ISNAP Seminars - Abstracts 2015 Spring

Topic: Investigation of transitional rare earth nuclei using light ion reactions: Nuclear Structure and Surrogates

Prof. Cornelius Beausang ( University of Richmond ) / January 19, 2015

For several years, the Richmond group has been investigating the low/medium spin structure of N ~ 90 rare earth nuclei utilizing light ion transfer reactions, such as (p,d), (p,t) and inelastic proton scattering: work relevant for both basic nuclear structure and for surrogate (n,γ) cross section measurements. Such light ion induced reactions populate a variety of states and structures from the ground state to very high excitation energies, up to and indeed beyond the neutron separation energy. Thus we are probing the relatively unexplored bound state quasi-continuum at high energy and medium spin. Our recent work has focused on Sm and Gd nuclei and on several aspects: Single particle structures of odd N nuclei: Measurements of the spin distribution in the high energy bound quasi-continuum: Precision cross section measurements for excited states: and the properties of a intense feature, called the peak-like-feature observed just above the pair gap in all five even-even Gd and Sm nuclei studied to date.

Topic: Stellar Neutron Sources and s-Process in Massive Stars

Rashi Talwar ( University of Notre Dame ) / January 26, 2015

Potential stellar neutron sources for the s-process in massive stars are associated with α-capture reactions on light nuclei. The capture-reaction rates provide the reaction flow for the buildup of the neutron source 22Ne during the helium-burning phase in stars. A critical influence on these reactions is expected to come from low-energy resonances at stellar energies below 800keV. Direct measurements of capture reactions to study these resonances are handicapped by the Coulomb barrier. Also, it is possible that some of these resonances correspond to pronounced cluster structures near the α-threshold. Hence, inelastic α-scattering on 26Mg has been used as an alternative tool to probe into the level structure. Also α-transfer technique has been used to extract α-strength information. In reference to this, the experiments performed using the Grand Raiden Spectrometer at RCNP, Osaka will be discussed and results will be presented.

Topic: Low energy nuclear physics research at LLNL

Dr. Jason Burke ( Lawrence Livermore National Laboratory ) / February 3, 2015

The low energy nuclear physics program at LLNL consists of a myriad of accelerator based and table top experiments. Studies include nuclear structure, neutron induced cross section measurements, determination of cross sections using the surrogate reaction technique, nuclear-atomic plasma induced interactions and beta decay studies. In this seminar, I will provide an overview of our low energy nuclear physics research program at LLNL. These include efforts at the Texas A&M Cyclotron Institute, Argonne National Laboratory, Lawrence Berkeley National Laboratory and Los Alamos National Laboratory. The Hyperion, Hydra and NeutronSTAR detector systems will also be discussed in detail.

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Topic: Measurement of the plasma astrophysical S factor for the 3He(d, p)4He reaction in exploding molecular clusters

Dr. Marina Barbui ( Texas A&M University ) / February 9, 2015

The plasma astrophysical S factor for the 3He(d, p)4He fusion reaction was measured for the first time at temperatures of few keV, using the interaction of intense ultrafast laser pulses with molecular deuterium clusters mixed with 3He atoms. D2 or CD4 molecular clusters were produced in the adiabatic expansion in vacuum of high pressure (52.5 bar) and low temperature gas, through a supersonic nozzle. The temperature of the gas was 86 K in the case of D2 and 200-260 K in case of CD4. Different proportions of D2 and 3He or CD4 and 3He were mixed in the gas jet target in order to allow the measurement of the cross-section for the 3He(d, p)4He reaction. The Texas Petawatt laser delivered 90-180 J per pulse with 150 fs duration to irradiate the clusters. The Coulomb explosion of deuterium molecular clusters provided energetic D ions capable of drive fusion reactions. The energy distribution of the deuterium ions was obtained from their time of flight measured with a Faraday cup. The yield of 14.7 MeV protons from the 3He(d, p)4He reaction was measured in order to extract the astrophysical S factor at low energies. Results of the experiment performed at Center for High Energy Density Science at The University of Texas at Austin will be presented [PRL, 111, 082502]. The possibility to use the same technique to investigate other reactions of astrophysical interest will be also discussed.

Topic: Inelastic Neutron Scattering Studies Relevant to Neutrinoless Double-Beta Decay

Dr. Benjamin Crider ( NSCL/MSU ) / February 16, 2015

The search for neutrinoless double-beta decay (0νββ) is a large international effort, with hopes of discovering physics beyond the standard model. One candidate for 0νββ, the decay of 76Ge to 76Se, is the focus of two major experiments, GERDA and MAJORANA. In these experiments, the signature of 0νββ would appear as a sharp peak in the energy spectrum at 2039 keV. Due to the high sensitivity of such a measurement, knowledge of background γ rays is critical. One such concern is the 2040.70(25) keV γ ray from the 3951.70(14) keV level in 76Ge, found in a study of 76Ga β- decay. This level could be populated via cosmic-ray-induced inelastic neutron scattering in the large experiments searching for the 2039 keV signature of 0νββ. The neutron-induced cross section of this level was measured at the University of Kentucky Accelerator Laboratory (UKAL). In this seminar, I will discuss the capabilities of UKAL and how we are using this facility to investigate topics related to 0νββ searches in 76Ge, ranging from the structure of the parent and daughter nuclei to potential backgrounds from decays in 76Ge itself. In particular, we have found that the 2039 keV region of the γ-ray spectrum following neutron scattering in 76Ge is more complex than was previously thought.

Topic: Study of fission dynamics and nuclear viscosity using particle emission as a probe

Dr. Yogesh Gupta ( University of Notre Dame ) / February 23, 2015

Understanding of fission dynamics in a wide energy regime has been a topic of continued research interest. Among many other degrees of freedom, the nuclear viscosity plays a crucial role in governing the fission dynamics. Several probes have been employed previously to study the fission dynamics and hence the nuclear viscosity.
Study of near scission particle emission (ternary fission), is a very good probe to provide information about the nuclear motion during saddle to scission points. I will discuss the systematic study of pre-scission and near scission ?-particle emission made for various systems over a wide range of Z2/A and excitation energies. It is seen that nuclear collective motion exhibits a changeover from dynamical emission to statistical one in going from very low excitation energy to the higher one which is a result of variation of viscosity with energy. Detailed experimental, procedure and results would be presented.

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Topic: Proton-capture reactions in thermonuclear supernovae and the p process

Dr. Kerstin Sonnabend ( Goethe University Frankfurt, Germany ) / February 26, 2015

Most of the elemental abundances beyond the so-called iron peak of the solar abundance distribution are produced by neutron-capture reactions in the r and s processes. However, there are about 35 isotopes on the proton-rich side of the valley of stability that are shielded against these reaction chains. These isotopes are usually referred to as ρ nuclei and are thought to be produced by different mechanisms in a number of astrophysical scenarios. One of these scenarios is a thermonuclear or type Ia supernova explosion. Thermonuclear supernovae provide a hot scenario where the ρ nuclei can be synthesized by the γ process and also by a series of proton-capture reactions.
The reactions producing the most abundant p nucleus 92Mo and their experimental investigation will be presented, e.g., recent results on the investigation of the 90Zr(ρ, γ) reaction using high-resolution γ-ray spectroscopy at Cologne, Germany, and steps towards the study of the 91Nb(ρ, γ) reaction in direct kinematics at FRANZ, Frankfurt, Germany. Future possibilities at different experimental facilities will be discussed in a detailed outlook.

Topic: Beta decay of deformed nuclei with the proton-neutron finite amplitude method

Thomas Shafer ( University of North Carolina ) / March 2, 2015

In the last few years the finite amplitude method (FAM) has emerged as an efficient technique to calculate nuclear transitions. While formally equivalent to the quasiparticle random-phase approximation (QRPA), the FAM removes the need to construct and diagonalize the large Hamiltonian matrix usually required in the QRPA. As a result, calculations performed with the FAM are significantly faster than their QRPA counterparts, reducing computational burden and enabling larger-scale studies.
At UNC we have recently extended the FAM to calculate beta-decay rates and other charge-changing observables of axially-deformed even-even nuclei. In this talk I will review mean-field theory and the QRPA before discussing the finite amplitude method and its application to beta-decay rates important for r-process nucleosynthesis.

Topic: Jet Modification and the Question of Quark Gluon Plasma in Small Collision Systems

Prof. Justin Frantz ( Ohio University ) / March 16, 2015

A recent development in the study of Relativistic Heavy Ion Collisions has been the observation of hydrodynamic flow in the collisions of "small" species, where at least one of the projectiles has only one, or just a few, nucleons. This flow shows many of the same properties and QGP signals previously found in A+A collisions, where the number of nucleons, A, is large for both projectiles in the collision. Now the signal seems to be present in p+Pb, d+Au, and He+Au at RHIC and LHC energies, and even in rare p+p collisions at the LHC. Such signals appear in events where the produced particle multiplicity is large. However, the question arises whether such small volumes should also quench jets in the same way as previously observed in A+A. So far no experimental evidence has been seen for this, but it is unclear how strong the quenching could be and thus whether it may be hidden in the current uncertainties. I present a summary of this situation include a recent PHENIX analysis which has been able to observe what appears to be a very small jet modification with high precision, which also does not seem to fit with any traditional "cold" nuclear matter explanations.

Topic: Optically Probing Nuclei Trapped in Cryogenic Solids: Opportunities for Nuclear Physics

Prof. Jaideep Singh ( NSCL/MSU ) / March 30, 2015

Inert gases frozen at cryogenic temperatures have been used to trap and study atoms and molecules for over 60 years. In particular, thin films of noble gas solids (NGS) are a promising medium for the capture, detection, and manipulation of atoms and nuclear spins. They provide stable, chemically inert, and efficient confinement for a wide variety of guest species. Because NGS are transparent at optical wavelengths, the guest species can be probed using lasers. Longitudinal and transverse nuclear spin relaxation times of a guest species can be made very long under well-understood and feasible conditions. Potential applications include measurements of rare nuclear reactions, long-term memory for quantum information processing, and tests of fundamental symmetries.
In this talk, I will summarize the results of our optical spectroscopic study of ytterbium atoms embedded in a frozen neon matrix, which was performed at Argonne National Lab. Furthermore, I will describe the planned activities of my new group at the NSCL at MSU, which includes the demonstration of optical single atom detection in-medium and the measurement of spin relaxation times of trapped nuclei. Our eventual goals are (1) to use single atom detection to measure the 22Ne(α,η)25Mg & 22Ne(α,γ)26Mg nuclear reactions, which are important for the s-process in stellar nucleosynthesis, and (2) to search for the permanent electric dipole moments of pear-shaped nuclei such as 225Ra and 229Pa, which have an enhanced sensitivity to time-reversal & parity violating interactions originating within nuclei.

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Topic: The TITAN facility at TRIUMF: precision experiments with ion traps

Dr. Ania Kwiatkowski ( TRIUMF, Canada) / April 13, 2015

TRIUMF’s Ion Trap for Atomic and Nuclear science (TITAN) facility deploys three ion traps for Penning trap massspectrometry of radionuclides and in-trap decay spectroscopy. The former focuses on very short-lived species,such as the halo nuclide 11Li and more recently in a detailed survey of the island of inversion at N = 20. The latterhas revealed the lowest shell gap of any magic nuclide (in 32Mg) and the only known overlap in two-neutronseparation energies on the chart of nuclides (at N = 21). Q-value measurements were performed to elucidate theso-called gallium anomaly in neutrino physics, in which the observed and expected solar neutrino fluxes arediscrepant. To investigate nucleosynthesis via the r-process mass measurements near A = 100 were performed onneutron-rich Rb and Sr isotopes.
Prior to the mass measurement, the Rb and Sr isotopes were charge bred in an electron beam ion trap (EBIT), which is playing an increasingly important role at TITAN. Originally, constructed to reduce beam-timerequirements, the EBIT has been used to improve beam purity (e.g. threshold charge breeding), to accumulateions (ion stacking), to recapture charged decay products, and for in-trap decay spectroscopy. The last has beendirected toward the measurement of branching ratios of intermediate nuclides of double electron-capturecandidates. The magnetic field separates the charged and neutral decay particles, thereby eliminating thebackground from annihilating β particles. The electron beam enhances the radial confinement, extending theobservation period to minutes. For x-ray detection seven Si(Li) detectors are placed radially around the EBIT. Anoverview of the ion manipulation techniques and the facility will be given as well as highlights from the researchprogram.

Topic: Covariant density functional theory: the journey across three-dimensional nuclear landscape

Prof. Anatoli Afanasjev ( Mississippi State University ) / April 20, 2015

Covariant density functional theory (CDFT) is well established theoretical tool for the description of nuclear systems. In CDFT, the nucleus is described as a system of nucleons which interact via the exchange of different mesons. In my talk, I will concentrate on a number of topics which have been addressed or satisfactorily resolved within its framework only recently. First, I will start from the global assessment of the description of ground state observables of even-even nuclei in the CDFT framework in the (proton number Z, isospin) plane of nuclear landscape [1]. Calculated binding energies, the deformations, radii, neutron skins, two-neutron separation energies and the positions of the proton-drip line will be compared in asystematic way with available experimental data. Such comparison allows to establish theoretical uncertainties in the description of physical observables in known regions of nuclear chart and extrapolate them towards neutron-drip line. I will also discuss the uncertainties in the position of two-neutron drip line and their sources [1,2]. A recent reassessment of the situation in the region of superheavy nuclei will also be presented.
The physics is not limited to spin zero and ground state observables of even-even nuclei. Single-particle degrees of freedom, rotational excitations and fission are examples which go beyond that limit. The analysis of the single-particle properties inspherical and deformed nuclei will be presented. The angular momentum represents the third dimension of nuclear landscape. Thus, the rotational response of nuclei will be illustrated by a recent detailed study of even-even and odd-massactinides [3]. Finally, the results of the study of fission barriers in actinides and superheavy nuclei will be presented.
It is clear that the current generation of covariant energy density functionals has some limitations. The possibilities of their extension and thus the improvement of the description of experimental data and the decrease of theoretical uncertainties will be discussed.
[1] S. E. Agbemava, A. V. Afanasjev, D. Ray and P. Ring, Phys. Rev. C89, 054320 (2014).
[2] A. V. Afanasjev, S. E. Agbemava, D. Ray and P. Ring, Phys. Lett. B 726, 680 (2013). [3] A. V. Afanasjev and O. Abdurazakov,Phys. Rev. C88, 014320 (2013).

Topic: The photonuclear cross section of Boron-10 from the No Core Shell Model

Dr. Michael Kruse ( Lawrence Livermore National Laboratory ) / April 27, 2015

In this talk I will outline how I have started to build a framework to calculate electroweak observables in the ab-initio No Core Shell Model. Electromagnetic probes provide stringent tests of the predictive power of our nuclear Hamiltonians and can be used to constrain potential models whereas weak observables can answer interesting questions about the nature of the universe. As a starting point, I have calculated the photonuclear cross section of Boron-10 using as input realistic nuclear interactions to determine the ground- and excited-states of the nucleus. By using the moment-generating method of Lanczos one is able to construct the electric-dipole strength function from which one can calculate the energy-weighted strength function. To determine the cross section one needs to take into account the finite width of the continuum states; in our case we use the physics of neutron-escape widths, which will be extensively discussed. Theoretical results are compared to experimental data and some results regarding the Brink hypothesis will be presented.

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Topic: Shell model nuclear level densities and applications in nuclear astrophysics

Prof. Mihai Horoi ( Central Michigan University ) / May 4, 2015

Nuclear level densities provide important information regarding the nuclear dynamics. Inparticular, they can be used to predict nuclear compound cross sections and reactionrates, which are of large interest in nuclear astrophysics. We developed a methodology ofcalculating the the spin- and parity-dependent nuclear level density using the interactingshell model, which takes into account the effect of the residual interaction amongnucleons. These new techniques are based on nuclear statistical spectroscopy, butcalculate spin- and parity-projected moments of the nuclear shell model Hamiltonian. Inmy talk I will review the model we use, and I will present few applications that could berelevant for the rp-process.

Topic: Experimental measurement of 16O+12C fusion at stellar energies

Xiao Fang ( University of Notre Dame ) / May 11, 2015

The total cross section of 12C+16O fusion has been determined by applying the statisticalmodel calculation to experimental data. Both emitted protons and γ-ray from this reactionwere measured simultaneously for c.m. energies from 3.64 to 4.93 MeV by using siliconand Ge detectors with high-intensity beam from the Santa Ana 5MV accelerator atUniversity of Notre Dame. By comparing statistical model and data of measurement, theclear correlation between them was observed. It results in more reliable extrapolation ofthe total 12C+16O fusion cross section for stellar nucleosynthesis.

Topic: Electromagnetic processes in few-nucleon systems with the Chiral Effective Field Theory

Dr. Dagmara Rozpedzik ( Jagiellonian University, Krakow, Poland ) / May 18, 2015

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