Rozenn Colleter
email: rozenn.colleter@inrap.fr

Benoît Clavel

Anita Pietrzak

Sylvie Duchesne

Ludovic Schmitt

Michael P Richards

Norbert Telmon

Éric Crubézy

Klervia Jaouen
email: klervia_jaouen@eva.mpg.de

Social status in late medieval and early modern Brittany: insights from stable isotope analysis

Keywords: Post medieval, Modern period, Brittany, Diet, Sulfur isotopes, Mobility, Convent

We document for the first time the diet of a privileged French population from Brittany, a region that was the center of battles between the Kingdoms of England and France until the end of the fifteenth century. We present here the results of stable isotope analyses of carbon, nitrogen, and sulfur of human and animal bone and tooth collagen for a late medieval to early modern Breton population. The isotopic values observed for the Dominican convent of Rennes, Brittany, are very similar to those reported for medieval archaeological populations in Great Britain, namely they have enriched δ 15 N values combined with almost entirely terrestrial carbon signals. We discuss the consumption of young animals in a diet made up of terrestrial, marine, and freshwater resources. We report dietary differences between socioeconomic groups and gender, with women and nobles (male and female) showing patterns consistent with high animal product consumption and lower mobility. The S isotope ratios of both humans and fauna are very homogeneous and generally have coastal δ 34 S values. The convent is known to have been an interregional pilgrimage site during the early modern period, but the isotope values indicate that the identified migrants were not pilgrims. Stable isotope analysis therefore complements the available historical information on human diets and mobility.

Introduction

Stable isotope studies of late medieval and early modern Western European populations Isotopic analyses are widely used to unravel dietary differences between social status in late medieval and early modern times in several European regions [START_REF] Polet | Reconstruction of the diet in a mediaeval monastic community from the coast of Belgium[END_REF]Müldner andRichards 2005, 2007;[START_REF] Fornaciari | Food and disease at the Renaissance courts of Naples and Florence: a paleonutritional study[END_REF][START_REF] Kjellström | Dietary patterns and social structures in medieval Sigtuna, Sweden, as reflected in stable isotope values in human skeletal remains[END_REF][START_REF] Mundee | Exploring diet and society in medieval Spain: new approaches using stable isotope analysis[END_REF][START_REF] Yoder | Let them eat cake? Status-based differences in diet in medieval Denmark[END_REF][START_REF] Lubritto | Radiocarbon chronology and paleodiet studies on the medieval rural site of Zaballa (Spain): preliminary insights into the social archaeology of the site[END_REF][START_REF] Quintelier | Isotopic examination of links between diet, social differentiation, and DISH at the post-medieval Carmelite Friary of Aalst, Belgium[END_REF][START_REF] Lamb | Multi-isotope analysis demonstrates significant lifestyle changes in King Richard III[END_REF][START_REF] Alexander | Diet, society, and economy in late medieval Spain: stable isotope evidence from Muslims and Christians from Gandía, Valencia[END_REF][START_REF] Olsen | Isotopic anthropology of rural German medieval diet: intra-and inter-population variability[END_REF][START_REF] Van Der Sluis | A palaeodietary investigation of a multi-period churchyard in Stavanger, Norway, using stable isotope analysis (C, N, H, S) on bone collagen[END_REF]. In France, the diet of urban poor individuals was documented [START_REF] Bocherens | Application de la biogéochimie isotopique (13C, 15N) à la détermination du régime alimentaire des populations humaines et animales durant les périodes antiques et médiévales[END_REF][START_REF] Herrscher | Comportements alimentaires au Moyen Âge à Grenoble: application de la biogéochimie isotopique à la nécropole Saint-Laurent (XIIIe-XVe siècles, Isère, France)[END_REF][START_REF] Herrscher | Alimentation d'une population historique: Analyse des données isotopiques de la nécropole Saint-Laurent de Grenoble (XIIIe-XVe siècle, France)[END_REF][START_REF] Herrscher | Comportements socioculturels liés à l'allaitement et au sevrage: le cas d'une population grenobloise sous l'Ancien Régime[END_REF], but differences between socio-economic groups have yet to be investigated. In other Western European regions, high δ 15 N values and low δ 13 C values of wealthy populations were generally reported, especially in Great Britain and Scandinavian Electronic supplementary material The online version of this article (https://doi.org/10.1007/s12520-017-0547-9) contains supplementary material, which is available to authorized users.

countries. The high δ 15 N values of European wealthy populations cannot be explained by the sole consumption of herbivore meat. The diet of ecclesiastics and privileged European populations therefore probably included the consumption of food with a higher trophic level such as omnivore meat (pig, fowl) or freshwater fish [START_REF] Müldner | Fast or feast: reconstructing diet in later medieval England by stable isotope analysis[END_REF]. The later hypothesis is generally preferred because Catholic fasting rules forbade the consumption of meat for one third of the year (Müldner andRichards 2005, 2007;[START_REF] Yoder | Let them eat cake? Status-based differences in diet in medieval Denmark[END_REF]). Nevertheless, the diversity of animal protein sources in medieval and modern period human diets (marine, migratory and freshwater fish, herbivore or omnivore meat, eggs, and to a lesser extent, dairy products) complicates the quantification of each food category. S isotope analyses are being utilized more often to investigate this issue, as they can trace the consumption of marine, terrestrial and sometimes freshwater food [START_REF] Nehlich | The application of sulfur isotope analyses in archaeological research: a review[END_REF][START_REF] Sayle | Deciphering diet and monitoring movement: multiple stable isotope analysis of the Viking Age settlement at Hofstaðir, Lake Mývatn, Iceland[END_REF]. However, in coastal regions, terrestrial and aquatic food products have similar S isotope signature [START_REF] Lamb | Multi-isotope analysis of the population of the lost medieval village of Auldhame, East Lothian, Scotland[END_REF][START_REF] Nehlich | The application of sulfur isotope analyses in archaeological research: a review[END_REF]. In this context, S isotopes are usually utilized to identify migrants coming from inland regions [START_REF] Richards | Sulphur isotopes in palaeodietary studies: a review and results from a controlled feeding experiment[END_REF].

Dietary habits in Brittany from the thirteenth to the eighteenth centuries

Between the thirteenth and the eighteenth centuries, Brittany is going through a wealthy period, first as an autonomous Duchy (medieval period) and then as a French Province (early modern period). This golden age of Brittany is however interspersed by military conflicts and epidemics. Little is known on the respective influence of the increased over time wealth and temporary difficulties on Bretons' diets. In medieval and early modern times, dietary habits of Bretons were supposed to be very similar to those of their European contemporaries and were mainly influenced by their socio-economic status. It is known that nobles and well-off individuals included a high proportion of meat into their diet [START_REF] Croix | La Bretagne aux 16e et 17e siècles: la vie, la mort, la foi[END_REF][START_REF] Quellier | La table des français. Une histoire culturelle (XVedébut XIXe siècle[END_REF]. Peasant diets were mostly cereal-based (wheat, rye, barley, buckwheat, and oats; [START_REF] Croix | L'âge d'or de la Bretagne[END_REF] while urban laborers had regular access to meat [START_REF] Croix | La Bretagne aux 16e et 17e siècles: la vie, la mort, la foi[END_REF][START_REF] Quellier | La table des français. Une histoire culturelle (XVedébut XIXe siècle[END_REF]. In wealthy and urban populations, fish usually replaced meat on fasting days [START_REF] Quellier | La table des français. Une histoire culturelle (XVedébut XIXe siècle[END_REF]. In Rennes, the consumption of fish can be considered a social marker: nobles and elites favored a diversity of species, including marine, migratory, and freshwater animals [START_REF] Clavel | Données archéozoologiques et fouilles d'hôpitaux: l'exemple de l'hôpital Sainte Anne[END_REF], whereas urban commoners complained of the monotony of cod consumption [START_REF] Croix | La Bretagne aux 16e et 17e siècles: la vie, la mort, la foi[END_REF].

Aim of the study

The Dominican convent of Rennes offers the unique opportunity to study dietary differences between socio-economic groups which were buried there (parliamentary nobility, ecclesiastics, soldiers, and commoners) in one of the three capital cities of Brittany (Fig. 1; [START_REF] Colleter | Procedures and frequencies of embalming and heart extractions in modern period in Brittany. Contribution to the evolution of ritual funerary in Europe[END_REF][START_REF] Cloirec | L'étude archéologique du couvent des jacobins de Rennes (35), du quartier antique à l'établissement religieux: rapport final d'opération archéologique[END_REF]. Moreover, the presence of a garbage dump contemporaneous and adjacent to the convent allows the direct study of the animals that were eaten by the local population. Using stable isotopes of carbon, nitrogen, and sulfur, we here investigate if the intense fish and meat consumption in urban Breton diets can be documented and if social status differences can be distinguished despite the substantial animal product consumption of the whole population. We present isotope data for human (85 individuals) and animal (34 terrestrial animals and fish) material collected during the rescue excavations at the Dominican convent of Rennes, France (Fig. 1;[START_REF] Figeac | Les noblesses de France[END_REF]-2014[START_REF] Cloirec | L'étude archéologique du couvent des jacobins de Rennes (35), du quartier antique à l'établissement religieux: rapport final d'opération archéologique[END_REF] as well as the sixteenth c. garbage dump of the Sainte-Anne Hospital situated nearby [START_REF] Van Klinken | Bone collagen quality indicators for palaeodietary and radiocarbon measurements[END_REF][START_REF] Clavel | Données archéozoologiques et fouilles d'hôpitaux: l'exemple de l'hôpital Sainte Anne[END_REF]. These data provide further evidence for an overall substantial consumption of animal products in the whole population, however nuanced by social status differences. We identified a mixed subsistence strategy of the privileged individuals, including marine and terrestrial foods, albeit dominated by herbivore and omnivore meat. The latter may be confused isotopically with proteins from freshwater or migratory resources (likely eels). We find that the relative amount of protein in the diet of Breton nobles is similar to those observed for other European courts [START_REF] Fornaciari | Food and disease at the Renaissance courts of Naples and Florence: a paleonutritional study[END_REF][START_REF] Lamb | Multi-isotope analysis demonstrates significant lifestyle changes in King Richard III[END_REF][START_REF] Jiménez-Brobeil | How royals feasted in the court of Pedro I of Castile: a contribution of stable isotope study to medieval history[END_REF]. Taking into account the historical context, we also discuss the meaning of the increase of δ 15 N values of ecclesiastics' teeth and bones between the late medieval and early modern periods and the higher δ 15 N values for females for both tooth and bone collagens. Finally, the documentation of δ 34 S values reveals a local origin of the parliamentary nobles and the women, whereas the soldiers all came from inland regions.

Material Human material

Six hundred and five burials corresponding to the medieval to early modern periods were the subject of a detailed study [START_REF] Cloirec | L'étude archéologique du couvent des jacobins de Rennes (35), du quartier antique à l'établissement religieux: rapport final d'opération archéologique[END_REF]. Three phases of burial are differentiated on the site (Le Cloirec 2016; Fig. 2). The first phase (thirteenth c. phase 1) predates the construction of the convent, and includes 12 subjects (2 children and adolescents and 10 adults including 8 men). The presence of these skeletons in a residential neighborhood out of a cemetery remains unexplained. The second (phase 2) and the third phase (phase 3) are contemporaneous to the convent. The phase 2 goes from the end of the fourteenth century to the sixteenth century with 137 (22 immature individuals and 115 adults), and the phase 3 covers the seventeenth and eighteenth centuries and therefore corresponds to the modern period (73 immature individuals and 383 adults) (Table 1). Between the end of the fourteenth and the eighteenth century, the settlement was an important place of pilgrimage-because of the presence of a Bmiraculous painting^in the convent-and burial, especially for the parliamentary nobility [START_REF] Croix | La Bretagne aux 16e et 17e siècles: la vie, la mort, la foi[END_REF][START_REF] Bordeaux | Moi, Claude Bordeaux… Journal d'un bourgeois de Rennes au 17ème siècle[END_REF][START_REF] Colleter | Procedures and frequencies of embalming and heart extractions in modern period in Brittany. Contribution to the evolution of ritual funerary in Europe[END_REF].

Historical archives and previous archaeological works document the existence of different socio-economic groups buried in the Dominican convent of Rennes (Table S1). For instance, the existence of privileged burials (church choir, adjoining chapels), characterized by ostentatious attributes (marking, situation, etc.), is a good indicator of the establishment of a favored socio-economic group in the Dominican convent [START_REF] Croix | La Bretagne aux 16e et 17e siècles: la vie, la mort, la foi[END_REF]. We therefore attributed a socioeconomic group to the different individuals depending on their burial location. The majority (319/605 subjects, i.e., 53%) correspond to the abovementioned privileged status (A group). Within this group, 26 aristocrats are identified based on specific funeral practices: embalming and/or lead coffins (A′ group; [START_REF] Colleter | Procedures and frequencies of embalming and heart extractions in modern period in Brittany. Contribution to the evolution of ritual funerary in Europe[END_REF]. A second group is identified as less favored as the A group (B group) and includes 188 subjects buried in the nave of the church, in the cloister garden, and in the immediate exteriors of the convent (188/605, i.e., 31% of subjects). The individuals from the nave (B′ group) might have a higher status than the others since they are buried within the convent walls. The west yard also contains an ossuary (group B″). In addition, 66 individuals from the chapter house are likely to be Dominican ecclesiastics (C group) (66/605, i.e., 11%). Finally, 32 subjects with blade injuries (and some lethal) were found in mass graves; all men and death between 15 and 50 years old were probably soldiers (D group) (32/605, i.e., 5%; Table 1). In total, 85 individuals were sampled (81 bones, 54 teeth). The distribution of the individuals sampled by socio-economic group, phase, age of death, and sex are given in the Table 1.

Faunal remains

The faunal remains consist of the most common species found in the garbage dump (teeth and bones of domesticated species and fish bones) from the hospital associated with, and contemporary to, the convent (Fig. 1), and are listed in the Table S2 of the Supplementary Information. The samples mostly come from the garbage dump (sixteenth century, end of phase 2; Fig. 1) but three of the fish bones were from the soil of the convent's refectory (end of the fourteenth, beginning of the phase 2; Fig. 2). Most of the fish data come from individual bones, but they are some exceptions such as the eel Fig. 1 Location of the site. On the left: The historical border of Brittany in the fifteenth century. On the right: Rennes in the sixteenth century. The locations of the convent and the garbage dump are outside of the walls (Supplementary Information). A previous zooarchaeological study of the garbage dump reported that the cattle, pork, and sheep/goat represent 90% of the bone remains collected by hand [START_REF] Clavel | Données archéozoologiques et fouilles d'hôpitaux: l'exemple de l'hôpital Sainte Anne[END_REF]. Cattle is the predominant species and are present in higher proportion than what is usually observed in the North of France [START_REF] Clavel | L'animal dans l'alimentation médiévale et moderne en France du Nord (XIIIe-XVIIe siècles)[END_REF][START_REF] Clavel | L'archéozoologie du Moyen Âge au début de la période moderne dans la moitié nord de la France[END_REF], and 50% of them were slaughtered before the age of 20 months. The majority of the sheep were slaughtered after the age of weaning. Half of the pigs were slaughtered before the age of 13 months. Contrary to the cattle and sheep, pigs were bred in yards within the city [START_REF] Croix | La Bretagne aux 16e et 17e siècles: la vie, la mort, la foi[END_REF]. The hospital accounts mention the presence of a pen with a sow, and her piglets and a boar were fed with food leftovers [START_REF] Clavel | L'animal dans l'alimentation médiévale et moderne en France du Nord (XIIIe-XVIIe siècles)[END_REF]. Domestic fowl and game acquisitions are also reported but to

th th

Fig. 2 Evolution of the convent structure and location of the burials from the thirteenth to the eighteenth centuries a lesser extent. In the garbage dump, the fish bones (from sieving) are abundant and mostly come from the marine environment (more than 40%) and migratory species (50%) [START_REF] Clavel | Données archéozoologiques et fouilles d'hôpitaux: l'exemple de l'hôpital Sainte Anne[END_REF]. The migratory species are dominated by eels, but the allis shad can also be found. Only 10% of the marine fishes belong to the Gadidae, the taxonomic group of the cod. These proportions of migratory, marine, and freshwater fish species nearly correspond to those found in the soil of the convent's refectory (Le Cloirec 2016). The number of taxa, both in the garbage dump and in the refectory's soil, is remarkable and includes what would have been expensive species, which argues for the wealthy status of the hospital patients [START_REF] Clavel | L'animal dans l'alimentation médiévale et moderne en France du Nord (XIIIe-XVIIe siècles)[END_REF], the caregivers, and the Dominicans. This wealth could notably explain why the codfish is underrepresented, whereas historical writings document its substantial consumption among urban workers. Dogs and cats can be found in the garbage dump. Most of the dogs were apparently killed, possibly during culling or because their owners could not keep them within the hospital [START_REF] Clavel | Données archéozoologiques et fouilles d'hôpitaux: l'exemple de l'hôpital Sainte Anne[END_REF].

Methods

Anthropological data and chronology

Sex assessment was performed using the coxal bone using morphoscopic [START_REF] Bruzek | A method for visual determination of sex, using the human hip bone[END_REF]) and morphometric methods [START_REF] Murail | DSP: a tool for probabilistic sex diagnosis using worldwide variability in hip-bone measurements[END_REF]. Results are given for a sex determination probability exceeding 95%, but we also mentioned Bprobable^sex for an assessment with a probability of 80 to 94.9%. When these bones were not preserved, we performed a secondary sex diagnosis using long bone measurements. Age at death was assessed using sacropelvic surface information for adults [START_REF] Schmitt | Une nouvelle méthode pour estimer l'âge au décès des adultes à partir de la surface sacro-pelvienne iliaque[END_REF], stages of dental development for children (Moorrees et al. 1963a, b), as well as bone maturation when dental data were inconclusive [START_REF] Palkama | Estimation of stature from radiographs of long bones in children. II. Children under one year of age[END_REF][START_REF] Virtama | Estimation of stature from radiographs of long bones in children. III. Children aged from ten to fifteen[END_REF][START_REF] Telkka | Estimation of stature from radiographs of long bones in children[END_REF][START_REF] Birkner | L'image radiologique typique du squelette: Aspect normal et variantes chez l'adulte et l'enfant par médecins, étudiants et manipulateurs[END_REF]. We used a combination of different methods (stratigraphic data, artifacts, grave pits, and radiocarbon dating) to establish the chronology of the different burials. Phase 2 (end of the fourteenth-sixteenth c.) A 4 (1) 4 0 7 (2) 1 4 (1) 16 (3) 20 (4) A′ 0 0 0 3( 1 ) 0 1( 1 ) 4( 2 ) 4( 2 ) B 12 (1) 32 (5) 0 15 (2) 0 12 ( 6) 59 ( 13) 71 ( 14) See the text for the description of the socio-economic groupings J juvenile, I indeterminate, M male, PM probable male, F female, P probable female

B′ 1 3 0 3( 1 ) 0 3 9( 1 ) 1 0( 1 ) C 3 (1) 4 (1) 2 (1) 1 0 4 (1) 11 (3) 14 (4) D 3 (

Collagen extraction and stable isotope analyses

The bone and tooth samples were first cleaned by a sandblasting technique. Then, the bone samples were pretreated using the protocol described by [START_REF] Richards | Stable isotope evidence for similarities in the types of marine foods used by Late Mesolithic humans at sites along the Atlantic coast of Europe[END_REF] adding an ultrafiltration step [START_REF] Brown | Improved collagen extraction by modified Longin method[END_REF]. The decision was later taken to radiocarbon date some of the samples. We therefore prepared the following samples with a protocol suitable for radiocarbon dating [START_REF] Talamo | A comparison of bone pretreatment methods for AMS dating of samples >30,000 BP[END_REF]). The protocol used for each sample can be found in the Tables S2 andS3 and does not impact final C and N isotope ratios.

Carbon and nitrogen isotope ratios were determined in duplicates using an elemental analyzer Flash EA 2112 coupled to a Delta XP continuous flow IRMS at the Department of Human Evolution of the Max Planck Institute for Evolutionary Anthropology (Leipzig, Germany). The sulfur isotope ratios of the first bone samples were measured at the Department of Archaeology of the University of British Columbia (UBC) on a EuroVector (HekaTech, Wegberg, Germany) elemental analyzer coupled to a Delta V-plus (Thermo-Finnigan). In addition, some samples were sent to commercial labs for measurement: Isodetect (IE; Germany) for the dentine analyses and Isoanalytical (IA; UK) for the remaining bone collagen samples (Table S3). To verify the collagen quality, the elemental composition and associated atomic ratios were measured and compared to the range of wellpreserved collagen [START_REF] Deniro | Postmortem preservation and alteration of in vivo bone collagen isotope ratios in relation to palaeodietary reconstruction[END_REF][START_REF] Van Klinken | Bone collagen quality indicators for palaeodietary and radiocarbon measurements[END_REF][START_REF] Nehlich | Establishing collagen quality criteria for sulfur isotope analysis of archaeological bone collagen[END_REF].

Results

Sulfur isotope analyses

Sulfur content

We were able to conduct S isotope analysis for 72 of the 81 bones and 34 of the 54 teeth we analyzed for carbon and nitrogen. The sulfur content of all the human bone collagen samples is equivalent to that of modern mammals (Supplementary Tables S3,S4, and S8). However, terrestrial animal bone collagen (9 samples out of 17) and some human tooth collagen samples were poorly preserved (4 out of 34), with 70% of the analyzed animal samples falling out of the acceptable range defined by [START_REF] Nehlich | Establishing collagen quality criteria for sulfur isotope analysis of archaeological bone collagen[END_REF] (Tables S2,S4, and S8). Fish bones and animal teeth were better preserved (Tables S2 andS5). The standards analyzed by the three laboratories (UBC, IA, IE) gave consistent results (Table S6). When possible, the samples were analyzed in duplicate. The standard deviation (SD) of these duplicates is acceptable for the IA and UBC data, with most of SD's being below 0.2‰. However, many IE duplicates had more variation. Since it seemed to us that these data were still meaningful, we chose to include them in our study, and the standard deviation for each sample is given in Fig. 3 and the Supplementary Information.

Definition of the local range and identification of the migrants

The 69 bone and 30 tooth collagen samples for which we obtained reliable δ 34 S values correspond to 75 individuals. The S isotope ratios of terrestrial animal bone and tooth collagen range from 11.4 ± 0.1 to 16.1 ± 0.2‰. The marine fish bones have δ 34 S values extremely close to the terrestrial animals (Figs. 3 and 10; δ 34 S = 12.2-19.1‰). The terrestrial local values are therefore impacted by the sea spray. The eels have a δ 34 S value of 11.3‰. The small amount of collagen extracted from freshwater fish bones did not allow us to measure the associated S isotope ratios. The sulfur isotope results of the terrestrial animals are consistent with them having been bred in the city or in the nearby country [START_REF] Croix | La Bretagne aux 16e et 17e siècles: la vie, la mort, la foi[END_REF], as the S isotope ratios of their collagen represent local values. We define the local animal range based on their average δ 34 S values ± 2SD (10.9 to 17.3‰). We defined a local range for humans, based on the animal values. Human values that fall outside of what we have defined at the local animal range can be then considered as Bnon-locals.^The individuals exhibiting Bnon-local^tooth values then would have spent the end of their childhood or the beginning of their adolescence in another region. If these non-local values are also present in the bone collagen, then the individual did not spend more than a few years in the area before death. The distribution of individuals with values matching the local range or showing non-local values is seen in the Table S7. Among the 24 individuals for whom bone and tooth values have been measured, half of them show an isotope shift exceeding 1‰ (Fig. 3). These isotope shifts can be positive or negative, and a significant part of it is possibly related to the analytical standard deviation, sometimes quite elevated for sulfur isotopes (Supplementary Information). All nobles (female and male) and all women tested (group 2A′ and 3A′) fall into the Blocalĝ roup, and all soldiers (group D) show non-local values (bones or teeth). The bones from the privileged groups also exhibited values comparable with the local range, but one individual had a non-local value, indicating a different childhood residency. Among the 75 individuals for which S isotope ratios have been measured in bone and/or tooth collagen, 19% have non-local values. Most (64%) of the non-local values can be found in the chapter house and the exteriors of the convent, whereas the number of individuals excavated and analyzed in these locations was lower than that of the church (Tables S1 andS3).

Carbon and nitrogen isotope analyses

Results for C and isotope ratios measured in animal and human bones and teeth are given for each individual in the Supplementary Information and are summarized in Table 2. All samples, which did not match quality criterion ensuring good collagen quality (more than 1% collagen yield and C/N ratio between 2.9 and 3.6), were removed from the study (two human bone samples, seven human teeth, and four animal bones). All details on C/N ratios, standard deviations, and collagen yields are given in the Supplementary Information.

Animal bone and tooth samples δ 13 C ratios of the terrestrial animals indicate a diet reliant on C3-plants. δ 15 N ratios for domestic animal bone samples tend to show higher values than what is usually observed in late medieval and early modern Europe fauna (δ 15 N bone = 6.3 to 8.8‰ for sheep and cattle, 9.5 to 13.9‰ for the pigs; Fig. 4), except for the dog and domestic fowl. This is particularly true for five of the six piglets, which cannot be isotopically distinguished from freshwater and migratory fish (Fig. 4). The dog value falls in the herbivore range, whereas the tooth, from another dog, falls within the human range (Fig. 4). The hare and fish isotope values are similar to those previously reported in the literature [START_REF] Bocherens | Application de la biogéochimie isotopique (13C, 15N) à la détermination du régime alimentaire des populations humaines et animales durant les périodes antiques et médiévales[END_REF][START_REF] Müldner | Fast or feast: reconstructing diet in later medieval England by stable isotope analysis[END_REF][START_REF] Olsen | Isotopic anthropology of rural German medieval diet: intra-and inter-population variability[END_REF]. Fish δ 15 N values are consistent with the trophic level of the various species, and the range observed in the Sainte-Anne garbage dump and the Dominican convent overlaps. The collagen of the freshwater fish was not well preserved (less than 1% of collagen for one of them, a C/N ratio of 3.7 for the other; Fig. 5). The rabbit, usually a wild animal in medieval Northern France [START_REF] Quellier | La table des français. Une histoire culturelle (XVedébut XIXe siècle[END_REF]), shows a very high δ 15 N possibly resulting from suckling or environmental-physiological effect [START_REF] Ugan | Variation in collagen stable nitrogen values in black-tailed jackrabbits (Lepus californicus) in relation to smallscale differences in climate, soil, and topography[END_REF]. Young rabbits were a choice dish in medieval and early modern cuisine, and their consumption was allowed during fasting days [START_REF] Adamson | Food in medieval times[END_REF]). This impact of suckling also explains the higher N isotope ratios observed in animal tooth collagen (Fig. 5; Supplementary Information), except for the dog tooth, as its δ 13 C value strongly differs from that of the herbivore and the other dog bones, which could reflect the occasional consumption of marine fish.

Human bone and tooth samples

The 80 bone and 48 tooth collagen samples for which we obtained reliable δ 15 N and δ 13 C values correspond to 82 individuals. The range of δ 13 C and δ 15 N bone and tooth data is similar to previously published data from Scandinavian or British late medieval human populations (e.g., Müldner andRichards 2005, 2007;[START_REF] Kjellström | Dietary patterns and social structures in medieval Sigtuna, Sweden, as reflected in stable isotope values in human skeletal remains[END_REF][START_REF] Lamb | Multi-isotope analysis of the population of the lost medieval village of Auldhame, East Lothian, Scotland[END_REF]; [START_REF] Buonincontri | Multiproxy approach to the study of medieval food habits in Tuscany (central Italy)[END_REF]Spain: Lubritto et al. 2013;[START_REF] Alexander | Diet, society, and economy in late medieval Spain: stable isotope evidence from Muslims and Christians from Gandía, Valencia[END_REF]. The late medieval French wine maker analyzed by [START_REF] Bocherens | Application de la biogéochimie isotopique (13C, 15N) à la détermination du régime alimentaire des populations humaines et animales durant les périodes antiques et médiévales[END_REF] however fits with the range we found in Rennes. δ 13 C values for the samples average -19.4 ± 0.4‰ (1 S.D.) for bones and -19.2 ± 0.4‰ tooth. δ 15 N values are 12.5 ± 1.2‰ (1 S.D.) for bones and 13.0 ± 1.1‰ for teeth. The mean results of the analysis are presented in relation to the socio-economic group in the Table 2.

Isotope differences between socio-economic groups

Kruskal-Wallis tests indicate that δ 15 N bone values, but not δ 15 N tooth values, are strongly related to socio-economic group Pairwise comparisons between groups indicate that significant differences are observed between 3A and 2B (p < 1 × 10 -3 ). The highest N isotope ratios of bones (δ 15 N bone = 15.1‰) and teeth (δ 15 N tooth = 15.9‰) can be found among the five identified nobles (group A′). The average δ 13 C and δ 15 N values of the A′ group are similar to that in royal medieval courts in Europe [START_REF] Fornaciari | Food and disease at the Renaissance courts of Naples and Florence: a paleonutritional study[END_REF][START_REF] Lamb | Multi-isotope analysis demonstrates significant lifestyle changes in King Richard III[END_REF][START_REF] Jiménez-Brobeil | How royals feasted in the court of Pedro I of Castile: a contribution of stable isotope study to medieval history[END_REF]. Of note, the four individuals from the 3C group show higher δ 15 N values in bones and teeth than the three individuals from the phase 2 (2C group), even though this difference does not appear significant because of the sample size (Fig. 6a). δ 13 C bone values are also related to socioeconomic groups and change over time, the privileged individuals from the phase 3 (3A) showing higher values than the individuals from the 1B, 2B, and 2C groups (χ 2 = 16.81, p = 0.01 without subgroups A′, B′, and B″). When the main burial location groups are considered (church, chapter house, and exteriors; Figs. 2 and7), it appears that individuals buried in the exteriors of the convent (groups B, B″, and D) have significantly lower δ 13 C bone and δ 15 N bone values than those buried in the church (groups A, A′, and B′; p = 0.04 for δ 13 C bone and p < 1.10 -3 for δ 15 N bone ) and in the chapter house (group C, p = 0.007 for δ 13 C bone , p = 0.016 for δ 15 N bone ). The patterns observed in bones are similar in teeth-albeit less clear-for δ 15 N teeth values but do not appear for δ 13 C teeth values.

Isotope differences between bones and teeth

C and N isotope ratios of M2 and M3 teeth roots (respectively formed between the ages of 7-16 and 12-25 years old) do not differ. The δ 15 N bone value is generally lower than the δ 15 N tooth value of the same individual (Fig. 6a). This is particularly true for the group 1B, 2B, 3B′, and 2D (Fig. 6b). [START_REF] Bocherens | Application de la biogéochimie isotopique (13C, 15N) à la détermination du régime alimentaire des populations humaines et animales durant les périodes antiques et médiévales[END_REF][START_REF] Polet | Reconstruction of the diet in a mediaeval monastic community from the coast of Belgium[END_REF][START_REF] Müldner | Fast or feast: reconstructing diet in later medieval England by stable isotope analysis[END_REF][START_REF] Herrscher | Alimentation d'une population historique: Analyse des données isotopiques de la nécropole Saint-Laurent de Grenoble (XIIIe-XVe siècle, France)[END_REF][START_REF] Yoder | Let them eat cake? Status-based differences in diet in medieval Denmark[END_REF][START_REF] Lubritto | Radiocarbon chronology and paleodiet studies on the medieval rural site of Zaballa (Spain): preliminary insights into the social archaeology of the site[END_REF][START_REF] Olsen | Isotopic anthropology of rural German medieval diet: intra-and inter-population variability[END_REF][START_REF] Castillo | Social complexity in early medieval rural communities[END_REF][START_REF] Alexander | Diet, society, and economy in late medieval Spain: stable isotope evidence from Muslims and Christians from Gandía, Valencia[END_REF], and square symbols correspond to local values. Error bars are giving the SD for each site t = δ 13 C bone -δ 13 C tooth ; Fig. 6). There are no statistically significant differences identifiable in the mean N isotope ratios across the adult age range, but children and individuals who died before the age of 40 years tend to show lower values than adolescent and individuals who died after the age of 40 years (Supplementary Information, Fig. S1).

Isotope differences between males and females

To compare male and female values, we only considered individuals from socio-economic groups where both men and women were represented. We therefore excluded the groups B, C, and D, that is too say, all the ecclesiastics and soldiers were not considered to avoid any bias related to different male social status. We found that, on average, δ 15 N bone and δ 15 N tooth female values are significantly higher than those of men (t test, p bone = 0.006 and p teeth = 0.007; Fig. 8). Individuals with non-local δ 34 S values tend to show lower δ 15 N values, except for one soldier that also exhibits a high δ 13 C value (Fig. 9). The δ 13 C bone and δ 15 N bone values of local individuals poorly but significantly correlate (r 2 = 0.34, p < 1 × 10 -3 ).

Discussion

A privileged burial place for Bretons

The S isotope ratios of terrestrial animals from the Sainte-Anne garbage dump correspond to coastal values [START_REF] Nehlich | The application of sulfur isotope analyses in archaeological research: a review[END_REF]. This observation can be explained by the fact that Brittany is a peninsula with an oceanic climate (Fig. 1). The absence of correlation between δ 34 S and δ 15 N or δ 13 C values of human bones and teeth indicates that S isotope variability is more related to location than marine (or freshwater) food consumption. In Rennes, being one of the most distant Breton cities from the coast (60 Fig. 1), we hypothesized that the whole region exhibits coastal δ 34 S values. Additionally, most of the population of Brittany was concentrated in the capitals (Vannes, coastal city, Nantes-50 km from the coast-and Rennes; Fig. 1) and within 15 km of the coast during this time period [START_REF] Nières | Les Villes de Bretagne au XVIIIe siècle[END_REF]. Consequently, individuals falling within the local range could originate from other Breton or coastal locations, whereas individuals who do not fit with this range are likely to originate from more inland regions in France or Europe. Interestingly, these non-local individuals are mostly soldiers (group D), people buried before the convent construction (phase 1), and ecclesiastics from the chapter house (group C). Eighty-one percent of the individuals analyzed are likely to come from Brittany, and this proportion goes up to 92% for those buried in the church, its chapels, and the cloister gallery. If the Dominican convent from Rennes was a pilgrimage place, it seems that most of the individuals that requested to be buried close to its miraculous painting lived in Brittany most of their life. Most of the nobles (85%) we studied were buried after the annexation of Brittany to the Kingdom of France. It seems that they spent more time in Brittany-in their fiefdoms or in the capital cities-than at the royal court, since individuals living in the Parisian Basin are expected to have different δ 34 S values. [START_REF] Bocherens | Preservation of bone collagen sulfur isotopic compositions in an early Holocene river-bank archaeological site[END_REF] indeed documented very low S isotope ratios for Mesolithic animals from Noyen-sur-Seine that lived under climatic conditions similar to historical times. This assumption is consistent with historical evidence that documents: (1) Non-local individuals are mostly buried during the first and second phases in mass graves (group 2D, for example). Several deadly conflicts destabilized the city in the end of the Middle Age and the beginning of the modern period, such as the capture of Rennes in 1491 and religious wars in the late sixteenth century. Foreign soldiers fought, either as enemies (e.g., the French Royal Army in 1491) or as allies (German Holy Roman Empire, British, and Spanish armies during the siege of the city and Spanish soldiers during the religion wars) [START_REF] Croix | L'âge d'or de la Bretagne[END_REF]. Some of these foreign soldiers could have possibly been buried in the west yard of the Dominican convent.

The importance of young animal consumption in the aristocratic diet

The C and N isotopic ranges for bones and teeth of the individuals buried in the convent overlap with what has been previously reported in the British Isles [START_REF] Müldner | Fast or feast: reconstructing diet in later medieval England by stable isotope analysis[END_REF] and indicate not only a minor contribution of marine fish into the diet but also an important consumption of migratory fish or omnivore meat. Given the low abundance of freshwater fish remains found in the garbage dump and the refectory soil, the isotope values are not likely to be explained by the consumption of these fishes. It seems that marine contribution in diets was more important during childhood given that the tooth values generally exhibited higher δ 15 N and lower δ 13 C values which correlate (Fig. 6b). We noticed in Rennes higher herbivore δ 15 N values than what is usually reported in Western Europe (Fig. 4). Interestingly, young cattle do not show higher δ 15 N values than one of the adult cows or the sheep, which were mostly killed after weaning age [START_REF] Clavel | Données archéozoologiques et fouilles d'hôpitaux: l'exemple de l'hôpital Sainte Anne[END_REF]. We can therefore speculate that the high δ 15 N values of these animals could be due to the consumption of manured plants [START_REF] Van Klinken | An overview of causes for stable isotopic variations in past European human populations: environmental, ecophysiological, and cultural effects[END_REF][START_REF] Fraser | Manuring and stable nitrogen isotope ratios in cereals and pulses: towards a new archaeobotanical approach to the inference of land use and dietary practices[END_REF][START_REF] Castillo | Social complexity in early medieval rural communities[END_REF] and would partially explain the high δ 15 N values of the humans. With one exception, the piglets exhibit extremely high δ 15 N values, falling into the range of freshwater fish (Fig. 5). Their δ 34 S values also fit that of the other terrestrial animals (Fig. 9) but so do the eels-albeit the δ 34 S eel value is much lower than the average local values (Figs. 3 and 10). One should expect similar or lower δ 34 S for other freshwater fish [START_REF] Nehlich | The application of sulfur isotope analyses in archaeological research: a review[END_REF]. The elevated δ 15 N and low δ 13 C values of the humans can be explained by nonweaned animal or migratory fish consumption.

Diet and social status

Insofar as the dining table is a place where social superiority is expressed [START_REF] Figeac | Les noblesses de France[END_REF], it is expected to observe more animal protein consumption for privileged individuals. The isotope compositions however do not permit the assessment of the quality of the food eaten by the different social categories. They cannot confirm whether the privileged groups had access to the choice cuts of the animals also eaten by the other social groups. The privileged group seems to have a similar diet between the late medieval (2A) and early modern periods (3A). The individuals buried in the nave (3B′) do not significantly differ from the other individuals buried in the church, although the δ 15 N bone values of this group are lower on average. They were also probably privileged individuals, slightly less well off than the members of the 3A group.

There is no burial in the exteriors of the convent in the seventeenth and eighteenth centuries. In the medieval period, the people buried in the exteriors exhibit the lowest δ 15 N values, sometimes overlapping with the range of the local herbivores.

Most of these individuals however originated from non-Breton locations. The urban population from Rennes is supposed to have an easy access to meat products, which were cheaper than in other French regions [START_REF] Croix | La Bretagne aux 16e et 17e siècles: la vie, la mort, la foi[END_REF]. Low δ 15 N values such as one of the local individuals (70,551) could indicate cerealbased diet of the poor individuals. Interestingly, none of the non-privileged individuals exhibit a δ 13 C values that could indicate a significant contribution of cod into the diet, contradicting historical documents [START_REF] Croix | La Bretagne aux 16e et 17e siècles: la vie, la mort, la foi[END_REF]. There is however a clear relationship between the amount of animal products consumed in the diet and the social status estimated from the burial location and the type of burial (lead coffin, mass grave, etc.; Fig. 6; Supplementary Information).

Among the individuals buried in the chapter house, the lowest δ 15 N values belong to individuals-including a priest-originating from other regions, according to the δ 34 S values of bones and teeth. These individuals had a richer diet in their childhood, which could indicate the respect of their poverty vow as ecclesiastics. The local individuals tend to show similar values in their childhood (δ 15 N tooth ) and adulthood (δ 15 N bone ). Despite a low sample size (n = 7), it seems that early modern Dominicans' diet contained a higher amount of young animals and/or fish than that of their medieval equivalents (Fig. 6). This trend can be seen for δ 15 N values of bones and teeth. The early modern Dominicans have a diet similar to the privileged group, whereas the medieval friar diet is closer to that of people buried in the exterior of the convent (Fig. 5). The enrichment of the convent during the sixteenth century could therefore have led the Dominicans to feast more than fasting: among the five individuals of the convent who suffered from DISH (diffuse idiopathic skeletal hyperostosis) [START_REF] Wendorf | Archaeology of NIDDM[END_REF], two of them were buried in the chapter house.

Women and juveniles' diets

The δ 15 N tooth values generally reveal a diet with either a similar or higher amount in animal products at the end of childhood and beginning of the adolescence than during the last years before death (85% of the individuals for which bones and teeth have been analyzed). The decrease between δ 15 N tooth and δ 15 N bone exceeds 1‰ for 27% of the individuals, which are likely to have known better life conditions in their childhood than the end of their adulthood. To our knowledge, this is the first time that such a pattern is reported (e.g., [START_REF] Hakenbeck | Diet and mobility in Early Medieval Bavaria: a study of carbon and nitrogen stable isotopes[END_REF][START_REF] Reitsema | Stable isotope evidence for sex-and status-based variations in diet and life history at medieval Trino Vercellese, Italy[END_REF][START_REF] Salazar-Garcia | Dental calculus is not equivalent to bone collagen for isotope analysis: a comparison between carbon and nitrogen stable isotope analysis of bulk dental calculus, bone and dentine collagen from same individuals from the Medieval site of El Raval (Alicante, Spain)[END_REF]. Most of these individuals belong to the 2B and the 3A groups. Breton custom is somewhat original in that it is based on the practice of Bnoble sharingŵ hich consists in bequeathing two thirds of its property to the first born, without discrimination between boys or girls, the last third being divided between the other children [START_REF] Chédeville | La Bretagne féodale[END_REF]. The 3A individuals showing such δ 15 N depletion through life may have been the ones disadvantaged by inheritance laws.

The privileged access of men to choir and chapel burials in churches is described by the historical sources on the convent where 58% of the individuals buried in the areas corresponding of the group A are males. From archaeological sources, women (80/276) and men (87/276) are equally represented in the chapels of the Rennes Dominican convent (Le Cloirec 2016). Moreover, we noticed significantly higher δ 15 N isotope values in female bones and teeth. This difference could either be due to a higher amount of animal proteins or a higher proportion of fish and/or young animals in their diets. The first hypothesis could indicate the burial of higher status women in the convent, possibly wealthy benefactresses such as widows with economic independence, who contributed to the clerical management and gained in recognition and emancipation. Nevertheless, young woman's isotope values do not differ from their seniors and the gender trend is more significant in the nave (group B) than in the choir or the chapels (group A). Women could also have more exceptions to fasting rules: eating more meat was for instance allowed to women in confinement [START_REF] Quellier | La table des français. Une histoire culturelle (XVedébut XIXe siècle[END_REF]). On the contrary, they could have been strictly observing the dietary rules and then eating more fish and young rabbits than men.

Conclusions

The bone and tooth stable isotope ratios of carbon and nitrogen isotope values of commoners, ecclesiastics, and privileged individuals buried in the Dominican convent of Rennes clearly highlight differences in diet related to social class. Surprisingly, we found that the C, N, and S isotope compositions of suckling pigs were similar to that of the eels. We also documented quite elevated N isotope signature of the local terrestrial animals. This indicates that the high δ 15 N and low δ 13 C isotope compositions of European medieval individuals could be explained by the consumption of young animals and not necessarily by an intense use of freshwater or migratory fish resources. Contrary to historical evidences, individuals belonging to the poorest socio-economic group do not have evidence of high amounts of cod fish consumption. The influence of this pilgrimage place does not seem to extend over Breton's borders, since the identified migrants mostly consist of ecclesiastics, soldiers, and individual predating the convent. Nobles buried in the convent seem to have spent their life in Brittany or coastal regions and were therefore not living at the French court.

Women buried in the convent are all locals and tend to have a diet rich in animal products. Finally, we documented an increase in the amount of animal products in ecclesiastical diets during the early modern period, but that needs to be confirmed with a future study with a larger sample size. The novel use of sulfur isotopes allowed us to further determine the origin of some of those individuals. As these studies move forward, more systematic sampling of freshwater fish is needed, as are measurements of young and adult terrestrial animals as well as the estimation of the slaughter age. The addition of results from other isotope systems (e.g., Zn) will also help to better decipher the importance of fish in medieval and early modern diets in Western Europe.

Fig. 3

 3 Fig. 3 Sulfur isotope compositions of the human (a grouped by gender; b grouped by socio-economic group) and animal bones (closed symbols) and teeth (open symbols). The grey area corresponds to the local range (see text)

Fig. 4

 4 Fig. 5 C and N isotope ratios of bones (on the left) and tooth collagen (on the right) of animals and humans from the Dominican convent and the Sainte-Anne garbage dump. SD of δ 13 C and δ 15 N values are of 0.1 and 0.2‰, respectively

Fig. 6

 6 Fig. 6 Compared N isotope ratios in human bones and teeth. a N isotope ratios depending on the phase and socio-economic group. Colored box plots correspond to bone values and associated box plot to tooth values. Blue colors, privileged individuals; grey, commoners; red, ecclesiastics;

Fig. 7

 7 Fig.7C and N isotope ratios in human bones and teeth depending on the burial location. Elite individuals are likely to be buried in the church, ecclesiastics in the chapter house, and commoners in the exteriors (west

Fig. 9 Fig. 8 N

 98 Fig. 9 C and N isotope ratios of human bones. SD of δ 13 C and δ 15 N values are 0.1 and 0.2‰, respectively

  Fig. 10 Bone S and N isotope ratios (a) and S and C isotope ratios (b) of animals and humans from the Dominican convent and the Sainte-Anne garbage dump. SD of δ 13 C and δ 15 -N values are 0.1 and 0.2‰,

PHASE 1 (XIII c.) PHASE 2 (End XIV -XVI c.)

		Cloister
	Refectory	Cloister garden
		Chapter
		house
		Chapel
	Church Nave
	group D	Church Choir
	0	50 m
	area of the study
	area of the dig
	path
	bulding and mansonry
	group A
	group B
	group B'
	group C
	grave
	grave tested
	secondary grave

th th th PHASE 3 (XVII -XVIII c.)

	Saint-Joseph
	Chapel	Chapter
	Notre-Dame	house
	Chapel
		Sacristy

Table 1

 1

	Number of individuals
	per sex and socio-economic	Number of individuals excavated (and sampled)
	group for each burial phase and
	associated number of sampled	Age/sex	J	M	PM	F	PF	I	Total adult	Total
	individuals (numbers given
	between brackets)	Phase 1 (thirteenth c.)
		B	2 (1)	8 (5)	0	1	0	1	10 (5)	12 (6)
		Total	2 (1)	8 (5)	0	1	0	1	10 (5)	12 (6)

Table 2

 2

	Average C, N, and S isotope values of different socio-		Material Number δ 15 N SD Number δ 13 C	SD Number δ 34 S	SD
	economic groups and faunal
	species	Animals
		Cat	B	1	11.2	-	1	-19.3 -	1	1 6 . 1	-
			T	1	1 2 . 4	-	1	-19.2 -	-	14.8	-
		Dog	B	1	8.0	-	1	-22.3 -	0	-	-
			T	1	1 1 . 0	-	1	-19.6 -	1	1 3 . 3	-
		Cow	B	2	7.4	1.6 2	-21.2 2.3 -	15.7	-
			T	1	8.0	-	-	-21.6 -	1	1 2 . 0
		Veal	B	1	7.8	-	1	-22.3 -	1	-
			T	1	8.6	-	1	-22.0 -	1	11.4
		Domestic	B	2	8.4	1.1 2	-21.8 1.2 1	16.1	-
		fowl
		Piglet	B	6	12.7	1.6 6	-21.0 0.6 1	14.3	0.8
		Eel	B	1	13.3	-	1	-20.8 -	1	11.3	-
		Marine fish	B	10	13.0	1.5 10	-13.3 1.3 6	16.0	2.0
		Sheep	B	2	8.1	1.0 2	-21.8 0.2 0	-	-
			T	1	9.6	-	-	-21.7 -	1	1 2 . 1	-
		Game	B	2	8.0	2.1 2	-23.3 0.7 -	-	-
		Humans
		1B	B	6	11.3	1.6 7	-19.4 0.4 3	5.2	11.7
			T	2	12.2	1.5 2	-19.1 0.4 1	-10.7
		2A	B	4	12.7	0.1 4	-19.6 0.1 3	14.3	1.5
			T	2	13.5	0.5 2	-19.4 0.3
		2B	B	9	11.3	0.6 9	-19.9 0.6 8	12.9	3.6
			T	6	12.6	0.7 6	-19.4 0.4 3	12.3	3.6
		2C	B	3	11.9	1.4 3	-19.4 0.5 3	10.3	6.9
			T	3	12.1	0.9 3	-19.4 0.1 3	12.4	4.8
		2D	B	5	11.8	1.9 5	-19.3 0.8 5	7.7	2.0
			T	3	1 1 . 7	1 . 4 3	-19.2 0.2 1	3.9
		2B″	B	5	12.1	0.4 5	-19.6 0.3 5	14.9	0.4
		3A	B	30	13.3	0.9 30	-19.3 0.4 28	13.3	0.9
			T	1 9	1 3 . 4	1 . 1 1 9	-19.2 0.4 15	13.1	1.7
		3B	B	11	12.4	0.9 11	-19.4 0.3 11	11.6	3.4
			T	9	13.0	1.1 9	-19.0 0.4 4	9.3	6.7
			B	7	13.2	0.7 7	-19.1 0.2 5	12.6	2.7
			T	4	13.6	0.2 4	-19.1 0.3 3	13.8	1.6
		ALL	B	80	12.5	1.2 80	-19.4 0.4 70	12.1	4.3
			T	4 8	1 3 . 0	1 . 1 4 8	-19.2 0.4 33	11.4	5.4
		Isotope values are expressed in per mille
		B bone, T tooth

(χ 2 = 31.3, p < 1 × 10 -4 with the A, B, C, and D groups, χ 2 = 33.3, p < 1 × 10 -3 with subgroups A′, B′, and B″) even when individuals with non-local individuals are excluded.

Acknowledgements Open access funding provided by Max Planck Society. We are grateful to Annabell Reiner for technical support. We would like to thank Dorothée Drucker and the two anonymous reviewers for their careful reading and suggestions, which greatly improved this paper.

Funding information

We are grateful to the Max Planck Institute and the Department of Archaeology, Faculty of the Environment, Simon Fraser University for funding this study.

Open Access This article is distributed under the terms of the Creative Comm ons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.


Student Member, IEEE S Rizzolo

A Boukenter

E Marin

T Robin

M Cannas

A Morana

J Périsse

J-R Macé

Y Ouerdane

B Nacir

Senior Member, IEEE P Paillet

C Marcandella

Member, IEEE M Gaillardin

Senior Member, IEEE S Girard

Evaluation of Distributed OFDR-Based Sensing Performance in Mixed Neutron/Gamma Radiation Environments

Keywords: Neutrons, optical fiber sensors, radiation, Rayleigh scattering

We report the study of a radiation resistant single mode optical fiber doped with fluorine exposed to mixed neutron and γ -radiation up to 10 17 n/cm 2 fluence and >2 MGy dose to evaluate its performances when used as the sensing element of a distributed Optical Frequency Domain Reflectometry (OFDR). The use of complementary spectroscopic techniques highlights some differences between the responses of solely γ -radiation (10 MGy) or mixed neutron and γ (10 17 n/cm 2 + > 2 MGy) irradiated samples. Those differences are linked to the defect generation rather than to structural changes of the a-SiO 2 host matrix. We show that a modification of the refractive index of ∼ 10 -5 is induced at the highest investigated neutron fluence. However, the feasibility of distributed temperature measurements along the irradiated fiber is demonstrated with an accuracy of 0.1 • C over a sensing length up to ∼ 130 m with the tested OBR4600 interrogator. These results are very promising for the integration of OFDR sensors in mixed neutron and gamma radiation environments.

I. INTRODUCTION

O PTICAL fiber sensors (OFS) are more and more consid- ered for integration in harsh environments characterized by moderate to high levels (doses and fluences) of radiation. Most of the studies have been devoted to the radiation effects on the transmission of optical fibers, highlighting that the radiation induced attenuation (RIA) is the most influential mechanism altering the signal propagation [START_REF] Girard | Radiation effects on silica-based optical fibers: Recent advances and future challenges[END_REF]. Other phenomena, such as radiation induced emission (RIE) or radiation induced refractive index change (RIRIC) affect also the performances of optical fibers in some applications [START_REF] Girard | Radiation effects on silica-based optical fibers: Recent advances and future challenges[END_REF]. While the RIA reduces the sensing length that can be used, the RIRIC will induce errors on the measured parameters due to the local evolution of the refractive index.

With the development of distributed sensors based on Raman, Brillouin or Rayleigh scattering, new radiation effects have been considered in addition to RIA, RIE and RIRIC. For Brillouin sensing, a Radiation-induced Brillouin Frequency Shift (RI-BFS) was observed and causes an additional error in the estimation of temperature or strain values by the systems, amplitudes of RI-BFS being, like the RIA, dependent on various parameters such as the fiber composition [START_REF] Phèron | High γ-ray dose radiation effects on the performances of Brillouin scattering based optical fiber sensors[END_REF]. Raman sensors have been shown to be sensitive to the RIA used to evaluate the temperature along the sensing fiber, which, if not corrected [START_REF] Francesca | Radiation hardened architecture of a singleended raman-based distributed temperature sensor[END_REF], is responsible of huge errors on temperature responses of single-ended systems. Recent results have shown that Rayleigh sensors are more robust against total ionizing dose (TID): the temperature and strain coefficients are not affected nor by permanent effects up to at least 10 MGy(SiO2) irradiation dose [START_REF] Rizzolo | Vulnerability of OFDR-based distributed sensors to high γ-ray doses[END_REF] either by steady-state irradiation up to, at least, 1 MGy(SiO2) [START_REF] Rizzolo | Radiation hardened optical frequency domain reflectometry distributed temperature fiber-based sensors[END_REF]. Nevertheless, fiber packaging influences the OFDR sensor response: radiation changes the mechanical properties of the coating surrounding the fiber core/cladding and can lead to errors in strain or temperature distributed measurements. Irradiations up to 3 MGy, as well as thermal treatments up to the maximum operating temperature of this surrounding coating, have been shown to improve the performances of the sensors under harsh environments [START_REF] Rizzolo | Radiation characterization of optical frequency domain reflectometry fiber-based distributed sensors[END_REF], [START_REF] Rizzolo | Investigation of coating impact on OFDR optical remote fiber-based sensors performances for their integration in high temperature and radiation environments[END_REF].

In this work we investigate the behavior of OFDR sensors to fission neutrons (0.8 MeV) at fluences up to 10 17 n/cm 2 . The explored fluence levels are representative of several applications within nuclear power plants (NPPs) covering both their nominal or accidental operations. The feasibility of OFDR temperature measurements was already demonstrated in nuclear reactor [START_REF] Sang | One centimeter spatial resolution temperature measurements in a nuclear reactor using Rayleigh scatter in optical fiber[END_REF], under a neutron flux of 1.2 × 10 13 n/cm 2 s and γ -dose rate of 1 MGy/h. Previous studies performed at comparable fluences but with 14 MeV neutrons [START_REF] Girard | 14-MeV neutron, γ-ray, and pulsed X-ray radiation-induced effects on multimode silica-based optical fibers[END_REF], demonstrate that X-rays, γ -rays and neutrons globally lead to the same defect generation mechanisms and the same class of defects contributes to the induced losses in fluorinedoped optical fiber i.e. E' center, ≡Si•, nonbridging oxygen hole center (NBOHC), ≡Si-O•; peroxy radical (POR), ≡Si-O -O•; and H(I), O =Si• -H. However, neutrons may affect more strongly the acrylate or polyimide coating material than γ -rays and, as already mentioned, coating alteration can impact the OFDR response by modifying the internal stress distribution in the fiber transverse cross section [START_REF] Rizzolo | Radiation characterization of optical frequency domain reflectometry fiber-based distributed sensors[END_REF]. Moreover, previous studies reveal that, above 10 17 -10 18 n/cm 2 , the silica amorphous structure is directly affected by displacement damage [START_REF] León | Neutron irradiation effects on the structural properties of KU1, KS-4V and I301 silica glasses[END_REF] that may act on the sensor performances exploiting the Rayleigh signature of the amorphous glass.

Consequently, the understanding of generation mechanisms of these defects is relevant to improve the radiation resistance of optical fibers. Previous studies have shown that pure silica core (PSC) and F-doped fibers exhibit lowest RIA levels in both UV-Visible [START_REF] Girard | Radiation effects on silica-based optical fibers: Recent advances and future challenges[END_REF] and infrared (IR) [START_REF] Rizzolo | Vulnerability of OFDR-based distributed sensors to high γ-ray doses[END_REF] spectral domains for high irradiation doses; they are, for this reason, of particular interest for the design of radiation hardened distributed fiber sensors for nuclear industry.

The purpose of this work is to study the response of fluorine doped single mode fiber as OFDR-based sensor in mixed γ and neutron environments. This radiation hardened optical fiber shows good OFDR sensing properties at MGy dose levels and for irradiation temperature range of [30 • C-100 • C] [START_REF] Rizzolo | Radiation hardened optical frequency domain reflectometry distributed temperature fiber-based sensors[END_REF].

The evaluation at the system level is combined to a spectroscopic analysis (absorption, confocal micro-luminescence and electron paramagnetic resonance -EPR) on the point defects generated under mixed neutrons/gamma and pure gamma environments allowing a better comprehension on the involved mechanisms at the basis of the radiation induced effects.

II. EXPERIMENTAL DETAILS

We tested a fluorine (F)-doped single mode fiber (SMF) developed by iXBlue manufacturer with an acrylate coating (maximum operating temperature 80 • C). The sizes of core and cladding diameters are ∼ 10 μm and ∼ 125 μm, respectively, and the fluorine concentration is about 0.2 wt.% in the core and ∼ 1.5 wt.% in the cladding. We investigated the properties of non-irradiated fiber as well as those exposed to a mixed neutron and γ -ray environment up to neutron fluences of 10 17 n/cm 2 and γ -doses >2 MGy.

The neutron irradiations were performed at:

• The PROSPERO reactor of the Commissariat à l'énergie atomique et aux énergies alternatives (CEA), (Valduc, France) to reach fluences between 10 13 n/cm 2 and 10 14 n/cm 2 of fission neutrons (0.8 MeV) (with an additional γ -dose in the kGy level). • The 2 MW pool-type TRIGA Mark II reactor of the CNESTEN, Centre National de l'Energie, des Sciences et des Techniques Nucléaires, (Maamoura, Morocco). The accumulated fluence of fast neutrons (>9 keV) varies between 10 15 n/cm 2 (with an additional γ -dose of 0.02 MGy) and 10 17 n/cm 2 (with an additional γ -dose higher than 2 MGy). Distributed temperature and strain measurements were carried out after irradiation (post-mortem) with an OBR 4600 from Luna Technologies equipped with a laser probe source centered at 1550 nm (accuracy of 1.5 pm) and tuned over a range of 21 nm. The spatial resolution was 40 μm for the scattering pattern along a maximum fiber length of 70 m, and was set to 1 cm and 0.2 cm for distributed temperature and strain measurements, respectively.

According to our previous studies [START_REF] Rizzolo | Radiation characterization of optical frequency domain reflectometry fiber-based distributed sensors[END_REF], [START_REF] Rizzolo | Investigation of coating impact on OFDR optical remote fiber-based sensors performances for their integration in high temperature and radiation environments[END_REF], we performed a pre-thermal treatment up to the maximum operating temperature of the acrylate coating, i.e. the maximal temperature it can resist without degrading the fiber properties, to stabilize the fiber/coating configuration and consequently the fiber temperature coefficient C T values. The temperature profile of the thermal treatment was set from 30 • Cto 80 • C with an incremental step Tof5 • C. The fibers were maintained at the same temperature for each step for half an hour.

Temperature calibration was done in stabilized oven controlled with a type K thermocouple; each fiber temperature sensitivity coefficient, C T , was determined from the linear slope response of the Rayleigh spectral shift with temperature. For such experiments, all the samples, both non-irradiated and irradiated ones, were spliced to each other in series to perform the temperature treatment at the same time on a sample segment of ∼ 12 m long.

Strain was induced by stretching the samples with a translation set up coupled to an adapted tensile gauge for both the evaluation of the applied constraint and its stability during the measurement. The strain applied along the fiber is given by the relation: ε = L/L, where L ( L) is the length (extension) of the stretched fiber (∼ 50 cm). Strain sensitivity coefficient, C ε , was determined from the slope of the Rayleigh spectral shift with strain in the range [∼ 0 με -4000 με].

RIA in the visible spectral range was measured with the cutback method; we injected light from a Halogen source into the sample; the signal transmitted through the fiber link was analyzed with a UV-Visible optimized spectrometer from 200 to 900 nm. The infrared contribution was evaluated at 1550 nm with an Optical Time Domain Reflectometer (OTDR, EXFOFTB-7400E) [START_REF] Henschel | Radiation induced loss measurements of optical fibres with optical time domain reflectometers (OTDR) at high and low dose rates[END_REF]. The measurements were performed with a signal pulse width of 10 ns and an exposure time of 180 s.

Confocal micro-luminescence and micro-Raman (CML-R) measurements were performed to investigate the distributions of neutron and gamma induced active optical centers and profiles along the fiber transverse cross sections by using an Aramis spectrometer associated with a He-Cd ion laser excitation line (energy 325 nm (3.8 eV)) and supplied with a CCD camera, micro translation stages and a ×40 objective. The employed experimental conditions lead to a spatial resolution of about 3μm.

The EPR was used to identify and quantify the paramagnetic centers induced by the radiation. E'-Si and H(I) defects are probed at room temperature (300 K), whereas NBOH and POR centers were checked at low temperature (77K) using a Bruker EMX-Micro Bay spectrometer working at 9.8 GHz with a magnetic-field modulation frequency of 100 kHz. The absolute concentration of defects was estimated by comparing the double integral of the EPR spectrum with a reference silica sample. To this aim, we used an irradiated silica bulk sample whose E' concentration was known by spin-echo measurements, the accuracy of the absolute concentration being estimated as 50% whereas the relative error among the different defect types is about 20% [START_REF] Agnello | Structural relaxation of E γ centers in amorphous silica[END_REF].

In order to understand the effect of the mixed radiation environment on OFDR-based sensors we studied the trend of temperature and strain coefficients as a function of the total deposited neutron fluence and gamma dose. To highlight a direct comparison with γ -rays, from the total accumulated fluence (expressed in n/cm 2 ) we calculate the equivalent deposited dose (in MGy) using the converting factor 1 Gy(SiO 2 ) equal to 2 × 10 12 n/cm 2 to sum up the additional γ -dose [START_REF] Witteles | Neutron radiation effects on MOSFETs: Theory and experiment[END_REF].

III. EXPERIMENTAL RESULTS

A. Distributed Sensing

In this section we report the results on OFDR distributed sensors to investigate their response to mixed exposure. The comparison with only-γ irradiated samples, investigated in [START_REF] Rizzolo | Coating impact and radiation effects on optical frequency domain reflectometry fiber-based temperature sensors[END_REF], will be discussed later.

The results of C T and C ε as a function of the equivalent total dose are reported in Fig. 1 (a) and Fig. 1 (b). In the case of C T , we note that mixed radiation affects C T s only for the highest dose (i.e. 10 17 n/cm 2 + > 2 MGy) where the coefficient varies of 5 % with respect to the others values. This variation, if not taken into account in temperature measurements, allows an error on distributed measurement due to the temperature calibration up to ∼ 3 • C at 80 • C. In the case of Cεs, the dose dependence is not as precisely determined due to the complexity of the measurements: we find that the values are indeed scattered within a variation of 5%, highlighted by the blue rectangle in Fig. 1 (b).

Once the C T is determined, the performances of temperature OFDR-sensors have been tested. The measurement as a function of the time is reported in Fig. 2 for the entire temperature profile. We can note that temperature, measured with the sample irradiated at the highest dose, is well determined during the experiment. The comparison with thermocouple results allows estimating the accuracy of our measurement: we obtain pretty good performances, being the temperature well determined with a precision of 0.1 • C in each step from 30 • C to 80 • C.

B. Spectroscopic Analysis

In this section we report the results obtained by coupling diverse spectroscopic techniques. The investigation was performed on both mixed and only-γ irradiated samples and the results are directly compared for each technique.

Fig. 3 (a) and Fig. 3 (b) display RIA in the visible spectrum and at 1550 nm as a function of the equivalent deposited dose, respectively. The measure of the RIA in the visible is compared for the two samples irradiated at higher doses (i.e. 10 17 n/cm 2 + > 2 MGy and 10 MGy). Our results highlight at least two different contributions to the attenuation. We find an UV-absorption tail which increases at shorter wavelengths and an absorption band centered at around 630 nm that is well associated with the NBOHC defects [START_REF] Morana | Origin of the visible absorption in radiation-resistant optical fibers[END_REF].

We can observe that RIA level is higher in the mixed neutron and γ irradiated sample than in the only-γ irradiated one with RIA levels of ∼ 6 dB/m and ∼ 3 dB/m respectively. In the IR, we observe that, for mixed neutron and γ -irradiated samples, RIA at 1550 nm starts to increase with radiation at 10 15 n/cm 2 and reaches 75 dB/km at the highest fluence, whereas for lower fluences (10 13 and 10 14 n/cm 2 ) RIA remains negligible in our samples being the attenuation similar to nonirradiated sample (∼ 0.4 dB/km). From the comparison with γ -irradiated samples (data are adapted from [START_REF] Rizzolo | Vulnerability of OFDR-based distributed sensors to high γ-ray doses[END_REF]), we note that at the highest fluence RIA is higher than after a 10 MGy γ -dose irradiation (55 dB/km) remaining within the same order of magnitude.

Thanks to the CML technique it was possible to investigate the photoluminescence (PL) properties of pre-existing and induced defects to highlight the differences in their generation during mixed or only gamma exposures. Fig. 4 reports the comparison between not-irradiated and irradiated samples with the highest doses: 10 17 n/cm 2 + > 2 MGy and 10 MGy. PL spectra in both core and cladding zones (Fig. 4 (a) and Fig. 4 (b) respectively) show a band peaked around 650 nm, associated with the NBOHCs [START_REF] Skuja | The origin of the intrinsic 1.9 eV luminescence band in glassy SiO 2[END_REF], [START_REF] Vaccaro | Influence of fluorine on the fiber resistance studied through the non-bridging oxygen hole center related luminescence[END_REF] for which their radial distribution is reported in Fig. 4 (c) demonstrating the impact of the F-co-doping on their concentration [START_REF] Vaccaro | Influence of fluorine on the fiber resistance studied through the non-bridging oxygen hole center related luminescence[END_REF]. These defects are mainly located in the fiber core, whereas in the cladding their concentration decreases towards the outside part of the fiber. It is worth noting that the concentration of NBOHC in the cladding decreases before in mixed neutron and γ irradiated samples than in only-γ irradiated ones, result better highlighted from the 2D maps in the whole fibers of Fig. 4 (e) and Fig. 4 (f), where we can note a higher luminescence signal of these centers in the cladding for the 10 MGy irradiated sample. Another PL band is observed in the green region with characteristics depending on the radiation nature: we find that it is peaked at 527 nm for 10 17 n/cm 2 + > 2 MGy irradiated sample, whereas it is centered at 515 nm for 10 MGy irradiated sample. We acknowledge that a similar band was observed in [START_REF] Morana | Influence of neutron and gamma-ray irradiations on rad-hard optical fiber[END_REF], [START_REF] Martìn | Photoluminescence in neutron irradiated fused silica[END_REF]; these finding suggest that neutrons induce an intrinsic peculiar defect characterized by this green photoluminescence. The radial distribution of the PL band at 530 nm, reported in Fig. 4 (d), reveals a difference between the two radiation sources. We find, indeed, that in mixed neutron and γ irradiated sample the luminescence is one order of magnitude higher than in only γ -irradiated one and we note that the concentration increases in the core in mixed neutron and γ irradiated sample, whereas it slightly decreases in γ irradiated ones.

Fig. 5 reports EPR spectra in the 10 MGy irradiated sample: in Fig. 5 (a) one of the most common intrinsic defects, the E', characterized by axial symmetry; its EPR spectrum was recorded at RT. A variant of the E' center can be also observed in EPR spectra, the H(I) defect, reported in Fig. 5 (b), consisting of a dangling Si bond with one neighboring O substituted by a H atom. The hyperfine interaction between the unpaired electron and the H nucleus, with spin of 1/2, originates the of 74 G. Also its spectrum was measured at RT. In contrast to these centers, the oxygen-related defect signals (NBOHC and POR) are large (∼ 30 G) and they overlap, so they cannot be distinguished at RT; so measurements at low temperature, (77 K liquid nitrogen), are needed to separate the two spectra, shown in Fig. 5 (c), whose relative contributions can be estimated by a deconvolution procedure.

The defect concentrations as a function of the equivalent dose are reported in Fig. 6. All defect concentrations increase with the dose, both in mixed neutron and γ irradiated sample and in only-γ irradiated ones.

We observe that POR and H(I) concentration is higher in mixed neutron and γ irradiated samples that in only-γ irradiated one, whereas we reach similar value of concentration in the case of E' and NBOH centers. E' centers exhibit the highest concentration of ∼ 10 17 cm -3 .

In Fig. 7 we report the Raman spectra recorded in the samples irradiated at the highest doses, i.e. 10 17 n/cm 2 + > 2MGy and 10 MGy. The comparison evidences that the spectra remain almost unchanged, thus suggesting that the explored fluence range is not high enough to observe structural modifications induced on the silica matrix. However, we observed slight differences on the bands at ∼ 800 cm -1 and ∼ 1050 cm -1 associated with the bending and the asymmetric stretching of Si-O-Si bridges [START_REF] Henderson | The structure of GeO 2 SiO 2 glasses and melts: A Raman spectroscopy study[END_REF].

IV. DISCUSSION

The investigation on distributed OFDR measurements in mixed neutron and γ radiation environment reveal differences for the fluence of 10 17 n/cm 2 and doses exceeding the MGy range. We have indeed observed a change in the C T of 5% at the highest dose. Comparing these results with those obtained in only γ -irradiated samples [START_REF] Rizzolo | Coating impact and radiation effects on optical frequency domain reflectometry fiber-based temperature sensors[END_REF] we notice that the coefficient value corresponding to the highest fluences is similar to the 1 MGy irradiated sample (values are (6.59± 0.02)•10 -6 • C -1 for the 1 MGy irradiated sample of reference [START_REF] Rizzolo | Coating impact and radiation effects on optical frequency domain reflectometry fiber-based temperature sensors[END_REF] and (6.58± 0.01)•10 -6 • C -1 in 10 17 n/cm 2 irradiated sample) thus suggesting that both mixed neutron and γ -radiation at these fluences and doses can change the fiber response. As a first approximation, the radiation, by acting on the coating stabilization, changes the coating/fiber configuration according to the studies on thermic and mechanical properties on acrylate coated optical fibers of [START_REF] Nakajima | A study for estimating thermal strain and thermal stress in optical fiber coatings[END_REF]. We have shown, however, that the C T change is not affecting temperature measurements that have shown good performances (accuracy of 0.1 • C) up to the highest temperature probed, i.e. 80 • C. To go further on the induced radiation effects, in Fig. 8 we have reported the spectral shift as a function of the fiber length path for different temperatures, acquired during the calibration.

We can observe that the spectral shift is pretty uniform along each sample, thus allowing a comparison of its value for the different doses. We note also that the value of recorded spectral shift is the same from non-irradiated sample to 10 16 n/cm 2 + 0.2 MGy and that it changes for the highest doses. We have calculated this spectral shift difference (SSD) Fig. 8. Spectral shift as a function of the sample path length at some different temperature steps during the temperature calibration. The non-irradiated, as well as the irradiated segments are spliced to each other to perform the calibration in the same conditions for all the samples. Fig. 9. Refractive index radial distributions in non-irradiated (black point) and 10 17 n/cm 2 + > 2MGy irradiated one (red points). The inset shows a zoom in the fiber core. The results are shown by taking the difference with respect the refractive index in the exterior part of the cladding. between non-irradiated and 10 17 n/cm 2 + > 2MGy obtaining that it is function of the temperature: 0.6 GHz for 40 • C up to 3.6 GHz at 80 • C.

It is well known that the Rayleigh signature of a given fiber is dependent on the refractive index fluctuation along its length [START_REF] Soller | High resolution optical frequency domain reflectometry for characterization of components and assemblies[END_REF], [START_REF] Kreger | High-resolution extended distance distributed fiberoptic sensing using Rayleigh backscatter[END_REF], fluctuation which also influences the spectral shift calculation by [START_REF] Leparmentier | Study of the hydrogen influence on the acoustic velocity of single-mode fibers by Rayleigh and Brillouin backscattering measurements[END_REF]:

- ν ν C = n n (1)
where ν C = c / λ C , with c the speed of the light and λ C = 1550 nm, and n the refractive index of the fiber core. Then, from the measured values of SSD we are able to estimate the refractive index variation in 10 17 n/cm 2 + > 2MGy irradiated sample finding values from ∼ 0.5 • 10 -5 at 40 • C to ∼ 3 • 10 -5 at 80 • C. These values of refractive index variations induced by mixed exposure are in agreement with those obtained from n radial measurements, reported in Fig. 9, from which we have extracted the variation of nwith ∼ 7•10 -5 in the fiber core, see inset of Fig. 8, (it is worth noting that obtained refractive index variation values are near the limit imposed by the experimental technique). The use of complementary spectroscopic techniques has allowed studying the main radiation effects at the microscopic scale and estimating the differences induced by the two environments here considered. We have found that both mixed neutron and γ and only-γ irradiations lead to the generation of the same defects as highlighted from absorption, luminescence and EPR measurements. We have observed that the radiation induced losses are higher in 10 17 n/cm 2 + > 2MGy than 10 MGy irradiated samples, even if they remain in the same order of magnitude. These findings agree with the study reported by Henschel et al., where it is shown that γ and neutron irradiations lead to the same induced losses if the fluences is <10 14 n/cm 2 whereas higher fluences of neutrons cause higher losses than γ irradiation [START_REF] Henschel | Comparison between fast neutron and gamma irradiation of optical fibres[END_REF].

V. CONCLUSION

In this work we have reported the characterization of an F-doped single mode optical fiber exposed to mixed neutron and γ -radiation to evaluate its performances as sensing element of distributed an OFDR-sensors.

The use of complementary spectroscopic techniques has allowed studying the principal radiation effects such as RIA, photoluminescence and EPR of induced point defects. We have found that the main effect induced by both kinds of radiations is the generation of point defects such as E', NBOHC, H(I) and POR which generate similar absorption and EPR properties. Luminescence investigation has evidenced that the presence of neutrons affects differently the radial distribution of the NBOHC. Moreover, a green luminescence band, centered at ∼ 527 nm, is peculiarly observed in mixed neutron and γ -irradiated samples.

Furthermore, the calibration measurement of an OFDR-sensor has shown that a modification of the refractive index of ∼ 10 -5 is induced by the exposure to mixed radiation environments. Nevertheless, the feasibility of distributed temperature measurements is demonstrated with an accuracy of 0.1 • C at the highest investigated doses of 10 17 n/cm 2 + > 2MGy, thus suggesting that the RIRIC acts on temperature coefficient, which became function of the refractive index:

C T → C T (n) (2)
Even if the sensing parameters dependence on refractive index has to be further investigated to improve and to predict their behavior radiation environments, reported results have shown that OFDR remains a promising solution for the development of distributed sensors also in mixed environments with presence of neutrons.

Fig. 1 .

 1 Fig. 1. (a) Temperature and (b) strain coefficients as a function of the total deposited dose (expressed in MGy[START_REF] Witteles | Neutron radiation effects on MOSFETs: Theory and experiment[END_REF]) for mixed neutron and γ -irradiated fibers. The associated neutron fluence is reported in the top horizontal-axis.

2 .

 2 Temperature evolution as a function of the time measured on the (10 17 n/cm 2 + > 2 MGy) sample (open red circles) and compared with thermocouple reference results (black line).

Fig. 3 .

 3 Fig. 3. (a) RIA spectra in UV-Visible range for mixed neutron fluence and γ -rays at 10 17 n/cm 2 + > 2 MGy (black dotted line) and with only γ -rays at 10 MGy (black solid line). (b) Radiation induced attenuation of studied optical fibers at 1550 nm as a function of the total deposited dose (expressed in MGy[START_REF] Witteles | Neutron radiation effects on MOSFETs: Theory and experiment[END_REF]) for mixed neutron and γ -irradiated fiber (open circles) and only ã-irradiated fiber (solid squares adapted from[START_REF] Rizzolo | Vulnerability of OFDR-based distributed sensors to high γ-ray doses[END_REF]).

Fig. 4 .

 4 Fig. 4. PL spectra emitted from (a) the core center and (b) the cladding of the fiber irradiated with mixed neutron fluence and γ -rays at 10 17 n/cm 2 + > 2 MGy (black dotted line) and with only γ -rays at 10 MGy (black solid line). (c) and (d) show PL intensity profiles along the diameter of 650 nm and 530 nm bands in 10 17 n/cm 2 + > 2 MGy (open circles) and 10 MGy (solid squares) irradiated samples; red points represent the F-profile. In (e) and (f) the 2D maps of the emission at 650 nm are reported for mixed neutron fluence and γ -rays at 10 17 n/cm 2 + > 2 MGy and only γ -rays at 10 MGy, respectively.

Fig. 6 .

 6 Fig. 6. Concentration of (a) Si-E', (b) H(I), (c) NBOHC and (d) POR point defects as a function total deposited dose [13]. Solid point indicated the only-γ irradiated samples, whereas open point are mixed neutron and γ irradiated ones.

Fig. 7 .

 7 Fig. 7.Raman spectra recorded in the core center of 10 17 n/cm 2 + > 2 MGy (black dotted line) and 10 MGy (black solid line) irradiated samples. The red line indicates the data obtained for non-irradiated sample.