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Open Access Publications from the University of California

Scholarly Works (197 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2020)

Positron science can shed light into deeper understanding of materials’ structures, surface electron energies, topology, porosity and other properties. To conduct this science, intense sources of positrons are required; a few source types are available, but some of them are costly and could require a lot of sophisticated apparatus. This work addresses overcoming the difficulties in the deposition of a uniform 22-NaCl isotope layer, with the goal of producing intense positron sources from radioactive isotopes at a much lower cost than currently available commercially. The first of its kind, as far as we know, apparatus for deposition of radioactive salt (22-NaCl) into a specially designed capsule is discussed in this work. This discussed design could serve as a substitute for the current 22-Na positron sources that are only available from iThemba labs in South Africa, where the deposition is done by hand pipetting the radioactive solution into the capsule designed for UHV use. Lastly, preliminary work based on isotope separation for the potential use of radioactive 79-Kr isotope as another means for intense positron source, will be discussed as well. Based on my measurements, the source designs tested in this work have shown promise to be an achievable goal. This will give labs, who are limited on funding, an ability to afford and conduct experiments utilizing positrons, thus expanding the field of positron science and its applications to labs with different specializations. The new designs will also make possible sources of very high intensities that could be the basis for a U.S. National Positron Facility.

    Cover page: Methods for the Production of a High-Activity Positron Sources: Na-22 Coffee-Ring Minimization and Kr-79 Isotope Separation
    • Article
    • Peer Reviewed
    LBL Publications (1998)
    A new class of self-consistent 6-D phase space stationary distributions is constructed both analytically and numerically. The beam is then mismatched longitudinally and/or transversely, and we explore the beam stability and halo formation for the case of 3-D axisymmetric beam bunches using particle-in-cell simulations. We concentrate on beams with bunch length-to-width ratios varying from 1 to 5, which covers the typical range of the APT linac parameters. We find that the longitudinal halo forms first for comparable longitudinal and transverse mismatches. An interesting coupling phenomenon - a longitudinal or transverse halo is observed even for very small mismatches if the mismatch in the other plane is large - is discovered.
      Cover page: Halo Formation in Spheroidal Bunches with Self-Consistent Stationary Distributions
      • Article
      LBL Publications (2003)

      The Spallation Neutron Source (SNS) accumulator ring is designed to accumulate, via H- injection, protons of 2 MW beam power at 1 GeV kinetic energy at a repetition rate of 60 Hz [1]. At such beam intensity, electroncloud is expected to be one of the intensity-limiting mechanisms that complicate ring operations. This paper summarizes mitigation strategy adopted in the design, both in suppressing electron-cloud formation and in enhancing Landau damping, including tapered magnetic field and monitoring system for the collection of stripped electrons at injection, TiN coated beam chamber for suppression of the secondary yield, clearing electrodes dedicated for the injection region and parasitic on BPMs around the ring, solenoid windings in the collimation region, and planning of vacuum systems for beam scrubbing upon operation.

        Cover page: Electron-cloud mitigation in the spallation neutron source ring
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2023)
        TURBOMOLE is a highly optimized software suite for large-scale quantum-chemical and materials science simulations of molecules, clusters, extended systems, and periodic solids. TURBOMOLE uses Gaussian basis sets and has been designed with robust and fast quantum-chemical applications in mind, ranging from homogeneous and heterogeneous catalysis to inorganic and organic chemistry and various types of spectroscopy, light-matter interactions, and biochemistry. This Perspective briefly surveys TURBOMOLEs functionality and highlights recent developments that have taken place between 2020 and 2023, comprising new electronic structure methods for molecules and solids, previously unavailable molecular properties, embedding, and molecular dynamics approaches. Select features under development are reviewed to illustrate the continuous growth of the program suite, including nuclear electronic orbital methods, Hartree-Fock-based adiabatic connection models, simplified time-dependent density functional theory, relativistic effects and magnetic properties, and multiscale modeling of optical properties.
          Cover page: TURBOMOLE: Today and Tomorrow.
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2023)
            Cover page: Neutron detection and application with a novel 3D-projection scintillator tracker in the future long-baseline neutrino oscillation experimentsCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Davis Previously Published Works (2013)
            Abstract Background The process of generating raw genome sequence data continues to become cheaper, faster, and more accurate. However, assembly of such data into high-quality, finished genome sequences remains challenging. Many genome assembly tools are available, but they differ greatly in terms of their performance (speed, scalability, hardware requirements, acceptance of newer read technologies) and in their final output (composition of assembled sequence). More importantly, it remains largely unclear how to best assess the quality of assembled genome sequences. The Assemblathon competitions are intended to assess current state-of-the-art methods in genome assembly. Results In Assemblathon 2, we provided a variety of sequence data to be assembled for three vertebrate species (a bird, a fish, and snake). This resulted in a total of 43 submitted assemblies from 21 participating teams. We evaluated these assemblies using a combination of optical map data, Fosmid sequences, and several statistical methods. From over 100 different metrics, we chose ten key measures by which to assess the overall quality of the assemblies. Conclusions Many current genome assemblers produced useful assemblies, containing a significant representation of their genes and overall genome structure. However, the high degree of variability between the entries suggests that there is still much room for improvement in the field of genome assembly and that approaches which work well in assembling the genome of one species may not necessarily work well for another.
            • 1 supplemental file
            Cover page: Assemblathon 2: evaluating de novo methods of genome assembly in three vertebrate species
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2022)
            The first measurement of lepton-jet momentum imbalance and azimuthal correlation in lepton-proton scattering at high momentum transfer is presented. These data, taken with the H1 detector at HERA, are corrected for detector effects using an unbinned machine learning algorithm (multifold), which considers eight observables simultaneously in this first application. The unfolded cross sections are compared with calculations performed within the context of collinear or transverse-momentum-dependent factorization in quantum chromodynamics as well as Monte Carlo event generators.
              Cover page: Measurement of Lepton-Jet Correlation in Deep-Inelastic Scattering with the H1 Detector Using Machine Learning for Unfolding
              • Article
              • Peer Reviewed
              LBL Publications (2017)
              Background: The electromagnetic form factors of the proton measured by unpolarized and polarized electron scattering experiments show a significant disagreement that grows with the squared four-momentum transfer (Q2). Calculations have shown that the two measurements can be largely reconciled by accounting for the contributions of two-photon exchange (TPE). TPE effects are not typically included in the standard set of radiative corrections since theoretical calculations of the TPE effects are highly model dependent, and, until recently, no direct evidence of significant TPE effects has been observed. Purpose: We measured the ratio of positron-proton to electron-proton elastic-scattering cross sections in order to determine the TPE contribution to elastic electron-proton scattering and thereby resolve the proton electric form factor discrepancy. Methods: We produced a mixed simultaneous electron-positron beam in Jefferson Lab's Hall B by passing the 5.6-GeV primary electron beam through a radiator to produce a bremsstrahlung photon beam and then passing the photon beam through a convertor to produce electron-positron pairs. The mixed electron-positron (lepton) beam with useful energies from approximately 0.85 to 3.5 GeV then struck a 30-cm-long liquid hydrogen (LH2) target located within the CEBAF Large Acceptance Spectrometer (CLAS). By detecting both the scattered leptons and the recoiling protons, we identified and reconstructed elastic scattering events and determined the incident lepton energy. A detailed description of the experiment is presented. Results: We present previously unpublished results for the quantity R2γ, the TPE correction to the elastic-scattering cross section, at Q2≈0.85 and 1.45 GeV2 over a large range of virtual photon polarization ?. Conclusions: Our results, along with recently published results from VEPP-3, demonstrate a nonzero contribution from TPE effects and are in excellent agreement with the calculations that include TPE effects and largely reconcile the form-factor discrepancy up to Q2≈2GeV2. These data are consistent with an increase in R2γ with decreasing ? at Q2≈0.85 and 1.45 GeV2. There are indications of a slight increase in R2γ with Q2.
                • Article
                LBL Publications (2023)
                The radiation pattern within high energy quark- and gluon-initiated jets (jet substructure) is used extensively as a precision probe of the strong force as well as an environment for optimizing event generators with numerous applications in high energy particle and nuclear physics. Looking at electron-proton collisions is of particular interest as many of the complications present at hadron colliders are absent. A detailed study of modern jet substructure observables, jet angularities, in electron-proton collisions is presented using data recorded using the H1 detector at HERA. The measurement is unbinned and multi-dimensional, using machine learning to correct for detector effects. All of the available reconstructed object information of the respective jets is interpreted by a graph neural network, achieving superior precision on a selected set of jet angularities. Training these networks was enabled by the use of a large number of GPUs in the Perlmutter supercomputer at Berkeley Lab. The particle jets are reconstructed in the laboratory frame, using the $k_{\mathrm{T}}$ jet clustering algorithm. Results are reported at high transverse momentum transfer $Q^2>150$ GeV${}^2$, and inelasticity $0.2 < y < 0.7$. The analysis is also performed in sub-regions of $Q^2$, thus probing scale dependencies of the substructure variables. The data are compared with a variety of predictions and point towards possible improvements of such models.
                  Cover page: Unbinned Deep Learning Jet Substructure Measurement in High $Q^2$ ep collisions at HERA
                  • Article
                  • Peer Reviewed
                  UC Riverside Previously Published Works (2023)
                  The radiation pattern within high energy quark- and gluon-initiated jets (jet substructure) is used extensively as a precision probe of the strong force as well as an environment for optimizing event generators with numerous applications in high energy particle and nuclear physics. Looking at electron-proton collisions is of particular interest as many of the complications present at hadron colliders are absent. A detailed study of modern jet substructure observables, jet angularities, in electron-proton collisions is presented using data recorded using the H1 detector at HERA. The measurement is unbinned and multi-dimensional, using machine learning to correct for detector effects. All of the available reconstructed object information of the respective jets is interpreted by a graph neural network, achieving superior precision on a selected set of jet angularities. Training these networks was enabled by the use of a large number of GPUs in the Perlmutter supercomputer at Berkeley Lab. The particle jets are reconstructed in the laboratory frame, using the kT jet clustering algorithm. Results are reported at high transverse momentum transfer Q2>150GeV2, and inelasticity 0.2
                    Creative Commons 'BY' version 4.0 license
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