale. By using the interface to generate virtual actors in our virtual environment augments the cognitive immersive capacity for the student as visualizing in real-time several urban realities. We set behaviors to objects of the 3d virtual environment and we observe the relations developed in the virtual environment.The application, as a tool of method, refers to the possibility of the digital tools of virtual/augmented/mixt reality, in terms of presence into the environment, to help cognitive operations about architectural conception and better understand the concept of scale in the urban environment.

WHY AN INTEGRATED EDUCATIVE PACK-AGE

A game between architecturological scales and models, as a process of cognitive activity of design [START_REF] Lecourtois | From Architecturology to Architecturological research[END_REF] in the virtual environment Insprired by theurapeutic games since games are one of the most ancient forms of designed human interactivity, from a "design" point of view. (example: Block Hood)The user to instantly recognize the navigational pattern (Larsen, 2006) and explore the environment fluidly. The urban environment must be created in a way that the user (player/student of architecture) is working with it and it does not become an obstacle for further exploration of the virtual environment, based on the criterions we set in the pattern design session. In addition, the technologies of virtual reality's display allow cognitive biases and illusions of evoked reality means (Pillai) to be implemented with the daily use of mobile phones, accessible to everyone. The understanding and the perception of virtual interaction by using virtual simultaneous presence as a cognitive operation, augments and merges with reality and stimulates imagination of the architecture's students (Totten). When the designers of games try to prototype a space, or levels or scales of the space, the strive to prototype it in an interactive form (Tottem, 2014) and this is an experiment we try with students of the Departement of Architecture of NTUA.

Gamification of architecture

The gamification of architecture can be beneficial for architects' skills. To play with models and different scales, by applying sciences of artificial is a way of questioning architecture [START_REF] Lecourtois | From Architecturology to Architecturological research[END_REF]Playing as architectural conception is an individual conceived cognitive activity [START_REF] Lecourtois | From Architecturology to Architecturological research[END_REF] and game design notions can offer this conception the new extended means to "qualify" reality. [START_REF] Lecourtois | From Architecturology to Architecturological research[END_REF]. The collective practice of architecture is translated in the multiuser real-time experience of the demo. This can become a systemic model of knowledge of conception in future version.Finally, the experiment includes the use of an 3D dynamic system interface for behaviors to help understand the concept of scale.

Didactic method. for architectural training based on game design principles about interaction and architectural scale analysis that can take form in virtual and augmented reality applications/builds for tablets.

Design patterns. in architectural conception provide no verbal indications in the buildings, no sound or wind.The analysis of the notion of the pattern deign (???) helps understanding the notion of scale, mapping examples of actors' behavior and the dynamic interface. In pattern design we can discover the hierarchy of a project. It reveals the complexity of prototyping and the key for this form of analysis is to set two criterions (Rodwin,1981). -Identify form qualities significant to the scale of the territory of implementation. These qualities can be controlled by scale. We visualize the urban characteristics of the selected area for implementation and set functions to objects as behaviors with control conditions of distance, collision and time counter.-The interactions of behavior must concern the physical form of the environment or the distribution of activities or functions as two separate things. By influencing the object behavior by size, hierarchy, functions, speed, the patterns of influence become strategic linkages and interactions within and between objects that interact. The aim of the pattern design pattern is to generate events as manifested forms of interaction.

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Results

Concept of merging realities to understand the notion of scale. The application consists an implication in practices of virtual reality, the theory of Fundamental Architecturology. Architectural process of conception as to qualification of reality [START_REF] Lecourtois | From Architecturology to Architecturological research[END_REF]Systemic model of knowledge and experimentation of parameters of conception through scale-Knowledge on Architecture to Architecture, pattern, self-evaluation

Problems and Future work

Not very intuitive-Not enough open data for the city of Athens. As a matter of fact, the digital representation of a single site, it is a manual texturing and 3D implementation of the setup environment.-Students could have reasons in advanced 3D modelling skills that will facilitate later the implementation of their models in virtual immersive applications-The construction of the virtual environment for a vr application, is not the same as the principles of construction of a 3D modeling, are not familiar with the constructions of virtual reality (from computer games, art or commercial vr applications)

CONCLUSION

Transmission of meaning through symbolic forms of buildings in gamespaces that can augment behavior (Totten, 2014). This is a formal interaction, signifies that form is communicated with the user by means of symbolic or metaphorical form. If we apply the Fundamentals of Architecturology [START_REF] Lecourtois | From Architecturology to Architecturological research[END_REF] in video games, precisely the concept of scale, it could be called formal symbolic interactive scale, proposing the criterions of interaction and presence as criterions for architectural research.By Applied Architecturology we put in evidence students' intentions for their conception of the space, by questioning the design in terms of the concept of scale. As well, visually represent cognitive decisions and evaluate them into immersive environment. By defining a scale or a combination of scales, our proposal integrates the possibility of designing (conceiving) the architectural environment as an interactive one. The environment is able to receive visualized interactions or behaviors that also reveal information about architectural dimensioning.Implementing actors / claiming the virtual space as your own A methodology toward building a dynamic system for learning Unity / possibility of application with many relevant technologies Offer possibilities of easy observe notions of complexity and randomness into the environmentHive -BII interface opensource to settle manipulation to dynamic objects. By setting behaviors to actors, the user can easily populate any virtual environment and create interactions among the actors. Animate the virtual environment, by giving fabrication of the living can in the first place increase the immersive capacity into a virtual environment (case studies my researches in the domain of video games) The design of the interface simulates the reality in question. Didactic proposal as post educational material, after the finish or the semester so as not to charge students with extra demands. Increase empathetic experience to the students (Alan Craig, 2009) Finally, the dialogue remains open about how digital applications, video games, 3D cinema are cultural phenomenon of communication that manifest strong visual representations, in the level that architects can search for new evoked methods of learning architecture and building realities.

Discussion. In questions of perception, phenomenology of architecture, architect Steven Holl describes the task of architecture as the concrete example that enable us a continuum of culture. The task of architecture is to shelter us, to become our home and domicile that provides us an identity and this identity is embodied in architecture as an existential metaphor, as body and being (Holl,2006). The phenomenological approach to modern digital tools by architects, arise the question of concepts of place and emplacement in relation to the phenomenon of globalization (Champion).

The profession of the architect has undergone a fundamental mutation in the last twenty years which has been based much on the widely used media in the world of work as a computer game. The advantages that CAO offers to architects are important in terms of speed of design production, schematics, synthesis images and even videos. Despite the increase in architectural designs and drawings, architect Rem Koolhaas, when interviewed, commented on a difficulty in architects communicating their ideas and nuanced the weakness of architects to interpret each cultural phenomenon that is based on its own system of values. That means that the perception of concepts as scale, dimension, are not always facilitated to be explained to students simply by exhaustive production of drawings. As a learning tool, immersive visualization of conception, can help the student to measure 'quality' understanding of those concepts by in a relation to dimensions. Creating model of mechanism of cognitive operation for integrating VR into architectural basic studies.Merging methodologies toward an autotelic educational experience of evaluation of design.

The urban virtual environment escapes its static nature, by dematerialization, as well as its locational coordinates and geographical constraints in terms of representational reconstitution. In the same time the First-Person Perspective view immerse into a virtual ephemeral identity. Several philosophers consider the place more as a spontaneous event rather than a static and neutral scenery. (Champion, Phenomenology of virtual and real Places). The conception of space consists a dynamic process of cognitive activity for designers with studies in architecture that can be observed and evaluated by in virtual reality environments.

  Figure 2 Educational integration. Layering Logic for learning process.

Figure 3

 3 Figure 3 Case study. Site of implementation.

Figure 4

 4 Figure 4 Case of study, Student's proposal model.

  Figure 5 Merged Scales detailing representation. Buildings and ground detailing.

  Figure 6 Define Navigation Mesh.

  Figure 8 Behavior Interactive Interface (BII). Set movement and create Spawn function.

  Figure 9 Spawn Function with Conditions related to time (counter time passed : 12 and distance : 10).

  Figure 10 Movement of virtual actors.

Table 1 Multistable

 1

	image.
	Figure 1
	Circular diagram

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| eCAADe RIS 2019 -Draft

ACKNOWLEDGMENTS

Matthaios Papavasileiou, Tina Mikrou, Manthos Santorineos, Elhem Younes, Calin Segal, Christina Zavlanou and for the students that accepted to participate Paulina Saade, Giannis and George.


Adrien Gruel

K Ambrožič

C Destouches

Vladimir Radulović

Alix Sardet

L Snoj

Gamma-Heating and Gamma Flux Measurements in the JSI TRIGA Reactor: Results and Prospects

Keywords: CaF2, gamma flux, ionization chamber, LiF, TRIGA, TLD K. Ambrožič, V

I. INTRODUCTION

HE ability of a research reactor to perform precise irradiations (for electronic components, biological samples, sensors,…) lies with its neutron and gamma fields characterization. In steady state operating mode, the neutron field of various irradiation positions of the TRIGA Mark II reactor of the Jožef Stefan Institute (JSI) has already been measured by means of neutron activation dosimetry and miniature fission chambers techniques [START_REF] Snoj | Experimental benchmarks at the JSI TRIGA Mark II reactor for validation of computer codes and nuclear data[END_REF]- [START_REF] Kaiba | Calculations to Support On-line Neutron Spectrum Adjustment by Measurements with Miniature Fission Chambers in the JSI TRIGA Reactor[END_REF].

In a reactor core, gamma rays can be emitted promptly after a reaction (from fission or capture for instance), or after a Submitted May, 14 th , 2019 A. Gruel, C. Destouches and A. Sardet are with the CEA, DEN, Cadarache, DER/SPESI, F-13108 Saint Paul lez Durance, France (e-mail: adrien.gruel@cea.fr). certain time, which can range from seconds to years, or more (decay of activation and fission products). From the radiation safety point of view, as well as for irradiation purposes, it is mandatory to characterize these contributions, especially shortly after reactor shutdown. Thus, a comprehensive gamma characterization campaign has been carried out, within the framework of the 10-year JSI-CEA collaboration agreement on nuclear instrumentation [START_REF]Review of 10 years of JSI -CEA collaboration on nuclear instrumentation development[END_REF]. The work presented here deals with the gamma measurements performed in late 2016 and 2017. The delayed gamma contribution importance has already been pointed out [START_REF] Radulović | Measurements of miniature ionization chamber currents in the JSI TRIGA Mark II reactor demonstrate the importance of the delayed contribution to the photon field in nuclear reactors[END_REF] [START_REF] Fourmentel | Delayed Gamma Measurements in Different Nuclear Research Reactors Bringing Out the Importance of Their Contribution in Gamma Flux Calculations[END_REF], and this study will bring new data to characterize this component into the TRIGA reactor. Complementary to his study, the dose rates were assessed with dosimetry techniques.

The JSI TRIGA Mark II reactor is a pool type reactor with a maximum steady state power of 250 kW. The core has 91 available positions in a concentric configuration for U-ZrH fuel elements, irradiation positions and control rod positions (Fig. 1). In between these positions, there are also positions with diameter of 10 mm, 8 mm and 6 mm, arranged along two lines across the reactor core, used for miniature detectors (Fig. 2).

0018-9499 (c) 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TNS.2020.2974968, IEEE Transactions on Nuclear Science #04-1337 2

A brief review of existing techniques for the characterization of gamma doses and flux levels in mixed field environments is presented in the first part. The second part of the paper describes the measurement techniques used, while the results and comparisons of measurements and calculations are discussed in the third part.

II. OVERVIEW OF EXISTING MEASUREMENT TECHNIQUES

Depending on the gamma flux level, as well as on the quantity of interest (flux, nuclear heating, dose), various types of gamma-sensitive (and to some extent, neutron-sensitive) sensors can be used.

In low dose and low dose rate environments, up to a few Gy.h -1 , such as in medical physics, personal and environmental dosimetry, space or in zero power research reactors, thermoluminescent or optically stimulated-luminescent detector (TLD and OSLD, respectively) techniques are widely used [START_REF] Guillou | State of the art on nuclear heating measurement methods and expected improvements in zero power research reactors[END_REF]. Depending on the detector material, as well as on its doping elements, they can have a linear response versus the integrated dose from 10 -7 Gy up to 10 Gy. Moreover, they appear not to be sensitive to the dose rate. Except for 6 Li-doped fluoride materials, they are less sensitive to neutrons than to gammas, and are therefore of great interest for the gamma dose characterization in mixed n- fields. For instance, in the center of a thermal zero-power reactor, the neutron contribution to the total dose in CaF2:Mn TLDs was estimated to be around 10% [START_REF] Amharrak | Analysis and recent advances in gamma heating measurements in MINERVE facility by using TLD and OSLD techniques[END_REF]. The gamma heating can be assessed in different materials with uncertainties better than 5%, providing valuable data for nuclear data improvement or experimental validation of calculation schemes [START_REF] Amharrak | Développement et optimisation de méthodes de mesures d'échauffements nucléaires et de flux gamma dans les réacteurs expérimentaux : identification, maîtrise, traitement et réduction des incertitudes associées[END_REF]- [START_REF] Lemaire | Validation des calculs d'échauffements photoniques en réacteur d'irradiation au moyen du programme experimental AMMON et du dispositive CARMEN[END_REF].

Recent efforts outline the possibility of using a single type of TLD to measure doses ranging from µGy to MGy, thus covering 12 orders of magnitude [START_REF] Obryk | Method of thermoluminescent measurement of radiation doses from micrograys up to a megagray with a single LiF:Mg,Cu,P detector[END_REF], and that it could be used in mixed fields, such as those encountered in research reactors [START_REF] Obryk | High-dose TL dosimetry of reactor neutrons[END_REF]. It has been shown that the luminescence curves of Mg,Cu,P-doped LiF TLD exhibit additional peaks at higher temperatures when subjected to high doses. With an adaptation of the reading protocol, it is then possible to assess very high doses.

Measurements performed with semiconductor radiationsensitive field effect transistors (RadFETs) have also proven useful for measurements of high dose radiation fields, due to their small size, possibility of active or passive measurements and adjustable sensitivity. Measurements were performed in different irradiation positions in conjunction with an absolutely calibrated PTW Farmer® Ionization Chamber Type 30010. Simulations were used to calculate the cross-calibration factors [START_REF] Ambrožič | Application of JSIR2S for dosimetry calibration of Nürfet semiconductor dosimeters at the JSI TRIGA reactor in-between reactor shutdowns[END_REF].

Another type of detector that could provide a broad range of measurements are ionization chambers (ICs). Gamma source dose-rates are usually calibrated against such sensors. However, they could be unsuitable for in-core measurements because of their relatively large size (> 1 cm). Miniature ionization chambers (MICs), such as those manufactured by CEA, could be appropriate. They are currently mainly used for gamma flux characterization, but not for dose or dose rate measurements. Their operational range depends on the size of the detector, since the signal originates from the particle interactions with the MIC structures [START_REF] Fourmentel | Measurement of photon flux with a miniature gas ionization chamber in a Material Testing Reactor[END_REF]. Therefore, the drawback remains the same as with commercial ICs for low dose environments.

Due to the very high neutron and gamma fluxes in Material Testing Reactors (MTRs), nuclear heating will be substantial and will lead to a temperature increase of the samples. In such environments, the nuclear heating is typically of 1 to 20 W.g -1 (1 to 20 kGy.s -1 ). In this case, the most used sensors are gamma thermometers (GTs) or differential calorimeters [START_REF] Carcreff | Last improvements of the CALMOS calorimeter dedicated to thermal neutron flux and nuclear heating measurements inside the OSIRIS reactor[END_REF] [START_REF] Ambrožič | Computational support of the development of nuclear heating calorimeter detector design[END_REF]. They both enable the measurement of nuclear heating, or nuclear absorbed dose rate, in different materials, according to the device design, by means of a thermocouple.

Because of their relatively low output current, self-powered gamma detectors (SPGDs), which work similarly to selfpowered neutron detectors, usually operate at medium or high dose rates, from 10 2 to 10 8 Gy.h -1 [START_REF] Nieuwenhove | Online gamma dose rate measurements by means of a self-powered gamma detector[END_REF]. Recently developed SPGDs were proven to show a linear response versus the dose between 10 and 5×10 3 Gy.h -1 [START_REF] Takeuchi | Development of a self-powered gamma detector[END_REF]. Their geometry can be adapted to suit smaller irradiation positions.

Knowing that one of the major constraints for the use of instrumentation in a reactor environment is accessibility, the detectors must be made small enough to fit in tight spaces, a property which ensures as localized as possible results for characterization or experimental validation purposes. Therefore, TLDs seem to be promising candidates to offer the possibility of using a single type of detector for a broad-range assessment of doses and dose rates in research reactors. It should also be noted that alanine dosimeters, based on electron paramagnetic resonance spectroscopy, seem to be able to cover a range of doses from 1 to 1.5×10 5 Gy, and to be dose-rate insensitive [START_REF] Schmitz | The alanine detector in BNCT dosimetry: Dose response in thermal and epithermal neutron fields[END_REF]. They also could be a good candidate for incore and environmental dose measurements.

The following part describes the sensors that were used during the measurement campaign in the present work, as well as their main characteristics.

III. DESCRIPTION OF THE EXPERIMENTAL SETUP

A. Ionization and fission chambers

We used two types of ionization chambers. The first one, developed by CEA, is a 3-mm diameter and 55-mm long MIC, sealed at the end of a mineral cable. The second one is a commercial PTW Farmer® Ionization Chamber Type 30010, labelled PTW IC (30-mm long sensitive length, 16-mm diameter) absolutely calibrated to dose in water. By modelling the calibration procedure [START_REF]in collaboration with World Health Organization: Calibration of Reference Dosimeters for External Beam Radiotherapy[END_REF], air kerma and absorbed dose calibration factors were obtained to 1% precision. These detectors are devoted to gamma radiation detection.

To monitor the irradiations we used a Miniature Fission Chamber (MFC), of same geometry as the MIC, but with a fissile coating of a few micrograms of 235 U on the anode, to have a very high sensitivity to thermal neutrons.

CEA-made MIC and MFC measurements were carried out using Keithley 6517B picoammeters, with a sampling time of 1 second.

B. Optically stimulated luminescent detectors and thermoluminescent detectors 1) Characteristics

To assess gamma doses ranging from a few Gy up to kGy we used TLDs. These detectors were inserted in 2-mm thick aluminium cylinders to ensure the charged particle equilibrium. Two types of TLD were used: TLD400 (CaF2:Mn) and TLD700 ( 7 LiF:Mg,Ti) obtained from ThermoFisher Scientific. The dose reader used was a Harshaw model 3500, which can heat a single TLD up to 400 °C. The readout process for the TLD400 is the following: pre-heating at 150 °C for 5 s, then heating up to 350 °C at a rate of 10 °C.s -1 (total readout time: 30 s). For TLD700, the read out consists of pre-heating at 140 °C for 10 s, then heating up to 280 °C at a rate of 15 °C.s -1 (total readout time: 23 s) [START_REF] Amharrak | Développement et optimisation de méthodes de mesures d'échauffements nucléaires et de flux gamma dans les réacteurs expérimentaux : identification, maîtrise, traitement et réduction des incertitudes associées[END_REF]. Prior to each irradiation, the residual dose in the detectors is annealed (1 h at 400 °C + 2 h at 100 °C).

In order to avoid the saturation of the photomultiplier tube while reading dosimeters with integrated doses higher than a few Gy, and also to avoid any electronic settings modification of the readout setup, we used neutral optical filters. The efficiency of the filter, 10%, 1% and 0.1%, corresponds to the fraction of the transmitted radiation. The filter efficiency according to the wavelength was assessed by transmittance measurements at JSI, between 190 and 1100 nm. Results are shown on Fig. 3. Transmittance exhibit a clear dependency to incident wavelength. The efficiencies stated by the manufacturer seem to be given for a reference value around 500 nm. To take into account potential wavelength shifts between measurements (for instance to compare measurements of a given detector irradiated at different doses, or to compare the results of different detectors irradiated the same dose) the calibration step was performed with each filter.

For the background doses in the different positions, nanoDots OSLDs from Landauer were also used. They are made of carbon-doped aluminium oxide discs (5-mm diameter, 0.15-mm height) encapsulated in polyester (total height of 0.35 mm). These discs are stored in rigid plastic carriers of dimensions 10×10×2 mm, so as to protect them. During the experiments, the OSLD discs were extracted from the plastic carriers in order to be irradiated inside aluminium pillboxes, along with the TLDs, to ensure charged particle equilibrium. The integrated dose is read thanks to light stimulation of the detector, with a Landauer microStar reader [START_REF] Landauer | InLight complete dosimetry system solution: nanoDot dosimeter[END_REF]. The read out process is non-destructive, so the integrated dose can be read several times, since the light stimulation keeps more than 99% of the information in the detector. On the other side, the detector cannot be annealed afterwards, the dose is then cumulated if the same detector is used several times. Since the detector sensitivity is strongly affected beyond a cumulated dose of 10 to 20 Gy, they are only used for the small dose range.

2) Calibration

The relation between the charge integrated in the detector (in nC) and the dose (in Gy) is determined by calibration measurements in a reference gamma field.

For doses below 2 Gy (measurements without filters), the calibration was carried out at the radiation protection division of CEA Cadarache. The aluminium pillboxes containing the detectors were placed in front of a 60 Co source, at a distance of 80 cm. The KERMA (Kinetic Energy Released per unit MAss) in air rate was around 500 mGy.h -1 , and measurements were carried out for various doses between 100 mGy to 2 Gy by adjusting the exposure time. Thus, irradiations were performed with a constant dose rate, except for the source radioactive decay. The distance is adjusted with a remote-controlled sliding table, and assumed to be perfectly reproducible. The irradiation time is programmed to the second. The uncertainty on the dose rate, calibrated against a standard ionization chamber, is 0.8%. The main uncertainty is on the TLD response reproducibility. It was experimentally assessed for TLD400 to be of 3.3% [START_REF] Amharrak | Développement et optimisation de méthodes de mesures d'échauffements nucléaires et de flux gamma dans les réacteurs expérimentaux : identification, maîtrise, traitement et réduction des incertitudes associées[END_REF]. As it was not measured for TLD700, it is assumed to be equal. However, it has been shown that at doses beyond a few grays, a permanent sensitivity loss can be observed for this type of TLD, even with optimized annealing process [START_REF] Bilski | Sensitivity loss and recovery for individual TL peaks in LiF:Mg,Ti and LiF:Mg,Cu,P after high-dose irradiation[END_REF]. Within this dose range, TLD responses appear to be proportional to the dose. The final uncertainty on the calibration coefficient, determined individually for each TLD, is around 1.9%.

The high dose calibration, carried out with filters, was performed at CEA Saclay on the PAGURE and POSEIDON 60 Co irradiators, where the dose rate can reach several kGy.h -1 . In order to be able to carry out one irradiation, the readout, and the annealing process in a single day, the dose rate was adjusted for each irradiation in order to keep exposure time around one hour. It is assumed that the TLD responses are not sensitive to the dose rate. For each one of the three filters, 3 to 4 irradiations were carried out (see Tab. I).

In the readout process, TLDs are heated in a nitrogen atmosphere. Despite this nitrogen flow in the reader, some parasitic peaks due to residual moisture in the reader may appear, which can significantly contribute to the charge read for the lower dose readings with filters. To overcome this effect, a Hampel filter [START_REF] Pearson | Generalized Hampel Filters[END_REF] is applied on the raw glow-curve before the integrated charge calculation (see Fig. 4). Afterwards, the background integrated signal coming from the photomultiplier tube (estimated every five readouts) is also subtracted from the TLD signals. It should be noted that the largest background contribution to the total signal is only of 1% for the TLD700 measurements at 1.5 Gy with the 0.1% filter. It has also been checked that non-irradiated detectors gave negligible signals compared to those measured.

The following uncertainties are taken into account into the calibration estimation:

-Dose rate uncertainty: it is provided by the calibrated IC used to determine the source dose rate at the measurement location, before irradiation. Depending on the distance (therefore on the dose rate), it amounts to 0.50% (below 50 Gy.h -1 ) or 0.65% (above 50 Gy.h -1 ). -The positioning uncertainty is evaluated by propagating an estimated 3-cm distance difference between the calibrated IC and the TLDs. It affects the dose rate by 0.05%. -Irradiation duration: in PAGURE and POSEIDON, the 60 Co sources are extracted from their shielding (respectively, storage pool) by mean of an elevator from below the irradiation room. As the counting time starts when the sources are fully extracted, there is an additional time to be added to the programmed one, linked to the sources movement. The equivalent additional time was experimentally assessed to be around 30 seconds in PAGURE, and below 10 seconds in POSEIDON. It is considered in the total integrated dose calculation. Considering rather high uncertainties on these transient times, it results in an additional uncertainty on the total integrated dose of 0.1%. -The last uncertainty is on the TLD response reproducibility (see above). Filtering and background subtraction operations are assumed not to contribute to the overall uncertainty.

To take into account the behaviour of the TLDs responses for the considered dose ranges, experimental data are fitted with a second order polynomial:

𝑄 = 𝑎𝐷 + 𝑏𝐷 ( 1
)
where 𝐷 is the integrated dose (Gy) and 𝑄 the charge read (nC), after filtering and background subtraction. Final calibration curves are shown on Fig. 5.

An orthogonal distance regression of the data is performed with the Python package scipy.ODR [START_REF] Boggs | Statistical analysis of measurement error models and applications[END_REF][27], in order to take into account both uncertainties on the integrated dose (dose rate, position, time) and the read charge (reproducibility). The dispersion of the linear coefficient 𝑏 for TLD400 is around 5%, and around 13% for TLD700. The dispersion of the quadratic coefficient 𝑎 for TLD400 is around 18% and around 15% for TLD700.

Indeed, the limitation of this calibration is the rather low number of measurements in a very large dose range. Therefore, local changes of the calibration factors can remain hidden. However, it has the advantage of enabling the measurement of doses on a very broad range and with a single detector with the same experimental procedure.

IV. RESULTS

Several steady state power levels were investigated with reactor power ranging from 50 W to 250 kW. After reactor shutdown by rapid control rod insertion (SCRAM) the delayed contribution was also measured with MICs, MFC and TLDs. The methodology to distinguish the delayed and prompt gamma component from MIC and MFC measurements is described in [START_REF] Ambrožič | Delayed gamma determination at the JSI TRIGA reactor by synchronous measurements with fission and ionization chambers[END_REF]. To assess the delayed contribution with TLDs, they were inserted at different times after a SCRAM, in the central channel.

The MFC was inserted in the MP17 position, the reference gamma detector PTW IC was into the F25 position, into an irradiation channel. To assess the gamma flux dependence against the distance to the core centre, the MIC was moved in measuring positions (Fig. 2). We measured the dose with TLDs in the central channel A1, the peripheral position F26 and in the IC40 position of the carousel, inside the graphite reflector. TLDs were introduced in aluminium containers (the same as during the calibration), and packed into a polyethylene cylinder (Fig. 6). They were inserted the bottom of the irradiation channels, while the reactor power was stable.

A. Prompt gamma flux characterization

Regarding the gamma flux dependence on the position inside the core, we observed a clear decrease of the delayed contribution, linear with the distance to the core center (Fig. 7).

carried out so far showed that the delayed contribution, estimated after 10 minutes of irradiation, obeys the following equation:

Delayed 𝛾 fraction = -6.25 × 10 . 𝑟 + 0.314 (2) 𝑟 being the distance (in centimeters) from the core center. This seems to originate from the environment of the measurement, since central measuring positions are surrounded by more fuel elements than the peripheral ones. The delayed contribution ranges roughly from 20 % (periphery) up to 30 % (inner of the total gamma flux after 10 minutes of reactor operation. The thorough description and complete results of the delayed gamma contribution analysis can be found in [START_REF] Ambrožič | Delayed gamma determination at the JSI TRIGA reactor by synchronous measurements with fission and ionization chambers[END_REF].

B. Gamma dose measurements

For gamma dose assessment in the different positions, a background measurement (3.5 min irradiation) and four irradiations at different power levels lasting different times were carried out:

-20 minutes at 0.2 kW -10 minutes at 4 kW -5 minutes at 80 kW -2 minutes at 200 kW The integrated doses during the last two irradiations should be equal, provided that the TLDs are not dose-rate sensitive.

For TLD700, the glow-curves after irradiations at higher power levels than 4 kW show a large shape difference with regard to the ones at lower power (Fig. 8). A second peak clearly appears, that was previously small compared to the main one. For low power irradiations, this second peak is assumed to be linked to the neutron contribution to the integrated dose. Indeed, the heights ratio can be used for neutron calibration of LiF TLDs [START_REF] Gambarini | Determination of gamma dose and thermal neutron fluence in BNCT beams from the TLD-700 glow curve shape[END_REF] [START_REF] Guillou | The CANDELLE Experiment for Characterization of Neutron Sensitivity of LiF TLDs[END_REF]. Here, this peak could also be linked to the glow-curve shape modification due to the high integrated dose. In this case, the heating law for the readout is inadequate to accurately assess these doses. The TLD400 glow-curve shape does not seem to change significantly according to the integrated power (Fig. 9). However, experimental results show the same saturation phenomenon for the last two irradiations. This saturation issue could be either intrinsic to the detector (sensitivity modification), or caused by an inappropriate readout protocol, since it was not modified for this dose range. Further studies should be carried out at higher dose range in the calibration geometry to explain this behavior.

Two corrections are applied to the raw measured doses: the cavity correction factor and the energy and anisotropy factor. The first one corresponds to the fact that the calibrating reference quantity (gamma KERMA in air) does not correspond to the real energy deposited inside the detector. This factor is calculated using a fine Monte Carlo modeling of the calibration setup, taking into account the detectors and their surroundings. In the geometry used here, this factor is 1.040 ± 0.2% [START_REF] Amharrak | Development and Optimization of Nuclear Heating Measurement Techniques in Zero Power Experimental Reactors[END_REF]. The second correction factor is meant to take into consideration the difference between the gamma calibration flux (monodirectional and with two energy peaks at 1.173 MeV and 1.332 MeV) and the reactor flux (isotropic with a continuous distribution from keV to several MeV). The value of this is taken from [START_REF] Amharrak | Development and Optimization of Nuclear Heating Measurement Techniques in Zero Power Experimental Reactors[END_REF], since our configuration is very close. This correction factor is worth 1.040 ± 0.1%. Those calculations were made with the Monte Carlo code MCNP [START_REF]X-5 Monte Carlo Team[END_REF].

The measured total dose 𝑄 is the sum of gamma and neutron contributions: 𝑄 = 𝜂 𝐷 + 𝜂 𝐷 (3) where 𝜂 (respectively 𝜂 ) is the gamma (respectively neutron) sensitivity and 𝐷 (respectively 𝐷 ) is the gamma (respectively neutron) dose. Therefore, the gamma dose can be written:

𝐷 = -𝐷 (4)
The second term is the neutron contribution, divided into two components, the first one for the thermal sensitivity, the second one for the epithermal sensitivity [9][11]. Assuming a cut-off energy 𝐸 , taken here at 0.625 eV, this neutron component can be written:

𝐷 = 𝑅𝜙 + 𝐷 (5)
with 𝑅 is a response factor, in Gy.cm 2 , 𝜙 is the thermal neutron fluence, 𝜂 𝜂 ⁄ is the relative neutron sensitivity between 𝐸 and 10 MeV, and 𝐷 is the calculated neutron dose between 𝐸 and 10 MeV. The two values 𝑅 and 𝜂 𝜂 ⁄ are taken from the literature, and for TLD400 are 0.45 Gy.(10 12 neutron.cm -2 ) -1 ± 100% [START_REF] Standard | Standard Practice for Application of CaF2(Mn) Thermoluminescence Dosimeters in Mixed Neutron-photon Environments[END_REF] and 0.288 ± 60%

respectively. The calculations of the thermal neutron fluence and of neutrons doses were carried out with MCNP model of the core and of the detectors and surroundings.

The absolute background doses and the doses measured at 0.2 kW and 4 kW by TLD400 and OSLDs are presented in Tab. II. OSLDs were only used for background measurements. The uncertainties (given at 1𝜎) correspond to the maximum between the standard uncertainty propagation and the discrepancy between detectors. For the background measurements in F26 and IC40, there is a good agreement between both types of sensors.

The total prompt gamma air KERMA has been calculated in various core positions in [START_REF] Ambrožič | Computational analysis of the dose rates at JSI TRIGA reactor irradiation facilities[END_REF]. By adding to the prompt component a delayed contribution, calculated as:

𝐾 ,tot = Delayed γ fraction 𝐾 ,prompt (6)
one can estimate the total gamma air KERMA rate in the dosimeter positions, per unit power. The delayed 𝛾 fraction is evaluated from Eq. ( 2). Results are presented in Tab. III. Measurement uncertainties are defined here as the quadratic sum of the standard deviation between the averaged dose rates of the two irradiations (around 10%) and of the uncertainty on the neutron dose evaluation (from Eq. ( 5)). The uncertainties be quite large, especially for the calculation in the core periphery, but there is a very good agreement between calculations and measurements. In the A1 position, the neutron contribution is 5% ± 55%, it is 6% ± 68% in the F26 position, and 8% ± 93% in the IC40 position.

C. Delayed contribution

The delayed gamma contribution was assessed after a 4 kW irradiation. TLDs were inserted at different times after SCRAM and spent different amount of time in the core (Tab. IV).

On the top plot of Fig. 10, one can see that the difference between MFC and MIC signal after SCRAM is related to the delayed neutrons. The decaying behavior of the TLD400 dose rate agrees with the behavior of the MIC. The bottom plot of Fig. 10 displays plots of relative MIC and TLD signals. The relative MIC signal is obtained by dividing the MIC signal with the constant value just before the reactor SCRAM. The TLD relative signal is obtained by normalizing the TLD values to the MIC signal averaged on the TLD time of integration, either on the first TLD measurement, from 2 to 122 seconds (red squares) or the second one, from 5.5 to 10.5 minutes (black triangles):

𝐷 ̇norm, = ̇ ̇ or ∫ 𝐼(𝑡)d𝑡 (7)
where 𝐷 ̇ is the TLD dose rate measurement 𝑖, 𝐼(𝑡) the MIC signal, 𝑇 the time of integration of measurement 𝑖. The relative signals of the MIC current and dose rates determined with TLD after a reactor SCRAM match one another very well, within 10% in both cases, except for the last point. The latter indeed shows a very high uncertainty linked to the discrepancy between TLDs. In addition, the MIC signal appears to be very noisy for currents below 1 nA, leading to a more difficult analysis. The slight difference between the two normalizations is attributable to the partial sensitivity of the TLDs to the neutron field, more specifically delayed neutrons, still present shortly after a reactor SCRAM. For the first TLD measurement after the SCRAM, the average neutron contribution is 0.1 Gy.min -1 , representing less than 1% of the total signal. However, the difference is of 1.5%, comparable to the measurement uncertainty, making it difficult to analyze. Dose rate calculations, including the electron and gamma sources due to decay of radioactive products were calculated using the JSIR2S code package [START_REF] Ambrožič | JSIR2S Code for delayed radiation field calculations: validation against measurements at the JSI TRIGA reactor[END_REF]. The reactor was simulated in 109 steps, following the measured reactor power to a logarithmically interpolated precision of 1% at 4 kW and at 10 W. The measured and modeled reactor power, as well as FC and IC signals in their respective positions as measured during and after 4 kW reactor power excursion, are displayed in Fig. 11. The absorbed dose calculations were performed in 10 steps during each TLD irradiation (Fig. 12). The transport part was performed using the ENDF/B-VII.1 nuclear data library [START_REF] Chadwick | ENDF/B-VII.1 Nuclear Data for Science and Technology: Cross Sections, Covariances, Fission Product Yields and Decay Data[END_REF], while the isotopic inventory calculations were performed using the JEFF 3.3 nuclear data library [START_REF]The Joint Evaluated Fission and Fusion File (JEFF), version 3.3[END_REF]. The comparison between calculated and measured doses during experiments is presented in Tab. V. The last experimental measurement (from 32 minutes after the SCRAM during 137 minutes) was not calculated. Uncertainties on the calculated values are between 3 to 4.5 % for each step. The experimental values and the calculated ones are in very good agreement. The dose rate calculations after shut down show a very good agreement with the measured current in the MIC (Fig. 12).

D. Note on temperature

It was supposed that the saturation of the charge read after irradiation could come from self-annealing of the detector during irradiation, caused by its temperature increase. Recently, temperature measurements were carried out by attaching thermocouples onto bare 7 LiF:Mg,Cu,P (MCP-7) and nat (MCP-N) TLDs, that show a clear increase during power excursion [START_REF] Ambrožič | Delayed gamma determination at the JSI TRIGA reactor by synchronous measurements with fission and ionization chambers[END_REF]. For MCP-7 TLDs, the temperature seems to stabilize around 35 °C after a few minutes 100 kW, and reached 70 °C at 250 kW (the pool temperature was stable around 25 °C). For MCP-N, the temperature reached 70 °C at 100 kW, and more than 120 °C at 250 kW, due to the larger amount of neutron interactions caused by the presence of 6 Li. Temperatures have not been measured for TLD700 nor TLD400, but they can be assumed to be similar to the ones of MCP-7 (rather low neutron sensitivity). Therefore, it seems unlikely that measurements carried out at 80 kW and 200 kW for 5 minutes and 2 minutes respectively, could have been affected by self-annealing. This study aimed at comparing measurements and calculations of gamma flux and dose rates in different irradiation positions of the JSI TRIGA reactor.

It is shown that TLD400 can be used for dose measurements with consistent results in the Gy to kGy range, without modifying the readout procedure. No recent studies were found to deal with high-dose measurements with TLD400 (CaF2:Mn) dosimeters. Irradiations carried out in front of a 60 Co source, up to 10 3 Gy indicate that they do not present any linearity change [START_REF] Burgkhardt | High-dose characteristics of CaF2 and CaSO4 thermoluminescent dosimeters[END_REF]. More recently, a 2008 study [START_REF] Teixeira | Performance of thermoluminescent materials for high dose dosimetry[END_REF] shows that they present a linear behavior from 0.1 Gy to 100 Gy. The work presented here seems to prove that this range could be extended. Nevertheless, the calibration process has to be adapted in order to reach a better accuracy on a very large dose range. Further studies should be conducted to see up to what dose the TLD400 detectors can be used with this readout protocol, or if the latter should be modified to assess higher doses. Nevertheless, for both prompt and delayed gamma dose assessment, a very good agreement is observed between measurements and calculations.

The TLD700 results were not presented since their readout and annealing protocol was not suited for such high integrated dose levels. Even background measurements with and without filters were not consistent. Further studies on the TLD700 readout protocol are to be performed, based on works on the adaptation of reading procedures for high dose measurement with LiF:Mg,Cu,P detectors [START_REF] Bilski | Characteristics of LiF:Mg,Cu,P thermoluminescence at ultra-high dose range[END_REF]- [START_REF] Obryk | High-Dose TL Dosimetry of Reactor Neutrons[END_REF].

This study emphasizes the complementarity between on-line and off-line techniques, and the interest of using TLDs for incore dose monitoring, thanks to their measurement range spanning over several orders of magnitude.

Fig. 1 .

 1 Fig. 1. Core configuration during the measurement campaign.

Fig. 2 .

 2 Fig. 2. Measurement positions inside the TRIGA core.

Fig. 3 .

 3 Fig. 3. Filter transmittance versus wavelength.

  This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TNS.2020.2974968, IEEE Transactions on Nuclear Science #04-1337 4

Fig. 4 .

 4 Fig. 4. Example of raw data filtering and correction on a TLD700 glow-curve, after a 15 Gy irradiation, read with the 0.1% filter.

Fig. 5 .

 5 Fig. 5. Calibration curves for the different filters for all detectors. Top: 10% filter, middle: 1% filter, bottom: 0.1% filter. On each figure, the lower set of curves correspond to the TLD700.

Fig. 6 .

 6 Fig. 6. TLDs experimental setup for in-core measurements. Each aluminum container encloses one TLD400 and one TLD700.

Fig. 7 .

 7 Fig. 7. Delayed gamma contribution measured with the MIC at 5 kW with regard to the position [28].

Fig. 8 .Fig. 9 .

 89 Fig. 8. Normalized TLD700 glow-curves after different irradiations. Total time of the readout: 23 s, time temperature profile in red.

  (c) 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TNS.2020.2974968, IEEE Transactions on Nuclear Science #04-1337 7

Fig. 10 .

 10 Fig. 10. Gamma and neutron signals decay after SCRAM. Top: MIC and MFC currents (left axis) compared to dose rates in the TLD400 (right axis). Bottom: comparison of the normalized MIC current and dose rates. Horizontal bars correspond to the time of dose integration for TLDs.

Fig. 11 .Fig. 12 .

 1112 Fig. 11. Measured and modeled reactor power, along with FC and IC measured signals during and after 4 kW power excursion.

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	#04-1337			9
	V. CONCLUSION AND OUTLOOKS
		TABLE V
	COMPARISON BETWEEN CALCULATED AND MEASURED DOSE AFTER THE 4 KW
		POWER EXCURSION (GY.MIN -1 )
	Integration time	Measurement	Calculation	Discrepancy
	[2-122] s.	17.6 ± 0.3	17.9 ± 0.2	(1.5 ± 1.7)%
	[5.5 -10.5] min.	8.2 ± 0.4	8.1 ± 0.1	(-0.3 ± 5.4)%
	[16 -26] min.	5.2 ± 0.4	5.3 ± 0.1	(-3.0 ± 8.0)%

0018-9499 (c) 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Authorized licensed use limited to: CEA. Downloaded on April 09,2020 at 08:16:22 UTC from IEEE Xplore. Restrictions apply.

ACKNOWLEDGMENT

The authors wish to thank the operator team of the TRIGA reactor, as well as the SPR/LRIR EDEPE group for the calibration phase at CEA Cadarache.

The present work was financially supported by the Research Agency of the Republic of Slovenia (ARRS) and the French Atomic and Alternative Energy Commission (CEA), under the reference n° BI-FR/CEA/16-18-001.


Henri Gouin
email: henri.gouin@univ-cezanne.fr

Witold Kosin Śki

BOUNDARY CONDITIONS FOR A CAPILLARY FLUID IN CONTACT WITH A WALL

Contact of a fluid with a solid or an elastic wall is investigated. The wall exerts molecular forces on the fluid which is locally strongly nonhomogeneous.

The problem is approached with a fluid energy of the second gradient form and a wall surface energy depending on the value of the fluid density at the contact. From the virtual work principle are obtained limit conditions taking into account the fluid density, its normal derivative to the wall and the curvature of the surface.

Introduction

The phenomenon of surface wetting is a subject of many experiments [START_REF] Van Giessen | Contact angles of liquid drops on low-energy solid surfaces[END_REF].

Such experiments have been used to determine many important properties of the wetting behavior for liquid on low energy surface [START_REF] Cahn | Critical point wetting[END_REF]. In fact the wetting transition of fluids in contact with solid surfaces is an important field of research both for mechanics and physical chemistry. In the recent paper [START_REF] Gouin | Energy of interaction between solid surfaces and liquids[END_REF], the 1 first author using statistical methods proposed an explicit form for the energy of interaction between solid surfaces and liquids. This energy yields a bridge connecting statistical mechanics and continuum mechanics. To obtain boundary conditions between fluid and solid it is also necessary to know the behaviour of the fluid as well as the solid.

We propose a mechanical model similar to that used in the mean-field theory of capillarity that leads to the second gradient theory of continuous media in fluid mechanics [START_REF] Germain | La méthode des puissances virtuelles en mécanique des milieux continus[END_REF]. The theory is conceptually more straightforward than the Laplace one to build a model of capillarity [START_REF]La théorie du second gradient et la capillarité[END_REF][START_REF] Casal | Sur les interfaces liquide-vapeur non isothermes[END_REF]. That theory takes into account systems in which fluid interfaces are present [START_REF] Gouin | Utilization of the second gradient theory in continuum mechanics to study the motion and thermodynamics of liquid-vapor interfaces[END_REF]. The internal capillarity is one of the simplest cases since we are able to calculate the superficial tension in the case of thin interfaces as well as in thick ones [START_REF] Dell'isola | Deduction of thermodynamic balance law for bidimensional nonmaterial directed continua modelling interface layers[END_REF]. It is possible to obtain the nucleation of drops and bubbles [START_REF] Dell'isola | Nucleation of spherical shell-like interfaces by second gradient theory: numerical simulations[END_REF].

It seems that the approximation of the mean-field theory is too simple to be quantitatively accurate. However, it does provide a qualitative understanding.

Moreover, the point of view, that the fluid in interfacial region may be treated as a bulk phase with a local free energy density and an additional contribution arising from the nonuniformity which may be approximated by a gradient expansion truncated at the second order, is most likely to be successful and perhaps even quantitatively accurate near the critical point [START_REF] Rowlinson | Molecular theory of Capillarity[END_REF].

In this paper we connect both the interaction of a solid surface and a fluid phase by means of the virtual work principle. The distribution of fluid energy in the volume and the surface density energy on the solid surface yield boundary conditions. The conditions are different from those obtained for a classical fluid within the theory of gas dynamics. We obtain an embedding effect for the density of the fluid; moreover, the conditions take into account the curvature of the surface. The result is extended to the case of an elastic wall.

A discussion is obtained depending on the value of the density of the fluid at If f (x) is a scalar function of the vector x associated with the Euler variables in the physical space, ∂f ∂x is the linear form associated with the gradient of f and

∂f ∂x i = ( ∂f ∂x ) i . Consequently, ( ∂f ∂x ) * = grad f 2.

Continuous mechanical model of capillary layers

We consider a fluid in contact with a solid. The fluid occupies the domain D and its boundary Σ which is common with the solid wall. Physical experiments prove that the fluid is nonhomogeneous in the neighbourhood of Σ [START_REF] Rowlinson | Molecular theory of Capillarity[END_REF]. It is also possible to consider the fluid as a continuous medium by taking into account a capillary layer existing in the vicinity of Σ and a form of its stress tensor [START_REF] Seppecher | Equilibrium of a Cahn-Hilliard fluid on a wall: influence of the wetting properties of the film upon the stability of a thin film[END_REF]. One way to present the behaviour of such a fluid is to consider the specific internal energy ε as a function of the density ρ as well as grad ρ. Such an expression is known in continuum mechanics as internal capillary energy, see [START_REF] Germain | La méthode des puissances virtuelles en mécanique des milieux continus[END_REF][START_REF]La théorie du second gradient et la capillarité[END_REF]. It is related to molecular models of strongly non homogeneous fluids in the frame of the mean field theory and is equivalent to the van der Waals model of capillarity (see the review by Rowlinson and Widom [START_REF] Rowlinson | Molecular theory of Capillarity[END_REF]). The energy ε is also a function of the specific entropy. In the case of isothermal media at a given temperature, the specific internal energy is replaced by the specific free energy.

In the mechanical case the entropy or the temperature are not concerned by the virtual variations of the medium. Consequently, for an isotropic fluid, it is assumed that

ε = f (ρ, β)
where β = ( --→ grad ρ) 2 = gradρ . gradρ (the dot . denotes the scalar product).

The fluid is submitted to external forces represented by a force potential Ω per unit mass as a function of Euler variables x.

We denote by x ∈ Σ -→ B(x) ∈ R the surface density of energy of the solid wall. The total energy E of the fluid in D and its boundary Σ is the sum of the three potential energies:

E = E f + E p + E S with, E f = D ρ ε(ρ, β) dv , E p = D ρ Ω(x) dv , E S = Σ B ds
Let us denote by δ a variation of the position of the fluid as in [START_REF] Serrin | Mathematical principles of classical fluid mechanics[END_REF]. The variation is associated with the virtual displacement

x ∈ D → δx = ζ(x)
We have the following results presented in Appendix, The scalar R m is the mean curvature of Σ and grad tg is the tangential part of grad relatively to Σ.

δE f = D (-div σ) . ζ dv + Σ -A dζ n dn + 2A R m n + grad tg A + σn . ζ ds (1)
Moreover,

δE p = D ρ ∂Ω ∂x ζ dv = D ρ grad Ω . ζ dv (2)
and using the results presented in Appendix,

δE S = Σ δB - 2B R m n + grad tg B . ζ ds (3)
One assumes that the volume mass in the fluid has a limit value ρ s at the wall Σ. One assumes also that B is a function of ρ s only. These hypotheses are confirmed by results presented in [START_REF] Gouin | Energy of interaction between solid surfaces and liquids[END_REF]. Then

δB = B ′ (ρ s )δρ s = -ρ s B ′ (ρ s ) div ζ Let us denote G = -ρ s B ′ ρs . Consequently, Σ δB ds = Σ G div ζ ds = Σ ( G dζ n dn - 2G R m n . ζ -grad tg G . ζ ) ds (see Appendix). Now, H = B(ρ s ) -ρ s B ′ ρs (ρ s )
is the Legendre transformation of B with respect to ρ s . Then,

δE S = Σ G dζ n dn -( 2H n R m + grad tg H ) . ζ ds (4)
The d'Alembert-Lagrange principle of virtuals works is expressed in the form [START_REF] Serrin | Mathematical principles of classical fluid mechanics[END_REF] :

∀ x ∈ D → ζ(x), δE = 0 (5)
Consequently, from the fundamental lemma of variation calculus, we obtain the balance equation in the fluid D and the boundary conditions on the solid wall Σ.

Equilibrium equations :

From any arbitrary variation

x ∈ D → ζ(x) such that ζ = 0 on Σ, we take first D ρ ∂Ω ∂x -div σ ζ dv = 0 Consequently, -div σ + ρ ∂Ω ∂x = 0 ( 6
)
This equation is the well known equilibrium equation [START_REF]La théorie du second gradient et la capillarité[END_REF][START_REF] Gouin | Utilization of the second gradient theory in continuum mechanics to study the motion and thermodynamics of liquid-vapor interfaces[END_REF][START_REF] Dell'isola | Nucleation of spherical shell-like interfaces by second gradient theory: numerical simulations[END_REF] Boundary conditions : a) Case of a rigid (undeformed) wall.

We consider a rigid wall. Consequently, the virtual displacements satisfy on Σ the condition n * ζ = 0 . Then, at the rigid wall

Σ (G -A) dζ n dn + 2(A -H) R m n + grad tg (A -H) + σn . ζ dσ = 0
Hence, we deduce the boundary conditions at the rigid wall

For x ∈ Σ, G -A = 0 (7)
and moreover, there exists a Lagrange multiplier

x ∈ Σ → λ(x) ∈ R such that 2(A -H) R m n + grad tg (A -H) + σ n = λ n (8)
b) Case of a elastic (non-rigid) solid wall.

In such a case the equilibrium equation( 6) is unchanged. On Σ the condition [START_REF] Gouin | Utilization of the second gradient theory in continuum mechanics to study the motion and thermodynamics of liquid-vapor interfaces[END_REF] is also unchanged. The only different condition comes from the fact that we do not have anymore the slipping condition for the virtual displacement (n * ζ = 0).

Due to the possible deformation of the wall, the virtual work of mechanical stresses on Σ is

δE e = Σ T * ζ ds
with T = Q n representing the stress (loading) vector, where Q is the value of the Cauchy stress tensor of the wall on the boundary Σ.

Relation ( 8) is remplaced by :

2 (A -H) R m n + grad tg (A -H) + σ n = -T (9)

Analysis of the boundary conditions

Relation (7) yields :

C dρ dn + B ′ ρ = 0 ( 10
)
and we obtain,

H -A = B.
Consequently, from the definition of σ,

σ n = P n -C dρ dn gradρ.
The tangential part of equation ( 8) is always verified and equation ( 8) yields the value of the Lagrange multiplier λ.

For an elastic (non-rigid) solid wall we obtain

T * tg = 0 and T n = 2B R m + P -B ′ ρ dρ dn (11)
where T * tg and T n are respectively the tangential and the normal components of T. Taking into account equation [START_REF] Rowlinson | Molecular theory of Capillarity[END_REF], 2 and equations [START_REF] Seppecher | Equilibrium of a Cahn-Hilliard fluid on a wall: influence of the wetting properties of the film upon the stability of a thin film[END_REF] yield the value of the stresses in the elastic (non rigid) medium.

T n = P + 2B R m + 1 C (B ′ ρ )
The only new condition comes from equation [START_REF] Rowlinson | Molecular theory of Capillarity[END_REF].

We have the consequences:

In [START_REF] Gouin | Energy of interaction between solid surfaces and liquids[END_REF] we propose the surface energy in the form B(ρ) = -γ 1 ρ + γ 2 2 ρ 2 with γ 1 , and γ 2 as two positive constants. We obtain the condition for the fluid density on the wall

C dρ dn = γ 1 -γ 2 ρ (12)
Denoting again ρ s the value of ρ on the wall Σ (the limit value of the volumic mass of the fluid on the wall Σ ), we obtain that dρ dn is positive (or negative) in the vicinity of the wall if

ρ s < ρ i (or ρ s > ρ i ) with ρ i = γ 1 γ 2
which is the bifurcation fluid density at the wall.

excess of fluid density at the wall.

Conclusion

For conservative medium, the first gradient theory corresponds to the case of compressibility. To take into account superficial effects acting between solids and fluids, we propose to use the model of fluids endowed with capillarity. The theory interprets the capillarity in a continuous way and contains Laplace's theory. The model corresponds for solids to elastic materials with couple stresses indicated by Toupin in [START_REF] Toupin | Elastic materials with couple stresses[END_REF].

We notice that the extension to the dynamic case is straightforward: by virtual work principle, equation ( 6) takes the form:

ργ * -div σ + ρ ∂Ω ∂x = 0,
where γ denotes the acceleration of the fluid. Equations ( 10), ( 11), [START_REF] Serrin | Mathematical principles of classical fluid mechanics[END_REF] and consequences in paragraph 3 are unchanged.

First of all we recall the following fact issued from differential geometry:

Let Σ be a surface in the 3-dimensional space and n its external normal. For any vector field ζ,

n * rot(n × ζ) = div ζ + 2 R m n * ζ -n * ∂ζ ∂x n.
Then, for any scalar field A, we obtain:

A div ζ = A dζ n dn - 2A R m ζ n -(grad * tg A) ζ + n * rot (An × ζ) = tr ∂A ∂x (nn * -1) - 2A R m n * ζ + A dζ n dn + n * rot(An × ζ) (A.1)
Let us calculate δE f : since D is a material volume,

E f = D ρ ε dv ⇒ δE f = D ρ
′ β = C.
In the mean-field molecular theory, the quantity C is assumed constant [START_REF] Rowlinson | Molecular theory of Capillarity[END_REF], but it is not necessary for our calculations. One can suppose the scalar C is a general function of ρ and even β. Then

ρ ε ′ β δβ = div(C grad ρ δρ) -div(C grad ρ) δρ -tr C gradρ grad * ρ ∂ζ ∂x
Due to the fact that δρ = -ρ div ζ (see [START_REF] Serrin | Mathematical principles of classical fluid mechanics[END_REF]),

ρ δε = div(C grad ρ δρ) -ρ 2 ε ′ ρ -ρ div(C grad ρ ) div ζ -div(C

  asterisk "*" to denote conjugate (or transpose) mappings or covectors (line vectors). For any vectors a, b we shall use the notation a * b for their scalar product (the line vector is multiplied by the column vector) and ab * or a ⊗ b for their tensor product (the column vector is multiplied by the line vector). The product of a mapping A by a vector a is denoted by A a. Notation b * A means covector c * defined by the rule c * = (A * b ) * . The divergence of a linear transformation A is the covector divA such that, for any constant vector a, div(A) a = div (A a ).

  with σ = -P I -C gradρ ⊗ gradρ = -P I -C( ∂ρ ∂x ) * ∂ρ ∂x where C = 2ρε ′ β and P = ρ 2 ε ′ ρ -ρ div(C grad ρ), ε ′ ρ denotes the partial derivative of ε with respect to ρ, ζ n = n * ζ where n is the external unit normal to Σ and A = Cρ dρ

DdivΣ

  grad ρ grad * ρ ζ) + div(C grad ρ grad * ρ) ζ ρ δε = div C grad ρ δρ -(C grad ρ grad * ρ)ζ -P ζ + ∂P ∂x ζ + div(C grad ρ grad * ρ) ζThenδE f = D ∂P ∂x + div(C grad ρ grad * ρ) ζ dv -Cρ grad ρ div ζ + C grad ρ grad * ρ ζ + P ζ dv = D -(div σ) ζ dv + Σ (-A div ζ + n * σ ζ) dsTaking into account (A.1), we deduce immediatelyδE f = D -(div σ) ζ dv + n * + grad * tg A + n * σ) ζ ds + Σ n * rot(A n × ζ)ds But Σ n * rot(A n × ζ) ds = Γ At . (n × ζ) dℓ = Γ A(t, n, ζ) dℓwhere Γ is the line boundary of Σ and t its tangent unit vector. If n ′ = t × n we obtain the relationδE f = D (-div σ) ζ dv + grad * tg A + n * σ) ζ ds + Γ A n ′ * ζ dℓ (A.2)In the following we assume that Σ has no boundary and consequently, the term associated with Γ vanishes.Let us calculate δES δBn * 2B R m + grad * B (1 -nn * ) ζ ds + Γ A n ′ * ζ dℓ (A.3)We notice that grad * B(1 -nn * ) belong to the tangent plane to Σ.Let us proof equation (A.3): If we write E S = Σ B det (n, d 1 x, d 2 x) , where d 1 x and d 2 x are the coordinate lines of Σ, we may write, E S = Σ0 B det F det (F -1 n, d 1 X, d 2 X)

Acknowledgment

The authors are grateful to the Polonium Program of Co-operation N 0 7075 between Polish KBN and French Foreign Office for the financial support of this research.

where Σ 0 is the image of Σ in a reference space in Lagrangian coordinates X and F the deformation gradient tensor ∂x ∂X .

Then, We assume that Σ has no boundary and consequently, the term associated with Γ is null.