ISNAP Seminars - Abstracts 2007 Spring

Topic: Calculations with Finite-range forces in Neutron-rich Nuclei

Nicolas Schunck (Oak Ridge National Laboratory, University of Tennessee) / April 23, 2007

With the on-going development of Radioactive Beam Facilities, proton- or neutron-rich nuclei have come under more intense scrutiny. From a theoretical point of view, these systems present a number of interesting features. First of all they are characterized by a very large isospin, whose influence on the density functional is not very well-known. Moreover their Fermi level is quite close to the continuum threshold, and residual interactions such as pairing correlations can scatter particles into it. The interplay of the nuclear system with the continuum is therefore a crucial ingredient of all serious theoretical approaches, and this has been the main argument to call for the development of more appropriate techniques than those used in stable nuclei. However, the description of pairing correlations in such systems, e.g. in Skyrme HFB approaches, has always been somewhat empirical. We therefore developed a technique to extend the realm of HFB calculations with finite-range forces like the Gogny force to weakly-bound nuclei. In addition to include continuum effects, this technique is amenable to go beyond the mean-field and to give a precise measure of the true role of the continuum in ground-state properties of atomic nuclei. I will describe this technique, present some of the results we obtained and discuss critically the extent of continuum effects in weakly-bound nuclei.

Topic: Use of the ¹⁵N(p,a,g)¹²C Reaction for Nanoscale Studies of Cement Hydration

Jeffrey S. Schweitzer (University of Connecticut) / April 19, 2007

A better understanding of the mechanisms and kinetics of cement hydration during the induction period is critical to improved concrete technology. During the induction period a characteristic pattern of reaction layers develops at and just below the surface of the cement grain. To understand the processes that occur, it is necessary to study the time dependence of the reactions on a nanometer scale. The ¹⁵N(p,a,g)¹²C reaction is particularly well-suited to measure the distribution of hydrogen with depth with a spatial resolution of a few nanometers. Time-resolved measurement of the hydration profile is achieved by stopping the chemical reactions at specific times. The mechanism controlling the induction period of tricalcium silicate is a semi-permeable layer on the grain surface. The diffusion of the hydrogen cannot be represented by a simple Fickian profile, indicating that multiple diffusion and reaction processes are occurring. The hydration profiles for other common components of cement are significantly different. The effects of additives on the chemical reactions have also been studied.

Topic: Molecular Picture in Heavy Ion Reactions at Low Energy

Alexis Diaz-Torres (Dept. of Nuclear Physics, Australian Nat'l University) / April 16, 2007

The molecular description of nuclear reactions is justified when the relative motion of the interacting nuclei is much slower than the motion of the nucleons in the volume of the system, which is expected to be valid at incident energies near and below the Coulomb barrier. As a consequence, the nucleons near the Fermi surface move in molecular orbitals when the colliding nuclei come into contact. The two-center shell model (TCSM) is a fundamental microscopic approach to describe this scenario. In my talk, I will introduce a new realistic TCSM based on Woods-Saxon potentials and discuss the impact of collective and single-particle molecular effects on fusion of light and heavy nuclear systems.

Topic: ISAC, DRAGON and Exploding Stars

John D'Auria (Simon Fraser University, Canada) / April 2, 2007

Experimental nuclear science took a quantum leap forward with the advent of using energetic exotic radioactive beams for a wide variety of studies in nuclear astrophysics, fundamental symmetries, nuclear structure at the limits of stability and condensed matter physics. Probably the best facility in North America is the TRIUMF-ISAC facility (Canada). Based upon the ISOL approach, it produces the most intense RB in the world. A key application are measurements of rates of reactions occurring in exploding stars, involving radioactive reactants. Such studies require an intense RB, a low velocity accelerator, an appropriate recoil mass separator, and experienced personnel. This talk will review some of the history of RB production, but the main focus will be the experimental program using the DRAGON facility to measure key reaction rates involving species such as ²¹Na, ²⁶gAl, ⁴⁰Ca, and others.

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Topic: ²⁴Mg(a,y)²⁸Si Resonance Parameters at Low Alpha Energies

Elizabeth K. Strandberg (University of Notre Dame) / March 26, 2007

²⁸Si is formed by successive alpha captures on ¹²C during the AGB (asymptotic giant branch) stage of stellar burning. The last reaction in this series, ²⁴Mg(a,y)²⁸Si, has not been examined with sufficient sensitivity at alpha energies below 1.5 MeV. Several ²⁸Si states appear favorable for formation by this reaction in the alpha energy range of 1.0 – 1.5 MeV, motivating a new study of this reaction at the University of Notre Dame. To maximize experimental sensitivity, a high efficiency coincidence detection system was developed. Several previously unknown resonances were observed between 1.1 and 1.5 MeV, and an upper limit for any lower energy resonances was obtained. Newly calculated resonance parameters and reaction rates will be presented.

Topic: Direct measurement of ⁴He(¹²C,¹⁶O)g cross section near stellar energy at KUTL

Kenshi Sagara (Kyushu University Tandem Laboratory, Fukuoka, Japan) / March 19, 2007

As is well known, ⁴He+¹²C→¹⁶O+g reaction is very important in helium burning in stars, however, the reaction rate at the stellar energy of E_cm = 0.3 MeV has been measured yet in spite of many attempts in the world for about 40 years.  Due to resonances in ¹⁶O the cross section (S-factor) varies sharply around 0.3 MeV.  Hence we have to measure the cross section at energy as low as possible, e.g., down to 0.7 MeV where the cross section is about 1 pbarn. 

In Kyushu University tandem accelerator laboratory (KUTL), direct measurement of ⁴He+¹²C→¹⁶O+g reaction cross section is in progress.  A ¹²C beam is injected onto a ⁴He windowless target, and all the ¹⁶O recoils in a charge state are separated from the ¹²C beam by a recoil mass separator and are detected by a Si detector.

To measure the ⁴He+¹²C→¹⁶ O+g cross section down to 0.7MeV, we need 1) to increase the beam intensity, target thickness and detection efficiency, because ¹⁶O counts at 0.7 MeV are estimated as about 5 counts/day at most, and 2) to thoroughly reduce backgrounds, because the number of ¹⁶O recoils at 0.7 MeV is 10^-18 smaller than the number of ¹²C beam particles.

I will talk on several methods and instruments at KUTL, such as a) a blow-in type windowless gas target and its cooling, b) a new accel-decel strong-focus operation of the tandem accelerator, c) a recoil mass separator, and d) a long-time chopper to reduce backgrounds by 10^-3.

Also our recent data on ⁴He(¹²C,¹⁶O)g cross section at E_cm = 2.4 MeV will be shown.

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Topic: Study of Beta-delayed Neutron Decay Near the Neutron Drip Line

Chandana Sumithrarachchi (Michigan State University) / March 1, 2007

The rapid change in the location of the neutron drip as one goes from carbon, to oxygen, to fluorine isotopes is still not understood. Recently, it has been suggested that the traditional single particle shell structure changes significantly in approaching the drip line. The study of nuclei in the region of the heaviest oxygen and fluorine isotopes can provide important information on the variation of nuclear shell structure leading to new magic numbers. Very little spectroscopic information is available on these neutron-rich nuclei. One way of exploring information on them is to combine the traditional study of the beta-delayed gamma-ray decay of exotic nuclei with beta-delayed neutron spectroscopy. The first measurement of beta-delayed neutrons and gamma-rays from the decays of ²²N and ²³O has been performed at the NSCL (National Superconducting Cyclotron Laboratory). The beta decay schemes of ²²N and ²³O will be presented and compared to shell model calculations.

Topic: Structural Studies of the Most Exotic Nuclei: First Results from The Stopped RISING Project

Paddy Regan (University of Surrey, UK) / February 26, 2007

Projectile fragmentation reactions at relativistic energies allow unpredecented experimental access to nuclei with the most extreme values of proton-to-neutron ratio. The RISING (Rare Isotope Spectroscopic Investigations at GSI) project couples the fragmentation production mechanism with high-resolution gamma-ray spectroscopy for the study of internal structure of nature's most exotic nuclear species. This talk will present the first experimental results from the RISING gamma-ray array in its 'Stopped Beam'configuration, which is designed for studies of spectroscopy of exotic nuclei following isomer and beta-delayed decay. I will specifically present isomer-spectroscopy identifying new metastable states in nuclei (i) along the N=Z line between the doubly magic systems ⁵⁶Ni and ¹⁰⁰Sn and (ii) neutron-rich nuclei associated with proton holes in the ¹³²Sn and ²⁰⁸Pb doubly-magic closed shells pertinent to the r-process path. These results reveal new physics associated with T=1,0 proton-neutron pairing competition along the proton drip line for N=Z nuclei and the persistence (or not) of the N=82 and 126 shell closures for large neutron excess.

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Topic: To chiral rotation and back again

S. Daniel Almehed (University of Notre Dame) / February 19, 2007

Several near degenerate rotational bands with the same parity have been found in the A=135 and A=105 mass regions where triaxiality of the nuclear shape appears. Some of these have been interpreted as chiral rotational bands within the Tilted Axis Cranking (TAC) model. Chiral rotation can appear in triaxial nuclei when proton and neutrons align along different principal axes and the collective rotation occurs along the third. These bands can some times be interpreted as a chiral vibration, which is a vibration of the orientation of the principal axes of the nucleus with respect to the angular momentum vector. Chiral vibration can be studied with the RPA plus TAC formalism. I examine the limits of chiral rotation as a function of particle number and angular momentum in the A=135 region. The properties of the RPA phonons are discussed and compared to experimental data.

Topic: A Cluster Model of ⁶He and ⁶Li

Jeremy Armstrong (NSCL, Michigan State University) / January 29, 2007

Small nuclei provide an ideal testing ground of few-body theories. ⁶He is particularly interesting in that it shows an extended particle distribution similar to a halo nucleus, is loosely bound, and is a Borromean system. We apply the Brink Formalism in secondary quantization to study the structure of ⁶He. This formalism allows for the proper treatment of Fermi statistics and correct projection into eigenstates of angular momentum. The alpha plus dineutron configuration and "cigar" (neutron, alpha, neutron chain) configuration were studied to obtain binding energies, charge radii, matter radii, and B(E2) for ⁶He. The same configurations were used to obtain the same observables for ⁶Li. We were then able to calculate the log ft value for the beta decay of ⁶He.

Topic: Low-Energy Nuclear Astrophysics - the Fascinating Region of A=7

Michael Hass (Weizmann Institute of Science, Rehovot, Israel) / January 10, 2007

We discuss results of low-energy reactions that play an important role in current nuclear astrophysics research and that happen to concentrate around the region of A=7. The ⁷Be(p,gamma)⁸B and the ³He(⁴He,gamma)⁷Be reactions are crucial for understanding the solar-neutrino oscillations phenomenon and the latter one plays a central role in the issue of cosmic ⁷Li abundance and Big-Bang Nucleosynthesis. We also present results regarding the host dependence of the half life of the electron-capture 7Be radio-nuclide and future plans for reactions with radioactive beams.

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