Introduction aux incertitudes de mesures

Les incertitudes évaluées en utilisant les normes internationales sont entrées dans les programmes de terminales scientifiques. Les livres de physique, destinés aux élèves, sont un peu justes pour les enseignants. Cet article a pour objectif de présenter les notions nécessaires à une bonne application des formules utiles

200 ans après Laplace, l’essor des méthodes bayésiennes d’analyse des données

International audienceInitiée par Pierre-Simon de Laplace il y a deux cents ans, l’analyse bayésienne des données expérimentales est en plein essor. Elle a fait son entrée dans certaines normes internationales de métrologie. Le but de cet article est d’introduire simplement le changement philosophique sous-jacent.Des applications à différents types de mesurages sont présentées

What can we learn from the fission of super-heavy elements?

The conference proceedings will be published by World Scientific PublishingNuclear shell model calculations predict the existence of super-heavy elements (SHE) that are tentatively synthesized through heavy-ion collisions. A complete description of the reaction to synthesize super-heavy elements is necessary to bridge these predictions with the experimental results on the fission time and residue cross sections. In this contribution, we will present the constraints that can be given on the shell correction energy from experimental data and the developments that are needed for the dynamical models. We will especially focus on the fission time of heavy elements and on the role of the isomeric potential pockets

Synthesis of superheavy elements: Uncertainty analysis to improve the predictive power of reaction models

13 pages, 13 figures, submitted for publication in PRCInternational audienceBackground: Synthesis of super-heavy elements is performed by heavy-ion fusion-evaporation reactions. However , fusion is known to be hindered with respect to what can be observed with lighter ions. Thus some delicate ambiguities remain on the fusion mechanism that eventually lead to severe discrepancies in the calculated formation probabilities coming from different fusion models. Purpose: In the present work, we propose a general framework based upon uncertainty analysis in the hope of constraining fusion models. Method: To quantify uncertainty associated with the formation probability, we propose to propagate uncertainties in data and parameters using the Monte-Carlo method in combination with a cascade code called KEWPIE2, with the aim of determining the associated uncertainty, namely the 95% confidence interval. We also investigate the impact of different models or options, which cannot be modeled by continuous probability distributions, on the final results. An illustrative example is presented in detail and then a systematic study is carried out for a selected set of cold-fusion reactions. Results: It has been rigorously shown that, at the 95% confidence level, the total uncertainty of the empirical formation probability appears comparable to the discrepancy between calculated values. Conclusions: The results obtained from the present study provide a direct evidence for predictive limitations of the existing fusion-evaporation models. It is thus necessary to find other ways to assess such models for the purpose of establishing a more reliable reaction theory, which is expected to guide future experiments on the production of super-heavy elements

KEWPIE2: A cascade code for the study of dynamical decay of excited nuclei

Submitted to Computer Physics CommunicationsInternational audienceKEWPIE — a cascade code devoted to investigating the dynamical decay of excited nuclei, specially designed for treating very low probability events related to the synthesis of super-heavy nuclei formed in fusion-evaporation reactions — has been improved and rewritten in C++ programing language to become KEWPIE2. The current version of the code comprises various nuclear models concerning the light-particle emission, fission process and statistical properties of excited nuclei. General features of the code, such as the numerical scheme and the main physical ingredients, are described in detail. Some typical calculations having been performed in the present paper clearly show that theoretical predictions are generally in accordance with experimental data. Furthermore, since the values of some input parameters cannot be determined neither theoretically nor experimentally, a sensibility analysis is presented. To this end, we systematically investigate the effects of using different parameter values and reaction models on the final results. As expected, in the case of heavy nuclei, the fission process has the most crucial role to play in theoretical predictions. This work would be essential for numerical modeling of fusion-evaporation reactions

From Di-Nucleus to Mono-Nucleus - Neck Evolution in Fusion of Massive Systems -

Dynamics of the neck degree of freedom during fusioning process between heavy ions is studied. Time scales of the three degrees of freedom (the relative distance, the neck and the mass-asymmetry) are studied, showing an early equilibration of the neck. This means that a di-nucleus formed by the incident combination of ions quickly forms a mono-nucleus with a superdeformation during the fusion process and that the other two degrees of freedom have to be solved in a coupled way. A brief introduction of Langevin approach and dissipation-fluctuation dynamics is also given and of the application to the synthesis of the superheavy elements.Comment: 6 pages, 3 figures, Proceedings of the Japanese French Symposium - New paradigms in Nuclear Physics, Paris, 29th September - 2nd October, to be published in Int. J. of Modern Physics

Di-nucleus dynamics towards fusion of heavy nuclei

This work has been first presented at : First Workshop on State of the Art in Nuclear Cluster Physics, 13/16-05-2008 Strasbourg (France)International audienceThe Two-Step Model for fusion of massive systems is briefly recapitulated, which clar- ifies the mechanism of so-called fusion hindrance. Since the neck changes the potential landscape, especially the height of the conditional saddle point, time evolution of the neck degree of freedom plays a crucial role in fusion. We analytically solve time-evolution of nuclear shape of the composite system from di-nucleus to mono-nucleus. The time- dependent distribution function of the neck is obtained, which elucidates dynamics of fusion processes in general, and thus, is useful for theoretical predictions on synthesis of the superheavy elements with various combinations of incident heavy ions

Fusion hindrance of heavy ions: role of the neck

International audienceFusion of heavy ions is largely hindered because of the appearance of an inner barrier between the contact point of the two colliding nuclei and the compound nucleus. But there are still quantitative ambiguities on the size of the barrier and on the role of the dissipation. In this paper we stress the importance of the neck of the composite system on the hindrance of the fusion of heavy nuclei. We show that the \denecking" process is very quick compared to the other collective degrees of freedom as the relative distance. This behavior of the neck will change the potential seen by the relative distance on the way to fusion and its e ective initial value through a dynamical coupling. Both e ects contribute to the hindrance of fusion

Origin and age of an ongoing radioactive contamination of soils near La hague reprocessing plant based on ^239+240Pu/²³⁸Pu and ²⁴¹Am/²⁴¹Pu current ratios and ⁹⁰Sr and Ln(III) soil contents.

Nuclear reprocessing plants are sources of environmental contamination by gaseous or liquid discharges. Numerous radionuclides are of concern, with actinides and <sup>90</sup> Sr being the most radiotoxic. Environmental radioactivity survey programs mostly use γ-spectrometry to track contaminations because γ-spectrometry is very cost effective and can be carried out on raw samples. On the other hand, the determination of β- or α-emitting radionuclides in environmental samples requires rather sophisticated analytical methods, and are thus dedicated to specific goals. However, measuring radionuclides such as Pu, Am, and Sr often provides more information about the presence of a current or prior contamination and on its origin, based on the isotopic composition of the samples. We found that the analysis of <sup>241</sup> Pu, <sup>239+240</sup> Pu, <sup>241</sup> Am, and <sup>90</sup> Sr of a few selected soil samples taken near the nuclear reprocessing plant of La Hague, France, revealed the presence of a previous environmental contamination originating from several incidents in La Hague site involving atmospheric transfer and leaks in flooded waste pits. The <sup>241</sup> Am- <sup>241</sup> Pu dating method indicated a contamination period prior to 1983. The presence of elevated levels of light non-radioactive lanthanides and yttrium in the soil samples confirmed the involvement of cold fuel. Our results demonstrate how long-lived actinides are likely to reveal a long-term contamination of the environment by spent fuel. Our study indicates that there is a requirement to use more sophisticated tools than γ-spectrometry when surveying the environments surrounding industrial plants for nuclear power and nuclear reprocessing with a potential for the accidental release of radioactivity into the environment

Dynamics of massive systems and synthesis of superheavy elements

International audienceFor the synthesis of superheavy elements, it is indisensable to divide process into two steps : overcoming the Coulomb barrier and passing over the conditional saddle or the ridgeline. To facilitate the understanding of the mechanism which explains the fusion hindrance, we first emply an analytic model with an inverted parabola for the saddle. Then, results by realistic calculations are given for the cold fusion. Ambiguities of the model are also discussed for future investigations. Since the model is general, it is applied to incident channels with neutron-rich projectile and/or targets. These are necessary for synthesis of nuclides on so-called superheavy island around Z=114 and N=18