ISNAP Seminars - Abstracts 2012 Spring

Topic: Streaked X-Ray Radiography Experiments on the National Ignition Facility

Amy Cooper ( National Ignition Facility, California ) / January 23, 2012

Project Searchlight was a campaign aimed at studying radiation transport through machined foam features on the National Ignition Facility (NIF). These experiments used a laser-irradiated gold cavity (hohlraum) to create a 200 eV x-ray source, which in turn irradiated a Ta₂O₅ foam, causing the foam to heat and evolve. The streaked radiography campaign developed an eight ns duration slot-apertured backlighter, and captured continuous images of the evolution of features in four different foam patterns on x-ray streak cameras. This backlighter created radiography images using a nickel x-ray source (8 keV), and had better than 20 μm resolution, while delivering a signal to noise of >10 until the backlighter signal was overwhelmed by the x-ray background from the hohlraum. The National Ignition Facility (NIF) is currently the largest and most energetic laser system in the world. It is currently capable of delivering up to 1.4 MJ of ultraviolet laser light to a millimeter-sized target over the course of nanoseconds. NIF can be used to probe previously inaccessible physical regimes, including creating highpressure systems (100 Mbar), intense radiation baths (350 eV hohlraums; 4 million K), and significant neutron production (predicted 1019 neutrons in a 100 ps burst, currently 4 x 1014 total neutrons).

Topic: Renaissance of the Discovery of Isotopes

Michael Thoennessens ( NSCL, Michigan State University ) / January 30, 2012

Ninety years after Soddy won the Nobel prize in chemistry for "his investigations into the origin and nature of isotopes", the quest to expand the nuclear horizon and discover new isotope has regained momentum. After about 10 years of very little progress the discovery rate has increased tremendously during last few years. In 2010 over 100 new isotopes were reported which is the largest number of isotopes discovered in any given year. This increase is primarily due to the beginning of the operation of RIBF at RIKEN and to the technical advances to separate and identify heavy neutron-rich isotopes following uranium fragmentation at GSI.
This latest trend and other interesting statistical data were extracted from a project to document the discovery of all isotopes. This project was recently completed analyzing the over 3000 isotopes presently known isotopes. An overview of the history and present status of the discovery of all presently known isotopes will be presented.

Topic: AMS detection system for actinides at the Argonne Fragment Mass Analyzer

Chithra Kumaran Nair ( Argonne National Laboratory ) / February 6, 2012

The AMS (Accelerator Mass Spectrometry) project at ATLAS complements the MANTRA (Measurement of Actinide Neutronic Transmutation Rates with Accelerator mass spectrometry) experimental campaign to obtain valuable integral information about neutron cross sections for very high mass actinides. The irradiated samples from the Advanced Test Reactor (ATR) at the Idaho National Laboratory are studied via AMS at ATLAS. The project progress as well as first results from the test run will be discussed.
A second, but different topic addressed will be results from the nuclear structure studies of the protonrich 179Tl and 180Tl nuclei. Both in-beam and decay properties of the Tl-nuclei were deduced in a Recoil Decay Tagging experiment using the Gammasphere array in tandem with the Argonne Fragment Mass Analyzer. The alpha decay fine structures as well as level schemes depicting the excited structures will be presented.

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Topic: Study of ⁹C via the d(¹⁰C,t)⁹C reaction

Scott Marley ( Western Michigan University ) / February 13, 2012

The structure of ⁹C is poorly known. Only a few excited states have been observed and little information exists on their single-particle proper-ties. These data are vital to test ab initio nuclear theories which have ex-celled in modeling light nuclear systems in the p-and sd-shells. To probe the structure of ⁹C the ¹⁰C(d,t)⁹C reaction was performed in inverse kinemat-ics at the ATLAS facility at Argonne National Laboratory. An “in-flight” radioactive ¹⁰C beam was developed at ATLAS through the p(¹⁰B,¹⁰C)n reaction using a 185-MeV ¹⁰B beam incident on a cryogenic hydrogen gas cell. The resulting 171-MeV ¹⁰C beam had an average intensity of 2.2x104 pps and bombarded a 660 μg/cm² deuterated polyethylene (CD²)n target. Tritons were detected and identified in an array of annular double-sided silicon detectors covering θ_lab between 8 and 42 degrees. Heavy beam-like recoils were detected in a set of forward-angle silicon detectors in a ΔE-E configuration. The results from the Distorted Wave Born Approximation (DWBA) analysis performed for the ground state transition are compared to those from ab initio (GFMC/VMC) calculations. Results on transfer to the proton-unbound excited states of ⁹C will also be presented.

Topic: The Beauty of the (n,n' γ) Reaction for Investigating Collective Excitations Across the Te Isotopic Chain

Sally Hicks ( University of Dallas ) / February 20, 2012

The Te nuclei are in a region rich in nuclear structure information. As has been evinced in the Cd nuclei, which mirror Te in proton number with respect to the Z=50 shell closure, and in the nearby Xe and Sn nuclei, intruder states, mixed-symmetry states, and normal collective vibrations all play an important role in the structures observed. To better understand these excitations and their evolution across the Te isotopic chain, excited levels of the stable even-even ^122-130Te nuclei have been investigated using γ-ray spectroscopy following inelastic neutron scattering. This reaction mechanism non-selectively excites all nuclear levels with spins ≤6, so such measurements provide a wealth of information on level and transition energies, spins, branching and mixing ratios, and the Doppler shift attenuation method can be used to determine the lifetimes of nuclear levels in the fs to few ps regime. Our investigations have revealed, for example, that the lowest 2⁺MS mixed-symmetry excitations in these nuclei are fragmented and do not appear to evolve in a consistent way across the isotopic chain. The beauty of this reaction mechanism occurs, however, because transition rates are determined for both ground-state and higher-lying transitions, which tests nuclear models in a very detailed way and allows one to look for specific higher lying structures, e.g.,1⁺MS and 3⁺MS states. This talk will focus on dipole transitions observed across the Te chain and the important role they play in identifying nuclear properties. Additionally, the importance of undergraduate participation in this research will be discussed.
* This work was supported by grants from the National Science Foundation.

Topic: Nuclear Configuration Interaction: the Golden Standard of Nuclear Structure Calculations

Mihai Horoi ( Central Michigan University ) / February 27, 2012

In the last few decades the Nuclear Configuration Interaction (NCI), widely known as the shell model, emerged as a golden standard of nuclear structure calculations, used not only for a comprehensive understanding of the data, but also for guiding expensive experimental endeavors. Although widely used, the understanding if its main ingredient, the effective Hamiltonian, is difficult to be understood in a purely reductionist manner. In my talk I will review the most recent development and understanding of the effective Hamiltonians used for NCI calculations, and I will show some recent applications of NCI to rp-process reaction rates, direct reactions, and double-beta decays.

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Topic: The Importance of Being Neutral

William Peters ( Oak Ridge National Lab ) / March 19, 2012

Experiments that detect ejected charged particles are a staple of low energy nuclear physics. Many of the same (or isospin-mirrored) results can be achieved through experiments that eject neutrons. The utility of neutrons as a probe of nuclear structure is just as extensive as their charged siblings. In fact, in some cases, their neutrality can be a plus. Generally, neutrons are much more difficult to detect, especially in tight geometries required in nuclear physics experiments. Modern neutron detector arrays like the Modular Neutron Array (MoNA) and the newly completed Versatile Array of Neutron Detectors at Low Energy (VANDLE) open the doors to a variety of new neutron-ejectile experiments for current and future rare ion beam facilities. I will discuss some of the highlights and future goals of both detector arrays, including structure experiments beyond the neutron dripline, transfer reactions of astrophysical importance near the proton dripline, and decay spectroscopy of neutron-rich isotopes.

Topic: Indications of deformation along N = 40 isotones

Sean Liddick ( NSCL, MSU ) / April 2, 2012

The rapid development of collectivity in the N = 40 region as protons are removed from the f_7/2 single-particle state is suggested by the dramatic drop in energy of the first excited 2⁺ state from ⁶⁸Ni to ⁶⁶Fe and the increase in B(E2) along the Fe isotopic chain. Recent experiment and theoretical work has suggested the nuclei below Ni become prolate deformed with some nuclei exhibiting indications of both prolate deformed and spherical structures in their low-energy level schemes. While numerous experiments have focused on even-even nuclei, including Cr and Fe, very little is known about the neighboring odd-Z isotopic chains. To explore the role of deformation in this region, the beta decays of the Cr and Fe isotopes into the respective Mn and Co daughter nuclei were studied at the NSCL. Low-energy level structures for odd-odd nuclei straddling N = 40 and their interpretation will be presented.

Topic: Where do unusual metal-poor stars abundances come from?

Fernando Montes ( NSCL, MSU ) / April 16, 2012

Observations of metal-poor stars provide insights into the nucleosynthesis processes active in the early universe. Among metal-poor stars observed so far, a significant number of them show an anomalous abundance signature that is different than the abundances observed in r-process-enriched stars. Those anomalous stars present a strong overproduction of strontium, yttrium, zirconium and probably also of heavier elements, at least up to silver. The unknown nucleosynthesis process producing these elements has been called stellar "Light Element Primary Process" (LEPP). Although little is known about such process recent nucleosynthesis studies suggest that LEPP elements could be synthesized in at least two astrophysical environments:
* fast rotating massive stars by the s-process, and
* core-collapse supernovae (in an incomplete r-process or in the vp-process).
Although neutrino driven wind nucleosynthesis calculations have traditionally focused on reproducing r-process nuclei, the necessary conditions to produce elements up to the third peak are difficult to obtain in state-of-the-art simulations. In this talk, I will discuss the observational evidence and status of nucleosynthesis studies focusing on the neutrino-wind in core collapse supernovae as the site of the Light Element Primary Process.

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Topic: Update of the Fukushima Accident: Physical Events and Radiation Effects

Jay LaVerne ( University of Notre Dame ) / April 23, 2012

This presentation will update the physical events occurring at the Fukushima nuclear reactor facility following the earth quake and tsunami of March 11, 2011. Most of the presentation is based on information obtained by a committee of nuclear engineers specifically set up to examine the destruction of the facility and release of radioactive materials. The focus will be on the events of the last year, but the long term clean up and radiation effects will be discussed.

Topic: Taming the computational scale explosion in the nuclear many-body problem

Mark Caprio ( University of Notre Dame ) / April 30, 2012

A major outstanding problem in nuclear theory lies in developing the connection between single particle and collective degrees of freedom, that is, predicting strong collective correlations which arise in the motion of nucleons within the nucleus. In principle, once the nucleon nucleon force is known, prediction of nuclear structure is simply an exercise in solving the many body Schr�ödinger equation. However, the nucleus is a strongly correlated, finite size quantum many body system, lying in the computationally challenging regime between the few body problem and the thermodynamic limit. Direct quantum solution is overwhelmed by a combinatorial explosion in the dimension of the model space: the dimension of the eigenproblem rapidly becomes intractable as the number of nucleons and relevant single particle states increases. This talk will introduce both the challenges in the nuclear problem and some approaches to overcoming them, through the use of group theory, symmetries, and optimized bases for the many-nucleon dynamics.

Topic: A New Spectroscopic Tool by the Radioactive-Isotope-Beam Induced Exothermic Charge-Exchange Reaction

Shumpei Noji ( NSCL/MSU ) / May 7, 2012

Charge-exchange reactions induced by radioactive heavy-ions have potentiality for studies of a variety of spin-isospin responses due to their unique reaction kinematics and selectivities.
Among them the (¹²N,¹²C) reaction has peculiar features:
This reaction can be exothermic (Q>0) owing to the large mass difference of about 17MeV between the ¹²N projectile and the ¹²C ejectile, and accordingly it can realize small momentum transfer even for highly-excited states. Moreover, since the final state in the ¹²C ejective can be identified by detecting the de-excitation gamma rays, the excitation modes with the transferred quantum numbers (S=1, T=1) and (S=0, T=1) can be selected. These features make this reaction suitable for the study of yet-to-be-discovered states such as the isovector spin-isospin monopole resonance (IVSMR).
We performed for the first time an experiment of this exothermic charge-exchange (¹²N,¹²C) reaction on a ⁹⁰Zr target at an incident energy of 175MeV/u. The experiment was carried out at the RI Beam Factory (RIBF) at RIKEN using the magnetic spectrometer SHARAQ and the gamma-ray detector array DALI2. The double differential cross sections were measured at the excitation energy of 0–-70MeV and at the scattering angles of 0—3 degrees for both of the (S=1, T=1) and (S=0, T=1) modes. In the former mode, peaks for the Gamow-Teller giant resonance (GTGR) and the isovector spin monopole resonance (IVSMR) were clearly observed at ~10 MeV and ~30 MeV, respectively. In the latter mode, a peak for the isobaric analog state was also clearly observed at ~5 MeV. Furthermore, we obtained the cross section of the GTGR per Gamow-Teller transition strengths of the target-residual and projectile-ejectile systems and that of the IAS per Fermi transition strengths, and found that their ratio was enhanced compared with the (p,n) reaction.
In this talk, I will present the details of the experiment and discuss the results.

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Topic: The Rare Earth Peak: An Overlooked r-Process Diagnostic

Matt Mumpower ( North Carolina State University ) / May 9, 2012

The r-process is thought to be responsible for approximately half of the neutron-rich elements above iron. While many studies of r-process environments have focused on early time behavior, e.g. conditions for sufficient neutron-to-seed ratio, less effort has been made studying late-time (low neutron-to-seed ratio) r-process dynamics. I first cover the formation and evolution of the rare earth 'peak' (A~160) which occurs during freeze-out as matter decays back to stability. Neutron capture rates and separation energies in the region are especially important for peak formation. I show that the rare earth peak is sensitive to nuclear physics input, thermodynamic evolution and the the interplay between the two. I will discuss the types of astrophysical conditions that produce abundance patterns that best match observational data. The success of this method in constraining the conditions is dependent on underlying uncertainties in the nuclear physics input. Lastly, I identify and discuss neutron capture rates of nuclei which are critical to the final structure of the rare earth abundances. These nuclei lie within 10-15 neutrons from stability and potentially could be measured at future radioactive ion beam facilities.

Topic: What's the NORM?

Patrick Regan ( Centre for Nuclear and Radiation Physics, University of Surrey, UK ) / May 24, 2012

Naturally Occurring Radioactive Material or 'NORM' is ubiquitous on Earth. The main components of this ambient background radiations to which humans are exposed every day of their lives, can be divided into (a) shorter-lived and constantly replenished cosmogenic nuclides such as ⁷Be and ¹⁴C, or (b) very long-lived, primordial species including ⁴⁰K, ^235,238U and ²³²Th. This talk will discuss the physical origins and decay mechanisms of NORM and demonstrate how high-resolution gamma-ray spectrometry measurements can be used to determine baseline environmental measurements of radioactivity levels and its potential health effects in the environment. Examples of recent studies of NORM measurements by the Surrey group across the State of Qatar [1], along beaches and rivers in Thailand [2,3] and in Libyan oil pipelines [4] will be discussed together with evidence for technically enhanced levels of NORM in specific cases.
[1] A preliminary report on the determination of natural radioactivity levels of the State of Qatar using high-resolution gamma-ray spectrometry, H.Al-Sulaiti et al., Nucl. Inst. Meth. Phys. Res. A619 (2010) 427-431; Determination of the natural radioactivity in Qatarian building materials using high-resolution gamma-ray spectrometry, H.Al-Sulaiti et al., Nucl. Inst. Meth. Phys. Res. A652 (2011) 915-919.
[2] An evaluation of the level of naturally occurring radioactive material in soil samples along the Chao Phraya river basin, T. Santawamaitre et al., Nucl. Inst. Meth. Phys. Res. A619 (2010) p452-456; Study of natural radioactivity in riverbank solids along the Chao Phraya river basin in Thailand, T. Santawamaitre et al., Nucl. Inst. Meth. Phys. Res. A652 (2011) p920-924.
[3] Measurements of NORM in beach sand samples along the Andaman coast of Thailand after the 2004 Tsunami, D. Malain et al., Nucl. Inst. Meth. Phys. Res. A619 (2010) p441-445; An Evaluation of the Natural Radioactivity in Andaman Beach Sand Samples of Thailand after the 2004 Tsunami D. Malain et al., Applied Radiation and Isotopes, DOI information: 10.1016/j.apradiso.2012.04.017
[4] The use of MCNP and gamma spectrometry in supporting the evaluation of NORM in Libyan oil pipeline scale, A.S. Habib et al., Nucl. Inst. Meth. Phys. Res. A619 (2010) p245-251

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