Hugo Lourenço-Martins

Mathieu Kociak

Sophie Meuret

François Treussart

Hong Yih

Lee

Xing Yi Ling

Huan-Cheng Chang

Henrique Luiz

Galvão Tizei
email: luiz.galvao-tizei@u-psud.fr

Probing plasmon-NV 0 coupling at the nanometer scale with photons and fast electrons

Keywords: Purcell effect, lifetime measurement, localized surface plasmon, neutral nitrogenvacancy defect, electron energy loss spectroscopy, cathodoluminescence

The local density of optical states governs an emitters' lifetime and quantum yield through the Purcell effect. It can be modified by a surface plasmon electromagnetic field, but such a field has a spatial extension limited to a few hundreds of nanometers, complicating the use of optical methods to spatially probe emitter-plasmon coupling.

Here we show that a combination of electron-based imaging, spectroscopies and photon-based correlation spectroscopy enables measurement of the Purcell effect with nanometer and nanosecond spatio-temporal resolutions. Due to the large variability of radiative lifetimes of emitters in nanoparticles we relied on a statistical approach to probe the coupling between nitrogen-vacancy centers in nanodiamonds and surface plasmons in silver nanocubes. We quantified the Purcell effect by measuring the nitrogen-vacancy excited state lifetimes in a large number of either isolated nanodiamonds or nanodiamond-nanocube dimers and demonstrated a significant lifetime reduction for dimers.

Note: this manuscript has been published in ACS Photonics: H. Lourenço-Martins, et al,ACS Photonics, 5, 324 (2018). This is the last version submitted before proof editions.

The radiative lifetime of a light emitter is intrinsically linked to the local density of optical states (LDOS), ρ( r, ω) ( r represents position in space and ω frequency). The presence of an optical cavity or a plasmonic structure leads to the increase of the LDOS which can be quantified by a transition rate enhancement factor, γ = ρ( r, ω)/ρ 0 ( r, ω) 1 (ρ and ρ 0 being the LDOS with and without the a cavity or plasmonic structure), what is known as Purcell effect 2 . Owing to the Fermi golden rule, the radiative lifetime of an emitter interacting with these photonic structures is modified by γ. Due to the short lifetime of surface plasmons (SP) 3 (typically few femtoseconds), their coupling with quantum emitters (QE) is usually weak. In this regime, the Purcell effect is predominant.

To date, only two strategies have been considered to measure the Purcell effect in QE-SP dimers. From a microscopic perspective, experiments were carried out on a single dimer [4][5][6][7] with a precise positioning of the QE. Alternatively, macroscopic measurements were performed simultaneously on a large ensemble of dimers 8,9 . Both approaches present limitations due to the large variability of the isolated QEs lifetimes [10][11][12][13] .

Here, to overcome this intrinsic variability we adopt a statistical method applied to individual nano-objects, where the lifetime of large sets of isolated QEs and dimers are measured. In our experiments, neutral nitrogenvacancy (NV 0 ) centers in nanodiamonds dipoles were coupled to surface plasmons (SP) on Ag nanocubes. To quantify the Purcell effect at the nanoscale we applied a combination of electron-based imaging, spectroscopies and photon-based correlation spectroscopy that allowed us to achieve the required spatial and temporal resolutions with large throughput.

Modern scanning transmission electron microscopes (STEM) are versatile, allowing the use of nanometer-wide electron probes to obtain complementary information from spectroscopic, diffraction and imaging techniques. Specifically electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) have shown their remarkable capability to measure absorption and emission spectra with nanometer spatial resolution [14][15][16][17] . Figure 1 shows a typical EEL spectrum for an isolated Ag nanocube and a CL spectrum for an isolated nanodiamond containing NV 0 centers. The EEL spectrum presents three peaks corresponding to different plasmon modes. The peak at 1.8 eV with a 500 meV width matches the energy range of the NV 0 emission, indicating the possibility of coupling between SP and the NV 0 centers. We note that SP spatial intensity distribution is highly anisotropic, as seen in Fig. 1(c-d) for the mode at 1.8 eV. In principle, this could play a role into where the nanodiamond should be positioned. As discussed later, this does not play a major role due to the large spatial extent of SP modes.

Lifetimes of isolated nanodiamonds and nanodia-arXiv:1906.00388v1 [cond-mat.mes-hall] 2 Jun 2019 mond/nanocubes dimers have been inferred from the second order correlation function (g (2) (τ )) of light emitted from NV 0 centers excited by the fast electrons, as already demonstrated for different systems [18][19][20][21] . In short, each electron excites more than one electron-hole pair above the bandgap of diamond. These pairs excite within a few picoseconds more than one localized defect. The end result is the emission of more than one photon within the emitter's lifetime, leading to the effective formation of a light pulse and a second order correlation function presenting a bunching behavior. This bunching peak has a time decay constant equal to the emitter's total lifetime, allowing its measurement. Note that lifetimes can also be measured using a pulsed electron source and a cathodoluminescence setup, but with limited spatial resolution (50 nm) up to now 22 . g (2) (τ ) was measured using a Hanbury Brown and Twiss (HBT) interferometer (see Fig. 1(a)) coupled to the light collection system 23 . Annular dark field (ADF) images allows us to determine the position of the nanodiamond with respect to the nanocube and the nanocube size with a nanometer resolution, giving us all the parameters necessary to estimate expected the enhancement factor.

Samples were prepared by sequentially drop casting solutions of Ag nanocubes and nanodiamonds containing multiple NV 0 centers onto a 15 nm thick Si 3 N 4 membrane. Nanodiamonds typically appear as aggregates (Fig. 1a). However, among these aggregates a larger and luminescent nanoparticle is always observed, which is the one considered in each measurement. Most of the nanodiamonds tend to be adsorbed on the nanocubes faces. Moreover, a distribution of sizes is observed in our nanocubes sample (100 nm average size with some variation). This influences the energy of a specific SP. But as a continuum of SP modes is observed for each given nanocube and the NV 0 emission is spectrally wide, coupling is always possible. Isolated nanodiamonds and dimers were identified using ADF images that are acquired simultaneously with wavelength filtered CL maps (first and second columns in Fig. 2). Quick access to this information allows an effortless identification. After a target isolated nanodiamond or dimer is selected, the g (2) (τ ) function of the emitted photons is measured using the HBT interferometer while the electron beam scans a fixed small area on the nanodiamond. The photon counting rate and the ADF image can be recorded live, allowing sample drift to be corrected by repositioning the scanning area. In total, the lifetime of 56 isolated nanodiamonds and 62 dimers were measured in the same sample in a single experimental run, ensuring identical experimental conditions.

Examples of measurements in two isolated nanodiamonds and two dimers are shown in Fig. 2(a-b) and Fig. 2(c-d), respectively. The isolated nanodiamond in Fig. 2(a) and the dimer in 2(d) have lifetimes 36 ± 5 ns and τ = 11 ± 1 ns, respectively, in agreement with an enhancement effect. However, isolated nanodiamonds and dimers (Fig. 2(b-c)) with similar lifetimes are also present (τ = 22 ± 2 ns and τ = 21 ± 1 ns in these examples). Such observations occur due to the large dispersion in NV 0 lifetimes in nanodiamonds already reported in the literature 11,24,25 . The lifetime for NV 0 in bulk diamond is 19 ns 26 and it is distributed between 10 and 40 ns 11 in nanodiamond. A similar behavior is known for the NV - (charged NV) center, for which the lifetime changes from 13 ns in bulk 27 to larger values (17 ns and 25 ns) in nanoparticles 24,25 , with a broad distribution 28 . The excited state lifetime increase is the result of the smaller LDOS in nanoparticle than in bulk (i.e. γ < 1, as predicted in the electrostatic regime of particle size smaller than the vacuum emission wavelength of the embedded emitter), while the lifetime dispersion is mainly due to nanoparticle size variability 29 .

To overcome this variability, a measurement of the Purcell effect can be performed by sequentially probing the NV 0 lifetime of an isolated nanodiamond, followed by coupling the nanodiamond to a plasmonic structure (either by mechanical movement or lithography), as shown by Beams et al 7 . However, this approach would normally involves taking the sample out of vacuum and performing a series of processes which can modify its local environment and, hence, its lifetime. Here we overcome the variability by measuring the excited state lifetime in a large ensembles of either isolated nanodiamonds or dimers.

The histograms (top) and a scattered plot (bottom) of the lifetime of isolated nanodiamonds (purple) and dimers (orange) are shown in Fig. 3 (see SI for the complete data set). The average lifetimes of isolated nanodiamonds and dimers are 24 ± 5 ns and 18 ± 4 ns (the most probable values are 22.5 ± 2.5 ns and 12.5 ± 2.5 ns). The two distributions overlap. However, they are significantly distinct, as confirmed by the Wilcoxon-Mann-Whitney statistical u-test (p = 1.91 × 10 -6 ). Therefore, the 40% reduction of lifetimes can be unambiguously associated to a Purcell effect with a spontaneous decay rate enhancement factor of 1.4, indicating some coupling, although weak, between NV 0 centers in nanodiamonds and SP in Ag nanocubes.

Lifetimes shorter than that in bulk are observed in both histograms, although with a small probability. These occur due to non-radiative decay channels involving e.g. surface defects, which do affect the excited state lifetime. Overall, the coupling leads to a rigid shift of NV 0 lifetime histogram to shorter values. These features are only accessible with a statistical approach which emphasizes the strength of our experimental method.

As recently pointed out 30 , the calculation of the luminescence enhancement of defects in presence of a plasmonic field is an intricate problem. A quantitative simulation of our experiment would require precise knowledge of a large set of parameters: the shape of the nanodiamonds, exact number of defects, their position in the nanoparticle, their respective lifetime and the profile of the plasmonic field within the nanodiamond. Although we do not tackle this problem in the current paper, we have performed numerical calculations to verify that the order of magnitude of the expected effect matches our observations. A key point for these calculations is the presence of numerous NV 0 s in our nanodiamonds. In principle, the NV 0 lifetime may vary with the position of the emitter within a nanoparticle for subwavelength-sized nanodiamonds (electrostatic regime) but experimentlly we have observed that it is constant throughout the nanodiamond, as pointed out by Greffet et al 29 . No variations within the same nanodiamond was observed, despite the nanometer spatial resolution provided by our experiments. Even if electron-hole diffusion in the nanodiamond could decrease our expected spatial resolution (as it is known to occur 31 ), variations of NV 0 s lifetime could be observed if they occurred in scales larger than 50 nm.

The expected radiative rate enhancement factor due to the Purcell effect was calculated by considering an isolated Ag nanocube, taken the Si 3 N 4 substrate into account. Calculations performed with the MNPBEM toolbox 32 show a SP mode centered at 2.1 eV, in agreement with the experimental value (Fig. 1(a)). Considering that most nanodiamonds were observed on a facet of the nanocubes, we calculated the LDOS enhancement factor at 2.1 eV along a line centered on a nanocube face and perpendicular to it, for an emitter at a distances between 10 to 100 nm from the nanocube surface (arrow on Fig. 4).

The enhancement factor is plotted on Fig 4 (purple). Taking into account the most probable lifetime for NV 0 in isolated nanodiamonds is 22.5 ns, we plotted in orange the expected NV 0 lifetime given the calculated enhancement factor. We see that the most probable lifetime measured in presence of a nanocube (12.5 ns) corresponds to a distance of 65 nm, also associated to the enhancement factor 1.4. This value is in qualitative agreement with the possible distance of a NV 0 center to the surface of a 100-200 nm nanodiamond (sizes typically observed in our sample). Although this result relies on the specific positioning of the emitters in the dimer, obtaining a consistent distance value is a strong evidence to support our conclusion.

We have used a combination of fast electron/photon techniques to quantify the Purcell effect resulting from the coupling of dipolar emitters embedded in nanoparticles to plasmonic structures. NV 0 in nanodiamond-SP coupling is evidenced by the reduction of the mean excited state lifetime of a distribution of individual isolated nanodiamonds and dimers measurements. This effect could have been masked if we had limited our study to a few objects because of the instrinsic lifetime dispersion. We have shown that combination of fast electron/photon techniques provide the required measurement throughput, spatial and temporal resolutions to disentangle the two effects. The ensemble of techniques described here can be applied seamlessly to any emitter with excited state lifetime in the 0.5 ns -50 ns range which emit light under electron irradiation, covering a wide range of nanoscale systems.

Methods

Experiments have been performed with a Vacuum Generator HB-501 STEM equipped with a cold field emission electron gun operating at 60 kV, a Gatan 666 EEL spectrometer and the Attolight Mönch 4107 Cl-system. An homemade HBT interferometer has been implemented on the nano-CL system (Fig 1(a)). The stage of the microscope is cooled with liquid nitrogen down to 150 K. EEL spectrum images were deconvolved using a Richardson-Lucy algorithm 33 , with a zero-loss peak (ZLP) width reduction from 0.33eV to 0.1eV. Lifetimes were measured as described by Meuret et al 18,19 , allowing measurements in a few tens of seconds. Nanocubes were synthesized via a polyol reduction route 34 . The g (2) (τ ) measurements were performed with sampling time of 512 ps, using τ -SPADs single photon avalanche photodiodes and a Pi-coHarp 300 from PicoQuant. The overall system has a response time of 130 ps.

,

  FIG. 1. (a) Schematic of the STEM and HBT interferometer setup. (b) EELS spectrum (absorption) of a Ag nanocube measured in region P indicated in (d). Each peak corresponds to a plasmon resonance. The purple rectangle indicates the energy window where the optical filter is active. (c) ADF image of a Ag nanocube. (d) Energy filtered (at 1.8 eV) EELS map of the silver nanocube showed in (c). Scale bar: 50 nm.

FIG. 2 .

 2 FIG. 2. (a-b)From left to right: ADF image, energy-filtered NV0 emission intensity image and g(2) correlation function of single nanodiamonds. (c-d) From left to right: ADF image, energy filtered CL maps (see Fig.1) and g(2) correlation function of nanodiamonds close to a silver nanocube. Scale bars: 50 nm.

FIG. 3 .

 3 FIG. 3. Distribution of nanodiamonds cathodoluminescence lifetime alone (purple) and close to a silver nanocube (orange) measured on a population of 118 diamonds. The two vertical lines indicate the maxima of the nanodiamonds' lifetime distribution alone (22.5±2.5 ns in purple) and close to a silver nanocube (12.5±2.5 ns in orange).

FIG. 4 .

 4 FIG.4. (Purple) LDOS enhancement factor , γ due to SP sustained by an Ag cube, at 2.1 eV calculated along the green arrow represented on the inset. (Orange) Corresponding expected NV 0 lifetime. When the LDOS enhancement factor is equal to one, the lifetime is assumed to 22.5 ns (as measured in Fig.3).

Acknowledgements

This work was financially supported by the National Agency for Research under the program of future investment TEMPOS-CHROMATEM with the reference ANR-10-EQPX-50.


Jean-Yves Fowler

G Chaufray

S Collinson

G Curry

M S Chaffin

C M Fowler

J.-Y Chaufray

G Collinson

S Curry

G Hanley

R Lillis

J Luhmann

R Ramstad

S W Stone

S Xu

KEY UNKNOWNS FOR VENUS ATMOSPHERIC EVOLUTION 1: PRESENT-DAY D/H ESCAPE RATE UNCERTAINTIES AND IMPLICATIONS FOR UPCOMING HDO/H2O MEASUREMENTS

research documents, whether they are published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction:

The highly enriched D/H ratio on Venus relative to the Earth indicates that the overwhelming majority of the planet's water has escaped to space [1]. Even if such loss occurred primarily during a period of hydrodynamic escape, a substantial fraction of Venus' initial water would likely remain in the atmosphere after hydrodynamic escape ceased [2,3]. Such water would need to be removed either by surface processes or escape to space in order to account for the present low abundance of atmospheric water [4]. Venus H and D escape in the post-runaway epoch must occur via nonthermal channels, a necessity imposed by the Earth-like mass of the planet combined with its cool, Mars-like thermosphere temperature [5,6]. However, a lack of knowledge regarding key loss mechanisms leads to significant uncertainties in the possible current and geologically recent escape rates of H and D [7], posing challenges to the interpretation of planned DAVINCI and EnVision HDO/H2O measurements. In this presentation we will provide a summary of these uncertainties and suggest new analysis and observations that could improve knowledge of Venus H and D loss to space and fractionation.

Importance of Contemporary D/H Escape Rates for Venus Evolution: Hydrodynamic loss has likely played a major role in removing water from Venus, either early in the planet's history before condensation of the primordial steam atmosphere, or later in its history following a period of relatively mild, potentially habitable surface conditions. In either case, hydrodynamic escape is the result of gas outflow from a hydrogen-dominated atmosphere unable to balance the incident solar energy in any other way. Hydrodynamic escape ceases once enough H has escaped that two and three atom molecules can effectively cool the upper atmosphere via radiative and conductive processes. This transition is likely to occur before all atmospheric H has escaped, so that a substantial amount of H can remain in the atmosphere as water, perhaps ~10 bars or ~100 m GEL [4]. Because the present atmosphere contains only 2x10 -3 bars of H 2 O, the dregs remaining after the conclusion of hydrodynamic escape must be removed by nonthermal loss processes, of which photochemical escape and ion outflow (via ambipolar electric fields) are thought to be dominant [7].

If the amount of water left behind after hydrodynamic escape and the nonthermal escape rates are both known, limits can be placed on the time at which hydrodynamic escape ended, with important implications for Venus evolution [8,9]. Unfortunately, significant uncertainties in both photochemical escape and ion loss at the present epoch limit the conclusions that can be drawn. For ion loss, early upper limits on loss rates suggested that the process might be strongly dominant [7,10], but later investigations found much lower rates, below what is expected via photochemical processes [11]. For photochemical loss, existing model calculations of H and D escape are limited by uncertainties in available cross sections and input ion densities/temperatures, and by the recent discovery of several potentially important photochemical processes. Taken together, these uncertainties mean that it is unknown whether H or D escapes predominantly as a neutral or ion, as both ion and neutral loss rates are similar and uncertain by a factor of several. The fractionation factor of D/H loss, equal to the ratio of relative escape fluxes to relative atmospheric abundances, is similarly uncertain (see Table ).

Need for Improved Knowledge to Interpret Planned HDO/H2O Measurements: DAVINCI will measure HDO/H 2 O in the lower atmosphere of Venus at very high precision, providing a vital and longneeded constraint for understanding the evolution of . Whether ion or neutral loss dominates depends on the assumed ion escape boundary condition, and the total H loss rate and the fractionation factor vary by a factor of several.

water on the planet. Together with constraints on volcanism provided by VERITAS and EnVision, and in tandem with middle atmosphere HDO/H 2 O measurements made with the high-resolution infrared spectrometer suite on EnVision, the stage will be set for a re-evaluation of water evolution since the last climate transition on Venus. This comprehensive set of lower and middle atmosphere D/H measurements must be paired with renewed study of upper atmosphere D and H loss to space to fully understand the history of water on Venus.

For example, 1D evolution models proposed by Grinspoon (1993) [8] and others to study the long-term evolution of the Venus atmosphere as a function of the bulk HDO/H2O ratio and the escape rate of H to space are critically dependent on the fractionation factor of escape to space (see Figure). In the primordial hydrodynamic escape scenario, the effect of existing uncertainties in the fractionation factor of escape frustrates any attempt to infer the long-term H loss rate. Conversely, in a late runaway greenhouse / hydrodynamic escape scenario, existing uncertainties in H loss rates produce a >1 Gyr uncertainty in the timing of the transition. For both scenarios, renewed study of existing H and D escape datasets is essential, as is planning for future Venus orbiter measurements capable of constraining the loss rates of H and D as ions and neutrals. Beyond Venus, such measurements are important for understanding planetary evolution in general: because thermal H loss at present day Venus is negligible, the planet is a unique laboratory for the study of nonthermal H loss processes likely to control the water inventory of planets throughout the cosmos with CO2-dominated atmospheres and Earth-like (or greater) gravity.

In this presentation we will outline a program of analysis that would tighten uncertainty bounds on D/H escape derived from existing datasets, and discuss the possibility of new spacecraft measurements capable of constraining H and D escape from Venus. Curves in each panel show the evolution of D/H in water in the well-mixed atmosphere as a function of initial enrichment, enrichment of outgassed water vapor resupplying the atmosphere, the loss rate of H to space, and the fractionation factor of that loss. Gray line and shading in each panel shows the PVO descent probe D/H measurement and measurement uncertainty (i.e., the composition of the present-day atmosphere). (upper panel) A primordial loss scenario in which early enrichment due to hydrodynamic escape is supplemented by later loss at one of three rates consistent with existing measurements. In this scenario, the long-term average H loss rate over Venus history can be inferred from the present-day HDO/H 2 O ratio, but only if the fractionation factor of D/H escape is known quite precisely, as indicated by the red curve. (lower panel) A catastrophic outgassing scenario in which late hydrodynamic escape enriches the atmosphere to a very high value, which falls over time as the atmosphere is resupplied by outgassing of unfractionated water. In this scenario, a combination of precise bulk HDO/H 2 O measurements and knowledge of the escape rate can be used to determine the time since the event, but existing H escape rate uncertainties lead to a >1 Gyr uncertainty in timing.

References:

[1] Donahue+1982, [2] Gillmann+2020, [3] Krissansen-Totton+2021, [4] Gillmann+2022, [5] Anderson1976, [6] Hodges1999, [7] Lammer+2006, [8] Grinspoon1993, [9] Donahue1999, [10] Hartle&Grebowsky1993, [11] Persson+2018

Figure :

 : Figure:Evolution of Venusian HDO/H2O as a function of time, adapted from Grinspoon+1993. Curves in each panel show the evolution of D/H in water in the well-mixed atmosphere as a function of initial enrichment, enrichment of outgassed water vapor resupplying the atmosphere, the loss rate of H to space, and the fractionation factor of that loss. Gray line and shading in each panel shows the PVO descent probe D/H measurement and measurement uncertainty (i.e., the composition of the present-day atmosphere). (upper panel) A primordial loss scenario in which early enrichment due to hydrodynamic escape is supplemented by later loss at one of three rates consistent with existing measurements. In this scenario, the long-term average H loss rate over Venus history can be inferred from the present-day HDO/H 2 O ratio, but only if the fractionation factor of D/H escape is known quite precisely, as indicated by the red curve. (lower panel) A catastrophic outgassing scenario in which late hydrodynamic escape enriches the atmosphere to a very high value, which falls over time as the atmosphere is resupplied by outgassing of unfractionated water. In this scenario, a combination of precise bulk HDO/H 2 O measurements and knowledge of the escape rate can be used to determine the time since the event, but existing H escape rate uncertainties lead to a >1 Gyr uncertainty in timing.

Table :

 : Photochemical and ion loss rates of D and H via computed by Hodges+1999 based on Pioneer Venus Orbiter measurements [6], and the fractionation factor of escape, 𝑓𝑓 =

	ϕ 𝐷𝐷 /ϕ 𝐻𝐻
	[HDO]/2[H2O]

8021.pdf 20th Meeting of VEXAG 2022 (LPI Contrib. No. 2688)


Guerres, conflits et collections. Les musées et l'obligation de restituer des biens culturels.

Publié dans : Marie Cornu, Jérôme Fromageau, Dominique Poulot (dir.), 2002. Genèse d'une loi sur les musées, éd. La documentation française/Comité d'histoire du ministère de la culture, 2022, pp. 73-82.

Guerres, conflits et collections. Les musées et l'obligation de restituer des biens culturels

Vincent NEGRI Chercheur HDR à l'Institut des sciences sociales du politique (UMR 7220) En miroir du concept de musée s'instille l'idée de faire collections. C'est à partir de cette dynamique qu'a été édifiée la définition du musée posée par la loi du 4 janvier 2002, dorénavant énoncée à l'article L. 410-1 du Code du patrimoine. En regard, restituer intègre l'idée de défaire la collection. Dans le jeu des oppositions, ce qui fait le musée contredit ce que restituer induit, surtout dans les énoncés d'un droit qui sanctuarise les collections. Loin d'être exorbitant, ce droit façonné par la domanialité publique -tout au moins pour la part prépondérante des musées publics -verrouille la disponibilité des collections par le double jeu de la règle de l'inaliénabilité et celle de l'imprescriptibilité. Cette double règle fonde un principe de conservation et d'accessibilité du public aux collections ; la propriété publique des collections muséales est une propriété spéciale qui, loin d'un modèle propriétaire exclusif entre les mains de la personne publique, garantit au public l'accès à ce qui constitue une part du patrimoine culturel de la nation. On entrevoit alors la densité normative de ce que vient percuter l'idée de restitution ; la restitution signe la privation du public, dans le territoire du musée.

Pour autant, l'inaliénabilité et l'imprescriptibilité, toute apparence d'absolu que projettent ces règles, ne constituent pas les unités de mesure intangibles pour évaluer les réponses apportées aux demandes de restitution. Le droit peut aussi signifier des conditions auxquelles la restitution devient une obligation à la charge des musées. C'est le mode de constitution des collections qui peut porter les germes de cette obligation, lorsque les collections témoignent de la violence de l'arrachement au lieu qui les abritait et au propriétaire légitime, par la guerre ou le pillage, par le vol ou les spoliations. L'idée même de musée peut être le symptôme de ces appropriations violentes. C'est cette trajectoire que tracent Louis Aragon et Jean Cocteau dans les Entretiens sur le Musée de Dresde 1 lorsqu'ils s'interrogent sur l'origine du musée : 1 Aragon, Jean Cocteau, Entretiens sur le Musée de Dresde, Paris, Éd. Cercle d'Art, 1957, p. 11. COCTEAU : Je voudrais te demander quelque chose. Je suis très inculte. De quand datent les musées ? Car jadis on ne pendait jamais les tableaux pour que les gens viennent les voir. Ils ornaient les maisons et les églises. On se les offrait et on les offrait à Dieu. L'enterrement du Comte d'Orgaz se trouve à Tolède dans un coin sombre. Qui pouvait le voir sans l'électricité ? Jamais on n'avait imaginé accrocher les tableaux les uns à côté des autres pour que les gens défilent devant leurs cadavres et les identifient. De quand datent les musées ? voilà ce que je te demande.

ARAGON : Je dois te dire que je n'en sais à peu près rien. Il y a bien eu des collections publiques en Grèce et à Rome. Mais autant que j'imagine, les musées, au moins en France, ailleurs j'ignore, sont une invention de la période moderne pour cette raison que les gens qui pouvaient acheter les tableaux, avant, étaient les gens puissants, des riches, il y en avait dans les palais des rois, au bout du compte les tableaux étaient faits pour les rois ou leurs fermiers généraux, mais par conséquent, pas pour qu'un très grand public y défile devant. Cet impérialisme signerait une violence originelle de l'institution muséale ; l'assertion renvoie peu ou prou à la notion de musée prédateur 2 et, dans une acception plus ethnographique, à celle de musée cannibale 3 , offrant un « espace pour l'ingestion de l'autre et un simulacre d'ouverture à l'altérité en laissant penser que cet autre devenu même est enfin assimilable4 ». Mais, désigner le musée comme le siège d'une violence dont témoigneraient la constitution des collections et leur appropriation univoque ne rend pas compte, aujourd'hui, de la diversité des formes muséales5 ; pour autant, le musée est en partie le fruit de cet impérialisme nourri des guerres et des conquêtes. C'est dans ce hiatus que s'insinue l'obligation de restituer des biens culturels, en contrepoint de la codification du droit de la guerre à la charnière des XIX e et XX e siècles.

Des manifestations de l'impérialisme, que Cocteau lie à la naissance du musée, perdurent sous 2

The Predatory Museum, ICOFOM Study Series, vol. 45, 2017. 3 Marc-Olivier Gonseth, Jacques Hainard et Roland Kaehr (dir.), Le musée cannibale, Neuchâtel, Éd. Alphil, 2002. une forme policée dans l'expression contemporaine de musée universel et dans la revendication de cette qualité par des musées occidentaux. En 2002 est signée la Déclaration sur l'importance et la valeur des musées universels par un panel de musées occidentaux 6 soucieux de légitimer des acquisitions anciennes en s'accordant sur « le fait que le musée offre lui aussi un contexte pertinent et précieux aux objets retirés de longue date de leur environnement original ». Il reste à interpréter ou, selon les cas, mésinterpréter cette dernière notion -« retirés de longue date de leur environnement original » -qui amorce le concept de musée universel. Ce concept, tout à la fois univoque et équivoque, est mobilisé par des musées occidentaux qui, en s'autoproclamant musées universels, entendent éteindre la propagation de revendications de collections, notamment archéologiques, dans le sillage des réclamations grecques sur les frises du Parthénon 7 . Il ne s'agit plus alors d'une obligation de restitution arrimée au droit international conventionnel, comme c'est le cas pour les pillages en temps de guerre depuis le début du XX e siècle, mais d'une obligation dont la charge juridique est moins dense, car fondée sur l'éthique armée par l'équité.

Une obligation de restituer pour réparer la prédation du patrimoine artistique des nations

L'article 47 des Conventions de La Haye de 1899 et de 1907 concernant les lois et coutumes de la guerre sur terre pose, en cinq mots, une règle du droit international dont la concision ajoute à la fermeté : « Le pillage est formellement interdit ». On objectera que le respect de cette règle a été pour le moins ténu dans les deux guerres mondiales du XX e siècle et dans les conflits régionaux qui éraillent l'histoire de ce siècle et se poursuivent encore. Cette lecture de la portée de la règle est légitime ; pour autant elle minimise par trop le double apport de la règle.

Le libellé de l'article 47 reprend l'article 39 du projet de Déclaration internationale concernant les lois et coutumes de la guerre, qui n'avait pu être adopté lors de la conférence diplomatique réunie à Bruxelles le 27 août 1874 à l'initiative du Tsar Alexandre II de Russie. L'article 47, doté de la force normative de l'obligation formulée dans une convention internationale, efface des siècles de droit au butin et de pratique des trophées, auxquels la tradition la plus ancienne n'a cessé d'attacher une dimension symbolique affermie par l'idée selon laquelle « l'appropriation des richesses artistiques d'un peuple, fruit de son génie national, apparait de tout temps comme un trophée qui contribue à entretenir chez les vainqueur l'exaltation de la victoire, chez le vaincu l'humiliation de la défaite 8 ». Suffolk Transnational Law Review, vol. 29, 2005-2006, p. 315-336. 8 Charles de Visscher, « Les monuments historiques et les oeuvres d'art en temps de guerre et dans les traités de paix », Recueil de législation comparée et de droit international, Office international des musées, n° 2, 1940, On a sans nul doute sous-estimé la conversion du droit international qu'opère cet article 47, effaçant un droit au butin multiséculaire. Les armées en campagne perdent le droit de s'accaparer les biens publics et privés, et les armées victorieuses ne peuvent plus se glorifier de leur puissance en exposant les « dépouilles opimes, comme des étendards pris à l'ennemi ». La guerre n'en est pas moins âpre, mais les prémisses de principes d'humanités, qui amorcent la formation d'un droit international humanitaire, se dessinent dans le sillage de l'initiative d'Henry Dunant, créant la Croix-Rouge, à la suite de la bataille de Solferino en 1859.

La prohibition du pillage est renforcée pour les biens culturels par l'article 56 des Conventions de La Haye de 1899 et de 1907 concernant les lois et coutumes de la guerre sur terre qui installe un principe d'immunité du patrimoine historique et artistique des nations, lors des conflits armés. Dorénavant, « toute saisie, destruction ou dégradation intentionnelle d'établissements consacrés à l'instruction, aux arts et aux sciences, même appartenant à l'État, de monuments historiques, d'oeuvres d'art et de science, est interdite et doit être poursuivie ».

Le respect de ce principe d'immunité des biens culturels dans les conflits armés est devenu une obligation primaire que doivent satisfaire les États belligérants ; cette obligation est aujourd'hui reconnue comme relevant du droit international coutumier et s'impose de jure à tous les États, qu'ils soient Parties ou non aux Conventions de la Haye de 1899 et 1907. De la violation de cette obligation découle une obligation secondaire de réparer les préjudices que provoquent la saisie, la destruction ou la dégradation intentionnelle. C'est alors sur le terrain de la responsabilité internationale que s'installe l'obligation de réparer qui, s'agissant des atteintes au patrimoine artistique, peut revêtir la forme d'une obligation de restitution pour reconstituer les collections dispersées. Cette articulation entre la violation d'une obligation primaire et l'obligation secondaire de restitution n'est pas fluide ; le principe de restitution est lié à la reconnaissance par son auteur de la violation de l'obligation primaire. À défaut, la restitution ne peut être fondée sur la responsabilité internationale de l'État, auteur de la violation du principe d'immunité des biens culturels. Cette question émergeait déjà dans les analyses du projet de Déclaration internationale concernant les lois et coutumes de la guerre, discuté à Bruxelles en 1874 ; au lendemain de l'échec de l'adoption de ce projet, l'Institut de droit international évoquait, dans un rapport daté d'août 1875, la reconnaissance d'un principe général de restitution.

La restitution des biens culturels accaparés par les belligérants dans les guerres et intégrés dans les collections publiques a toujours été une question âprement discutée. Il y a loin des considérations du duc de Wellington formulées en 1815 à la satisfaction du principe de restitution alors énoncé. Le 23 septembre 1815, alors que les semaines précédentes avaient bruissé des réclamations introduites par les souverains européens alliés contre la France, le duc de Wellington écrit à Lord Castlereagh : « Les mêmes sentiments qui font désirer au peuple français de garder les tableaux et les statues des autres nations, doivent faire désirer aux autres nations, maintenant que la victoire est de leur p. 16. côté, de voir restituer ces objets à leurs légitimes propriétaires et les souverains alliés doivent favoriser ce désir. Il est de plus à désirer, pour le bonheur de la France et pour celui du Monde, qu['] on […] fasse sentir [au peuple français] que quelque grands qu'aient pu être ses avantages partiels et temporaires sur une ou plusieurs des puissances de l'Europe, le jour de la restitution doit arriver à la fin 9 ».

Le jour de la restitution doit arriver à la fin : on aurait pu attendre des Conventions de la Haye de 1899 et 1907, et de leur universalité, qu'elles installent un principe de restitution des patrimoines pillés dans l'économie de la responsabilité collective des États envers le patrimoine culturel de toutes les nations 10 . Cette question a été éludée, et les premières traces de cette élision peuvent être observées dans l'assertion d'Euripide Foundoukidis, relevant en 1931 : « la nouvelle conception qui se fait jour depuis quelque temps et qui tend à considérer certains monuments d'art comme appartenant au patrimoine commun de l'humanité. Il semble qu'il y a là en formation un nouveau principe de droit international dans le domaine artistique 11 ».

L'ordre culturel international que façonne l'UNESCO à partir de 1945 est adossé à cette conception, qui ménage un équilibre entre l'idée de « protection d'un patrimoine universel de livres, d'oeuvres d'art et d'autres monuments d'intérêt historique ou scientifique 12 » et la souveraineté des États -horizon infranchissable -qu'un principe de restitution, dès lors qu'aurait été violée une obligation internationale de respect et de conservation du patrimoine, viendrait dérégler. portant deuxième application de l'ordonnance du 12 novembre 1943 sur la nullité des actes de spoliation accomplis par l'ennemi ou sous son contrôle et édictant la restitution aux victimes de ces actes de ceux de leurs biens qui ont fait l'objet d'actes de spoliations14 . Sur ce fondement, les actions en revendication des biens culturels spoliés pendant la Seconde guerre mondiale sont imprescriptibles.

S'obliger à restituer : les restitutions volontaires comme paradigme muséal ?

Envisager la restitution des biens culturels comme une obligation que satisferaient volontairement les musées va à rebours d'un droit des musées et d'une pensée muséale concentrée sur le statut des collections. Pour autant, la question de la restitution n'est pas dans un angle mort de la politique des musées ; elle est prise en charge par les normes déontologiques qui produisent un ordre parallèle à celui de la fabrique du droit par les organes de l'État15 . C'est alors une autorégulation de la profession qui génère les principes essentiels fondateurs d'une pratique professionnelle. Sur certaines questions -dont celle de la restitution des biens culturels -la déontologie se déploie dans les jachères du droit du patrimoine culturel ; s'agissant des revendications, elle investit le terrain de la légitimité de la détention et des acquisitions.

Le Code de déontologie professionnelle du Conseil International des Musées (ICOM), adopté en 1986, puis révisé en 2001 et en 2004, énonce les préceptes déontologiques communs à l'ensemble des professionnels de musées, répartis sur les cinq continents, et dont le respect est alors « considéré comme une condition minimale pour pouvoir être membre de la profession muséale16 ». Le Code de l'ICOM prolonge et développe l'Éthique des acquisitions, adoptée par l'ICOM en avril 1970, alors que se dessinaient les contours de la convention qui sera adoptée par l'UNESCO quelques mois plus tard17 .

En 1970, l'Éthique des acquisitions délimite le périmètre des missions des musées, répartis entre recherche scientifique, éducation, préservation et mise en valeur de l'héritage national et international, naturel et culturel. Les principes affirmés sont concentrés sur les conditions de constitution des collections, la provenance et la signification, culturelle, rappelant à ce titre que « l'objet n'a une signification (culturelle et scientifique) que s'il est complètement documenté » et qu'« aucune acquisition ne devrait être faite en l'absence de cette documentation ». L'orientation de ces principes ne ménage qu'un espace restreint pour une articulation avec les pays de provenance des collections et le dialogue avec les musées hors du territoire national. Ces questions sont ramassées dans un point 9 de l'Éthique des acquisitions : « 9. La responsabilité du muséologue dans les musées qui ont comme mission essentielle la préservation du patrimoine national est triple : a) acquérir et préserver pour le pays en question des collections exhaustives illustrant tous les aspects du patrimoine naturel et culturel de la nation ; b) aider au contrôle du mouvement international des objets appartenant à ce patrimoine ; c) coopérer avec les musées étrangers et les autres institutions scientifiques afin d'assurer une représentation correcte de sa culture au plan international. » Il serait hâtif de considérer que l'objectif « d'assurer une représentation correcte de sa culture au plan international » dévoile une problématique de restitution. C'est dans la dernière version du Code de déontologie de l'ICOM -révision lors de la Conférence générale de l'ICOM tenue à Séoul en 2004 -que cette problématique trouve son expression, dans une double entrée associant les concepts de retour et de restitution, aux articles 6.2 et 6.3 : « 6.2 Retour des biens culturels Les musées doivent être disposés à engager le dialogue en vue du retour de biens culturels vers un pays ou un peuple d'origine. Cette démarche, outre son caractère impartial, doit être fondée sur des principes scientifiques, professionnels et humanitaires, ainsi que sur la législation locale, nationale et internationale applicable (de préférence à des actions à un niveau gouvernemental ou politique).

Restitution de biens culturels

Si une nation ou une communauté d'origine demande la restitution d'un objet ou spécimen qui s'avère avoir été exporté ou transféré en violation des principes des conventions internationales et nationales, et qu'il s'avère faire partie du patrimoine culturel ou naturel de ce pays ou de cette communauté, le musée concerné doit, s'il en a la possibilité légale, prendre rapidement les mesures nécessaires pour favoriser son retour. » Le vocabulaire reprend une distinction classique en droit international, chargeant le terme retour par la recherche d'un rapport culturel et scientifique équilibré et d'une instance de dialogue, et projetant sous le vocable restitution la réparation d'un acte illicite. Cette distinction, dont la pertinence peut être discutée, s'érode aujourd'hui.

L'abandon d'une telle distinction est d'autant plus souhaitable qu'envisager la restitution dans cette acception juridique repose sur un postulat d'universalisme des Conventions de La Haye qui, à la charnière des XIX e et XX e siècles, ont interdit le pillage ainsi que la saisie des biens culturels dans les guerres. Or, en cette matière, il convient de « résister aux raisonnements ex post facto qui projettent dans le passé la configuration actuelle des relations internationales et du système étatique18 » et, ce d'autant plus, que ces Conventions de La Haye aménage un système de règles de droit public liant les nations civilisées d'Europe à une époque où le qualificatif civilisée était déniée à une partie du monde. Les articles 22 à 24 du Pacte de la Société des Nations adopté en 1919 sont un des symptômes de cet arbitrage de l'universalisme.

Il est dès lors loisible de s'interroger si une approche volontaire, initiée et maitrisée par les

6

  Déclaration du 8 décembre 2002 sur l'importance et la valeur des musées universels, signée par les directeurs de l'Institut d'Art de Chicago, du Musée bavarois à Munich (Alte Pinakothek, Neue Pinakothek), des Musées d'État à Berlin, des Musées d'Art de Cleveland, du Musée Getty à Los Angeles, du Musée Guggenheim à New-York, du Musée d'Art du comté de Los Angeles, du Musée du Louvre à Paris, du Musée d'Art métropolitain à New York, du Musée des Beaux-Arts de Boston, du Musée d'Art moderne à New York, de l'Opificio delle Pietre Dure à Florence, du Musée d'Art de Philadelphie, du Musée du Prado à Madrid, du Rijksmuseum à Amsterdam, du Musée de l'Ermitage à Saint-Pétersbourg, du Musée Thyssen-Bornemisza à Madrid, du Musée Whitney d'art américain à New York, du British Museum à Londres. 7 Christine K. Knox, « They've lost their Marbles: 2002 Universal Museum's Declaration, The Elgin Marbles and the Future of the Repatriation Movement »,

  Je suppose que c'est après la Révolution française que le Musée est réellement né. Après un essai sans lendemain au XVIII e siècle, le Directoire a fondé les musées de province. Il y a eu le musée Napoléon … COCTEAU : ... Et lorsque les tableaux ont été mis en circulation par les guerres, par le déséquilibre des guerres, par les pillages ... ARAGON : … Oui, naturellement, avec Napoléon ; et le Musée Napoléon, qui a été la première forme du Louvre, a été démembré lorsqu'en 1815, Canova est venu récupérer les oeuvres d'art pour le compte des Alliés … Ensuite beaucoup de choses ont été réparties entre le Louvre et les Tuileries. C'est en fait la Révolution de 1830, et surtout la Révolution de 1848 qui dans notre pays ont créé les Musées […].

	COCTEAU : En somme, on raflait les tableaux après la victoire et on les montrait au peuple
	comme des dépouilles opimes, comme des étendards pris à l'ennemi. C'est de cet impérialisme
	que dut naître le musée.

Ibid., p. 13.

Voir : François Mairesse, « Musée », in André Desvallées et François Mairesse (dir.), Dictionnaire encyclopédique de muséologie,Paris, Éd. Armand Colin, 2011, p. 271-320.

Mais un autre écueil se dresse vers l'expression d'un principe de restitution : l'acte source de la dispersion ou de la spoliation des biens culturels doit constituer une violation d'une obligation internationale posant un principe reconnu par la communauté internationale. En d'autres termes, la restitution fondée sur la responsabilité internationale ne peut intervenir qu'en réparation d'une action illégale au regard du droit international ; ce qui exclut du champ de la restitution les spoliations coloniales sauf à ce qu'elles soient intervenues lors d'un conflit armé.

JO 22 avril 1945JO 22 avril , p. 2283JO 22 avril -2285. .

Vincent Négri, « Les figures déontologiques du droit du patrimoine culturel », in Philippe Blachèr (dir.), Déontologie et droit public,Paris, LGDJ, 2014, p. 153-171.

Extrait du Préambule du Code de l'ICOM en 1986.

Convention sur les moyens d'interdire et d'empêcher l'importation, l'exportation et le transfert de propriété illicites de biens culturels, adoptée par la Conférence générale de l'UNESCO le 14 novembre 1970.

Isabelle Schulte-Tenckhoff, « L'Autre et le traité. Pour une anthropologie des traités autochtones », in Un droit pour des hommes libres : études en l'honneur du professeur d'Alain Fenet, Paris, Litec, 2008, p. 245.


Montserrat Gomà-I-Freixanet
email: montserrat.goma@uab.cat

Anna Muro

Does the sensation-seeking trait differ among participants engaged in sports with different levels of physical risk?

Keywords: personality, risk taking, sports participation, sensation seeking, athletes

Does the sensation-seeking trait differ among participants engaged in

Does the sensation-seeking trait differ among participants engaged in sports with different levels of physical risk?

Our ancestors, when they were organized in hunter-gatherer societies, explored new territories in the pursuit of food and water to have better opportunities for mating behaviour and for the growing of their offspring. This exploratory behaviour entailed gains (like new resources and increased survival of the group) as well as risks that were mainly physical in nature (increased probability of being injured or death). Today, human beings do not have the immediate need of exploring new territories in search of food and water, but they still engage in exploratory behaviour that entails risks. With the complexity of our contemporary societies, these risks are not anymore limited to physical risks, but also entail legal, economic, social, and political risks.

Therefore, the concept of risk is studied in different scientific disciplines, lacking a unitary meaning and interpretation. For example, the field of international relations and politics relies on prospect theory [START_REF] Levy | Prospect theory and international relations: theoretical applications and analytical problems[END_REF][START_REF] Levy | Prospect theory, rational choice and international relations[END_REF] which states that decision makers accept risks to avoid "losses" but refuse to take risks to make comparable gains. That is, most people are risk averse when facing an opportunity for a gain but are risk acceptant when facing the prospect of a loss. Another theory devoted to risk is that of [START_REF] Weinstein | Unrealistic optimism about future life events[END_REF]. This author drew attention to what he called the "popular belief" which states that people tend to perceive themselves as invulnerable. This "optimistic bias" holds for a wide range of health and other outcomes, and he coined the term "unrealistic optimism" to define it. In the field of risky sports research, a useful theory is that of reversal theory [START_REF] Smith | A theory of psychological reversals[END_REF]. Of particular interest for the behavioural analysis of risky sports is the theory's concept of "paradoxical behaviour" which refers to conduct that is inessential for human survival but is "voluntarily undertaken and yet (...) appears to militate against the health, well-being, and even survival of the individual concerned, exposing him or her to gratuitous risk" (Apter & Batler, 1997, p.119-120). Such conduct includes the performance of potentially harmful behaviours (to self and/or others), for instance, participation in dangerous sports.

The three theories described above have in common that they try to describe behaviour related to risk taking. However, none of these theories give much importance to individual differences in personality. This is surprising, given the important influence of psychological processes in risk perception that are propelled by stable personality traits. A personality theory inserted in the trait theory tradition and devoted to the study of individual differences in risk taking is Zuckerman's theory of Sensation Seeking (SS; [START_REF] Zuckerman | Sensation Seeking: Beyond the optimal level of arousal[END_REF][START_REF] Zuckerman | Behavioural expressions and biosocial bases of sensation seeking[END_REF]. The Sensation Seeking theory is based on a model that has social, biological and psychophysiological underpinnings (e.g., [START_REF] Duaux | Homozygosity at the dopamine D3 receptor gene is associated with opioid dependence[END_REF][START_REF] Fowler | Platelet monoamine oxidase activity in sensation seekers[END_REF][START_REF] Legrand | Does sensation seeking predict binge drinking in college students? Some ecological data collected in "open bar" parties[END_REF][START_REF] Stacy | Personality, problem drinking, and drunk driving: Mediating, moderating, and direct-effect models[END_REF] that influence behaviour, and the manifestation of attitudes and preferences. The Sensation Seeking trait is defined as "the seeking of varied, novel, complex, and intense sensations and experiences, and the willingness to take physical, social, legal, and financial risks for the sake of such experience" (Zuckerman, 1994, p. 27).

Zuckerman and colleagues [START_REF] Zuckerman | Development of a sensation seeking scale[END_REF] developed the Sensation Seeking Scale (SSS) to assess this trait based on the idea that consistent individual differences exist in optimal levels of arousal and stimulation. Research over a period of three decades led to several revisions of the questionnaire from Form I (1964) to VI (1984). Although Form VI of the SSS [START_REF] Zuckerman | Experience and Desire: A new format for Sensation Seeking Scales[END_REF] is the most updated version with only slight modifications from the previous one, Form V (1978) is by far the most widely used. The SSS-V has a total of 40 dichotomized items divided into four 10-item subscales. The Thrill and Adventure Seeking (TAS) scale contains items expressing a desire to engage in sports or other physically risky activities that provide unusual sensations of speed or defiance of gravity, such as parachuting, scuba diving, or skiing. Because most of the activities are not common, the majority of the items are expressed as intentions ("I would like...") rather than reports of experience. An attitude item that summarizes the factor is: "I sometimes like to do things that are a little frightening". The Experience Seeking (ES) scale encompasses items measuring the seeking of novel sensations and experiences through the mind and senses, as in arousing music, art, and travel, and through social nonconformity, as in association with groups on the fringes of conventional society (e.g., artists). The Disinhibition (Dis) scale contains items describing seeking sensation through social activities like parties, social drinking, and sex. The Boredom Susceptibility (BS) scale measures intolerance for repetitive experiences of any kind, including routine work and boring people. A total score (Total) is obtained by summing the four subscales. Finally, in the specific context of sports, an additional TAS-OUT score was computed in some studies [START_REF] Gomà-I-Freixanet | Personality profile of participants engaged in high physical risk sports[END_REF], 1995[START_REF] Gomà-I-Freixanet | Prosocial and antisocial aspects of personality in women: a replication study[END_REF][START_REF] Jack | Sensation seeking among high-and low-risk sports participants[END_REF] by summing all SSS subscales but TAS (thus ES, Dis, and BS). As some of the items from the TAS subscale are concerned with physically risky sports and activities that athletes can actually be participating in, the TAS-OUT scale controls for the possibility of variance in Total SS scores being due solely to sports participation.

One implication of the SS construct in the context of sports is that the particular sport discipline one is more likely to participate in may be associated with whether one is high or low on the SS trait. Of course, additional factors such as physical ability, age, gender, and economic status are important determinants as well but are beyond the scope of this paper. In 1983, Zuckerman published the first paper reviewing the literature on SS and sports. Since then a great deal of new empirical data has accumulated and methodological improvements have been applied that merit a new review of the literature. Thus, the purpose of this paper consists of reviewing the SS trait in the context of physically risky sports in order to establish whether data show that the SS trait differs among participants engaged in sports with different levels of physical risk.

Method

The editorial on research literature review published in Psychological Bulletin (Cooper, 2003) provided guidelines for methods of data collection of this literature survey.

Identification of studies

To identify pertinent studies on the relationship between the SS trait and participation in physically risky sports, a systematic search of published references was executed in the PsycINFO and Web of Knowledge databases from January 1964 until December 2007 inclusive. Two independent investigators did this search to control for possible mistakes and oversights regarding the papers selected for this review. We used the following keywords: "risk(s)", "sport(s)", "sensation(-)seeking", "behaviour(s)", and "personality", to cover the whole range of risky sports and personality.

Inclusion criteria

Only peer-reviewed articles were considered. We did not include neither dissertations and theses, nor conference proceedings or book chapters. Studies were included only if they provided empirical data and compared groups with different levels of physical risky sports (e.g., a control group and a sport one, or comparing different sports). Since there can be confusion concerning the meaning of the term "sport", we must precise that we used such a term to design all physical leisure pursuits, whether or not they are federative or competitive practices. The database searches produced 122 article titles. Of these articles, only 35 met our inclusion criteria.

Method of review

Papers were classified into high-medium-and low-risk sports and presented in three different tables, one for each level of risk. To make a systematic data extraction, we devised a form to describe the general characteristics of the paper. The data extracted from the 35 papers is presented in chronological order and in the following manner: in the first column named "sport" we have placed the kind of physically risky sport; on the second column named "author" the authors and year of publication are stated. At the third column, there is the sport practiced by the experimental group along with the sex and number of participants. At the fourth column, there is stated the kind of participants, sex and number of them that acted as a control group. The fifth column illustrates the statistical significant differences encountered among the compared groups including the sense of the differences. Finally, the last column "notes", states the version of the questionnaire used and some other observations that can be of interest when interpreting the results obtained in the specific study.

Classification criterion

Furthermore, since few consensus exists on how to best categorize the different kinds of sports, in the present study, we opted to use the same classification Zuckerman used. Thus, sports were grouped into categories with high, medium, and low physical risk according to the associated risks involved.

High physical risk sports were those considered with a high probability of serious injury or death as a consequence of practicing such a sport. Sports like climbing, parachuting, speleology or white water kayaking where participants have to struggle with the strong and unpredictable forces of nature are examples of this classification. We identified 26 papers devoted to high physical risky sports embracing different sports. The most studied sports category was parachuting/parasailing/hang-gliding. Medium physical risk sports were those with a higher probability of being injured than encountering death, the arena where the sport takes place is limited and the environment is static. Sports like boxing, karate, rugby or American football are good examples for this category. We found 6 studies on medium physical risky sports, such as karate and rugby and one category of miscellaneous sports. Finally, low physical risk sports were those with a very low probability of being seriously injured or having a fatal injury occurrence. Sports such as running, gymnastics, bowling or golf fit into this classification. We identified 4 studies devoted to these sports, studying sports like athletes.

Analysis and interpretation of the results

In general, studies compare SS scores either between groups practising sports with similar or different levels of risk, or with control groups. The gender of the athletes, the kind of sports practised, the level of competition, and the classification of individuals (elite versus subelite sportspersons, etc.) vary considerably across the studies. This hardly comes as a surprise taking into account that the first paper meeting inclusion criteria was published in 1974. Some studies differentiate genders while others do not. Some use team sports like rugby while others use individual sports such as parachutism. Some studies differentiate between competition levels (using a host of varying criteria such as expert nominations or membership of a national team) while others do not. Authors also use different criteria to assign individuals to a given level of risk (e.g., number of accidents, level of uncertainty, the dynamics of the sports situation, or international grading systems as in kayaking or climbing). Control groups also vary considerably since sports students, college students, sportsmen with a different level of risk, normative data, participants specifically selected for not practising any risky sport, and the general population are used.

The high heterogeneity encountered in the categorizations used by the different authors along these 34 years of research made us conclude that a mere statistical combination of the results encountered might mask important differences in research findings, and thus might drive us to invalid conclusions (Cooper, 2003). Thus, we opted for a systematic search, followed by an attempt to draw unbiased conclusions as a more valid method to deal with such a diversity of studies. This methodology still falls under the umbrella of a systematic review, given that the literature is searched systematically and the data extracted in a predetermined way.

Results

The reviewed studies are presented in Tables 1, 2 and 3. In order to integrate the empirical data obtained and draw summary statements of this highly heterogeneous data, we will adopt the following presentation of results in this paper. First sensation seeking is reviewed in high, medium, and low physically risky sports (individual Tables 123). Then, by cross-comparing the three tables, we can further analyse how the distribution of sensation seeking is affected when sports of similar and different physical risk are compared. Finally, SS in sports with different physical risk is compared to controls.

Sensation seeking in high physical risk sports

A total of 26 studies were identified devoted to SS in high physical risk sports (Table 1). These high physical risk sports were compared to either sports with similar physical risk, low physical risk, or with controls.

INSERT TABLE 1 ABOUT HERE

When we compare SS in high risk sports with SS in sports with a similar level of risk, a large majority of studies show that sportspersons do not differ from each other on the SSS, neither on the specific subscales nor on the Total score. Studies such as those of Gomà-i-Freixanet (1991), [START_REF] Rossi | The sensation seeking in mountain athletes as assessed by Zuckerman's Sensation Seeking Scale[END_REF], [START_REF] Breivik | Personality, sensation seeking and risk taking among Everest Climbers[END_REF], [START_REF] Breivik | Personality, psychological states and heart rate in novice and expert parachutists[END_REF], and [START_REF] Lafollie | Détection des personnalités à risque dans les sports à sensations fortes [Detection of high-risk personalities in risky sports[END_REF] illustrate these findings. These are endeed the results expected when comparing participants practicing sports with similar level of risk. We found only one study [START_REF] Cazenave | Psychological profiles and emotional regulation characteristics of women engaged in risk-taking sports[END_REF] showing a difference between a group of sportswomen engaged in high-risk sports for leisure purposes, and one in high-risk sports as professional, since the former scored higher on SSS than did the latter.

When we compare high risk sports versus low risk sports, participants differ on all subscales of the SSS as well as on the Total score [START_REF] Cantón | The sensation of risk and motivational tendencies in sports: an empirical study[END_REF][START_REF] Jack | Sensation seeking among high-and low-risk sports participants[END_REF][START_REF] Rainey | Sensation Seeking and competitive trait anxiety among college rodeo athletes[END_REF][START_REF] Wagner | Sensation seeking and trait anxiety in hang-glider pilots and golfers[END_REF][START_REF] Zarevski | Contribution of Arnett's inventory of sensation seeking and Zuckerman's sensation seeking scale to the differentiation of athletes engaged in high risk and low risk sports[END_REF]. Participants also differed on the variable named TAS-OUT in the studies that included it. That is, high-risk sportspersons score significantly higher on TAS, ES, Dis, BS, Total and TAS-OUT compared to low-risk sportspersons.

When we finally compare high risk sports with controls (either college students, sports students, general population or simply non-athletes), the above described tendency is still maintained with high significant scores on TAS, ES and Total, and even TAS-OUT. They do not differ significantly on BS and on Dis. Regarding Dis however, when we take a closer look at the studies, we can observe that when comparing high risk sportspersons to the general population and controlling for age [START_REF] Gomà-I-Freixanet | Personality profile of participants engaged in high physical risk sports[END_REF], 1995[START_REF] Gomà-I-Freixanet | Prosocial and antisocial aspects of personality in women: a replication study[END_REF][START_REF] Zaleski | Sensation seeking and risk taking behaviour[END_REF], the former do score significantly higher on Dis, but this effect is less clear when using sports students as a control group and it disappears totally when the contrast group is college students. This is probably due to the fact that these studies did not control for age and, in general, college students are younger than professional sportspersons. It is known that all SS scales scores decline with age [START_REF] Zuckerman | Sensation seeking in England and America: cross-cultural, age and sex comparisons[END_REF]. Therefore, the older professionals (in general being in the 30-39 age range), by not differing significantly from college students, actually score higher than their age group. Thus, professional sportspersons actually score higher on Dis as well. All in all, high-risk sportspersons either compared to athletes practicing sports with lower level of risk or to controls (controlling for age) show higher scores on the Total SSS and on all its subscales, except BS when compared to controls.

These results suggest that sportspersons who practise sports with a high level of risk enjoy extreme environments with great opportunities for encountering stimulation through the mind and senses such as height, depth, speed, a great amount of light or darkness, and changes in climatic conditions (wind, temperature); they like new, exciting and unconventional but not necessarily illegal experiences, and they have an intolerance for repetitive and routinely experiences. They do not feel high levels of anxiety that could interfere with the highly skilled performance required at these extreme conditions and they plan these actions very carefully. This is reflected in the planning of expeditions that may last for years since the equipment is very expensive and obtaining permissions is a lengthy process. Although they are unconventional, they follow the social rules and are well socialized. We must not forget that in general high risk sports are practiced in little groups (alpinism, climbing, speleology, parachuting) where one's behaviour can interfere with the group and sometimes one's life relies on the companionship and loyalty of the group.

Sensation seeking in medium physical risk sports

Six studies were identified that studied SS in medium physical risk sports (Table 2). Medium risk sports were either compared to low risk sports, sports students or controls.

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When compared to athletes practicing low risk sports, participants engaged in medium risk sports score significantly higher on TAS and Total [START_REF] Potgieter | Sensation seeking among medium and low-risk sports participants[END_REF]. This pattern was not replicated in the study of [START_REF] Cantón | The sensation of risk and motivational tendencies in sports: an empirical study[END_REF] probably due to the sample was constituted by male and female participants and this circumstance could have masked the differences since female participants score lower on SS trait. If we compare medium risk sports to sports students [START_REF] Davis | Some personality correlates of interest and excellence in sport[END_REF][START_REF] Rossi | The sensation seeking in mountain athletes as assessed by Zuckerman's Sensation Seeking Scale[END_REF], only Rossi and Cereatti found a significant difference on the ES subscale. Finally in comparison to controls [START_REF] Rossi | The sensation seeking in mountain athletes as assessed by Zuckerman's Sensation Seeking Scale[END_REF], athletes score significantly higher on TAS, Dis and Total.

The comparisons seem to indicate that when we compare medium risk sports with either lower risk sports or controls, the former score higher on TAS and Total. When participants in medium risk sports are specifically compared with controls, the Dis subscale appears as significantly different as well. These results probably could be explained by the following reasoning: when we compare medium versus low risk sports, we are still comparing scores between athletes, even though they are practicing different level of risky sports. Thus, both groups as athletes do not necessarily differ on Dis, while by practicing sports with a different risk level it can be expected they do differ on the TAS subscale as this specifically measures thrill seeking. As control participants are not engaged in any sports, the results on the Dis subscale could probably be related to the fact that athletes generally have social habits which are more open than those of non-athletes. In summary, sportspersons practicing medium risk sports compared to low risk sports and controls are thrill and adventure seekers and, specifically, when compared to participants not practicing any given sport they exhibit the general pattern of athletes as being unconventional and open minded.

Sensation seeking in low physical risk sports

In Table 3, we have summarized the available empirical data about the relationship between SS and low risk sports.

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The results obtained from these three [START_REF] Gundersheim | Sensation seeking in male and female athletes and nonathletes[END_REF][START_REF] Hartman | Differences in and correlates of sensation seeking in male and female athletes and nonathletes[END_REF][START_REF] Schroth | A comparison of sensation seeking among different groups of athletes and nonathletes[END_REF] highly homogeneous studies merit much credit since: 1) the selected sports were team sports, 2) all participants were college students, 3) the samples consisted of male and female participants and 4) the number of participants was high. Those practicing a low risk sport as compared to those not practicing any sport score significantly higher on Total, TAS and Dis. These results seem to indicate that when we compare athletes practicing low risk sports (in these studies those were all team sports) with non-athletes (controlling for age, sex, type of sport and years of education), the former are sensation seekers, high on thrill and adventure seeking, and more disinhibited. In relation to the Dis scale, as we said before, probably the difference is caused because team sports are related to competitions, and competitions entail frequent travel, meeting new people and celebrating of the winning matches, especially in college settings where parties are so popular and welcome. Therefore, the results suggest that, regardless of gender, college athletes are higher sensation seekers than college non-athletes, although the sports practised entail low levels of risk.

Comparing sports with similar level of risk

A large majority of studies showed that when comparing groups engaged in similar levels of risk, there is no significant difference on any of the subscales of the SSS neither on the Total score. This holds regardless if they belong to an elite or subelite group [START_REF] Davis | Some personality correlates of interest and excellence in sport[END_REF], if they have made summits at heights higher than 8.000 m. compared to lower altitudes [START_REF] Breivik | Personality, sensation seeking and risk taking among Everest Climbers[END_REF][START_REF] Gomà-I-Freixanet | Personality profile of participants engaged in high physical risk sports[END_REF], or if they belong to extreme as compared to high risk takers [START_REF] Slanger | Motivation and disinhibition in high risk sports: sensation seeking and self-efficacy[END_REF]. These results suggest that when sportspersons are at the top or upper level, they do not differ on the sensation seeking trait, i.e., probably all of them are attracted by variation and complexity, intense sensations, risks and adventures. Perhaps the difference between elite and near-elite groups should not be attempted to be explained by the trait level of SS alone, but rather using other variables such as constitutional features (i.e. muscle fibre type) as well. We found only one study [START_REF] Cazenave | Psychological profiles and emotional regulation characteristics of women engaged in risk-taking sports[END_REF] showing a difference between two groups actively engaged in high-risk sports, albeit in different ways; the first group practiced a risk-taking sport for leisure purposes whereas the second had a profession based on a risk-taking sport. Since the professional group was relatively low in both impulsivity and sensationseeking, the authors conclude that the professional group approach risk-taking sports in a more structured, less impulsive manner.

Comparing sports with different levels of risk

By contrasting different levels of risk in sports, we can distinguish three levels, although comparing both extremes of risk shows the most clear cut image. When comparing high risk sports versus low risk sports, those participating in high-risk sports differ on all subscales as well as on the Total score. This suggests that high risk sportspersons are genuine sensation seekers compared to low risk sportspersons. When comparing SS in high risk versus medium risk levels [START_REF] Gomà-I-Freixanet | Personality profile of participants engaged in high physical risk sports[END_REF], the difference has reduced to TAS, ES and Total scales. Finally, when comparing SS in medium versus low levels of risk the only difference found is on TAS and Total scale. These results suggest that high risk takers as compared to medium or low risk takers are sensation seekers that like the thrill and adventure, somehow unconventional and like new experiences involved in the practice of high risk sports as parachuting, climbing or motor racing.

Comparing sports with different levels of risk with controls

When sportspersons, either practicing high-, medium-, or low-risk sports, are compared to control groups, a general pattern appears despite the different criteria used to classify the control groups (either college students, sports students, or general population). Compared to control participants, risky sportspersons (either high, medium or low) score high on TAS, Dis and Total score. This indicates that in comparison to control participants, risky sportspersons (either men or women), score high on the Sensation Seeking trait, on TAS and on the Dis subscale (e.g. [START_REF] Gomà-I-Freixanet | Personality profile of participants engaged in high physical risk sports[END_REF], 1995[START_REF] Gomà-I-Freixanet | Prosocial and antisocial aspects of personality in women: a replication study[END_REF].

Thus athletes, compared to controls, like sports that provide unusual experiences and sensations, the opportunity of frequent travels, meeting new people and the social activity around the participation in sporting activities. In the same way, they would not like sports characterized by routines, long waiting times and slow pace.

Conclusion

Overall, some general findings regarding the Sensation Seeking Scale and its subscales in relation to physically risky sports can be drawn from this review of the literature. High scores in Thrill and Adventure Seeking (TAS) seem characteristic of participants engaged in sports with high and medium levels of risk, and even in low risk sports although to a lesser extent. High scores in Experience Seeking (ES) through the mind and senses (notably cognitive) seem characteristic only of participants engaged in sports with high levels of risk. The Boredom Susceptibility (BS) subscale only differs when comparing participants engaged in extreme levels of risk, that is, high-risk versus low-risk sport practitioners. High Total score and high score in Disinhibition (Dis) seem characteristic of athletes at any level of risk as compared to control groups, and of high-risk sport practitioners as compared to lowrisk sport practitioners. Thus, what the Total score measures seems to apply to sportspersons practicing sports with a given level of risk. In relation to Dis we suggest it relates to the liking of social activities around the sport, like meeting new people, celebrating the summit or the winning game, being nonconformist and unconventional. This contrasts with the more "asocial" aspect of the Dis subscale like disregard for social rules and taking drugs and alcohol. Thus, not only the SS scale as a whole but the different subscales themselves are useful to differentiate participants engaged in sports with different levels of physical risk. These results, in general, match previous findings, but add a new finding in relation to the Dis subscale. Since the last revision by [START_REF] Zuckerman | Sensation seeking and sports[END_REF], a great deal of empirical research has been published on sports and especially on high risk sports. Some of the caveats that limited previous research and findings have been improved since the 1990s. For example, the number of participants has been increased as some of the selected sport activities samples had small (less than 15) numbers of participants. Second, since that decade most studies use Form-V of the SSS which has been improved and thus the use of the same version facilitates comparisons and generalization of results. Third, with the improvement of statistical analysis, age differences between groups can be controlled for as a covariate. Finally, the refinement of methodology and the increase in the number of participants has allowed separate analyses for males and females, and thus replicate them in both genders.

All in all, the results obtained in the research conducted since the 1990s seem to be more accurate compared to the earlier results and suggest that high scores on TAS are mostly characteristic of participants engaged in sports with high and medium levels of risk; high scores on ES are only characteristic of participants engaged in sports with high levels of risk; BS scores only differ when comparing participants engaged in high risk sports versus those engaged in low risk sports; and high scores on Total and Dis scales seem characteristic of athletes at any level of risk as compared to controls, or athletes engaged in high risk sports as compared to those engaged in low risk sports. From the above, we can conclude that the SS scale is a useful tool to assess and interpret individual differences that exist between sportspersons practising sports with different levels of risk.

The findings obtained from this review have important implications for prevention. If high sensation seekers can be reliably identified, some prevention measures could be implemented to reduce the incidence of their putative high-risk behaviours while practicing high-risk sports. Indeed, a same high-risk sport (e.g., climbing) may be practised in several ways (e.g., soloing, leading, top-roping, etc.), each of which involves a different degree of risk exposure. High sensation seekers, identified at school or at University, could be required to complete an educational program focused on deterring them from engaging in risk-taking behaviours while practicing high-risk sports.

Table 1. Note. Table presentation based on [START_REF] Zuckerman | Sensation seeking and sports[END_REF]. Studies are ordered by year. If sex, n or SSS form are not mentioned, this information is not provided in the reviewed studies. (a) Numbers in the Group Differences column refer to the numbers in superscript next to physical risky categories for compact presentation of statistical significant results (e.g., TAS 1>0 indicates that group 1 scored higher on the TAS subscale than group 0). SSS, Sensation Seeking Scale; TAS, Thrill and Adventure Seeking;

Studies relating Sensation

ES, Experience Seeking; Dis, Disinhibition; BS, Boredom Susceptibility; Total, total score.

Table 3. are not mentioned, this information is not provided in the reviewed studies. (a) Numbers in the Group Differences column refer to the numbers in superscript next to physical risky categories for compact presentation of statistical significant results (e.g., TAS 1>0 indicates that group 1 scored higher on the TAS subscale than group 0). SSS, Sensation Seeking Scale; TAS, Thrill and Adventure Seeking; ES, Experience Seeking; Dis, Disinhibition; BS, Boredom Susceptibility; Total, total score.

Table 2 .

 2 Table presentation based on Zuckerman (1983). Studies are ordered by year. If sex, n or SSS form are not mentioned, this information is not provided in the reviewed studies. Studies relating Sensation Seeking to medium-risk sports

(a)

Numbers in the Group Differences column refer to the numbers in superscript next to physical risky categories for compact presentation of statistical significant results (e.g., TAS 1>0 indicates that group 1 scored higher on the TAS subscale than group 0). SSS, Sensation Seeking Scale; TAS, Thrill and Adventure Seeking; ES, Experience Seeking; Dis, Disinhibition; BS, Boredom Susceptibility; TAS-OUT, total score minus TAS; Total, total score.

  Table presentation based on Zuckerman (1983). Studies are ordered by year. If sex, n or SSS form