Alpha decay provides a very selective tool to investigate the decays of nuclei above 100Sn. The alpha decays of nuclei close to the N = Z line provide an unique opportunity to study the effects of protons and neutrons occupying identical orbitals [1]. Correlations between protons and neutrons could lead to the observation of ³superallowed² alpha decay [2]. Additionally, the observation of fine structure in the alpha decay allows for an inference of the energy separation between different single-particle orbits. Finally, from the alpha decay Q value, the mass difference between the decaying nucleus and its daughter can be determined. Small changes in mass could have an affect on the termination of the rp-process in which alpha decay figures prominently [3].

To investigate these varying aspects of alpha decay in this region 109Xe, 105Te and 109I have been studied using fusion evaporation reactions with at the HRIBF at Oak Ridge National Laboratory. The recoiling products were separated by means of the Recoil Mass Separator [4] and implanted into a Double-sided Silicon Strip Detector. Recoil and decay signals were analyzed using a digital data acquisition system based on XIA DGF modules [5].

The lightest mass alpha-radioactivity identified to date, 105Te, was detected through the 109Xe -> 105Te -> 101Sn alpha decay chain and marks the closest approach to the N = Z line above 100Sn. Additionally, fine structure in the millisecond alpha decay of 109Xe to 105Te was identified and the energy difference between the nd5/2 ground state and the ng7/2 first excited state was determined to be around 150 keV in 105Te. The identification of a minuscule 109I alpha decay branch enabled an indirect measurement of the 105Sb proton separation energy with possible implications for the termination of the rp-process.

[1] S.N. Liddick et al., Phys. Rev. Lett. 97, 082501 (2006); B. Hadinia et al., Phys. Rev. C 70, 064314 (2004); B. Hadinia et al., Phys. Rev. C 72, 041303(R) (2005).; Z. Janas et al., Eur. Phys. J. A 23, 197 (2005); A.A. Hecht et al., AIP Conf. Proc 819, 355 (2006).
[2] R.D. MacFarlane, A. Siivola, Phys. Rev. Lett. 14, 114 (1965).
[3] H.Schatz et al., Phys. Rev. Lett. 86, 3471 (2001).
[4] C.J.Gross et al., Nucl. Instr. Meth. Phys. Res. A 450, 625 (2000).
[5] R.Grzywacz et al., Nucl. Instr. Meth. Phys. Res. B 204, 649 (2003).

October 2

Piotr Borycki, University of Tennessee

Regularization of the Particle-Particle Interaction in the Nuclear Density Functional Theory
PhD Defense Presentation, 1:00 pm

Since its introduction in the 1960s, the Hohenberg-Kohn-Sham formulation of the Density Functional Theory has became a popular many-body technique in quantum chemistry, physics of correlated electronic systems, and low-energy nuclear physics. In this presentation, we show how to regularize the ultraviolet divergences appearing in the local pairing term commonly used in Skyrme-Hartree-Fock-Bogoliubov calculations. Our approach, entirely rooted in the framework of the Density Functional Theory, is based on the regularization of local densities and currents and can be applied to various classes of energy density functionals. We demonstrated, that for the particular choice of the pairing term of energy density functional, our procedure gives the same regularization scheme to the one obtained earlier by the means of the pairing gap regularization.

We have also investigated the non-unitarity of the Bogoliubov tranformation due to the energy cutoff in the quasiparticle space. We developed and tested the method of restoring the unitarity of the Bogoliubov transformation. We demonstrated that by applying this method one can perform HFB calculations for significantly lower cutoff energies, without losing accuracy.

October 9

Rob Mahurin, University of Tennessee

Measurement of the parity-violating asymmetry in polarized cold neutron capture

Of the interactions that occur in nuclear matter, only the weak interaction fails to conserve parity. Observations of parity violation in low-energy nuclear systems thus provide a window on their structure at length scales much smaller than the nucleon size. The NDPGamma experiment aims to measure the correlation between neutron spin and photon direction in the radiative capture of cold neutrons on hydrogen; this correlation is simply related to the hadronic weak neutral current. The collaboration has installed a liquid parahydrogen target at the Los Alamos Neutron Science Center and taken roughly one month of neutron beam. I will describe the present status of this measurement.

October 16

Juan Pablo Urrego-Blanco, University of Tennessee

Development of Polarized Proton Targets for Reactions with Radioactive Ion Beams

Soon after the first experiments with polarized protons were proposed, more than fifty years ago, polarized probes became powerful tools for investigating nuclear structure and reaction mechanisms. In particular, polarized proton and deuterium beams at low energies have been widely used in conjunction with stable or long lived target nuclei for spectroscopic studies involving elastic scattering and transfer reactions. In order to extend those studies to regions further away from stability, we are currently developing polarized proton targets for reactions with radioactive ion beams in inverse kinematics. The targets are 20-200um thick plastic foils, where protons are polarized according to the dynamic nuclear polarization method, reaching polarizations up to ~90% at temperatures of ~100mK and magnetic fields of 2.5T. In this talk a description of the target system and of the first attempts to characterize it by elastically scattering 12C ions on protons will be delivered.

October 23

Josh Hamblen, University of Tennessee

Collective Flow Measurements from the PHOBOS Experiment at the Relativistic Heavy Ion Collider Beams

PHOBOS is one of four experiments that study the hot and dense matter created at the Relativistic Heavy Ion Collider. The azimuthal correlations of the produced particles, also known as collective flow, serve as a useful probe in studying the dynamics of the matter created by giving insight into the early moments of the collision as well as the degree of thermalization of the system. The PHOBOS detector features a highly segmented silicon multiplicity array that covers nearly the entire azimuth and is capable of measuring collective flow over a broad range of pseudorapidity. After describing the detector, I will show results for the first and second harmonic azimuthal particle distributions for gold-ion collisions at four different energies, and discuss the evolution of the results both as a function of collision energy and pseudorapidity.

November 6

Stephen Libby, Lawrence Livermore National Laboratory

Inertial Fusion Diagnostics and Prospects for Investigating Nuclear Reactions Relevant to Astrophysics Using the National Ignition Facility

Informal 45 min discussion of nuclear physics possibilities at the National Ignition Facility.

The standard capsule design and other laser plasma targets at the National Ignition Facility offer the possibility of generating and studying thermal rates for significant astrophysical fusion reactions such as 3He(3He,2p)alpha, 7Be(p,gamma)8B, and 15N(p,alpha)12C. Presently, the S-factors for these reactions are determined either by extrapolation from higher energy scattering data or by underground low event rate experiments (LUNA) on neutral atoms with concomitantly large screening corrections. The ability to directly generate thermal astrophysical fusion reactions could complement "no core" shell model predictions for these light ion reactions. In addition, the expected enormous fluence of neutrons from the main d + t -> alpha + n burn reaction can drive 10-20% of seeded spectator nuclei into excited states via (n,n') reactions. The ~2% 'minority' neutrons with E< 2 Mev due to d + d -> 3He + n and (n, n') can furthermore drive reactions pertinent to r, s, and p process heavy element nucleosynthesis. Both classes of reactions can be studied using particle spectroscopy and radiochemistry. Additionally, for the first time, it may be possible to measure the effects of plasma screening on thermonuclear reactions, addressing the extent of quantum corrections to Salpeter screening. Radiochemistry measurements of noble gas end species can be made with very high efficiency with only ~ 10^4-5 atoms required. Solid collection systems are being developed as well. Because the capsule is essentially thin to neutrons, their reaction rate on advected sets of marker nuclei is a linear functional of the neutron source distribution. Determining this source function is thus computationally analogous to similar problems in medical imaging.

November 13

Hendrik Schatz, Michigan State University

Nuclear physics in X-ray bursts

Informal discussion on nuclear physics issues of the rp-process. In particular: uncertainties concerning the waiting points and the endpoint of the rp-process.

November 27

Carlos Bertulani, University of Arizona

Photon Physics at the Large Hadron Collider at CERN

Moving highly-charged ions carry strong electromagnetic fields that act as a beam of photons. In collisions at large impact parameters, hadronic interactions are not possible, and the ions interact through photon-ion and photon-photon collisions known as ultra-peripheral collisions (UPCs). Hadron colliders like the Relativistic Heavy Ion Collider (RHIC), the Tevatron, and the Large Hadron Collider (LHC) produce photonuclear and two-photon interactions at luminosities and energies beyond that accessible elsewhere; the LHC will reach a photon-proton energy ten times that of the Hadron-Electron Ring Accelerator (HERA). Reactions as diverse as the production of anti-hydrogen, photoproduction of the rho0, transmutation of lead into bismuth, and excitation of collective nuclear resonances have already been studied. At the LHC, UPCs can study many types of new physics processes.

December 4

Neil Summers, University of Tennessee

Extracting nuclear structure from nuclear reactions

Nuclear reactions offer a way to obtain information on the exotic structures of short-lived radioactive nuclei. The information we wish to extract from the reaction is usually not an observable, but other quantities such as spectroscopic factors and B(E1) strengths. This means that reaction theory has to bridge the gap between the structure input we wish to study and the observable measured in the experiment. Often many approximations are applied to the reaction theory in order to obtain a direct relationship between the structure input and the observable. 11Be is a one-neutron halo nucleus which has been extensively studied both theoretically and experimentally. Ab-initio calculations of 11Be are now possible and a range of different reactions involving 11Be have been performed. This provides a good testing ground for reaction theory. One major assumption of traditional reaction theory is that the core is inert during the reaction. Recent advances now allow for excited core contributions to be included consistently within the reaction theory. I will discuss various reactions, such as breakup, transfer, and Coulomb excitation, the approximations made in extracting information from such reactions, and how including core excitation can improve theoretical predictions of the experimental observables.

Previous Nuclear Physics Seminars

• Spring 2006