ISNAP Seminars - Abstracts 2007 Fall

Topic: Electron Screening Effect

Francesco Raiola ( University of Notre Dame ) / December 3, 2007

Understanding the electron screening effect in the laboratory is critical for a correct interpretation of low-energy nuclear reactions in stars. The theory of electron screening in stellar plasma is still based on the physics model introduced in 1954 by Salpeter, although the mathematical treatments have been improved significantly in the meantime. Extensive studies of the electron screening effect in different nuclear reactions have shown always the same discrepancy i.e. the experimental Ue adiabatic Ue. In the recent years extensive studies of the electron screening effect in deuterated metals (54 metals and 4 insulators) and other environments have been carried out in Bochum. Experimental results of anomalous enhancements have been interpreted in terms of the powerful verified prediction of this model one can consider it as a good parameterization of the experimental data.

Topic: Laser Trapping and Probing of Exotic Helium Isotopes

Zheng-Tian Lu ( Argonne National Lab ) / November 19, 2007

We have succeeded in laser trapping and cooling of the exotic helium isotopes, both ⁶He (t_1/2^ = 0.8 sec) and ⁸He (t_1/2 = 0.1 sec), and have performed precision laser spectroscopy on individual trapped atoms. Based on the atomic isotope shifts measured along the isotope chain ³He - ⁴He - ⁶He - ⁸He, and on the precise theory of the atomic structure of helium, the nuclear charge radii of ⁶He and ⁸He are determined for the first time tin a method independent of nuclear models. The results are compared with the values predicted by a number of nuclear structure calculations and test their ability to characterize these neutron rich, loosely bound halo nuclei. The ⁶He measurement was performed at ATLAS of Argonne, and the ⁸He measurement at GANIL, France.

Topic: Activation Measurements of Neutron Capture Cross Sections at Astrophysical Energies

Ethan Ueberseder ( University of Notre Dame ) / November 12, 2007

The Karlsruhe Van de Graaff accelerator has a long established importance in the field of Nuclear Astrophysics. Over the past few decades, neutron capture measurements have been performed with the goal of constraining our knowledge of s- and r- process nucleosynthesis. A strength of the experimental technique utilized in Karsruhe is its ability to closely reproduce stellar neutron energy distributions. As a result, Maxwellian averaged cross sections (MACS) of astrophysical interest can be directly measured within the laboratory. The method of cross section measurements will be discussed in the context of their astrophysical significance.

Topic: Explosions in Mass Transferring White Dwarf Stellar Binaries

Dean Townsley ( University of Chicago ) / November 5, 2007

White Dwarf (WD) stars are the most common stellar remnant, being produced by all stars below about 8 times the mass of our sun. Put in a close binary, a WD can gain mass from a companion and undergo a variety of bright, dynamic outburst phenomenon driven by both accretion and thermonuclear explosion. I will discuss how we study the WDs in these systems, during both outburst and quiescence, and what we are learning about their population as a whole and how these binaries form and evolve. I will highlight the ignition of the brightest outbursts they make: Classical Novae - due to thermonuclear runaway in hydrogen-rich material on the WD surface - and Supernovae - due to carbon-burning thermonuclear runaway in the deep interior. Particular attention will be paid to how the features of the evolution of the binary might determine aspects of the explosions. These systems are thought to produce the Supernovae of Type Ia, which have remarkably regular characteristics and are therefore of great interest as standard candles for cosmology. Interesting puzzles remain in our theoretical understanding of these systems and their remarkable outbursts.

Topic: Interplay between Spherical and Deformed States in the Neutron-deficient Pb-isotopes

Veerle Hellemans ( University of Ghent, Belgium ) / October 29, 2007

The neutron-deficient Pb-isotopes (Z=82) provide a unique laboratory to study the phenomenon of shape coexistence in atomic nuclei. Whereas shell-model excitations dominate the spectrum near the doubly closed-shell nucleus ²⁰⁸Pb, collective modes of motion govern the properties of the Pb-isotopes in the vicinity of neutron midshell. Microscopically, the appearance of the collective bands can be related to 2p-2h and 4p-4h proton excitations across the closed Z=82 proton shell. The interacting boson model (IBM), a symmetry-based truncation of the nuclear shell model, provides a framework in which the model space for the Pb-isotopes (including 0p-0h, 2p-2h, and 4p-4h proton excitations) becomes tractable. In our IBM calculations including configuration mixing, we focused on the ^186-196Pb–isotopes. The results of these calculations will be extensively discussed and confronted with recent experimental data.

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Topic: Measurements with the recoil separator ERNA

Daniel Schuermann ( University of Bochum, Germany ) / October 8, 2007

The study of radiative capture nuclear reactions by direct detection of the reaction products can be performed using a recoil separator. In this technique recoil ions and beam particles (both having the same momentum) are separated by velocity or energy filters. Focussing elements counter the opening angle induced on the recoil ions by the y-ray emission. Such a device, the European Recoil Separator for Nuclear Astrophysics (ERNA) was set up in the Dynamitron Tandem Laboratory of the Ruhr-University Bochum. In the talk, I will discuss several experimental key points like gas target properties, charge state distributions, acceptance measurements and suppression. The first two reactions studied with the recoil separator were ¹²C(a,y)¹⁶O and ³He(⁴He,y)⁷Be – key reactions in nuclear astrophysics. The peculiarities in the measurement of those reactions and their results are presented.

Topic: The Notre Dame Recoil Separator St. George

Manoel Couder ( University of Notre Dame ) / October 1, 2007

Studies of (alpha,gamma) and (alpha,gamma) reactions at low energies provide crucial information to improve our interpretation of the observed isotopic abundances, to predict the energy production and the time scale of nucleosynthesis processes during the stellar evolution and explosive events. While many radiative captures measurements have been made using various setup in direct kinematics, the very small cross section at astrophysically interesting energy of these reactions and the beam induced background limit the possible range of measurements. Reversing the kinematics and using a recoil separator to reject the beam ions which did not react in the target and to guide the reaction products to a detector is a good solution. Such a device aimed at low energy (alpha, gamma) measurements with stable beams is under development at the University of Notre Dame . In this talk I will present the concept of this new facility that is based on proven principles in existing devices. I will discuss the challenges encountered in low energy measurements and the solutions that we are pursuing.

Topic: Transiently accreting neutron stars and the nature of superdense matter

Dima Yakovlev ( Joffe Institute, St. Petersburg ) / September 24, 2007

Soft X-ray transient sources are binary systems containing a neutron star and a low-mass star. Neutron stars undergo periods of accretion from low-mass components and periods of quiescence. The accreted matter can penetrate deeply into the neutron star crust and warm up the entire star due to nuclear transformations in this matter, particular due to pycnonuclear reactions. Radiation from the warmed up surface is detectable during quiescent periods and carries information on the physical state of superdense matter in neutron star cores.
The physics of heating in the accreted matter is outlined. The theory is compared with observations of thermal radiation from neutron stars in soft X-ray transients during quiescent periods. The constraints on the properties of superdense matter which result from this comparison are summarized.

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Topic: Two key radioactive isotopes of gamma-ray astronomy: The past present, and future of Al²⁶ and Fe⁶⁰

Frank Timmes ( Arizona State University ) / September 17, 2007

Radioactive isotopes are common by-products of nucleosynthesis in stellar sources and constitute important probes of the underlying physical processes since they can be studied through their characteristic gamma-ray emission. Direct detection of these photons by instruments on ballons and satellites provides unique information on the stellar production sites, which cannot be obtained through observations at other wavelengths. Because the lifetimes of Al²⁶ and Fe⁶⁰ are long enough to allow sufficient amounts of these isotopes to accumulate in the Milky Way and give rise to a detectable signal, these two isotopes have long been prime candidates for gamma-ray astronomy. During this talk we will discuss the colorful history, the exciting present state, and the bright future of our attempts to understand the nuclear astrophysics of these two radioactive isotopes.

Topic: Turning Points in the Evolution of Isolated Neutron Stars' Magnetic Field

Ulrich Geppert ( German Aerospace Center, Institute for Space Systems ) / September 10, 2007

The neutron star (NS) magnetic field determines almost all our observations of those compact objects and affects crucial their evolution. Thus, in order to understand the basic physics of NSs, we have to study their field's structure, strength, and decay as well as its mutual interaction with a NS's thermal and rotational evolution. During the life of isolated NSs their magnetic field passes through a variety of evolutionary phases. Depending on its strength and structure and on the physical state of the NS (e.g. cooling, rotation), the field looks qualitatively and quantitatively different after each of these phases. Three of them, the phase of MHD instabilities immediately after NS's birth, the phase of fallback which may take place hours to months after NS's birth, and the phase when strong temperature gradients may drive thermoelectric instabilities, are concentrated in a period lasting from the end of the proto--NS phase until 100, perhaps 1000 years, when the NS has become almost isothermal. The further evolution of the magnetic field proceeds in general inconspicuous since the star is in isolation. However, as soon as the product of Larmor frequency and electron relaxation time, the so--called magnetization parameter, locally and/or temporally considerably exceeds unity, phases, also unstable ones, of dramatic changes of the field structure and magnitude can appear. An overview is given about that field evolution phases, the outcome of which makes a qualitative decision regarding the further evolution of the magnetic field and its host NS.

Topic: Nucleosynthesis in AGB stars

Maria Lugaro ( University of Utrecht, The Netherlands ) / August 27, 2007

Asymptotic Giant Branch (AGB) stars play a main role, together with supernova explosions, in producing the elements in the Universe thus shaping the composition of stars and galaxies. Different nucleosynthetic processes occur in AGB stars leading to the production of carbon, nitrogen, fluorine and heavy elements such as barium and lead. I will give an overview of these processes, describe how the theorist models them, and summarize the vast range of exciting applications of this kind of study. I will demonstrate that the study of AGB nucleosynthesis requires an interdisciplinary approach, which means that I always need to look at things from different prospectives and gain knowledge in different research fields. I will give some examples of how the study of AGB stars encompasses nuclear physics, stellar physics, cosmochemistry, astronomy and cosmology.

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Topic: High-precision (p,t) reactions to determine reaction rates of explosive stellar processes

Andrija Matic ( Kernfysisch Versneller Instituut, The Netherlands) / August 14, 2007

The aim of studying ²⁴Mg(p,t)²²Mg and ²⁸Si(p,t)²⁶Si reactions was to investigate the nuclear structure of ²²Mg and ²⁶Si. These two nuclei play a significant role in stellar reaction processes at high temperatures. We performed ²⁴Mg(p,t)²²Mg and ²⁸Si(p,t)²⁶Si experiments at RCNP by using the Grand Raiden spectrometer and the WS beam line. In this experiment we achieved an unprecedented resolution of 13 keV for (p,t) experiment and we were able to resolve 12 new levels in ²²Mg and 14 new levels in ²⁶Si. On base of the obtained nuclear structure we calculated the stellar reaction rates for the following reactions: ¹⁸Ne(a,p)²¹Na, ²¹Na(p,y)²²Mg, ²²Mg(a,p)²⁵Al and ²⁵Al(p,y)²⁶Si.

Topic: One-proton, two-proton, and a emission from ¹⁴O+a interactions

Changbo Fu ( Cyclotron Institute, Texas A & M University ) / August 13, 2007

In this work, by using radioactive beam ¹⁴O (> 99% pure), one-proton, two-proton, and a emission from ¹⁴O + a interactions were studied with the modified thick target inverse kinematics approach.
For the ¹⁴O(a,2p) interaction, it was found that the resonance excited states in 18Ne are populated, and most of them decay sequentially to the ground state of ¹⁶O, i.e. ¹⁸Ne* ® ¹⁷F* + p ® ¹⁶O + p + p. Two proton events corresponding to the excited state of ¹⁸Ne* (8.45MeV), were found to have strong 2p correlation, which is the fingerprint of ²He-decay. Several models were used to explain this decay process, and evidence suggests that this state decays by ²He-emission.
By measuring the Time-Of-Flight for protons through the thick target, the location of ¹⁴O(a,p) reactions occurring at different places in the helium gas target were identified. With this information, we were able to measure a spectrum of protons corresponding to the population of ¹⁷F particle stable states in the ¹⁴O(a,p)¹⁷F reaction. This method provides a new way to measure astrophysically important reactions which involve radioactive nuclei and a particles.

Topic: A Study of the Fusion Reactions ¹²C + ¹²C Toward the Gamow Energy

Timothy Spillane ( University of Connecticut ) / August 13, 2007

The set of fusion reactions ¹²C + ¹²C play a critical role in nuclear astrophysics. Several processes, including quiescent carbon burning and type Ia supernovae depend intimately on the reaction rate of the ¹²C + ¹²C reactions at energies far below the Coulomb barrier. In addition to their astrophysical relevance, these reactions play an ongoing role as both catalyst and test case for developments in nuclear structure. These developments include the theory of quasi-molecular nuclei, the hypothesis of absorption under the barrier, and very recently, the phenomenon of sub-barrier fusion hindrance. In a recent experiment, the fusion reactions ¹²C + ¹²C have been studied at ECM = 2.10 to 4.75 MeV by g-ray spectroscopy using a C target of ultra-low hydrogen contamination. The deduced astrophysical factor exhibits previously unknown resonances at E=3.0 MeV, in particular a strong narrow resonance at E = 2.14 MeV, which lies at the high-energy tail of the Gamow peak. The experiment and results are described, and the implications to astrophysics and nuclear structure are surveyed.

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