ISNAP Seminars - Abstracts 2013 Spring

Topic: Exotic Modes of Collective Nuclear Excitations-Nuclear Tidal Waves and the phenomenon of Multiple Chiral Bands

Daniel Ayangeakaa ( University of Notre Dame ) / January 21, 2013

Many aspects of low energy nuclear structure are characterized by the collective degrees of freedom, namely vibration and rotation. For the group of nuclei in the transitional region between spherical and well deformed nuclear shapes, the low-lying structure may be described semi-classically as quadrupole running waves (tidal waves) and the observed vibrational-rotational behavior can be thought of as resulting from a rotating condensate of interacting d-bosons. In the first part of my talk, I will discuss our recent lifetime measurements that led to the first experimental identification of a seven-phonon yrast state in ¹⁰²Pd. I will then describe the structural composition of these states in terms of a rotating condensate of d-bosons.
The second part of my talk will be based on the new concept of multiple chiral doublets (M_XD): the existence of more than one pair of chiral doublet bands in a single nucleus, a phenomenon previously predicted in the framework of the relativistic mean eld (RMF) theory. Two distinct pairs of chiral partner bands have been identified in ¹³³Ce. The chiral nature of these bands will be discussed in the context of the established fingerprints of nuclear chirality along with theoretical calculations that support this interpretation.

Topic: Halo World: The Story of Nuclear Halos and the Efimov Effect

Indranil Mazumdar (Tata Institute of Fundamental Research, Mumbai, India and TUNL, Durham, NC) / January 28, 2013

The advancement in the production of energetic radioactive ion beams (RIB) have opened up new vistas in contemporary nuclear physics. On one hand it provides the means to explore the structural properties and reaction dynamics of nuclei near the drip lines. On the other hand some of the light, neutron-rich nuclei with their 2-neutron halo structure, characterized by large spatial extension and very low separation energy of the neutrons, have emerged as ideal candidates to search for exotic quantum mechanical effects like the Efimov Effect. The first part of this talk will be devoted to an introduction to the discovery of halo nuclei and the global efforts in this direction. Thereafter, we shall present an overview of the different theoretical approaches with particular emphasis on a three-body model to understand the key structural properties, namely, binding energy, and RMS matter radii of these nuclei. The momentum distributions of the core and the valence neutrons and the beta-decay of 2-n halo nuclei will also be touched upon.
2-neutron halo nuclei are the ideal candidates for studying the Efimov effect in atomic nuclei. After a brief introduction to the Efimov effect and its universal features in three-body systems we shall discuss the search for Efimov states, within the framework of our model calculations, in 2n-halo nuclei, like, ¹⁴Be, ¹⁹B, ^20,22C. The important finding of the evolution of bound Efimov states into resonances with increasing 2-body (neutron-core) binding energy will be discussed. Finally, we will tie up the emergence of the asymmetric resonances with the Fano phenomenon and discuss its implication for the possible experimental observation of Efimov states in the atomic nuclei like ²⁰C, ³⁶Mg and ³²Ne.

Topic: Aspects of the Structure of Exotic Nuclei and New Opportunities with GRETINA

Augusto O. Macchiavelli ( Nuclear Science Division, Lawrence Berkeley National Laboratory ) / February 4, 2013

The structure of nuclei far from the stability line is a central theme of research both in experimental and theoretical nuclear physics. Radioactive beam facilities and novel detector systems are unique tools to produce and study these nuclei, and together with new developments in nuclear theory they provide a framework to understand the properties of these exotic nuclei.
In this talk I will present recent results related to the quadrupole collectivity in neutron rich Carbon isotopes, and the evolution of the N=28 and 40 shell closures with isospin.
I will also review the gamma-ray tracking technique, give a status report of GRETINA, and discuss some aspects of the exciting physics campaign being carried out at NSCL/MSU.

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Topic: Nuclear Astrophysics in China

Xiaodong Tang ( University of Notre Dame ) / February 11, 2013

Topic: Experimental Study of Resonant States in ²⁶Si and ²⁷P to Investigate the ²⁵Al(p, γ)²⁶SI and ²⁶Si(p,γ)²⁷P Reactions

Hyo Soon Jung ( University of Notre Dame ) / February 18, 2013

Proton resonant states in ²⁶Si and ²⁷P were studied by the resonant elastic scattering of ²⁵Al+p and ²⁶Si+p with ²⁵Al and ²⁶Si radioactive ion beams bombarding a thick H₂ gas target with the inverse kinematics method at CRIB (CNS Radioactive Ion Beam separator) facility. CRIB is a low-energy RI beam separator at Center for Nuclear Study (CNS), the University of Tokyo, installed in the RIBF facility of RIKEN. CRIB can produce low-energy (<10 MeV/u) RI beams in flight method, using primary heavy-ion beams from the AVF cyclotron (K=70). Studies on proton and alpha resonant scatterings, and (a,p) reactions have been performed using RI beams at CRIB, motivated by interests on astrophysical reactions and exotic nuclear structure. The properties of ²⁶Si and ²⁷P resonance states are important to better constrain the production rates of the ²⁵Al(p, γ)²⁶Si and ²⁶Si(p, γ)²⁷P reactions. These are astrophysically important reactions needed to understand proton-rich nucleosynthesis such as the galactic production of ²⁶Al and energy generation in explosive stellar environments. The details of these experiments and their results will be presented.
In addition, the progress and the recent several physics campaigns of the EURICA (Euroball-RIKEN Cluster Array) project for studying a wide range of exotic nuclei will be introduced.

Topic: Exploring the Nuclear Physics Opportunities of CARIBU at ANL

Scott Marley ( University of Notre Dame ) / February 25, 2013

The study of neutron-rich nuclei far from the valley of beta stability has led the development and construction of dedicated rare isotope facilities all over the world. The Californium Rare Isotope Beam Upgrade (CARIBU) to the ATLAS facility at Argonne National Laboratory presents a unique opportunity to study intermediate mass neutron-rich nuclei. Fission fragments from a ~350 mCi ²⁵²Cf source are cooled, focused, and extracted using a gas catcher and selected using an isobar separator. Ion beams can then be transported to a low-energy beam line (<2 keV) or re-accelerated to energies up to 6 MeV/A. A major component of the low-energy scientific program at CARIBU is the precision mass measurement of nuclei in this region using the Canadian Penning Trap (CPT) many of which play a critical role in determining the final r-process abundance pattern [1]. These mass measurements are challenging due to the short half lives (t_1/2<200 ms) and weak beam intensities making it a necessity to optimize the transmission and efficiency of the CARIBU-CPT systems. Many of the additional low-energy CARIBU experiments will be discussed including a forthcoming program for measurements of beta-delayed neutron emission using recoil-ion spectroscopy.
[1] S. Brett, I. Bentley, N. Paul, R. Surman, and A. Aprahamian. Eur. Phys. J. A (2012) 48:184.

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Topic: Improved Rate Calculations for Low Mass Reactions

R. James deBoer ( University of Notre Dame ) / March 4, 2013

Experimentally measured stellar reaction rates are crucial for a proper description of the nuclear burning processes which occur in stellar, big bang, and supernovae environments to name a few. Improved measurements of low energy cross sections have led to a much better understanding of these rates for a wide variety of low mass reaction. However, standardized uncertainty calculations of these rates have not been widely implemented. This talk will discuss the broader issue of determining more accurate and precise cross sections for low mass nuclei, and a technique will be presented for determining accurate uncertainties for the corresponding reaction rates.

Topic: The Nucleosynthesis of Heavy Elements in AGB Stars

Sara Bisterzo ( INAF-Astronomical Observatory Turin / University of Torino, Italy ) / March 18, 2013

The aim of the stellar nucleosynthesis is to explain the origin of the elements studying the main processes responsible for the production of individual isotopes.
About half of the heavy elements from Sr-Y-Zr up to Pb-Bi are synthesized through the s (slow) neutron capture process, occurring in Asymptotic Giant Branch (AGB) stars during their thermally pulsing phase. The s-process abundances observed today in the Solar System are the result of a complex Galactic evolution mechanism, which accounts for the pollution of several AGB stellar generations (see review by Käppeler et al. 2011, Rev. Mod. Phys., 93, 157).
In this scenario, neutrons are released by two key reactions, ¹³C(α, n)¹⁶O and ²²Ne(α, n)²⁵Mg.
In low mass AGB stars (M ~ 1.2 to 3 M_Θ), the ¹³C(α, n)¹⁶O reaction burns radiatively at T ~ 0.9×10⁸ K during the interpulse periods. It constitutes the major neutron source and provides more than 90% of the total neutron exposure. The ²²Ne(α, n)²⁵Mg neutron source is partially activated during the recurrent convective thermal pulses, when the maximum temperature at the bottom of the He-burning shell reaches T ~ 3×10⁸ K. This second reaction mainly affects the abundances close to the branching points that are sensitive to temperature and neutron density.
In intermediate mass AGB stars (M ~ 4 to 8 M_Θ), ²²Ne(α, n)²⁵Mg is efficiently activated owing to the large temperature reached during thermal pulses (T ~ 3.5×10⁸ K), while the contribution of ¹³C(α, n)¹⁶O becomes marginal.
Theoretical predictions for AGB models with different initial masses, metallicities and s-process efficiencies will be discussed.
AGB models are computed with a full network, from hydrogen to bismuth, updated by the latest experimental results.

Topic: Weak Decay Properties from QRPA Calculations with Realistic Forces

Dong-Liang Fang ( NSCL, Michigan State University ) / March 19, 2013

The r-process simulation is an interesting topic in Nuclear Astrophysics, as the r-process together with other astrophysical process provides the possible explanation of the element abundance of our solar system and even our universe. As the inputs, weak decay rates including the half-lives and beta-delayed neutron emission probabilities play important roles in the simulation, and accurate determination of these properties are crucial for the final product of the simulation. To calculate the weak decay rate, various methods have been applied, from the exact shell model calculations to different approximations. Exact solution as shell model can only deal with limited numbers of nuclei due to the large configuration space. So methods with approximation are needed, of which QRPA methods with various version are widely used in this calculations. In our calculation, we adopt the QRPA with realistic forces for both deformed and spherical nuclei. We obtain acceptable agreement with experiments and make predictions for nuclei which are currently out of the reach of experiments. The future plan is to make r-process simulations with the new data sets and investigate the effect of weak decay rate on the element abundance.

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Topic: Measuring the t+t Neutron Spectrum at Inertial Confinement Fusion Facilities

Jac Caggiano ( Lawrence Livermore National Laboratory ) / March 25, 2013

The t+t reaction is an interesting one to study because it is a mirror for the astrophysically relevant ³He+³He reaction and because the spectrum of emitted neutrons is sensitive to final state interactions. The low mass of the nuclei involved make it suitable for ab initio theoretical comparisons. There are two possible reaction branches leading to distinct final states: ⁵He+n and ⁴He+2n, populating the ground and excited state in the ⁵He nucleus. The three-body final state gives rise to a broad energy spectrum of neutrons where as the two body final states give rise to un-bound resonances that appear as peaks in the neutron energy spectrum. For several years, the Wong and Anderson (Nucl. Phys. 71, 106 (1965)) reactions have served as the standard data for the neutron energy spectrum.There has been limited experimental evidence suggesting that the shape of the spectrum changes depending on Ecm. In particular, there is some evidence that the ⁵He[g.s.]+n channel changes strength, despite the S-factor being relatively flat over that energy range. Most recently, low Ecm measurements with low-resolution spectrometers indicate no presence of this ground state peak in the spectrum. The goal of our measurement was to carry out a much higher resolution experiment such that the ground state was easily visible if it was populated. Through experiments at OMEGA/U.Rochester and the National Ignition Facility at LLNL, both inertial confinement fusion facilities, we have been able to extract energy spectra for En>~1 MeV, and temperatures of 3.5, 7, and 13 keV, including clear evidence for the ground state peak at all temperatures. The data have been fit using an R-Matrix framework. The facilities will be described briefly and preliminary results will be presented, including a description of how to extract a meaningful neutron spectrum from a measurement in which all of the reactions take place in 100ps.

Topic: First Direct Measurement of the O-17(p, gamma)F-18 Reaction Cross Section at Gamow Energies for Classical Novae

Gianluca Imbriani ( University of Naples, Federico II, INFN, Naples, Italy ) / April 1, 2013

Classical novae are important contributors to the abundances of key isotopes, such as the radioactive F-18, whose observation by satellite missions could provide constraints on nucleosynthesis models in novae. The O-17(p, gamma)F-18 reaction plays a critical role in the synthesis of both oxygen and fluorine isotopes, but its reaction rate is not well determined because of the lack of experimental data at energies relevant to novae explosions. In this study, the reaction cross section has been measured directly for the first time in a wide energy range E-c.m. similar or equal to 200-370 keV appropriate to hydrogen burning in classical novae. In addition, the E-c.m. = 183 keV resonance strength, omega gamma = 1.67 +/- 0.12 mu eV, has been measured with the highest precision to date. The uncertainty on the O-17(p, gamma)F-18 reaction rate has been reduced by a factor of 4, thus leading to firmer constraints on accurate models of novae nucleosynthesis.

Topic: Prospects for Nuclear Astrophysics Experiments in Korea

Kevin I. Hahn ( Ewha Womans University, Korea ) / April 4, 2013

Korea will build the next generation of the radioactive ion beam accelerator facility called RAON, which is a new name for KoRIA. It will produce various types of beams, for example, uranium at 200 MeV/nucleon, from a superconducting linear accelerator as well as 70 MeV protons from a 70 kW cyclotron. We will produce radioactive beams from both the projectile fragmentation method and the ISOL method. The RAON facility in Korea is expected to play an important role for understanding the mechanism and origin of heavy elements. The challenges and prospects of nuclear astrophysics experiments and the current status of RAON in Korea will be discussed.

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Topic: Toward Realistic Description of Low-energy Fusion of Light Ions for Astrophysics

Guillaume Hupin ( Lawrence Livermore National Laboratory ) / April 8, 2013

The description of nuclear reaction observables from first principles, which provides many insights into the nuclear Hamiltonian, has been a long-standing goal in nuclear physics. The ab initio no-core shell model combined with the resonating-group method (NCSM/RGM) [1, 2] is capable of addressing both structural and reaction properties of light-nuclei. While promising results have already been achieved starting from a two-body Hamiltonian, a truly realistic prediction of nuclear observables requires the treatment the three-nucleon interactions. Using similarity-renormalization-group evolved two- and three-nucleon interactions [3, 4], I will present N-4He and d-4He scattering processes when accounting for the chiral two- plus three-nucleon interaction versus the chiral two-nucleon interaction. This work paves the way to modeling of light-ion fusion reactions with realistic nuclear forces, that are important for understanding nuclear astrophysics processes.^a
References
[1] S. Quaglioni and P. Navr_atil, Phys. Rev. Lett. 101, 092501 (2008); Phys. Rev. C 79, 044606 (2009).
[2] P. Navr_atil and S. Quaglioni, Phys. Rev. C 83, 044609 (2011).
[3] E. Jurgenson, P. Navr_atil, and R. J. Furnstahl, Phys. Rev. C 83, 034301 (2011).
[4] R. Roth, J. Langhammer, A. Calci, S. Binder, and P. Navr_atil, Phys. Rev. Lett. 107, 072501 (2011).
^a Prepared in part by LLNL under Contract DE-AC52-07NA27344. Support from the U.S. DOE/SC/NP (Work Proposal No. SCW1158), the NSERC grant 401945-2011 and the DFG through contract SFB 634 is acknowledged.

Topic: Electron-induced Nuclear Excited State Population Effects on NIF Thulium Radiochemistry

Rob Hoffman ( Lawrence Livermore National Laboratory ) / April 15, 2013

We explore the inclusion of electron induced plasma modifications to nuclear excited state level populations in the ablator region of the NIF Rev5 ignition capsule. Two such processes, NEEC and NEET (along with photo-absorption), can populate low-lying nuclear excited states of each isotope in a Thulium radiochemical reaction network. Also included are plasma modifications to the internal conversion rates that de-populate these same states. We will demonstrate the influence these processes have on Thulium nuclear level populations and compare them to the competing influence attributed to various neutron-induced reactions during the temperature-density evolution of the Rev5 capsule for two fuel loadings: DT (leading to simulated ignition), and pure DD.
Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security LLC for the U.S. Department of Energy National Nuclear Security Administration under Contract DE-07NA27344. Lawrence Livermore National Laboratory P.O. Box 808, Livermore, CA 94551-0808

Topic: Nuclear Physics and r-Process Nucleosynthesis

Matt Mumpower ( University of Notre Dame ) / April 22, 2013

The rapid neutron capture process or "r-process" is believed to be the origin of approximately half of the heavy elements above iron found in the solar system. The thousands of nuclei that participate in this process are short-lived and so present a great challenge to experimental nuclear physics. Quantities such as nuclear masses, beta-decay rates and neutron capture cross-sections are key ingredients for the r-process; however, limited data currently exists. Sensitivity studies play a pivotal role in determining the relative importance of these properties and are used to motivate new experimental campaigns. I'll discuss recent results and provide an update of our progress towards more robust sensitivity studies.

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Topic: Superdeformation and alpha - cluster structure in ³⁵Cl

Abhijit Bisoi ( Saha Institute of Nuclear Physics, India ) / April 26, 2013

A superdeformed (SD) band has been identified in a non - alpha - conjugate nucleus ³⁵Cl. It crosses the negative parity ground band above 11/2- and becomes the yrast at 15/2-. Lifetimes of all relevant states have been measured to follow the evolution of collectivity as well as formation of a cluster structure. Energetics as well as enhanced B(E2) and B(E1) values provide evidences for superdeformation as well as its relation to parity doublet cluster structure for the first time in A ˜ 40 region. Large scale shell model calculations assign (sd)¹⁶(pf)³ as the origin of these states. Calculated spectroscopic factors also correlate the SD states in ³⁵Cl to those in ³⁶Ar.

Topic: Nuclear Diagnostics and the Search for Fusion

Stephanie Lyons ( University of Notre Dame ) / April 29, 2013

The search for fusion continues at the National Ignition Facility at Lawrence Livermore National Laboratory. Nuclear physics is used to help understand the components of indirect-drive fusion and diagnosis problems through various tools. The ¹²C(n, n’γ) signature gives scientists at LLNL a view into the neutron output through a Cerenkov radiation detector, called the Gamma-Ray History (GRH) detector. A comprehensive simulation of the capsule and generation of the 4.45 MeV gammas to the signature from the GRH was performed in various steps. MCNP was used to create a simplistic model of the NIF capsule. The gammas created as neutrons travel through the CH-ablator shell of the capsule were tracked and analyzed. The gamma output from these capsule cases was used as the source in a Geant4 simulation of the GRH detector and the data analyzed. Additionally, the GRH simulations allowed for a comparison of actual shot data to that of the best models, thereby progressing the NIF models to be more physically accurate. Results from the MCNP case study will be presented along with an update of the current status of the Geant4 GRH simulations.

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