Spring 2006 Nuclear Physics Seminar Schedule |
Unless otherwise noted, the nuclear physics seminars are held on Mondays, at 1:15 p.m. in Room 201 of UTK's Nielsen Physics Building. Abstracts are included below the schedule. The UTK Physics Colloquium Schedule and ORNL Physics Division Seminar Schedule might also be of interest. Professor Witek Nazarewicz is chair of the seminar program. He may be contacted via e-mail at: witek@utk.edu. |
Andras Kruppa, Institute of Nuclear Research of the Hungarian Academy of Sciences, Debrecen, Hungary
Scattering States and Complex Scaling
A standard approach of the quantum mechanics is that the unknown wave function is expressed as a linear superposition of some basis functions. The basis is generaly square integable. For bound states such a method works well. The wave function of resonance states, describing unstable decaying systems, however, are not square integrable and the simple expansion method does not work. The complex scaling, from a physicist point of view, represents a technical device to calculate resonance states using square integrable basis. The complex scaling means a special non-unitary transformation of the Hamiltonian. The transformation is given by the formula r->r exp(i*a) where r is the particle coordinate and a is the scaling parameter. The very special properties of this transformation will be discussed. The complex scaling has been used in physics since 1970 mainly for description of resonance states. In the last six years, there have been very promissing application of the exterior complex scaling for the calculation of the scattering S-matrix. It will be shown that the standard complex scaling can be used also for the calculation of the S-matrix. The great advantage of the method that it uses only square integrable wave functions.
Steve Pain, Rutgers University/ORNL
An Overview of the Center for Radioactive Ion beam Studies for Stewardship Science
The properties of fission fragments are of interest to stewardship science, as well as to nuclear astrophysics and nuclear structure. In particular, (n,gamma) cross sections for those neutron-rich fission fragments near shell closures are relatively low, with an increased importance of direct capture. However, as it is not currently possible to measure these cross sections directly, measuring (d,p) reactions can yield information on low lying levels, which are of importance for determining direct capture rates.
The Stewardship Center has embarked on a campaign to measure (d,p) reactions on fission fragments at the HRIBF, where nuclei up to A~132 can be accelerated to around the Coulomb barrier. To facilitate these measurements, a new array of silicon detectors (the Oak Ridge Rutgers University Barrel Array, ORRUBA) is being constructed. Techniques for measuring gamma rays in coincidence with protons are also being developed. In addition, the Center is involved efforts to develop new beams of fission fragments, and improve the intensity/purity of current beams of interest. The results of previous experiments and the development of new instrumentation will be discussed, along with an outlook for beam development.
Ted Barnes, ORNL/University of Tennessee
Models of the NN Force and Other Baryon-Baryon Forces and How to Test Them
QCD suggests several possible mechanisms for the strong interaction between nucleons, including pion exchange, heavier meson exchange, and direct quark-quark interactions. All these mechanisms presumably have some range of validity. In this talk I will discuss these mechanisms, how they can be tested by comparison to experimental NN phase shifts, some of our theoretical results for these phase shifts calculated using algebra programs, and our recent attempts to calculate meson-baryon coupling constants directly from the quark model. Since this is recent research, this talk is basically a "progress report", and suggestions from the audience regarding future research directions are strongly encouraged.
A Needle in a Haystack
At ORNL's Holifield Radioactive Ion Beam Facility (HRIBF) we are exploring the Accelerator Mass Spectrometry (AMS) capabilities of the 25-MV Tandem. Two main characteristics of the 25-MV Tandem provide unique opportunities for pushing the limits of sensitivity of AMS; namely (i) the highest operating voltage in the world, and (ii) a folded geometry which involves a 180 degree magnet in the terminal. We have concentrated in exploring two important isotopes 14C and 36Cl for AMS applications that require the highest sensitivity. Several important AMS applications that would benefit from enhanced sensitivity and future directions will be discussed.
Superheavy Nuclei in Relativistic Mean Field Theory
The possible existence of shell-stabilized superheavy nuclei has been a driving force behind experimental and theoretical efforts to investigate the superheavy nuclei. Unfortunately, theoretical predictions for such nuclei differ considerably. The review of present status of our understanding of superheavy nuclei within microscopic theories (in particular, relativistic mean field theory) will be presented. I will concentrate on the importance of single-particle degrees of freedom in establishing the reliability of particular parametrization of the model and on the role of self-consistency effects in superheavy nuclei.
Beta-decay Near 100Sn - an Unsolved Puzzle of Missing Gamow-Teller Strength
Beta-decays in nuclei lying south-east of doubly-magic 100Sn are governed by a transformation of a g9/2 proton to a g7/2 neutron. This region forms a unique testing ground for theoretical and experimental ideas. Comparison between calculations and experiment shows a big discrepancy known as a 'puzzle of missing Gamow-Teller strength'. In my talk, I will focus on results of Gamow-Teller studies at the GSI's on-line mass separator.
Identification of Mixed-symmetry States in an Odd-mass Nearly Spherical Nucleus
Mixed-symmetry (MS) states are collective vibrational phenomena which are not fully symmetric with respect to the proton-neutron (pn) degree of freedom, presenting pn symmetry departures from ground-state symmetry. These excitations are of isovector character, i.e., proton and neutron spin contributions are additive in the vector part of the M1 operator and can lead to strong M1 transitions from the MS state. The Interacting Boson Model-2 (IBM-2) gave a systematic description of these collective nuclear states. Recently, MS states have also been examined from a shell model approach, where large isoscalar E2 matrix elements have been found between states with MS assignments, and interpreted as states having similar pn symmetry. Whereas MS assignments are common and well studied in even-even nuclides, I will show both experimental and theoretical evidences for the first identification of MS states in an odd-mass nearly-spherical nucleus.
An Overview of the Latests Results from the Relativistic Heavy Ion Collider
An overview of the latests results from the Relativistic Heavy Ion Collider
Type Ia Supernova Explosions: Nuclear Burning in Degenerate Matter and Strong Gravity
Type Ia supernovae (SNe) are a class of stellar explosions that are distinguished by a lack of hydrogen in the observed spectra. The observed brightness of type Ia SNe suggest the universe is undergoing an accelerated expansion caused by a mysterious "dark energy." A number of current programs and future projects plan to use type Ia SNe to constrain the properties of the dark energy. These observational projects depend on the notion that type Ia SNe are standard candles, requiring a thorough knowledge of the explosion mechanism in order to properly calibrate their intrinsic brightness. I will discuss some of the issues associated with modeling these explosions and matching these simulations with observations.
Density Functional Theory for Fermions Close to the Unitary Regime
We consider interacting Fermi systems close to the unitary regime and compute the corrections to the energy density that are due to a large scattering length and a small effective range. Our approach exploits the universality of the density functional and determines the corrections from the analytical results for the harmonically trapped two-body system. The corrections due to the finite scattering length compare well with the result of Monte Carlo simulations. We also apply our results to symmetric neutron matter.
Relativistic Heavy Ion Collider, PHENIX, and Perturbative Quantum Chromodynamics
The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory is a scientific research facility dedicated to the study of Quantum Chromodynamics. PHENIX is one of the two major detectors at the collider and is optimized for rare processes. Perturbative Quantum Chromodynamics makes quantitative predictions of some of those rare processes and is a useful tool to probe the hadronic matter produced in the collisions. Current status of these efforts will be discussed.
Density Matrix Renormalization Group Approach for Many-Body Open Quantum Systems
We have applied the DMRG method in the context of the Gamow Shell Model (GSM). In this model, which describes the configuration mixing in open quantum many-body system, the completness relation is resolved in the Berggren ensemble consisting of bound states, resonant states (Gamow states) and complex energy continuum states. The eigenvalue problem is inherently non-hermitian and given by complex-symmetric matrix with complex eigenvalues. Inclusion of resonances and associated continuum states leads to an explosive growth of the size of the multiconfigurational space and the standard diagonalization methods cannot be applied. To deal with this difficulty, we have solved the GSM problem using the DMRG technique in the J-scheme. This choice ensures the total angular momentum conservation at each step in the iterative procedure. The GSM+DMRG model has then been applied for the description of weakly bound or unbound states (the multi-nucleon resonance configurations) in the He isotopes.
The Hadronic Weak Interaction and Parity Violation in np→dγ
Weak interactions within stable nuclei are mediated by the exchange of heavy bosons, with a length scale (~0.002 fm) much shorter than the size of a nucleon. Observations of nuclear parity violation therefore offer an opportunity to study the high-energy properties of ordinary, low-energy nuclear matter. In this talk I'll describe the effort underway at the Los Alamos Neutron Science Center (LANSCE) to measure one such observable: the correlation Aγ between neutron spin and photon direction in the capture of cold neutrons on (spin-zero) hydrogen. The goal of the np→dγ experiment is a measurement of Aγ with precision 5 parts per billion, or 10% of the expected value, which will provide a theoretically clean measurement of the weak pion-nucleon coupling.