M. A. McMahan

University of California, Berkeley, Berkeley, California, United States

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Publications (167)

  • Source
    [Show abstract] [Hide abstract] ABSTRACT: The surrogate nuclear reaction method is being applied in many efforts to indirectly determine neutron-induced reaction cross sections on short-lived isotopes. This technique aims to extract accurate (n,γ) cross sections from measured decay properties of the compound nucleus of interest (created using a different reaction). The advantages and limitations of a method that identifies the γ -ray decay channel by detecting any high-energy ("statistical") γ ray emitted during the relaxation of the compound nucleus were investigated. Data collected using the Silicon Telescope Array for Reaction Studies and Livermore-Berkeley Array for Collaborative Experiments silicon and germanium detector arrays were used to study the decay of excited gadolinium nuclei following inelastic proton scattering. In many cases, this method of identifying the γ -ray decay channel can simplify the experimental data collection and greatly improve the detection efficiency for γ -ray cascades. The results show sensitivity to angular-momentum differences between the surrogate reaction and the desired (n,γ) reaction similar to an analysis performed using low-lying discrete transitions even when ratios of cross sections are considered.
    Full-text available · Article · May 2012 · Physical Review C
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    [Show abstract] [Hide abstract] ABSTRACT: a b s t r a c t Experimental results for the minor decay channels of fusion-evaporation in light projectile plus light target systems are presented. These new data were obtained during test campaigns to measure the opening of different decay channels. Experiments were designed to provide relative cross-section information on weakly populated channels for gamma-ray spectroscopy experiments in coincidence with charged-particles. The results are compared to publicly available fusion-evaporation codes. The data follow a simple estimate which is useful in predicting experimental conditions to make the fusion-evaporation reaction a viable nuclear structure tool to study weakly populated light neutron-rich nuclei. & 2011 Elsevier B.V. All rights reserved.
    Full-text available · Article · Jul 2011 · Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
  • [Show abstract] [Hide abstract] ABSTRACT: A segmented Si telescope and HPGe array is used to study the 156Gd(p,d-γ)155Gd direct reaction by d-γ and d-γ-γ coincidence measurements using 25-MeV protons. The present investigation is the first time that this N=91 nucleus and the N=90 region—which is known for a rapid change from vibrational to rotational character, several low-lying 0+ states in the even-even nuclei, and large Coriolis (ΔΩ=1) plus ΔN=2 mixing in the even-odd nuclei—have been studied by particle-γ coincidence following a direct reaction with light ions. Gamma-ray energies and branches, excitation energies, angular distributions, and cross sections are measured for states directly populated in the (p,d) reaction. A new low-energy doublet state at 592.46 keV (previously associated with the K=0⊗3/2-[521] bandhead) and several new γ-ray transitions (particularly for states with excitation energies >1 MeV) are presented. Most notably, the previous ν 7/2+[404] systematics at and around the N=90 transition region are brought into question and reassigned as ν 5/2+[402]. This reassignment makes the ν 1/2+[400], ν 3/2+[402], and ν 5/2+[402] orbitals, which originate from the 3s1/2, 2d3/2, and 2d5/2 spherical states, respectively, responsible for the three largest cross sections to positive-parity states in the (p,d)155Gd direct reaction. These three steeply upsloping orbitals undergo ΔN=2 mixing with their N=6 orbital partners, which are oppositely sloped with respect to deformation. The presence of these steeply sloped and crossing orbitals near the Fermi surface could weaken the monopole pairing strength and increase the quadrupole pairing strength of neighboring even-even nuclei, which would bring ν 2p-2h 0+ states below 2Δ. Indeed, this could account for a large number of the low-lying 0+ states populated in the (p,t)154Gd direct reaction.
    Article · Jun 2010 · Physical Review C
  • [Show abstract] [Hide abstract] ABSTRACT: The surrogate nuclear reaction method can be used to determine neutron-induced reaction cross sections from measured decay properties of a compound nucleus created using a different reaction and calculated formation cross sections. The reliability of (n,γ) cross sections determined using the Weisskopf-Ewing and ratio approximations are explored for the 155,157Gd(n,γ) reactions. Enriched gadolinium targets were bombarded with 22-MeV protons and γ rays were detected in coincidence with scattered protons using the Silicon Telescope Array for Reaction Studies/Livermore-Berkeley Array for Collaborative Experiments (STARS/LiBerACE) silicon and germanium detector arrays. The γ-emission probabilities for the 154,156,158Gd compound nuclei were measured at excitation energies up to 12 MeV. It is found that the approximations yield results that deviate from directly measured 155,157Gd(n,γ) cross sections at low energies. To extract reliable cross sections, a more sophisticated analysis should be developed that takes into account angular-momentum differences between the neutron-induced and surrogate reactions.
    Article · Mar 2010 · Physical Review C
  • [Show abstract] [Hide abstract] ABSTRACT: Short-lived isomers in fission fragments following bombardment of 45-MeV 6Li on 232Th were examined. Isomers in the A~95,122, and 132 mass regions were observed. New isomeric decays were observed in 121In [T1/2=17(2) μs], 123In (T1/2≳100 μs), and 125Sb [T1/2=25(4) μs]. These isomers are suggested to arise from ν(h11/2⊗d3/2)7- and ν(h11/2⊗s1/2)5- neutron core excitations coupling with the valence proton.
    Article · Jan 2010 · Physical Review C
  • [Show abstract] [Hide abstract] ABSTRACT: Yrast states in ^34P were investigated using the ^18O(^18O,pn) reaction at energies of 20, 24, 25, 30, and 44 MeV at Florida State University and at Lawrence Berkeley National Laboratory. The level scheme was expanded, γ-ray angular distributions were measured, and lifetimes were inferred with the Doppler-shift attenuation method by detecting decay protons in coincidence with one or more γ rays. The results provide a clearer picture of the evolution of structure approaching the ``Island of Inversion'', particularly how the 1 and 2 particle-hole (ph) states fall in energy with increasing neutron number approaching inversion. Shell model calculations made using a small modification of the WBP interaction reproduce the negative-parity, 1-ph states rather well.
    Article · Oct 2009
  • [Show abstract] [Hide abstract] ABSTRACT: Yrast states in 34P were investigated using the 18O(18O,pn) reaction at energies of 20, 24, 25, 30, and 44 MeV at Florida State University and at Lawrence Berkeley National Laboratory. The level scheme was expanded, γ-ray angular distributions were measured, and lifetimes were inferred with the Doppler-shift attenuation method by detecting decay protons in coincidence with one or more γ rays. The results provide a clearer picture of the evolution of structure approaching the “island of inversion,” particularly how the one- and two-particle-hole (ph) states fall in energy with increasing neutron number approaching inversion. However, the agreement of the lowest few states with pure sd shell model predictions shows that the level scheme of 34P is not itself inverted. Rather, the accumulated evidence indicates that the 1-ph states start at 2.3 MeV. A good candidate for the lowest 2-ph state lies at 6236 keV, just below the neutron separation energy of 6291 keV. Shell model calculations made using a small modification of the WBP interaction reproduce the negative-parity, 1-ph states rather well.
    Article · Jul 2009 · Physical Review C
  • J.V. Branson · B.L. Doyle · G. Vizkelethy · [...] · M. A. McMahan
    [Show abstract] [Hide abstract] ABSTRACT: Radiation effects microscopy (REM) has evolved into an essential tool for the study, diagnostics and remedy of single event effects (SEE) in microelectronics devices, However, we are entering an era where the ion energies of the current systems are becoming inadequate for diagnosing SEE problems in modern ICs due to the great thickness of interlevel dielectric, metallization and passivation layers found on top of the active radiation-sensitive Si. Our solution is the ion photon emission microscope (IPEM), which eliminates the need to focus several GeV heavy ions. A tabletop IPEM is currently in use at Sandia National Laboratories (SNL), operating with alpha particles, and showing 4μm resolution. We have recently developed a second system, and installed it on one of the SNL nuclear microprobe lines to demonstrate the principle and prove its potential as a portable radiation effects microscope that can be installed at the LBNL GeV cyclotron facility. The microprobe system is currently operating with ∼2μm resolution. The determined advantages of installing a similar system at the LBNL cyclotron facility will be discussed, in addition to recently measured optical characteristics of the various phosphor materials being investigated.
    Article · Jun 2009 · Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms
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    [Show abstract] [Hide abstract] ABSTRACT: The surrogate reaction 238U(3He, tf) is used to determine the 237Np(n, f) cross section indirectly over an equivalent neutron energy range from 10 to 20 MeV. A self-supporting ∼761 μg/cm2 metallic 238U foil was bombarded with a 42 MeV 3He2+ beam from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory (LBNL). Outgoing charged particles and fission fragments were identified using the Silicon Telescope Array for Reaction Studies (STARS) consisted of two 140 μm and one 1000 μm Micron S2 type silicon detectors. The 237Np(n, f) cross sections, determined indirectly, were compared with the 237Np(n, f) cross section data from direct measurements, the Evaluated Nuclear Data File (ENDF/B-VII.0), and the Japanese Evaluated Nuclear Data Library (JENDL 3.3) and found to closely follow those datasets. Use of the (3He, tf) reaction as a surrogate to extract (n, f) cross sections in the 10–20 MeV equivalent neutron energy range is found to be suitable.
    Full-text available · Article · Jun 2009 · Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms
  • [Show abstract] [Hide abstract] ABSTRACT: The internal surrogate ratio method allows for the determination of an unknown cross section, such as (n,γ), relative to a better-known cross section, such as (n,f), by measuring the relative exit-channel probabilities of a surrogate reaction that proceeds through the same compound nucleus. The validity of the internal surrogate ratio method is tested by comparing the relative γ and fission exit-channel probabilities of a 236U* compound nucleus, formed in the 235U(d,p) reaction, to the known 235U(n,γ) and (n,f) cross sections. A model-independent method for measuring the γ-channel yield is presented and used.
    Article · May 2009 · Physical Review C
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    [Show abstract] [Hide abstract] ABSTRACT: In the Surrogate Method, the measured decay probability of a compound nucleus formed via a direct reaction is used to extract the cross section for a reaction with a different entrance channel that proceeds through the same compound nucleus. An extension of the Surrogate Method, the Surrogate Ratio Method (SRM), uses a ratio of measured decay probabilities to infer an unknown cross section relative to a known one. To test the SRM we compare the direct-reaction-induced fission probability ratio of 234 U(α, α f) to 236 U(α, α f) with the ratio of cross sections of 233 U(n, f) to 235 U(n, f). These ratios were found to be in agreement over an equivalent neutron energy range of 0.4–18 MeV.
    Full-text available · Article · Apr 2009 · Physical Review C
  • C. C. Jewett · M. McMahan · J. Cerny · [...] · M. Johnson
    [Show abstract] [Hide abstract] ABSTRACT: We have built a Gas Electron Multiplier (GEM) detector for detection of fast neutrons at Lawrence Berkeley National Laboratory. The detector consists of a 0.0625 inch thick polypropylene neutron converter, three GEM foils and a grid of 16 readout pads on a printed circuit board. In this talk, we present images of the GEM detector, the results of tests with ^60Co, AmBe sources and our neutron beam, and a comparison between the proposed fast neutron GEM detector and a fast neutron ^238U fission chamber we purchased. One of the advantages of the GEM detector over the fission chamber is the fact that it provides the x-y position information of the neutrons.
    Article · Oct 2008
  • Jeffrey S. George · Rocky Koga · Margaret A. McMahan
    [Show abstract] [Hide abstract] ABSTRACT: The Lawrence Berkeley National Laboratory cyclotron offers broad-spectrum neutrons for single event effects testing. We discuss results from this beamline for neutron soft upsets in Xilinx Virtex-4 and -5 field-programmable-gate-array (FPGA) devices.
    Conference Paper · Aug 2008
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    Margaret A. McMahan · Ewart Blackmore · Ethan W. Cascio · [...] · Harvey Eisen
    [Show abstract] [Hide abstract] ABSTRACT: Representatives of facilities that routinely deliver protons for radiation effect testing are collaborating to establish a set of standard best practices for proton dosimetry. These best practices will be submitted to the ASTM International for adoption.
    Full-text available · Conference Paper · Aug 2008
  • [Show abstract] [Hide abstract] ABSTRACT: The fusion-evaporation reaction 9Be(11B,2p) was used to populate excited states in 18N. New gamma-ray transitions were added to the 18N level scheme. The mean lifetime of the first excited state was measured to be 582(165) ps and its transition rate to the ground state was determined to be B(M1)=0.036(10) W.u. Shell model calculations in the full p-sd model space were used to investigate the low-lying configurations in 18N and in the N=11 isotones 17C and 19O. It was found that the role of the proton-neutron interaction is important in determining the ground state and low-lying excited state properties. The ground state spin inversion in these isotones is attributed to the increased importance of the quadrupole relative to the pairing interaction and is discussed within the framework of a schematic pairing + quadrupole model.
    Article · May 2008 · Physical Review C
  • [Show abstract] [Hide abstract] ABSTRACT: We measured the ratio of the fission probabilities of 234U∗ relative to 236U∗ formed via an (α,α′) direct reactions using the STARS array at the 88‐inch cyclotron at the Lawrence Berkeley National Laboratory. This ratio has a shape similar to the ratio of neutron capture probabilities from 233U(n,f) and 235U(n,f), indicating the alpha reactions likely formed a compound nucleus. This result indicates that the ratios of fission exit channel probabilities for two actinide nuclei populated via (α,α′) can be used to determine an unknown fission cross section relative to a known one. The validity of the External Surrogate Ratio Method (ESRM) is tested and the results support the conclusions of Burke et al. [1].
    Article · Apr 2008
  • [Show abstract] [Hide abstract] ABSTRACT: Over the past three years we have studied various surrogate reactions (d,p), (3He,t), (α,α′) on several uranium isotopes 234U, 235U, 236U, and 238U. An overview of the STARS∕LIBERACE surrogate research program as it pertains to the actinides is discussed. A summary of results to date will be presented along with a discussion of experimental difficulties encountered in surrogate experiments and future research directions.
    Article · Apr 2008
  • Margaret McMahan · C. C. Jewett · Lawrence Heilbronn
    [Show abstract] [Hide abstract] ABSTRACT: The Berkeley Accelerator Space Effects (BASE) Facility provides heavy ions and protons for radiation effects testing by government laboratories (Defense, Energy and NASA) and contractors, private U.S. companies and international companies and laboratories. The combination of state-of-the-art ion sources for heavy ion running and relatively high intensities (up to 10 microamps) for protons makes it a very versatile `one-stop-shop' for the radiation testing community. To add to this capability, a fast neutron capability has been developed using the d(Be,n) reaction in stopping targets. The choice of deuteron energy, ranging from 5 -- 65 MeV, gives a broad energy spectra with some tunability. The commissioning of this facility will be discussed including energy and flux measurements, dosimetry and initial experiments. In the future, two off-line neutron generators will also be in operation at the BASE facility, providing thermal neutrons as well as monoenergetic neutrons at 2.5 and 14 MeV. These sources, running independently of the Cyclotron, will complement the broad spectra neutrons at higher energies, providing a unique and versatile neutron capability.
    Article · Apr 2008
  • Simarjit Kaur · M. A. McMahan · L. A. Bernstein · D. Schneider
    [Show abstract] [Hide abstract] ABSTRACT: Two neutron generators - one 14 MeV and one thermal - will be installed at the 88-Inch Cyclotron at LBNL. The 14 MeV (d, t) source has an integrated neutron output of 10^11 neutron/sec at maximum power. The thermal source generates 10^7 neutrons/sec in a 116 cm^2 field. These neutron generators will be used in diagnostic studies and cross section measurements for the National Ignition Facility at LLNL, and will also be utilized for radiation effects testing. The project scope will be presented including facility layout, neutron flux, shielding calculations and timeline. This new capability will add to the versatility of the 88-inch cyclotron facility and provide exciting new opportunities for both basic and applied nuclear science and national space security missions.
    Article · Oct 2007
  • Michael B. Johnson · Margaret A. McMahan · Michelle Galloway · [...] · Raymond K. Thatcher
    [Show abstract] [Hide abstract] ABSTRACT: The 4.5 MeV/nucleon heavy ion cocktail at the 88-Inch Cyclotron has been expanded by incorporating beams from solid material to fill in the linear energy transfer curve. This supercocktail is available by special request and is useful when only normal incidence between the beam and the device under test is possible or desirable.
    Conference Paper · Aug 2007

Publication Stats

3k Citations

Institutions

  • 1986-2010
    • University of California, Berkeley
      Berkeley, California, United States
  • 1989-2008
    • Lawrence Berkeley National Laboratory
      • Nuclear Science Division
      Berkeley, California, United States
  • 2005
    • University of Brighton
      Brighton, England, United Kingdom
  • 1999
    • Kent State University
      Кент, Ohio, United States