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Recent Submissions

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    Effects of nuclear structure on quasi-fission
    (2012) Simenel, Cédric; Wakhle, Aditya; Avez, B.; Hinde, D. J.; Du Rietz, R.; Dasgupta, M.; Evers, M.; Lin, C. J.; Luong, D. H.
    The quasi-fission mechanism hinders fusion of heavy systems because of a mass flow between the reactants, leading to a re-separation of more symmetric fragments in the exit channel. A good understanding of the competition between fusion and quasi-fission mechanisms is expected to be of great help to optimize the formation and study of heavy and superheavy nuclei. Quantum microscopic models, such as the time-dependent Hartree-Fock approach, allow for a treatment of all degrees of freedom associated to the dynamics of each nucleon. This provides a description of the complex reaction mechanisms, such as quasi-fission, with no parameter adjusted on reaction mechanisms. In particular, the role of the deformation and orientation of a heavy target, as well as the entrance channel magicity and isospin are investigated with theoretical and experimental approaches.
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    Gemini south adaptive optics imager (GSAOI)
    (2004) McGregor, Peter; Hart, John; Stevanovic, Dejan; Bloxham, Gabe; Jones, Damien; Van Harmelen, Jan; Griesbach, Jason; Dawson, Murray; Young, Peter; Jarnyk, Mark
    The Gemini South Adaptive Optics Imager (GSAOI) is the science camera and commissioning instrument for the Multi-Conjugate Adaptive Optics (MCAO) system on the Gemini South telescope. GSAOI is required to deliver diffraction-limited performance at near-infrared wavelengths over a 85×85″ field of view. It must be delivered on a short timescale commensurate with MCAO delivery. GSAOI will use a high throughput, all-refractive optical design and a mosaic of four HAWAH-2RG detectors to form an imager focal plane of 4080×4080 pixels with a fixed scale of 0.02″/pixel. The On-Detector Guide Window (ODGW) capability of the HAWAII-2RG detectors will be used for flexure monitoring and as near-infrared substitutes for MCAO natural guide star wave front sensors. The imager will include a pupil viewer for accurate alignment to MCAO and defocus lenses to measure wave front phase errors at the science detector using the curvature technique. Non-common path wave front errors will be nulled by setting the base shapes of the three MCAO deformable mirrors. The science drivers, performance predictions, optical design issues, and detector system for the instrument are described.
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    Reassessment of ¹⁸²Hf AMS measurements at VERA
    (2011-12-15) Forstner, O.; Gnaser, H.; Golser, R.; Hanstorp, D.; Martschini, M.; Priller, A.; Rohlén, J.; Steier, P.; Vockenhuber, C.; Wallner, A.
    The radioisotope 182Hf (t1/2 = 8.9 Ma) is of great interest for astrophysical applications as a chronometer for the early solar system or as possible live supernova remnant on earth. However, AMS measurements of 182Hf are seriously influenced by the presence of the stable isobar 182W, which cannot be separated at typical AMS energies. Previous studies revealed a possible suppression of 182W against 182Hf by extracting the negatively charged pentafluoride HfF5- from the ion source, leading to a detection limit for 182Hf/ 180Hf in the order of 10-11. However, this suppression behavior is in contrast to theoretical calculations of the electron affinity and recent measurements using SIMS instruments, where the achieved suppression cannot be reproduced. The aim of our study is to determine the effects of ion source background as well as further investigate the suppression of tungsten against hafnium by extracting negatively charged fluoride ions from different sample materials. The previously reported suppression factor of about 6000 could be increased to 36000 by careful tuning of the ion source using HfF4 as sample material. The trend of the theoretical electron affinities could be reproduced using atomic tungsten and hafnium instead of HfF4 as sample material. This supports the assumption that the major contribution of the tungsten background is not sputtered from the target matrix but comes from somewhere else in the ion source. Measurements from the second ion source show a higher background of tungsten and a lower suppression factor, i.e. careful design of the ion source is crucial. Moving the sputter beam over the target surface extending over the wheel holding the targets revealed the highest tungsten background was detected outside the sputter target position. Further investigations are necessary to locate the origin of the tungsten background in the ion source. Possible sources are the material used for the ion source construction or contaminations in the cesium used for sputtering.
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    The StaggerGrid project: A grid of 3-D model atmospheres for high-precision spectroscopy
    (2011) Collet, Remo; Magic, Zazralt; Asplund, Martin
    In this contribution, we present the STAGGERGRID, a collaborative project for the construction of a comprehensive grid of time-dependent, three-dimensional (3-D), hydrodynamic model atmospheres of solar- and late-type stars with different effective temperatures, surface gravities, and chemical compositions. We illustrate the main characteristics of these 3-D models and their effects on the predicted strengths, wavelength-shifts, and shapes of spectral lines, highlighting the differences with respect to calculations based on classical, one-dimensional, hydrostatic models, and discuss some of their possible applications to elemental abundance analysis of stellar spectra in the context of large observational surveys.

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