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 ( Hubert Curien Multidisciplinary Institute in 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: Measuring the Half-life of 60Fe for stellar and early Solar System models

Karen Ostdiek ( University of Notre Dame ) / November 2, 2015

Radioisotopes, produced in stars and ejected through core collapse supernovae (SNe), are important for constraining stellar and early Solar System (ESS) models. The presence of these isotopes, specifically 60Fe, can identify progenitors of SN types, give evidence for nearby SNe, and can be a chronometer for ESS events. The 60Fe half-life, which has been in dispute in recent years, can have an impact on calculations for the timing for ESS events, the distance to nearby SN, and the brightness of individual, non-steady state 60Fe gamma ray sources in the Galaxy. To measure such a long half life, one needs to simultaneously determine the number of atoms in and the activity of an 60Fe sample. We have undertaken a half-life measurement at Notre Dame and have successfully measured the activity of our 60Fe sample using the isomeric decay in 60Co rather than the traditional 60Co grow-in decay. This will then be coupled with the results of the 60Fe concentration measurement of our sample using Accelerator Mass Spectrometry (AMS). I will present the most recent results of both the activity and the AMS measurements.

Topic: On the Evolution of Weak Nuclear Statistical Equilibrium

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

Nuclear reaction networks are collections of nuclei and reactions among them. In high temperature and high density environments in stellar explosions, the nuclear reactions inthese networks operate on different timescales. In particular, the strong and electromagnetic reactions are typically much faster than the weak reactions. This means that the network can achieve nuclear statistical equilibrium (NSE), which is an equilibrium with respect to the strong and electromagnetic reactions, much more quickly than it achieves weak nuclear statistical equilibrium (WSE), which is a full equilibrium under all reactions, including weak reactions. In this talk, I will explore the transition from NSE to WSE and will discuss consequences for astronomical displays from core-collapse supernovae and for isotopic anomalies in primitive solar system condensates.

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Topic: Searching for New Physics at RIB Facilities using Beta Decay

Prof. Kyle Leach ( Colorado School of Mines ) / November 16, 2015

Despite its success, the Standard Model is known to be incomplete, and providing limits on possible physics beyond the Standard Model (BSM) is crucial to our understanding of the physical universe. Although they are generally complex, unstable nuclear systems provide some of the best venues for these experiments through detailed studies of nuclear beta decay. As a part of this work, searching for possible scalar currents in the weak interaction and attempting to extract the effective Majorana mass of the neutrino are at the forefront. In this seminar I will present some of the methods in which we can attack these issues using radioactive beam experiments. Further, I will also discuss how some of these techniques can allow us to look at second order processes in electron-capture (EC) decay in the future.

Topic: β-Oslo method: A technique to determine the (n, γ) cross sections

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

Neutron-capture reaction rates are one of the key components in predicting the path of astrophysical rapid neutron-capture process (r-process). Due to the short half-lives of the nuclei involved in r-process, experimental data on the neutron-capture cross sections on these nuclei is very scarce. A new technique called the β-Oslo method has been developed to extract the nuclear level density and γ-strength function in neutron-rich nuclei. The (n, γ) cross sections are then constrained using the inputs from the measurement and Hauser-Feshback approach. The experiment focusing on the Co isotopes relevant for the weak r-process was performed at the National Superconducting Cyclotron laboratory (NSCL). The γ-summing technique was employed using the Summing NaI ( SuN ) detector from NSCL. Results from the first measurement on the neutron-rich nuclei and their astrophysical implication will be discussed.

Topic: Excited state lifetimes for A=109 nuclei via electronic timing with LaBr3(Ce) detectors

Mallory Smith ( University of Notre Dame ) / November 30, 2015

The evolution of nuclear structure across isotopic and isobaric chains is of great interest to nuclear structure and for structure applications to nuclear astrophysics. A particularly interesting region are the neutron-rich nuclei around A~110 region, which is characterized by rapid the onset of deformation. Shape-phase transitions, triaxial deformations, shape coexistence and oblate configurations have all been reported within a small window of the nuclear landscape.

In this talk, I will describe our studies of the A=109 β-decay chain including 109Tc, 109Pd, and 109Ru, produced from the fission of 238U at the University of Jyväskylä Ion Guide Isotope Separator Online (IGISOL) facility. Level lifetimes and gamma-ray transitions were measured with a multi-detector array consisting of two HPGe detectors, two LaBr3(Ce) scintillators and one NE111A plastic scintillator to detect β-decays. Triple coincidence β-γ-γ events were recorded and used to construct/check both level schemes, as well as extract level lifetimes via the fast-timing method pioneered by Henryk Mach. Results will be presented on the low energy structure of 109Ru, in context of the odd-Ru and odd-Pd isotopic chains, including our recent results in 109Pd. In each case we have found new levels, placed new transitions, and measured level lifetimes for the first time: including 3 lifetimes in 109Tc; 5 levels, 22 transitions and 8 lifetimes in 109Pd; and in 109Ru, 3 levels, 28 transitions, and 7 level lifetimes. Interpretation of the evolution of structure in this region is ongoing.

Topic: The Many Paths of Nuclear Science at LANL

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

Nuclear physics and nuclear measurement techniques play awide ranging role in the science and technology studies at Los AlamosNational Laboratory (LANL). From measuring neutron capture reactionsfor the synthesis of the heavy elements to determining the neutronexposure on commercial airlines, from the basic science studies thatunderpin the nation's nuclear stockpile to developing and producingnew radioisotopes for nuclear medicine, from designing detectors fornext-generation satellites to training IAEA inspectors, expertise in nuclear science is essential.

I will discuss recent developments in neutron capture, both directmeasurements at LANSCE and indirect measurements at Argonne, that play arole in both basic and applied nuclear physics. I will then expand thediscussion to give a larger picture of the range of ways that nuclearphysics expertise contributes to the scientific and technical mission atLos Alamos.

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