Spring
2008 Nuclear Physics Seminar Schedule
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WARNING !!! CHANGE OF SCHEDULE ! Unless otherwise noted, the nuclear physics seminars are held on Mondays, at 2:10 p.m. in Room 307 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. Dr. Robert Grzywacz is chair of the seminar program. He may be contacted via e-mail at: rgrzywac@utk.edu. |
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Date |
Speaker |
Title |
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M. Stoitsov |
New Efficient Method of Hartree-Fock-Bogoliubov Calculation for Weakly Bound Nuclei |
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L. Riedinger |
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L.
Townsend |
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J.Beene
(ORNL) |
Upgrading HRIBF: Justification, Technical Background and Prospects for Realization |
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F. Wietfeldt (Tulane University) |
Precision Neutron Scattering Length Measurements Using Neutron Interferometry |
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J.C.
Batchelder |
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J.N. Orce |
Pairing-gap vibrations as a new mechanism of collectivity in nuclei |
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T. Papenbrock |
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Matthew
Dimmock |
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R. Hix |
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J. Sheikh |
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J. Hamblen |
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K.
Chae |
Spin assignments of 22Mg through a 24Mg(p,t)22Mg measurement |
Abstracts
M. Stoitsov
New Efficient Method of Hartree-Fock-Bogoliubov Calculation for Weakly Bound Nuclei
We propose a new method to solve the Hartree-Fock-Bogoliubov equations for weakly bound nuclei, which works for both spherical and axially deformed cases. In this approach, the quasiparticle wave functions are expanded in a complete set of analytical P\"{o}schl-Teller-Ginocchio and Bessel/Coulomb wave functions. Correct asymptotic properties of the quasiparticle wave functions are endowed in the proposed algorithm. Good agreement is obtained with the results of the Hartree-Fock-Bogoliubov calculation using the box boundary condition for a set of benchmark spherical and deformed nuclei.
L. Riedinger
Searching for new shape effects in upcoming experiments
The purpose of this talk is to give an overview of new directions in the ‘high-spin’ experimental program at UT. One direction is a March experiment on the search for wobbling triaxial bands in 167Ta at Gammasphere (Argonne National Lab). This is an outgrowth of the PhD work of Martin Djongolov on superdeformed bands in 174Hf, structures we thought initially to be triaxial but probably are not. Another emerging direction for our program is to look for evidence of newly predicted tetrahedral shapes in deformed nuclei, and we are considering experiments that we might propose in the N = 90 region.
L. Townsend
Methods of Modeling Cosmic Ray Interactions and Transport
Interactions of high energy cosmic rays with
spacecraft shielding and crewmembers' bodies include both atomic and
nuclear collisions. The former result in energy losses and are well
described by a Bethe-Bloch formalism. The latter also result in
energy losses, but more importantly, nuclear fragmentation and other
particle production processes that result in
identity changes and
increases in particle multiplicities. These alterations in particle
multiplicities, identities and energies are described by radiation
transport codes, which "solve" a form of the Boltzmann
Transport Equation, or use Monte Carlo methods to simulate the
transport of the radiation fields. I will briefly discuss methods of
modeling radiation transport and interactions, and give an overview
of how they are applied to space shielding problems and astronaut
radiation protection. One recent application, also to be discussed,
is to use these methods to model the detector response for the CraTER
(Cosmic Ray Telescope for the Effects of Radiation) detector, which
is scheduled for launch in late 2008 as one of the instruments on the
LRO (Lunar Reconaissance Orbiter) spacecraft. CRaTER is a
collaborative effort involving the Astronomy Department at Boston
University, The Kavli Institute for Astrophysics and Space research
at MIT, the UT Nuclear Engineering Department, National Space Weather
Prediction Center, and the Aerospace Corporation.
Upgrading HRIBF:
Justification, Technical Background and Prospects for Realization
The Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory hosts a dedicated user program in nuclear physics using exotic beams. Radioactive species are produced by intense light-ion beams from the Oak Ridge Isochronous Cyclotron and post-accelerated by the 25 MV tandem. Vigorous and innovative research programs concentrating on nuclear astrophysics, nuclear structure and nuclear reactions are based at HRIBF, along with a Center of Excellence for Stewardship Science operated by Rutgers University and the UNIRIB consortium. Recent work has concentrated on investigations employing beams of neutron-rich exotic nuclei beyond the N=50 and N=82 closed shells. Projects to implement improvements in facility efficiency and reliability are proceeding well at the present time. However, HRIBF will require additional investments to remain productive and competitive over the decade between now and the completion of the long-planned next-generation U.S. exotic beam facility. There are several additional ways in which a comparatively modest-cost upgrade could substantially improve HRIBF performance and operation. The most promising and cost-effective of these appears to be addition of a high-power electron accelerator for production of neutron-rich species by photo-fission. I will discuss the scientific and technical basis of the electron driver upgrade path, why this particular path is favored, and what the capabilities of the resulting facility will be in the context of planned international developments in exotic beam facilities over the next decade.
Precision Neutron Scattering Length Measurements Using Neutron Interferometry
The neutron interferometer, developed by Werner and Rauch in the 1970's, splits the neutron matter wave into two paths by Bragg diffraction in a silicon crystal, then recombines them coherently to produce a interference signal measured by a neutron counter, thereby directly obtaining an interaction amplitude via the phase shift. It has been used to make famous demonstrations of quantum phenomena that are now found in many textbooks. It is also an ideal instrument for precision studies of low-energy neutron scattering lengths. I will describe a program at the NIST Neutron Interferometry and Optics Facility of few-body nuclear scattering length measurements that are important for comparing models of nuclear forces and nucleon substructure.
U(5)
symmetry in the even-even heavy Cd isotopes
The
neutron-rich even-even Cd isotopes are often cited as textbook
examples of vibrational nuclei and the best examples of U(5)
symmetry. Nuclei that are close to the limits of U(5) symmetry would
be expected to exhibit harmonically spaced multi-phonon states,
complicated by low-lying intruder states (caused by the elevation of
2 protons across the Z=50 shell gap). The experimental signature of
these N-phonon states is a strong preference (lower B(E2) values) for
decay of a N-phonon state to the (N-1)-phonon states over other lower
energy states. The expected contribution of intruder states to the
B(E2) values of the transitions from the N-phonon states can be
accounted for by calculations in the IBM-2 framework. The observation
of the complete set of 3-phonon states has been reported up to 118Cd.
However, this simple picture is at odds with the experimental data
(both previously published and our work at HRIBF) for 112,116,118Cd.
In this talk I will examine how well the known even-even Cd isotopes
fit within U(5) symmetry description by comparing experimental B(E2)
values from possible N-phonon states with calculations done in the
IBM-2 framework.
Pairing-gap vibrations as a new mechanism of collectivity in nuclei
Recent Coulomb excitation measurements in inverse kinematics have suggested an enhancement of E2 strengths for the first 2+ to ground state transitions in the neutron-deficient Sn isotopes. The enhancement of E transition strengths in the light Sn isotopes is in disagreement with the parabolic trend B(E2; 2+ → 0+ ) values between the N=50 and N=82 shell closures predicted by large-scale shell model calculations. A similar situation seems to occur for neutron-deficient Ni isotopes. As the N=28 and N=50 shell closures are approached in the light Ni and Sn isotopes, the role of proton-core excitations in N~Z nuclei become more important. In order to agree better with experiment, the inclusion of an enhanced quadrupole strength in the effective interaction seems essential in shell model calculations, whereas a stronger pairing interaction is necessary in relativistic mean-field predictions. The combination of both is taken into account in this work to explain the collectivity of the first 2+ states in the neutron-deficient Ni and Sn isotopes. The introduction of pairing-gap vibrations provide a simple interpretation of such phenomena by means of the proximity of both proton and neutron pairing fields.
Recent results (weakly bound He isotopes, three-nucleon forces, and benchmarks) and future directions (medium-mass nuclei, densities, saturation properties of microscopic interactions) within the ORNL/UT coupled-cluster effort.
M.R. Dimmock
Results from the Characterisation of Advanced GAmma Tracking Array prototype detectors and their consequences for the next generation nuclear physics spectrometer
The Advanced GAmma Tracking Array (AGATA) is a European project that is aiming to construct a complete 4π High Purity Germanium (HPGe) gamma-ray spectrometer for nuclear structure studies at future Radioactive Ion Beam (RIB) Facilities. The proposed array will utilise digital electronics, Pulse Shape Analysis (PSA) and Gamma-Ray Tracking (GRT) algorithms, to overcome the limited efficiencies encountered by current Escape Suppressed Spectrometers (ESS), whilst maintaining the high Peak-to-Total ratio.
Two AGATA symmetrical segmented Canberra Eurisys (CE) prototype HPGe detectors have been tested at the University of Liverpool. A highly collimated Cs-137 (662keV) beam was raster scanned across each detector and data were collected in both singles and coincidence modes. The charge sensitive preamplifier output pulse shapes from all 37 channels (one for each of the 36 segments and one for the centre contact) were digitised and stored for offline analysis. The shapes of the real charge and image charge pulses have been studied to give detailed information on the position dependent response of each detector. The singles data has enabled the investigation of the detector response through the bulk crystal. The coincidence data has been utilised to validate the electric field simulation code Multi Geometry Simulation (MGS). The precisely determined 3D interaction positions allow the comparison of experimental pulse shapes from single site interactions with those generated by the simulation. It is intended that the validated software will be used to calculate a basis data set of pulse shapes for on-line PSA in the AGATA array. The results from this validation, along with those from the investigation into the position sensitivity of each detector will be presented.
P. Hausladen
Portable fast-neutron imaging using D-T neutrons.
The differing contrast between materials that
neutrons provide (compared to x
rays) makes neutron imaging desirable for some applications. I
will present some progress toward a portable
fast-neutron imaging system being developed at ORNL, and point out
some of the functional imaging capabilities
enabled by the D-T reaction and the associated particle
method.
W.R. Hix
The Roles of Nuclear Physics during Stellar Core Collapse
Nuclear
electron capture and the nuclear equation of state play important
roles during the collapse of a massive star and the subsequent
supernova. The nuclear equation of state controls the nature of the
bounce which initially forms the supernova shock while electron
capture determines the location where the shock forms. Advances in
nuclear structure theory have allowed a more realistic treatment of
electron capture in supernovae to be developed. With this
improvement, we have shown that electron capture on nuclei with
masses larger than 50 dominates electron capture on free
protons,producing significant changes in the hydrodynamics of core
collapse and bounce. We will present explorations of the impact of
weak interactions with heavy nuclei in supernovae, focusing on the
consequences across the range of supernova progenitors. Examination
of the sensitivity of these effects to variations in the electron
capture rates will also be presented. Additionally, we will present
simulations showing the impact of a variety of nuclear equations of
state on supernova shock propagation and the interplay between
electron capture and the equation of state.
J. Sheikh
Pairing correlations in atomic nuclei
Pairing correlations derived from mean-field and spherical shell model approaches shall be discussed. It will be demonstrated that pairing correlations for even-even and odd systems have similar behavior as a function of temperature (excitation energy). Further, it will be shown that the ground-state in N=Z odd-odd systems is a neutron-proton paired state.
J. Hamblen
Physics
Prospects with the ALICE Detector at CERN
The
ALICE experiment is the only dedicated heavy ion detector at CERN's
Large Hadron Collider. The LHC is scheduled to begin proton-proton
collisions this summer, and Pb ion collisions will follow in 2009.
ALICE is focused on studying QCD matter created under extreme
conditions in heavy ion collisions. The LHC will provide a new
frontier in nucleus-nucleus collisions with over a factor of 25 gain
in collision energy when compared to the Relativistic Heavy Ion
Collider. I will discuss the detector setup as well as the physics
goals to be studied with ALICE.
K. Chae
Spin assignments of 22Mg through a 24Mg(p,t)22Mg measurement."
X-ray bursts are explained as thermonuclear explosions in the atmosphere of an accreting neutron star in a close binary system. When critical values for density and temperature are reached in the neutron star atmosphere, the freshly accreted hydrogen and helium ignites and burns via the hot CNO cycles at a constant rate. Depending on the strength of the 15O(alpha,gamma)19Ne and 18Ne(alpha,p)21Na reactions, break-out from the hot CNO cycles will occur, fueling the rapid proton capture (rp)-process. To understand the observed light curves of X-ray bursts and the subsequent heavy-element production in the rp-process, one must understand the rates of these breakout reactions. The 18Ne(alpha,p)21Na reaction plays a crucial role in the (alpha,p) process, which leads to the rapid proton capture process in X-ray bursts. The reaction rate depends upon properties of 22Mg levels above the alpha threshold at 8.14 MeV. Despite recent studies of these levels, only the excitation energies are known for most with no constraints on the spins.
To
better constrain the spins of important levels, we have studied the
24Mg(p,t)22Mg reaction with a 41-MeV proton beam at the Oak Ridge
National Laboratory (ORNL) Holifield Radioactive Ion Beam Facility
(HRIBF). By measuring the angular distributions of outgoing tritons,
we hope to provide the first experimental constraints on the spins of
astrophysically-important 18Ne(alpha,p)21Na resonances. Details of
the experiment and a report of the current stage of the analysis will
be presented.
Previous Nuclear Physics Seminars