Joshua Machacek

Bio: Joshua Machacek is an academic researcher from Australian National University. The author has contributed to research in topics: Positron & Positronium. The author has an hindex of 15, co-authored 48 publications receiving 567 citations. Previous affiliations of Joshua Machacek include California Institute of Technology & University of Nebraska–Lincoln.

Radiation tolerance of two-dimensional material-based devices for space applications

Abstract: Characteristic for devices based on two-dimensional materials are their low size, weight and power requirements. This makes them advantageous for use in space instrumentation, including photovoltaics, batteries, electronics, sensors and light sources for long-distance quantum communication. Here we present a comprehensive study on combined radiation effects in Earth's atmosphere on various devices based on these nanomaterials. Using theoretical modeling packages, we estimate relevant radiation levels and then expose field-effect transistors, single-photon sources and monolayers as building blocks for future electronics to γ-rays, protons and electrons. The devices show negligible change in performance after the irradiation, suggesting robust suitability for space use. Under excessive γ-radiation, however, monolayer WS2 shows decreased defect densities, identified by an increase in photoluminescence, carrier lifetime and a change in doping ratio proportional to the photon flux. The underlying mechanism is traced back to radiation-induced defect healing, wherein dissociated oxygen passivates sulfur vacancies.

Forward angle scattering effects in the measurement of total cross sections for positron scattering

Abstract: Measurements of total scattering by positron impact have typically excluded a significant portion of the forward scattering angles of the differential cross section. This paper demonstrates the effect that this can have on measurements of the total cross section. We show that much of the apparent disagreement between experimental measurements of positron scattering from atoms and molecules may be explained by this excluded angular range. It is shown that this same effect may also lead to an anomalous energy dependence of some cross sections.

Positron Scattering From neon and argon

Abstract: High-resolution measurements of positron interactions with Ne and Ar are presented, as well as theoretical treatments. The data extend over a range of 0.3 to 60 eV and comprise measurements of the grand total, positonium formation, and grand total minus positronium formation cross sections. Theoretical treatments of scattering from Ne and Ar are performed under the relativistic optical potential approach, as well as calculations using the convergent close-coupling method. Comparisons of the present measurements and theories are made with previous theoretical and experimental work.

Scattering data for modelling positron tracks in gaseous and liquid water

Abstract: We present in this study a self-consistent set of scattering cross sections for positron collisions with water molecules, in the energy range 0.1-10 000 eV, with the prime motivation being to provide data for modelling purposes. The structure of the database is based on a new model potential calculation, including interference terms, which provides differential and integral elastic as well as integral inelastic positron scattering cross sections for water molecules over the whole energy range considered here. Experimental and theoretical data available in the literature have been integrated into the database after a careful analysis of their uncertainties and their self-consistency. These data have been used as input parameters for a step-by-step Monte Carlo simulation procedure, providing valuable information on energy deposition, positron range, and the relative percentages of specific interactions (e.g. positronium formation, direct ionisation, electronic, vibrational and rotational excitations) in gaseous and liquid water.

Total, elastic, and inelastic cross sections for positron and electron collisions with tetrahydrofuran

Abstract: We present total, elastic, and inelastic cross sections for positron and electron scattering from tetrahydrofuran (THF) in the energy range between 1 and 5000 eV. Total cross sections (TCS), positronium formation cross sections, the summed inelastic integral cross sections (ICS) for electronic excitations and direct ionization, as well as elastic differential cross sections (DCS) at selected incident energies, have been measured for positron collisions with THF. The positron beam used to carry out these experiments had an energy resolution in the range 40–100 meV (full-width at half-maximum). We also present TCS results for positron and electron scattering from THF computed within the independent atom model using the screening corrected additivity rule approach. In addition, we calculated positron-impact elastic DCS and the sum over all inelastic ICS (except rotations and vibrations). While our integral and differential positron cross sections are the first of their kind, we compare our TCS with previous literature values for this species. We also provide a comparison between positron and electron-impact cross sections, in order to uncover any differences or similarities in the scattering dynamics with these two different projectiles. © 2013 American Institute of Physics.

Monthly Notices of the Royal Astronomical Society

Abstract: Monthly Notices is one of the three largest general primary astronomical research publications. It is an international journal, published by the Royal Astronomical Society. This article 1 describes its publication policy and practice.

Density Matrix Theory and Applications

Abstract: — 70°C (1 Hz) in PE (auf S. 245 wird von -20 °C bei derselben Frequenz gesprochen) als Glasübergang der amorphen Bereiche des PE anzusehen ist, der (als Maximum des tan oder G\") deutlicher hervortritt, wenn man durch physikalisch-chemische Effekte, die den Kristallisationsgrad nicht beeinflussen (z.B.: Quellung in einem unpolaren, niedermolekularen Quellungsmittel), Ànderungen des Zustands der nichtkristallinen Phasen erreichen kann. Auf diese Weise kann z.B. durch Quellung in CClj oder C„H,„ die Relaxationsstarke der nichtkristallinen Phasen verstârkt werden. Die genannten Quellungsmittel zeigen keine eigenen Dispersionsgebiete im interessierenden Bereich, sofern sie molekular dispers vorliegen [ . H. Illers, Kolloid . 190 (1963) 16, 250 (1972) 426, 231 (1967) 622J.

Plasma and trap-based techniques for science with positrons

Abstract: In recent years, there has been a wealth of new science involving low-energy antimatter (i.e., positrons and antiprotons) at energies ranging from 10 to less than 10 eV. Much of this progress has been driven by the development of new plasma-based techniques to accumulate, manipulate and deliver antiparticles for specific applications. This article focuses on the advances made in this area using positrons. However many of the resulting techniques are relevant to antiprotons as well. An overview is presented of relevant theory of single-component plasmas in electromagnetic traps. Methods are described to produce intense sources of positrons and to efficiently slow the typically energetic particles thus produced. Techniques are described to trap positrons efficiently and to cool and compress the resulting positron gases and plasmas. Finally, the procedures developed to deliver tailored pulses and beams (e.g., in intense, short bursts, or as quasi-monoenergetic continuous beams) for specific applications are reviewed. The status of development in specific application areas is also reviewed. One example is the formation of antihydrogen atoms for fundamental physics [e.g., tests of invariance under charge conjugation, parity inversion and time reversal (the CPT theorem), and studies of the interaction of gravity with antimatter]. Other applications discussed include atomic and materials physics studies and study of the electron-positron many-body system, including both classical electron-positron plasmas and the complementary quantum system in the form of Bose-condensed gases of positronium atoms. Areas of future promise are also discussed. The review concludes with a brief summary and a list of outstanding challenges. ∗ Contact information: jrdanielson@ucsd.edu † Contact information: ddubin@ucsd.edu ‡ Contact information: rgreaves@fpsi.edu § Contact information: csurko@ucsd.edu

Single-spin resonance in a van der Waals embedded paramagnetic defect.

Abstract: A plethora of single-photon emitters have been identified in the atomic layers of two-dimensional van der Waals materials1–8. Here, we report on a set of isolated optical emitters embedded in hexagonal boron nitride that exhibit optically detected magnetic resonance. The defect spins show an isotropic ge-factor of ~2 and zero-field splitting below 10 MHz. The photokinetics of one type of defect is compatible with ground-state electron-spin paramagnetism. The narrow and inhomogeneously broadened magnetic resonance spectrum differs significantly from the known spectra of in-plane defects. We determined a hyperfine coupling of ~10 MHz. Its angular dependence indicates an unpaired, out-of-plane delocalized π-orbital electron, probably originating from substitutional impurity atoms. We extracted spin–lattice relaxation times T1 of 13–17 μs with estimated spin coherence times T2 of less than 1 μs. Our results provide further insight into the structure, composition and dynamics of single optically active spin defects in hexagonal boron nitride. The optically detected magnetic resonance of a single defect in hexagonal boron nitride is reported.