Low Energy Threshold Research Articles

Plasma etching of high aspect ratio features in SiO2 using Ar/C4F8/O2 mixtures: A computational investigation

Plasma etching of high aspect ratio (HAR) features, typically vias, is a critical step in the fabrication of high capacity memory. With aspect ratios (ARs) exceeding 50 (and approaching 100), maintaining critical dimensions (CDs) while eliminating or diminishing twisting, contact-edge-roughening, and aspect ratio dependent etching (ARDE) becomes challenging. Integrated reactor and feature scale modeling was used to investigate the etching of HAR features in SiO2 with ARs up to 80 using tri-frequency capacitively coupled plasmas sustained in Ar/C4F8/O2 mixtures. In these systems, the fluxes of neutral radicals to the wafer exceed the fluxes of ions by 1–2 orders of magnitude due to lower threshold energies for dissociation compared with ionization. At low ARs (<5), these abundant fluxes of CFx and CxFy radicals to the etch front passivate the oxide to form a complex which is then removed by energetic species (ions and hot neutrals) through chemically enhanced reactive etching, resulting in the formation of gas phase SiFx, COx, and COF. As the etching proceeds into higher ARs, the fractional contribution of physical sputtering to oxide removal increases as the fluxes of energetic species to the etch front surpass those of the conduction constrained CFx and CxFy radicals. The instantaneous etch rate of oxide decreases with increasing aspect ratio (ARDE effect) due to decreased fluxes of energetic species and decreased power delivered by these species to the etch front. As the etch rate of photoresist (PR) is independent of AR, maintaining CDs by avoiding undercut and bowing requires high SiO2-over-PR selectivity, which in turn requires a minimum thickness of the PR at the end of etching. Positive ions with narrow angular distributions typically deposit charge on the bottom of low AR features, producing a maximum in positive electric potential on the bottom of the feature. For high AR features, grazing incidence collisions of ions on sidewalls depositing charge produce electric potentials with maxima on the sidewalls (as opposed to the bottom) of the feature.

Opto-electronic properties of twisted bilayer graphene quantum dots

The electronic and interband optical properties of vertically coupled stacked graphene quantum dots are investigated using the tight-binding method. Both zigzag and armchair edge configurations are taken into account. In particular, the effect of the geometrical shape (triangular or circle-like) and, most prominently, of the angle of twisting between layers is mainly addressed. The optical response is analyzed from the calculated imaginary part of the dielectric function. It is found that the interband absorption threshold is highly dependent on the dot size and geometry: For armchair triangular bilayer graphene dots the optical gap exhibits a moderate increase for smaller angles of twisting, and the structure behaves as an intermediate to a wide gap semiconductor; whereas zigzag triangular bilayer graphene dots are small gap systems in which the twisting causes the appearance of zero-gap states associated with the variation of HOMO and LUMO states resulting from the breaking of zero-energy degeneracy. In the latter case, it is shown that the low-energy transitions between those states are responsible for the main optical response of the structures which indicates possible applications in the THz optoelectronics. Circular dots are chosen in commensurable configurations and also show stronger low-energy absorption thresholds. A particular feature appearing in this case is the presence of Bravais-Moiré patterns in the two-dimensional probability density distributions for large enough dot radii.

Reduction of Metal Artifacts and Improvement in Dose Efficiency Using Photon-Counting Detector Computed Tomography and Tin Filtration.

The aim of this study was to investigate the impact on metal artifacts and dose efficiency of using a tin filter in combination with high-energy threshold (TH) images of a photon-counting detector (PCD) computed tomography (CT) system. A 3D-printed spine with pedicle screws was scanned on a PCD-CT system with and without tin filtration. Image noise and severity of artifacts were measured for low-energy threshold (TL) and TH images. In a prospective, institutional review board-approved, Health Insurance Portability and Accountability Act-compliant study, 20 patients having a clinical energy-integrating detector (EID) CT were scanned on a PCD-CT system using tin filtration. Images were reviewed by 3 radiologists to evaluate visualization of anatomic structures, diagnostic confidence, and image preference. Artifact severity and image noise were measured. Wilcoxon signed rank was used to test differences between PCD-CT TH and EID-CT images. Phantom TH images with tin filtration reduced metal artifacts and had comparable noise (32 HU) to TL images (29 HU) acquired without tin filtration. Visualization scores for the cortex, trabeculae, and implant-trabecular interface from PCD-CT TH images (4.4 ± 0.9, 4.4 ± 1.0, and 4.4 ± 1.0) were significantly higher (P < 0.0001) than EID-CT images (3.3 ± 1.3, 3.3 ± 1.2, and 3.3 ± 1.6). A strong preference was shown for PCD-CT TH images due to improved diagnostic confidence and decreased artifact severity. Noise in PCD-CT TH images (93 ± 41 HU) was significantly lower than that in EID-CT images (133 ± 92 HU, P < 0.05). Threshold high images acquired with tin filtration on PCD-CT demonstrated a substantial decrease in metal artifacts and an increase in dose efficiency compared with EID-CT.

Spectrum of Radiation of a Bunch of Positrons Channeled in Densely Packed Nanotubes

The problem of the radiation of a relativistic bunch of positrons channeled in densely packed nanotubes was solved. A model parabolic potential was used for the average nanotube field. The channeled positrons oscillate with the same frequency, but with different amplitudes equal to the distance from the point of entry of the positron in the nanotube to its axis. The polarization of the nanotube environment results in the appearance of a lower energy threshold for the formation of radiation. The boundaries of the frequency interval of radiation also depend on the amplitude of the oscillating positron. For the first time an analytical expression for the spectrum of the total radiation is obtained in the dipole approximation. At a zero angle, both hard (gamma) and soft (X-ray) photons are generated. Directed photon beams have important practical applications.

Escape-suppression shield detector for the MINIBALL $ \gamma$γ-ray spectrometer

A bismuth-germanate (BGO) escape-suppression shield for the high-purity germanium triple-cluster detector of the MINIBALL \( \gamma\)-ray spectrometer was designed and built. Monte Carlo simulations with the simulation code GEANT4 were performed to guide the construction and to determine the detector geometry of the new BGO shield. After the first measurements concerning mechanical properties of the BGO housing and the performance of the photomultiplier tubes at the Institut de Physique Nucleaire, Orsay, the prototype BGO escape-suppression shield was combined with a MINIBALL triple-cluster detector at the Institut fur Kernphysik, Cologne. A dedicated electronics and digital data-acquisition system was put into operation in order to determine timing properties of the combined coincidence measurement and to measure values for the energy resolution of the BGO detectors, for the BGO low-energy threshold, and for the crucial peak-to-total ratio (P/T). The measured P/T value for a standard 60Co \( \gamma\)-ray source compares well with expectations and will allow to proceed with the amendment of the MINIBALL triple-cluster detectors with an escape-suppression shield for improved in-beam \( \gamma\)-ray spectroscopy especially at the new HIE-ISOLDE accelerator for radioactive ion beams at CERN.

Search for low-mass dark matter with CDMSlite using a profile likelihood fit

The Cryogenic Dark Matter Search low ionization threshold experiment (CDMSlite) searches for interactions between dark matter particles and germanium nuclei in cryogenic detectors. The experiment has achieved a low energy threshold with improved sensitivity to low-mass (<10 GeV/c$^2$) dark matter particles. We present an analysis of the final CDMSlite data set, taken with a different detector than was used for the two previous CDMSlite data sets. This analysis includes a data "salting" method to protect against bias, improved noise discrimination, background modeling, and the use of profile likelihood methods to search for a dark matter signal in the presence of backgrounds. We achieve an energy threshold of 70 eV and significantly improve the sensitivity for dark matter particles with masses between 2.5 and 10 GeV/c$^2$ compared to previous analyses. We set an upper limit on the dark matter-nucleon scattering cross section in germanium of 5.4$\times$10$^{-42}$ cm$^2$ at 5 GeV/c$^2$, a factor of $\sim$2.5 improvement over the previous CDMSlite result.

Dissociative electron attachment to vibrationally excited CO2

Dissociative electron attachment to molecules in vibrationally excited states is associated with lower threshold energies and higher cross sections than for cold targets. To date, fewer studies have been performed for polyatomic molecules than for diatomic molecules, due to the experimental challenges and theoretical difficulties in describing the complicated dynamics. Here we report an observation of the dissociative electron attachment to $\mathrm{C}{\mathrm{O}}_{2}$ in the vibrationally excited states, where the vibrationally excited $\mathrm{C}{\mathrm{O}}_{2}$ targets are prepared by the electron-impact excitation at energies close to the ${}^{2}{\mathrm{\ensuremath{\Pi}}}_{u}$ resonant state of $\mathrm{C}{{\mathrm{O}}_{2}}^{\ensuremath{-}}$. The high-resolution anion momentum images of the ${\mathrm{O}}^{\ensuremath{-}}$ yield indicate that the rovibrational state distributions of the coproduct CO are dependent on the vibrationally excited states of symmetric bond stretching and bending modes of $\mathrm{C}{\mathrm{O}}_{2}$.

Distinguishing axion models with IAXO

Helioscopes, such as the proposed International Axion Observatory (IAXO), have significant discovery potential for axions and axion-like particles. In this note, we argue that beyond discovery they can resolve details of the model. In particular, in the region suggested by stellar cooling anomalies, there is a good chance to measure the mass of the particle and separately its couplings to electrons and photons. This can give crucial information on the nature of the underlying model. To achieve this, energy resolved detectors and a setup with low energy threshold are needed.

Proton-produced radionuclides for radiodiagnostic modalities in cancer studies

With increasing rates of cancer cases globally, scientists and technologists have proposed cancer diagnosis using radioisotope-labelled chemical compounds. In this paper, using the TALYS 2017 codes we calculated excitation functions of several (p,n), (p,α), (p, 2n) and (p,3n) nuclear reactions. The TALYS-calculated results indicated that a few radionuclides such as 18F, 11C, 64Cu, I24I could be produced at relatively low threshold energies ranging from as low as 2.7 MeV to 4 MeV, whereas other radionuclides such as 123I, 201Tl, 111In and 67Ga could only be generated with protons of over 10 MeV. While 18F, 15O, 64Cu and 124I radionuclides could be produced via (p,n) nuclear reactions, other radionuclides including 99mTc, 123I, 111In and 67Ga could be produced through (p,2n) nuclear reactions . In addition, (p,α) nuclear reaction was found to suit 11C and 13N radionuclide production, whereas (p,3n) fit 201Tl production. In general, most radionuclides used for Single Photon Computed Tomography (SPECT) modality requires higher proton incident energies compared to those for Positron Emission Tomography (PET) modality in nuclear medicine.

Generation and characteristics of shock optical pulses based on a fiber-loop time-lens system

&lt;sec&gt;The shock ignition scheme has the advantages of low ignition energy threshold, high gain, and good hydrodynamic stability, which has become one of the key schemes for the potentially successful ignition of inertial confinement fusion. The crucial element of shock ignition is how to achieve a highly efficient shock laser pulse. We propose a new scheme based on a time-lens system combining the fiber-loop phase modulation and the grating-pair compression to generate a highly controllable shock pulse. Based on the asymmetric phase modulation in time-domain followed by linear dispersion compensation in frequency domain, the shock pulse can be actively controlled with high precision in both pulse duration and pulse contrast (peak power ratio of the compression part to the shock part of the pulse). We construct a theoretical model based on the nonlinear Schrödinger equation to simulate the evolution of the spectrum and temporal shape of the shock laser pulse. The influences of various key parameters of the proposed system on the characteristics of the generated shock pulse are analyzed in depth. &lt;/sec&gt;&lt;sec&gt;The time lens system consists of three parts, i.e. the seed pulse carving part, the phase modulation loop, and the chirp-compensating grating pair. The operation principle of this system for generating shock pulse is as follows. First, a single-mode continuous wave 1053 nm distributed feedback seed laser is chopped into pulses with a Mach-Zehnder intensity modulator. Then the pulses enter into a fiber-loop for phase modulation. Owing to different modulation frequencies exerted on the left and right side of the pulse, the amount of spectral broadening of these two sides of the spectrum are also different after phase modulation. The spectrally broadened pulses are linearly chirped when the phase-modulation function has a parabolic shape. Finally, the pulse transits through a grating pair system for chirp compensating. Just like an anomalous dispersion delay line, the grating pair applies an anomalous group velocity dispersion to the passing optical pulse. When the chirp is compensated for appropriately, the pulse will be compressed. What the target pulse can be finally shaped into is dependent on the combined optimization of all the above processes.&lt;/sec&gt;&lt;sec&gt;The simulation results show that by systematically designing the parameters such as chopping function, phase modulation function, modulation depth, modulation frequency, and chirp compensating, the target shock pulse can be actively controlled with high-precision in the pulse width, pulse rising edge, and peak-power contrast. In addition, we can also tune only one parameter (such as the pulse width) of the pulse, with the other parameters kept unchanged. This new design idea and the proposed system can actively and independently adjust the two key parameters (the peak power contrast and the pulse width) of the generated shock pulse, which is not only helpful in deepening our understanding of the principle of laser-pulse shaping, but also significant for the subsequent practical implement of shock ignition of inertial confinement fusion.&lt;/sec&gt;

E- discrimination at high energy in the JUNO detector

Cosmic Ray and neutrino oscillation physics can be studied by using atmospheric neutrinos. JUNO (Jiangmen Underground Neutrino Observatory) is a large liquid scintillator detector with low energy detection threshold and excellent energy resolution. The detector performances allow the atmospheric neutrino oscillation measurements. In this work, a discrimination algorithm for different reaction channels of neutrino-nucleon interactions in the JUNO liquid scintillator, in the GeV/sub-GeV energy region, is presented. The atmospheric neutrino flux is taken as reference, considering $\mathop {{v_\mu }}\limits^{( - )} $ and $\mathop {{v_e}}\limits^{( - )} $. The different temporal behaviour of the classes of events have been exploited to build a timeprofile-based discrimination algorithm. The results show a good selection power for $\mathop {{v_e}}\limits^{( - )} $ CC events, while the $\mathop {{v_\mu }}\limits^{( - )} $ CC component suffers of an important contamination from NC events at low energy, which is under study. Preliminary results are presented.

Кислород на цилиндрическом поликристалле вольфрама

AbstractUsing low-energy (0–25 eV) threshold electron spectroscopy and contact potential difference in vacuum not worse than 2 × 10^–8 Pa, oxygen films obtained at 295, 325–1425 (in steps of 50), and 1575 K, including in subsequent heating from the adsorption temperature to 1925 ± 50 K, on ​​the cylindrical surface of a gas-phase tungsten coating 30 μm thick on NbC-1 alloy (Nb – 1% Zr), used as a collector material for thermal emission in space-borne nuclear power plants, have been studied.

Dark Matter implications of DAMA/LIBRA-phase2 results

Recently, the DAMA/LIBRA collaboration released updated results from their search for the annual modulation signal from Dark Matter (DM) scattering in the detector. Besides approximately doubling the exposure of the DAMA/LIBRA data set, the updated photomultiplier tubes of the experiment allow a lower recoil energy threshold of 1keV electron equivalent compared to the previous threshold of 2keV electron equivalent. We study the compatibility of the observed modulation signal with DM scattering. Due to a conspiracy of multiple effects, the new data at low recoil energies is very powerful for testing the DM hypothesis. We find that canonical (isospin conserving) spin-independent DM-nucleon interactions are no longer a good fit to the observed modulation signal in the standard halo model. The canonical spin-independent case is disfavored by the new data, with best fit points of a DM mass of ∼8GeV, disfavored by 5.2σ, or a mass of ∼54GeV, disfavored by 2.5σ. Allowing for isospin violating spin independent interactions, we find a region with a good fit to the data with suppressed effective couplings to iodine for DM masses of ∼10GeV. We also consider spin-dependent DM-nucleon interactions, which yield good fits for similar DM masses of ∼10GeV or ∼45GeV.

Time-dependent rate of multicomponent dark matter: Reproducing the DAMA/LIBRA phase-2 results

The current paradigm for dark matter direct detection is to assume that the dark sector is solely composed of a single particle species. In this short paper, we make the observation that dark matter comprising both a light and a heavy component that modulate out of phase leads to interesting phenomenology in annual modulation experiments. For an illustrative example, we use the recently released DAMA/LIBRA phase-2 results with a lower energy threshold. Immediately after, it was argued that a one-component spin-independent dark matter explanation of the observed annual modulation is strongly disfavored or excluded unless isospin-violating couplings are invoked. We show that a simple two-component extension can reproduce the observed spectrum without the need to invoke fine-tuned couplings. Using the publicly available DAMA/LIBRA data, we perform a fit of the DAMA/LIBRA energy spectrum of the annual modulation amplitude to a scenario with two dark matter components. We also take into account how gravitational focusing affects the phases of the light and a heavy components differently, which leads to nontrivial effects in the total time-dependent rate. Our results show that there exists a unique solution in agreement with the data in the simplest case of isospin-conserving couplings with equal cross sections. The distinctive features found in this work are crucial for a dark matter interpretation of any observed annual modulation.

A data acquisition and control system for large mass bolometer arrays

During the last couple of decades, the use of arrays of bolometers has represented one of the leading techniques for the search for rare events. CUORE, an array of 988 TeO2 bolometers that is taking data since April 2017 at the Laboratori Nazionali del Gran Sasso (Italy), exploits the large mass, low background, good energy resolution and low energy threshold of these detectors successfully. Thanks to these characteristics, they could be also sensitive to low energy rare processes, such as galactic dark matter interactions. In this paper we describe the data acquisition system that was developed for the CUORE experiment. Thanks to its high modularity, the data acquisition here described has been used in different setups with similar requirements, including the pilot experiment CUORE-0 and the demonstrator for the next phase of the project, CUPID-0, also taking data at LNGS.

Neutron detection and [formula omitted]-ray suppression using artificial neural networks with the liquid scintillators BC-501A and BC-537

In this work we present a comparison between the two liquid scintillators BC-501A and BC-537 in terms of their performance regarding the pulse-shape discrimination between neutrons and γ rays. Special emphasis is put on the application of artificial neural networks. The results show a systematically higher γ-ray rejection ratio for BC-501A compared to BC-537 applying the commonly used charge comparison method. Using the artificial neural network approach the discrimination quality was improved to more than 95% rejection efficiency of γ rays over the energy range 150 to 1000 keV for both BC-501A and BC-537. However, due to the larger light output of BC-501A compared to BC-537, neutrons could be identified in BC-501A using artificial neural networks down to a recoil proton energy of 800 keV compared to a recoil deuteron energy of 1200 keV for BC-537. We conclude that using artificial neural networks it is possible to obtain the same γ-ray rejection quality from both BC-501A and BC-537 for neutrons above a low-energy threshold. This threshold is, however, lower for BC-501A, which is important for nuclear structure spectroscopy experiments of rare reaction channels where low-energy interactions dominates.

Neutrino Physics and Astrophysics with the JUNO Detector

The Jiangmen Underground Neutrino Observatory (JUNO) is a 20 kton liquid scintillator multi-purpose underground detector, under construction near the Chinese city of Jiangmen, with data collection expected to start in 2021. The main goal of the experiment is the neutrino mass hierarchy determination, with more than three sigma significance, and the high-precision neutrino oscillation parameter measurements, detecting electron anti-neutrinos emitted from two nearby (baseline of about 53 km) nuclear power plants. Besides, the unprecedented liquid scintillator-type detector performance in target mass, energy resolution, energy calibration precision, and low-energy threshold features a rich physics program for the detection of low-energy astrophysical neutrinos, such as galactic core-collapse supernova neutrinos, solar neutrinos, and geo-neutrinos.

Impact of Sb and Na Doping on the Surface Electronic Landscape of Cu2ZnSnS4 Thin Films

Open-circuit voltage deficiency is the key limiting factor in Cu2ZnSnS4 (CZTS) thin-film solar cells, which is commonly associated with band tails and deep gap states arising from elemental disorder. The introduction of dopants such as Na and Sb has led to improvement in device performance, yet their effects on the optoelectronic properties of CZTS are yet to be fully elucidated. In this Letter, we unraveled the effect of Sb and Na:Sb co-doping on the surface energy landscape of solution-processed CZTS films employing energy-filtered photoelectron emission microscopy. In the absence of the additives, 150 nm resolution photoemission maps reveal oscillations in the local effective work function as well as areas of low photoemission energy threshold. The introduction of dopants substantially reshapes the photoemission maps, which we rationalize in terms of Cu:Zn and Sn disorder. Finally, we establish unprecedented correlations between the photoemission landscape of thin films and the performance of over 200 ...

Threshold displacement energy in Ni, Al and B2 NiAl

For simulation of radiation effects and defects in the Ni-Al system, a new many body potential was developed by joining the equilibrium part of the Mishin’s EAM potential with the universal function of Ziegler, Biersack and Littmark at a suitable interatomic spacing and the corresponding pairwise energy from the DFT calculations. To assess the qualities of this potential, we performed molecular dynamics simulations to calculate the threshold displacement energy of pure metals Ni, Al and their NiAl B2 phase superalloy at ambient temperature. It was found that the threshold energy of displacement of nickel in nickel is higher than that in the B2 long-range ordered alloy NiAl in contrast to aluminium, which has lower threshold energy than B2 NiAl. The threshold displacement energy ranges between 14±2 eV and 189±2 eV depending on the crystallographic direction. The lowest threshold energy of two fcc metals (14±2 eV for Al and 28±2 eV for Ni) was found in the direction <101>, these values correspond to the mean value in the easiest directions of atomic displacements and they are in good agreement with many reports from the literature.

Statistical Study of the Swift X-Ray Flash and X-Ray Rich Gamma-Ray Bursts

Abstract We build a comprehensive sample to statistically describe the properties of X-ray flashes (XRFs) and X-ray riches (XRRs) from the third Swift Burst Alert Telescope (BAT3) catalog of Gamma-ray bursts (GRBs). We obtain 81 XRFs, 540 XRRs, and 394 classical GRBs (C-GRBs). We statistically explore the different properties of the γ-ray prompt emission, the X-ray emission, the X-ray light-curve type, the association with supernovae (SNe), and the host galaxy properties for these sources. We confirm that most XRFs/XRRs are long GRBs with low values of peak energy and they are low-luminosity GRBs. XRFs, XRRs, and C-GRBs follow the same E X,iso–E γ,iso–E peak,z correlations. Compared to the classical GRBs, XRFs are favorable to have the association with SN explosions. We do not find any significant differences of redshift distribution and host galaxy properties among XRFs, XRRs, and C-GRBs. We also discuss some observational biases and selection effects that may affect our statistical results. The GRB detectors with wide energy range and low energy threshold are expected for the XRF/XRR research in the future.