ISNAP Seminars - Abstracts 2015 Fall

Topic: ANCs at sub-Coulomb energies to constrain key α-capture reaction rates

Dr. Melina Avila ( Argonne National Laboratory ) / August 24, 2015

Many important α-particle induced reactions can only be measured indirectly due to small cross section at energies of astrophysical interest. Extracting the Asymptotic Normalization Coefficients (ANCs) using sub-Coulomb α-transfer reactions can be used as an effective method to determine properties of near-threshold resonances. This will constrain and drastically limit the uncertainties related to extrapolations procedures for key astrophysical reactions. We have applied this valuable tool to investigate the important α-capture reactions 12C(α, γ)16O and 13C(α, n)16O. Results and the implication to the astrophysical rates will be discussed.

Topic: Special nuclear seminar: Accelerator Mass Spectrometry at the University of Cologne

Dr. Alfred Dewald ( University of Cologne, Germany ) / September 18, 2015

CologneAMS is a new Centre for Accelerator Mass Spectrometry (AMS) at the University of Cologne which is designed to measure all standard cosmogenic nuclides (10Be, 14C, 26Al, 36Cl, 41Ca, 129I). It became operational in October 2011. The AMS spectrometer is based on a 6 MV TANDETRON accelerator (HVEE) equipped with an all solid-state power supply, foil and gas stripper. Since 2011 effort was spent to increase the number of nuclides which can be measured routinely at CologneAMS, e.g. the plutonium isotopes 239,238,240,242Pu.
In this seminar I will report on the general performance of the total AMS system and on the quality of the AMS measurements which has been achieved for different nuclides. Examples of research work which is based on AMS measurements performed at CologneAMS, will be presented.
In addition I will report on a new project which aims for AMS measurements for intermediate mass nuclides, e.g. 53Mn and 60Fe at the Cologne FN tandem accelerator where higher beam energies enable isobar separation.

Topic: Synthesis and Applications of Stable Isotopically Labeled Saccharides

Prof. Anthony Serianni ( Dept. of Chemistry and Biochemistry, University of Notre Dame ) / September 21, 2015

Methods to prepare carbohydrates (sugars) containing site-specific labeling with carbon-13, hydrogen-2, nitrogen-15 and/or oxygen 17/18 isotopes will be discussed, and some examples of the applications of these labeled reagents to investigate saccharide structure and reactivity in chemistry and biochemistry will be presented.

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Topic: The Ion Conveyor for the Cyclotron Gas Stopper

Dr. Antonio Villari ( Facility for Rare Isotope Beams, MSU ) / September 28, 2015

The Ion Conveyor is a new apparatus devoted to transport ions fast and efficiently under moderate gas pressure. Such a device is particularly useful for long transition regions from relatively high pressures into vacuum. At the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University an Ion Conveyor will be used to extract rare isotopes from a new device to thermalize fast ions produced by the A1900 separator, called the Cyclotron gas Stopper [1,2]. Fast Ions (E/u ~ 100 MeV/u) produced by the A1900 are energy degraded and stopped in a gas-filled reverse-cyclotron filled with helium at ~100 mbar pressure, where they are collected and guided to a small exit orifice by traveling radio-frequency (RF) electric fields [3]. The transport of ions between the center of the cyclotron chamber and the external surface of the magnetic yoke, approximately 1 m distance with a strongly decreasing magnetic field, will use an Ion Conveyor with entrance and exit RF-carpets to span pressures between 100 and ~0.1 mbar. The concept of the Conveyor is based on similar devices used in mass spectrometry of heavy biochemical clusters [4]. The Ion Conveyor we developed for light and heavy ions is made by concentrical electrodes with central opening of 10 mm spaced by 1.4 mm and fed with a RF electric field in the range of 200 to 1000 kHz in traveling wave mode. This allows the ions to be transported rapidly and efficiently through the decreasing magnetic field, over the required distance. The present contribution describes the simulations, the mechanical design, the electronic circuitry, as well as the results obtained in off-line tests of the full-size Ion Conveyor for alkali ions. Measured efficiency in excess of 80% was demonstrated.

This work was supported by the National Science Foundation under Grants PHY-09-58726 and PHY-11-02511.

[1] G.K. Pang et al., Proceedings of PAC07, Albuquerque, New Mexico, USA (2007) 3588
[2] S. Schwarz et al., Nuclear Instruments and Methods in Physics Research B 317 (2013) 463
[3] M. Brodeur, et al., Intl. J. Mass Spec. 336 (2013) 53
[4] A.W. Colburn, et al., European Journal of Mass Spectrometry 10 (2004) 149

Topic: Measurement of the 17F(d,n)18Ne reaction using RESONEUT

Mr. Sean Kuvin ( Florida State University ) / October 5, 2015

The 17F(p,γ)18Ne reaction of astrophysical importance has been studied using the surrogate reaction 17F(d,n)18Ne in inverse kinematics. The usefulness of this type of approach has been demonstrated in previous experiments at the RESOLUT facility[1]. In this work we have developed a compact neutron detector array, RESONEUT, which is specialized for (d,n) reactions in inverse kinematics. The threshold and efficiency properties of the neutron detectors were characterized using the 12C(d,n)13N reaction. Spectroscopy of the 18Ne nucleus was accomplished using two methods. The first was by neutron time of flight spectroscopy and the second was by kinematic reconstruction of the unbound compound nucleus by detecting the emitted proton and heavy ion. We compared our results with those obtained from 17F + p elastic scattering measurements and from the direct 17F(p,γ) measurement conducted at Oak Ridge[2,3].

[1] Peplowski et al, PRC 79, 032801 (2009)
[2] Bardayan et al, PRL 83, 45(1999)
[3] Chipps et al, PRL 102, 152502 (2009)

Topic: Heavy-ion fusion reactions below the Coulomb barrier : structure effects and astrophysics implications

Prof. Sandrine Courtin ( Institut Pluridisciplinaire Hubert Curien, Strasbourg, France ) / October 12, 2015

Fusion-evaporation is the dominant reaction mechanism in medium-mass heavy-ion collisions around the Coulomb barrier. At these energies and at moderate sub-barrier energies, enhancement of the fusion cross-sections was observed whereas hindrance of the fusion cross-section has been identified in many systems at deep sub-barrier energies. Fusion cross-sections around the Coulomb barrier have been discussed extensively to be driven by couplings of the relative motion of the colliding nuclei to their low energy surface vibrations and/or stable deformations. The corresponding coupled-channel calculations and the distributions of barriers have revealed to be a powerful tool to better understand the role of couplings to collective degrees of freedom of the target and projectile.

The strong sensitivity of the sub-barrier fusion probability to the structure of the colliding nuclei will be discussed as well as recent results on the influence of particle transfer channels on the fusion cross-sections in medium mass systems like Ca+Ca and Ca+Ni.

At extreme sub-barrier energies, a surprising dependence of the process on fundamental properties of the nuclear matter is found, such as its incompressibility. In this energy region, for lighter systems like C+C and C+O, nuclear fusion is strongly connected to astrophysics, as it is an essential step in the synthesis of the chemical elements in stars. Experimental work on resonances observed in these systems will be presented.

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Topic: Discovery of Supernova-produced 60Fe in the Earth's Fossil Record

Dr. Shawn Bishop ( Technical University of Munich, Germany ) / October 19, 2015

Approximately 1.8 to 2.8 Myr before the present our planet was subjected to the debris of a supernova explosion. The terrestrial proxy for this event was the discovery of live atoms of 60Fe in a deep-sea ferromanganese crust [1]. The signature of this supernova event should also reside in magnetite (Fe3O4) magnetofossils produced by magnetotactic bacteria [2], which live in the ocean sediments, extant at the time of the Earth-supernova interaction. We have conducted accelerator mass spectrometry (AMS) measurements, searching for live 60Fe atoms in the magnetofossil component of Pacific Ocean sediment cores (ODP cores 848 and 851). We find a time-resolved 60Fe signal in both sediment cores, above background, centered at approximately 2 Myr ago and spanning approximately 700 kyr duration (full width half maximum), which will require eventual astrophysical interpretation to understand.
The production of elements beyond Fe occurs partly in what is known as the "r-process". This process involves the rapid capture of neutrons on time scales of milliseconds, temperatures of GK and densities of 109 g/cm3. The global physics of how the r-process works is largely understood; what is not known, however, is where in the universe it occurs. Candidate sites for the r-process are core collapse supernovae or binary neutron star mergers. The former is theoretically and observationally known to produce 60Fe; the latter is theoretically expected to produce negligible amounts of 60Fe. The heavy actinides, for example, are themselves r-process "only" nuclides; that is, they can only be made through the r-process. Present theoretical models favour r-process production in neutron star mergers over core collapse supernovae. Therefore, any future finding of a short-lived r-process "only" isotope in terrestrial reservoirs, coincident in time with the observed 60Fe signal, would show that core collapse supernovae are at least one site, in our cosmos, in which the r-process occurs.
This talk is designed to be accessible to a broad audience.
[1] Knie et al., Phys. Rev. Lett. 93, 171103 (2004).
[2] S. Bishop and R. Egli, Icarus 212, 960 (2011).

Topic: TBA

Prof. Bradley Meyer ( Clemson University ) / November 9, 2015

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Topic: TBA

Dr. Kyle Leach ( TRIUMF, Canada ) / November 16, 2015

Topic: TBA

Dr. Farheen Naqvi ( NSCL / MSU ) / November 23, 2015

Topic: TBA

Dr. Aaron Couture ( Los Alamos National Laboratory ) / December 7, 2015

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