Date	Speaker	Title
08/24/09	Chiara Mazzocchi	Nuclear astrophysics deep underground: the LUNA experiment
08/31/09	Gaute Hagen	Microscopic Coupled-Cluster approach to medium mass and neutron rich nuclei
09/07/09	Labor Day	None
09/14/09	David Miller	Intermediate-energy knockout reactions to probe single-particle structure in the "island of inversion”
09/21/09	Rodolfo IdBetan	Applications of coupling to the continuum in nuclear many-body systems
09/28/09	Miguel Madurga	Study of the beta-delayed charged particle emission of the halo nucleus 11Li
10/05/09	cancelled
10/12/09	Robert Grzywacz	The trouble with 101Sn
10/19/09	McDonnell, Jordan D	Report on the 4th International Workshop on Nuclear Fission and Fission-Product Spectroscopy
10/26/09	Lee Riedinger	Low-lying negative parity states - tetrahedral or octupole vibrational in structure?
11/02/09	Christine Nattrass	System size and energy dependence of the near-side of high-pT triggered correlations in STAR
11/09/09	Thomas Lesinski	Non-empirical pairing functional from low-momentum two- and three-body interactions
11/16/09	Walter Reviol	Octupole collectivity near N=126
11/23/09	TBA	TBA

Abstracts

August 24

Chiara Mazzocchi

Nuclear astrophysics deep underground: the LUNA experiment

Nuclear reactions that generate energy and synthesize elements take place inside the stars in a relatively narrow energy window: the Gamow peak. In this region, which is in most cases below 100 keV, far below the Coulomb energy, the reaction cross-section drops almost exponentially with decreasing energy. The extremely low value, from pico to femto-barn and even smaller, has always prevented its measurement in a laboratory at the Earths surface, where the signal to background ratio would be too small because of the background generated by cosmic ray interactions. Therefore, the observed energy dependence of the cross section at high energies is usually extrapolated to the low energy region, leading to substantial uncertainties. In particular, a possible resonance in the unmeasured region is not accounted for by the extrapolation, but it could completely dominate the reaction rate at the Gamow peak.The LUNA (Laboratory for Underground Nuclear Astrophysics) collaboration has exploited the low-background enviroment of the Gran Sasso National Laboratories in Italy to measure nuclear reactions that take place in stars at astrophysically relevant energies, at or close to the Gamow peak. Recent results will be reviewed.

August 31

Gaute Hagen

Microscopic Coupled-Cluster approach to medium mass and neutron rich nuclei.

As new and existing radioactive beam facilities probe the very limits of stable nucleonic matter, a solid theoretical framework that can verify and predict the location of the drip lines and study phenomena like the evolution of shell structure as the neutron to proton ratio increases is called for.
Coupled-Cluster theory is a promising candidate for extending the ab-initio program to medium mass and neutron rich nuclei. It is an ideal compromise between accuracy on the one hand and computational cost on the other. We show that the Coupled-Cluster wave function factorizes into a product of an
intrinsic part and a Gaussian for the Center of Mass coordinate. This finding allows us to compute accurately the binding energy of neutron-rich oxygen isotopes and medium mass Calcium isotopes by employing the coupled-cluster method and chiral nucleon-nucleon interactions. Our calculations
indicate that the stability of Oxygen-28 cannot be ruled out from ab-initio calculations. Three-nucleon forces and continuum effects will play a dominant role in deciding this question.

September 14

David Miller

Intermediate-energy knockout reactions to probe single-particle structure in the "island of inversion"

The ``island of inversion'' isotopes $^{31,33}$Mg were investigated through a proton-knockout reaction from 32,34Al at 90 MeV/u at National Superconducting Cyclotron Laboratory (NSCL). Single-particle states, with no neutron excitations across the N=20 shell gap from the sd-shell, were identified through the determination of the partial cross sections in the residue. The single-particle states identified lie above the ground state agreeing with the placement of these magnesium isotopes in the island of inversion.

Nuclear spin alignment is evident following the reaction as determined by the angular distribution of emitted gamma rays. Angular-distribution analysis constrained by the outgoing residue longitudinal momentum allows for the determination of multipolarity when combined with linear polarization measurements. Incorporating momentum constraints factors out possible systematic uncertainties of determining the in-beam gamma-ray detection efficiency.

September 21

Rodolfo IdBetan

Applications of coupling to the continuum in nuclear many-body systems

The overview of the treatment of the nuclear models which include coupling to the continuum spectrum of states will be given in this talk. Ways to obtain a single-particle representations which include continuum states will be sketched. Some applications of using two-particle systems which involve the continuum coupling will be discussed (borromean nuclei, isobaric analogue states, giant pairing resonance). Furthermore, an approximate many-particle treatment will be also presented. Applications to the pairing Hamiltonian will be shown. The role of the continuum effects in alpha-decay will also be covered.

September 28

Miguel Madurga

Study of the beta-delayed charged particle emission of the halo nucleus ¹¹Li

The beta-delayed charged particle emission of the halo nuclei 11Li has been inclusively studied using a highly segmented set up at ISOLDE, CERN. The previously known 2-body (8Li+t), 3-body (n-alpha6He) and 5-body (2alpha3n) channels have been studied, obtaining new branching ratios. A new channel involving the ground state resonance of 7He has been deduced from the study of the kinematics of the breakup. The study of this decay channel in full kinematics allowed for the reconstruction of the excitation energy the states in11Be participating in the decay channel, a previously unobserved state at 16.3 MeV and the previously known state at 18 MeV. The angular correlations in this decay channel favor an spin and parity assigment of 3/2− for both states. The 11Li beta-strength distribution for states in 11Be above the charged particle threholds is compared to shell model calculations.

October 12

Robert Grzywacz

The trouble with 101Sn

The island of alpha radioactivity near 100Sn provides an opportunity to study properties of tin isotopes using the extreme selectivity of charge particle decay spectroscopy. In an experiment which used the most advanced experimental spectroscopic techniques we have studied the 109Xe->105Te->101Sn alpha decay chain at the Holifield Radioactive Ion Beam Facility at Oak Ridge. Surprisingly, we have discovered that the majority of the alpha decay branching ratio of the 105Te populates the first excited state in 101Sn leading us to the conclusion that the first excited state in 101Sn is a d5/2 neutron single particle state, while the ground state is a g7/2 state. The in-depth analysis of this result with the state-of-the-art shell model calculations lead us to surprising conclusions on the role of the pairing correlations in the lightest tin isotopes.

October 19

Jordan D. McDonnell

Report on the 4th International Workshop on Nuclear Fission and Fission-Product Spectroscopy

Since its discovery in the early twentieth century, nuclear fission has been rapidly and thoroughly studied through a variety of experimental methods. With an abundance of data and phenomenological models to characterize fission, the description of the fission process from microscopic considerations remains an area of active research, accompanied by continuing measurements of increasing accuracy and reliability. After a brief overview of fission theory for the case of thorium-232, the proceedings of a May 2009 workshop on nuclear fission taking place in Cadarache, France, will be summarized. Special attention will be paid to the sessions on mass, charge, and energy distribution of fission fragments; the dynamical aspects of nuclear fission; fission fragment spectroscopy; new experimental developments and facilities; and the role of nuclear physics in present energy challenges.

October 26

Lee Riedinger

Low-lying negative parity states - tetrahedral or octupole vibrational in structure?

We have performed in the last year two measurements of rotational bands in ¹⁵⁶Dy using carbon and xenon beams at the Argonne Atlas accelerator using Gammasphere, the 4π array of gamma-ray detectors. Some of the ‘missing E2’ transitions were found in our first measurement, and now we have good E2/E1 branching ratios for the decay of the low-lying negative-parity band to members of that band compared to decays to the ground-state band. These branching ratios are very small, which means that the E2 transition within the band is small (due to tetrahedral shape) or the E1 is large (octupole vibrational). We performed in September a preliminary lifetime measurement with a backed target to deduce the absolute E2 and E1 matrix elements. No line shapes were seen in this preliminary experiment, which necessitates a plunger measurement of the lifetimes as the next step. As we analyze these data, I have studied the systematics of the K = 0, 1, and 2 octupole vibrational bands across the rare-earth region of deformation, trying to understand how ¹⁵⁶Dy might fit into that picture.

November 2

Christine Nattrass

System size and energy dependence of the near-side of high-pT triggered correlations in STAR

QCD predicts a phase transition in nuclear matter at high energy densities. This matter, called a Quark Gluon Plasma (QGP), should have very different properties from normal nuclear matter due to its high temperature and density. Jets can act as a calibrated probe to examine the QGP, however, reconstruction of jets in a heavy ion environment is difficult. Therefore jets have been studied in heavy ion collisions by investigating the spatial correlations between two intermediate to high-pT hadrons in an event.

Previous studies have indicated that the near-side peak of high-pT triggered correlations can be decomposed into two parts, the jet-like correlation and the Ridge. The jet-like correlation is narrow in both azimuth and pseudorapidity and has properties consistent with vacuum fragmentation, while the Ridge is narrow in azimuth but broad in pseudorapidity and roughly independent of pseudorapidity within STAR's acceptance. We present data from Cu+Cu and Au+Au collisions at sqrt(s_NN) = 62 GeV and 200 GeV, which should allow more robust tests of models. The yields of the jet-like correlation and Ridge components are presented in both systems and at both energies. The trends in energy, system, and particle type dependence of the jet-like correlation and Ridge present a more accessible test for models than the absolute yields. Attempts have been made to explain the production of the Ridge component as coming from recombination, momentum kicks, Glasma flux tubes, and a plasma instability. However, few models have attempted to quantitatively calculate the characteristics of the Ridge. The wealth of data should help distinguish models for the production mechanism of the Ridge.

November 9

Thomas Lesinski

Microscopic Coupled-Cluster approach to medium mass and neutron rich nuclei.

In nuclear structure, emphasis has been recently put on improving the predictive power of Energy Density Functional (EDF) models, particularly through ab-initio derivations of such a functional. I present systematic calculations of semi-magic nuclei across the nuclear chart using the EDF method, with a functional whose pairing part has been derived non-empirically, consisting of lowest-order contributions in the low-momentum, vacuum two-nucleon interaction, including the Coulomb force, and three-nucleon interaction derived from chiral Effective Field Theory. Energies of odd-mass nuclei are calculated self-consistently in the equal-filling approximation, which allows a direct comparison to odd-even mass difference data. I will thus assess the relative contributions of two-body and three-body direct terms, blocking, and missing higher-order terms of the perturbative expansion, to pairing in finite nuclei.

November 16

Walter Reviol

Chemistry Department, Washington University, St. Louis, MO 63130

Octupole collectivity near N=126

Recent experiments in the actinide region, using Gammasphere and the evaporation residue detector Hercules, have covered the territory between N = 126 and the center of static octupole deformation at N =134. The 220Th nucleus (N = 130) shows a multi-phonon like behavior, over a large spin range (ΔI > 20ħ). It has been described as a tidal-wave mode for a reflection-asymmetric nuclear surface. The neighboring odd-mass isotopes show the following features. In 219Th, parity-doublet structures (pairs of states nearly degenerate in energy with the same spin but opposite parity) appear at medium spin. In 221Th, an off-yrast octupole band is newly observed. With these findings, a rule for parity-doublet bands is established, which is related to the K component of the angular momentum carried by the unpaired nucleons.

Previous Nuclear Physics Seminars

·      Spring 2006

·      Fall 2006

·      Spring 2007

·      Fall 2007

·      Spring 2008

·      Fall 2008

·      Spring 2009