Pre-main-sequence isochrones -- I. The Pleiades benchmark

We present a critical assessment of commonly used pre-main-sequence isochrones by comparing their predictions to a set of well-calibrated colour-magnitude diagrams of the Pleiades in the wavelength range 0.4 to 2.5 microns. Our analysis shows that for temperatures less than 4000 K the models systematically overestimate the flux by a factor two at 0.5 microns, though this decreases with wavelength, becoming negligible at 2.2 microns. In optical colours this will result in the ages for stars younger than 10 Myr being underestimated by factors between two and three. We show that using observations of standard stars to transform the data into a standard system can introduce significant errors in the positioning of pre-main-sequences in colour-magnitude diagrams. Therefore we have compared the models to the data in the natural photometric system in which the observations were taken. Thus we have constructed and tested a model of the system responses for the Wide-Field Camera on the Isaac Newton Telescope. As a benchmark test for the development of pre-main-sequence models we provide both our system responses and the Pleiades sequence.Comment: 15 pages, 12 figures, 7 tables, accepted for publication in MNRAS. All tables are available online at the Cluster Collaboration homepage http://www.astro.ex.ac.uk/people/timn/Catalogues

Application of monte carlo calculation for the virtual calibration of a low-energy in vivo counting system

Internal dose assessment can be derived from the measurement of retained activity in the whole body or in an organ at a given time. In radiation protection, this assessment, so-called in vivo measurement, is performed by an external measurement of the subject with germanium detectors (in most cases). Calibration of these detectors is ensured by anthropomorphic phantoms which, for technical reasons, can only provide rough representations of human. It is especially the case for the chest phantoms used in lung counting, subject of this paper. This leads to substantial corrections on calibration factors that are particularly crucial and delicate in low-energy in vivo measurements, resulting in important systematic errors. In order to improve calibration, former work based on numerical phantoms associated with Monte Carlo computing techniques has already proven its benefits. To go further, a Graphical User Interface called "OEDIPE", a French acronym for "tool for internal personalized dose assessment", has been developing at the IRSN internal dose assessment laboratory, simulating real measurements using person-specific computational phantoms in association with MCNP calculation code. The study presented here is dedicated to the implementation and validation of a real in vivo monitoring system (AREVA/COGEMA Marcoule, France) equipped with 4 high purity germanium (HPGe) detectors. After modeling the facility and measurement geometry using OEDIPE (design and positioning of the detectors. . .), validation with different configurations was carried out in two steps: first with point sources (different nuclides, different source-detector distances) and then with the Livermore calibration phantom (different overlay plates, lungs contaminated with 241Am and a mixture of actinides). The final goal is to approach a personalized numerical calibration of the facilities in order to improve dose assessment, as the use of physical phantoms for calibration induces large uncertainties. Such application could be an opening door on a better activity assessment in nuclear medicine, especially in personalized dosimetry in radioimmunotherapy. © 2005 IEEE

La mesure de l'excrétion urinaire de l'uranium par spectrofluorimétrie laser à résolution temporelle

La quantification directe rapide de l'excrétion urinaire de l'uranium est souvent perturbée par des incertitudes métaboliques et par des interférences analytiques. Ces phénomènes conduisent à des limites de détection ou à des incertitudes trop élevées. La technique proposée associe à la fois un traitement rapide de l'échantillon et un système optimisé de mesure. L'association d'un laser solide de puissance utilisé comme source d'excitation et d'un fluorimètre modifié du commerce permet d'atteindre les objectifs de l'étude : rapidité de réponse et facilité de mise en oeuvre, précision et exactitude inférieures 10 %. Les stades analytiques selon deux modalités (mesure directe et après minéralisation de l'échantillon) sont décrits. Les résultats expérimentaux portant sur 120 mesures sont comparés aux résultats obtenus par chromatographie d'extraction. Les avantages et inconvénients de la technique sont commentés. Enfin les valeurs de l'excrétion urinaire naturelle de l'uranium chez 80 travailleurs non-exposés de la région de Marcoule sont présentées en fonction de la technique analytique choisie