LIBRA Seminars
Predrag Ujić (GANIL, France)
Wednesday, 1 December 2010, 14:00
Alpha-particle capture reactions in inverse kinematics relevant to p-process nucleosynthesis
Abstract
Some of the very few (α,γ) data show that the phenomenological α-particle optical potentials, used in astrophysics calculations can be wrong by a factor of ten or more. Hence, systematic cross‐section measurements of α-particle capture reactions at energies between 1 and 3.5 MeV/u are necessary. The measurement of the alpha capture reaction in inverse kinematics is the only possible solution when the radioactive nuclei are involved. Nevertheless, this was not ever performed for the medium-mass nuclei due to the small massdifference between the beam particles and the recoils which sets tight experimental conditions for their separation. For the first time we performed the inverse kinematics experiment with the medium-mass nuclei 78Kr(α,γ)82Sr reaction. The aim was to prove feasibility of this type of experiment. The energy of the reaction was E(CM)=6.5 MeV and the difference of velocity of 78Kr and 82Sr was about of 5%. The LISE3 spectrometer that includes a block of Wien Filters was used for the separation of the 82Sr from the primary beam – 78Kr. Considering very small difference of velocities between the primary beam and compound nucleus, very good rejection factor was achieved – almost 1011. The conclusion of the experiment is that the cross section measurement of the (α,γ) reactions is feasible regarding high rejection rate of the primary beam of factor, however the use of gas-jet target instead of solid one seems to be the solutions.
Anne Sauerwein (IKP, Universität zu Köln, Germany)
Wednesday, 1 December 2010, 13:00
In-beam measurement of the 74Ge(p,γ)75As reaction for the astrophysical p process
Abstract
Due to the absence of experimental data, p-process network calculations are almost completely based on theoretically predicted reaction rates. The accuracy of these predictions strongly depends on the adopted nuclear models for the optical-model potentials, photonstrength functions and nuclear level densities. A comprehensive experimental data base for these reaction rates is strongly required to improve the accuracy of these models. It has been shown that proton and α-particle capture reactions are particularly well suited to constrain theoretical calculations. Therefore we are currently performing an in-beam measurement of the astrophysical relevant reaction 74Ge(p,γ)75As at the tandem accelerator of "Demokritos".
Prof. Steven Karataglidis (University of Johannesburg, South Africa)
Wednesday, 1 September 2010, 12:00
Recent results from MCAS (Multi-Channel Algebraic Scattering)
Abstract
With the rush to build new radioactive-ion-beam (RIB) facilities, notably FRIB, the descriptions of weakly-bound systems, and the scattering there from, are becoming increasingly important. A central aspect of this, particularly in coupled-channels approaches to scattering, is the need to include target excited states that are unbound, with particle-emission widths that are significantly different from zero. One question which arises is how the particle-emission widths affect the formation of compound states, as well as the scattering cross sections. This is explored within the framework of the Multi-Channel Algebraic Scattering (MCAS) method. We consider a range of light-mass, particle unstable targets, and compare the results with, and without, the inclusion of the target resonance widths. We find that the compound spectra and evaluated cross sections vary markedly with the resonance character of the target. A proper description of the energy dependence of the decay widths is needed, to ensure a correct specification of both the sub-threshold bound and scattering states. Also, MCAS has been applied to hypernuclei, with potentials found to describe states in 9ΛBe and 13ΛC. I shall describe how the potentials describing hypernuclear states are found and compare results with those found from the shell model.
Prof. G.A. Lalazissis (Aristotle University of Thessaloniki, Greece)
Wednesday, 30 June 2010, 12:00
Covariant Density Functionals: Application to Nuclear Structure and Nuclear Astrophysics
Abstract
The framework of relativistic nuclear energy density functionals has been applied to the description of a variety of nuclear structure phenomena, not only in spherical and deformed nuclei along the valley of β-stability, but also in exotic systems with extreme isospin values and close to the particle drip-lines. Dynamical aspects of exotic nuclear structure have been explored using the fully consistent quasiparticle random-phase approximation based on the relativistic Hartree-Bogoliubov model. We present recent applications of energy density functionals with explicit density dependence of the meson-nucleon couplings: relativistic Hartree-Bogoliubov and QRPA calculations of nuclear ground-states and properties of excited states, calculation of masses, and modeling input for astrophysical calculations.
Dr. Róbert Huszánk (INP, NCSR Demokritos, Greece)
Friday 27 November 2009, 13:30
Interdisciplinary application of ion micro- and macrobeam
Abstract
Due to the more and more wide-spread presence of low energy accelerators in the world, the applications based on accelerated ions are more and more common not only in physics research but in many other fields of science like chemistry, biology, optics, medicine, micro engineering, space science and so on. The most common purposes to use ion beams related to ion beam applications are for ion beam analytics, ion beam lithography and ion implantation/irradiations. The methods which used in these analytical applications among others are the followings: Rutherford backscattering analysis (RBS), particle induced X-ray emission (PIXE), elastic recoil detection analysis (ERDA), scanning transmission ion microscopy (STIM), nuclear reaction analysis (NRA), deuteron or particle induced gamma ray emission (DIGE, PIGE). With these techniques, determination of the total mass, the sample morphology, major, minor and trace element content and distribution, and if necessary particle depth profile can be realised of, for example, thin layers, archaeological, biological or aerosol samples. The ion beam lithography or micromachining techniques are used to create microstructures in polymer, glass or silicon substrates. The applications which can also be created by this technique are microreactor and microfluidic devices, micro-electro-mechanical and lab-on-a-chip systems (MEMS), medical applications, micro-optical devices, biosensors or. With ion implantation or irradiation, the properties of a material can be changed which can be a base of many application like polymer surface modification, refractive index change, microelectronics, etc.
Dr. Stephen F. Ashley (University of Kentucky, USA)
Tuesday, 06 October 2009, 14:30
Picosecond Lifetime Measurements of Excited Nuclear States Relating to Shape-Phase Transformations in Atomic Nuclei
Abstract
In nuclear structure physics, the determination of the lifetimes of excited nuclear states greatly enhances our understanding of the modes of excitation associated with atomic nuclei. In particular, for nuclei that can generate excitations in ''collective'' ways, information such as the deformation of the nucleus can be inferred and direct comparisons to various theoretical models can be performed. In terms of the collective pictures in atomic nuclei, 106Cd is an interesting case. The low-lying excited states in cadmium purvey vibrational-like excitations, which are described both in terms of anharmonic quasi-particle RPA calculations [1] and the algebraic U(5) and E(5) models [2,3]. However, a similar cranked shell model analysis (associated with deformed, collective nuclei) on 106Cd [4] show good agreement and suggests that the Iπ = 10+ is a collective-band built on a two-quasiparticle ν(h11/2)2 configuration. Furthermore, numerous N=58 isotones also purvey this ''shape-phase'' transition at higher-spin and have been parameterised with the E-GOS plot [5]. This talk will discuss how the determination of lifetimes, using plunger techniques, have further furnished our understanding of this nucleus and will highlight some oddities which still need to be explained!
[1] J. Kotila, J. Suhonen and D.S. Delion, Phys.Rev. C 68, 014307 (2003)
[2] J. Kern, et al., Nucl. Phys. A 593, 21 (1995)
[3] F. Iachello, Phys. Rev. Lett. 85, 3580 (2000)
[4] P.H. Regan et al., Nucl. Phys. A 586, 351 (1995)
[5] P.H. Regan et al., Phys. Rev. Lett. 90, 152501 (2001)
Dr. Georgios Tsiledakis (University of Heidelberg, Germany)
Tuesday, 01 Sep 2009, 13:00
Scale dependence of mean transverse momentum fluctuations at top SPS energy and azimuthal momentum correlations of heavy-quarks as a sensitive probe for thermalization
Abstract
Non-statistical event-by-event fluctuations of mean transverse momentum, pT, have been proposed as a possible signature for the QCD phase transition, in particular for the critical point. A differential study of mean pT fluctuations is performed, which, by allowing to discriminate among various correlation sources, provides the sensitivity to the fluctuations related to the vicinity of critical point. For the first time at SPS energy, the charge-dependent mean pT fluctuations have been analyzed as a function of the angular pair separation, Δφ, and of the separation in pseudorapidity, Δη. An important contribution is a broad maximum at Δφ=180 degrees (`away-side') originating from back-to-back (dijet-like) correlations. Concerning the observed away-side structure, we demonstrate that it comes from high-pT correlations that cannot be attributed to elliptic flow.
In high-energy nuclear collisions at LHC, where a QGP might be created, the degree of thermalization at the partonic level is a key issue. Due to their large mass, heavy quarks are a powerful tool to probe thermalization . Following the same method, we propose to measure azimuthal momentum correlations of heavy-quark hadrons and their decay products.
Dr. Matthias Müller and Dr. Michael Kolbe (Physikalisch-Technische Bundesanstalt, Berlin, Germany)
Wednesday 26 August 2009, 12:00 (A joint seminar)
Nanolayer characterization and surface contamination analysis by reference-free X-ray spectrometry with synchrotron radiation
Abstract
The reliable determination of inorganic and organic contamination on semiconductor surfaces requires quantitative and sensitive analytical methods. X-ray spectrometry (XRS) is an effective non-destructive tool revealing information concerning the elemental composition and binding states. Reference-free quantitation in XRS is based on the knowledge of both the instrumental and fundamental atomic parameters. In total-reflection geometry only surface contamination and the surface-near layer of a few nm contribute to the fluorescence spectra. High-end investigations in the R&D of semiconductor samples require reference-free methods, especially for novel materials where not enough appropriate reference materials are available. The Physikalisch-Technische Bundesanstalt (PTB) operates its own laboratory employing well characterized and calibrated instrumentation at the electron storage ring BESSY II in Berlin, where silicon wafers up to 300 mm diameter as well as smaller semiconductor wafers can be investigated.
Here, various mass depositions on wafer surfaces have been investigated in both the hard X-ray range, as used in table-top devices, and in the soft X-ray range, allowing for higher dynamics in the subnanometer range. Furthermore, grazing-incidence angular scans in these energy regions were performed to discriminate between particle and layer-type mass depositions being additionally quantified by reference-free XRS.
The present joint presentation reports on the progress of the aforementioned PTB activities.
Prof. Dr. Birgit Kanngiesser (TU-Berlin, Institut für Optik und Atomare Physik, Germany)
Wednesday 29 April 2009, 13:00
What Conrad Röntgen would see today – Probing condensed matter with X-rays
Abstract
Since Wilhelm Conrad Röntgen discovered his “X-Strahlen” in the year 1895, X-rays have become an indispensable probe for the investigation of the structure of condensed matter and its electronic properties. Nowadays a multitude of X-ray methods exist relying on the basic interactions of X-rays with matter. They form the optical tool for such diverse fields like Micro- and Nanotechnologies, research of biological structure and agents and analytics of matter and processes. Examples of this enormous spectrum are the tomographic imaging of components of a single cell, the investigation of buried nanolayers, the determination of the elemental distribution in historic parchment and the stroboscopic imaging of oscillating nanolayers. A considerable portion of this research can be found in the Berlin area. The lecture will present an overview of the current research and its application.
Dr. Masahiko Katsuma (Institute d’ Astronomie et d’ Astrophysique, ULB, Belgium)
Thursday 9 April 2009, 13:00
Low-energy cross sections from direct reaction models
Abstract
Thermonuclear reactions are crucial for the Big-Bang nucleosynthesis and for stellar evolution and concomitant nucleosynthesis. However, the measurement of the cross sections at low energies is difficult because of the Coulomb barrier. To cope with the difficulties, theoretical approaches on evaluation of the cross section data have been required, to make the extrapolation of the experimental data down to the astrophysical energies. In this talk, we show the applicability of our potential model and distorted-waves Born approximation (DWBA) to the low-energy nuclear reactions relevant to nuclear astrophysics. The derived reaction rates are compared with the so-called NACRE (Nuclear Astrophysics Compilation of REaction rates).
Dr. Michael Axiotis (LNL, Legnaro-Padova, Italy & NTUA, Greece)
Wednesday 22 January 2009, 14:15
Spectroscopic Study of Nucleus 52Mn and Study of the β-Decay of the nucleus 52Fe
Abstract
The nucleus 52Fe (N=Z=26) has been a particular experimental challenge. Most of the known excited states, observed in (3He,n), (α,2n) and (p,t) reactions, were at relatively low spin (below 6 h-bar). Many attempts to extend the 52Fe yrast structure to higher spins in fusion-evaporation reactions induced by heavy ions have failed due to the presence of a 12+ isomer which acts as a “trap” for the de-exciting γ-ray flux. In some recent works with high efficiency γ-ray detectors, the level scheme of 52Fe has been extended up to the 10+ state, thereby confirming the predicted inversion of the yrast 10+ and 12+ states and placing the latter one with accuracy at the excitation energy of 6493 keV.
In the present work the Gamow-Teller strength (B(GT)) of the β-decay was identified within the QEC window, arriving to energies as close as possible to the QEC, and the week feeding to high lying states was measured by using a total absorption spectrometer. The investigation of the B(GT) distribution within the full QEC window will allow us to test the validity of large-scale SM (LSSM) calculations far from the yrast line, as well as to test the prediction capability of the LSSM concerning Gamow-Teller strength functions. For this study the knowledge of the level scheme of the daughter nucleus 52Mn would be important and so it was also studied using spectroscopic techniques. New experimental data on the odd-odd N=Z+2 nucleus 52Mn are presented. High-spin states of both positive and negative-parity have been observed up to excitation energy of ~16 MeV. Prior to the present work, these studies were limited to the band-terminating 11+ state at 3836 keV.
Dr. Tania Avgoulea (University of Manchester, UK)
Wednesday 21 January 2009, 13:00
Exploring the nuclear structure with Laser Spectroscopy
Abstract
High resolution Laser Spectroscopy provides a sophisticated means to measure the nuclear charge distribution, moments and spins of radioisotopes reaching far from the valley of stability. The current experimental status and future challenges will be presented along with work focused around the region of the Z=20 and Z=28 magic numbers.