Critical Excitation Energy Research Articles

Preparation and Thermal Stability of Nano-Sized HMX-Based Polymer Bonded Explosives

ABSTRACT In this study, M-HMX-based PBX (M-HMX-PBX) and N-HMX-based PBX (N-HMX-PBX) were prepared by industrial micron-sized HMX (M-HMX) and nano-sized HMX (N-HMX) via solution-water suspension method, respectively. The critical electronic excitation energy (Ec ) of M-HMX particle and N-HMX particles was calculated by combining with field emission scanning electron microscopy (FE-SEM) and Image Pro Plus (IPP) software. Meanwhile, the average critical electron excitation energy () of the entire particle group was calculated. Those results shown that of N-HMX was higher than that of M-HMX, which indicated that the stability of N-HMX was better than that of M-HMX. In addition, the reaction activation energies (Ea ) also indicated that N-HMX and N-HMX-PBX were more stable than M-HMX and M-HMX-PBX, which were calculated by the Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) methods. The above results explained the reasons for the high thermal decomposition stability of N-HMX-PBX from the micro and macro perspectives. And those provided theoretical basis for exploring the application of N-HMX in improving the thermal stability of HMX-PBX.

Non-stationary statistics and formation jitter in transient photon condensation

While equilibrium phase transitions are easily described by order parameters and free-energy landscapes, for their non-stationary counterparts these quantities are usually ill-defined. Here, we probe transient non-equilibrium dynamics of an optically pumped, dye-filled microcavity. We quench the system to a far-from-equilibrium state and find delayed condensation close to a critical excitation energy, a transient equivalent of critical slowing down. Besides number fluctuations near the critical excitation energy, we show that transient phase transitions exhibit timing jitter in the condensate formation. This jitter is a manifestation of the randomness associated with spontaneous emission, showing that condensation is a stochastic, rather than deterministic process. Despite the non-equilibrium character of this phase transition, we construct an effective free-energy landscape that describes the formation jitter and allows, in principle, its generalization to a wider class of processes.

Charge/mass yields in the fission of highly excited heavy actinides

The charge (mass) distributions of fission fragments resulting from low- and high-energy fission of the isotopes of Fm and No are studied with the statistical scission-point fission model. The calculated results are compared with the available experimental data. The transformation of the shape of the charge distribution with increasing isospin and excitation energy occurs gradually. At some critical excitation energy the the saturation of the symmetric component of the charge (mass) yield is demonstrated.

Characterization and formation of NV centers in 3C, 4H , and 6H SiC: An ab initio study

Fluorescent paramagnetic defects in solids have become attractive systems for quantum information processing in the recent years. One of the leading contenders is the negatively charged nitrogen-vacancy defect in diamond with visible emission but alternative solution in technologically mature host is an immediate quest for many applications in this field. It has been recently found that various polytypes of silicon carbide (SiC), that are standard semiconductors with wafer scale technology, can host nitrogen-vacancy defect (NV) that could be an alternative qubit candidate with emission in the near infrared region. However, it is much less known about this defect than its counterpart in diamond. The inequivalent sites within a polytype and the polytype variations offer a family of NV defects. However, there is an insufficient knowledge on the magneto-optical properties of these configurations. Here we carry out density functional theory calculations, in order to characterize the numerous forms of NV defects in the most common polytypes of SiC including 3C, 4H and 6H, and we also provide new experimental data in 4H SiC. Our calculations mediate the identification of individual NV qubits in SiC polytypes. In addition, we discuss the formation of NV defects in SiC with providing detailed ionization energies of NV defect in SiC which reveals the critical optical excitation energies for ionizing this qubits in SiC. Our calculations unravel the challenges to produce NV defects in SiC with a desirable spin bath.

Spinodal vaporization – a critical and imposing decay mode of highly excited nuclear systems

A novel prompt decay mode of highly excited nuclear systems is shown to set in with necessity as a certain critical excitation energy per nucleon is reached. It is driven by a peculiar, open-ended type of spinodal instability, unique to self-bound open systems, and consists in parts of the system undergoing spontaneous indefinite thermal expansion ending in vaporization into the surrounding open space. The mode, named here spinodal vaporization, is distinctly different from all known decay modes of excited nuclei and faces no competition from the latter. It sets a natural upper limit for the excitation energy that can be thermalized by compound nuclear systems, while setting also a limit to the applicability of thermodynamics to the description of highly excited nuclear systems.

Tight-binding molecular dynamics simulations of radiation-inducedC60fragmentation

The radiation-induced fragmentation of the ${\text{C}}_{60}$ fullerene was investigated by tight-binding molecular dynamics simulations based on the parametrization of Papaconstantopoulos et al. [Tight-Binding Approach to Computational Materials Science, edited by P.E.A. Turchi, A. Gonis, and L. Colombo, M.R.S. Symposia Proceedings No. 491 (Materials Research Society, Pittsburgh, 1998), p. 221] and employing novel models for nonadiabatic excitation and charge redistribution. The resulted fragment size and fragment charge distributions, averaged over large ensembles of trajectories corresponding to total ionization states up to $+24e$ and excitation energies up to 1000 eV, have been used to analyze the fragmentation statistics in terms of several derived quantities. For moderate excitation energies, the fragment size profiles reproduce the experimentally observed U shape and bimodal dependence. Even though for high excitation energies and high total charges, predominantly multifragmentation occurs, a genuine power-law dependence sets in only beyond 1000 eV. A well-defined phase-transition region is found in the total charge-excitation energy plane, which appears to be delimited by a roughly parabolic critical line. The overall average critical excitation energy estimated from the simulations amounts to 55 eV and agrees with the experimental findings.

Evolution of the giant dipole resonance properties with excitation energy

The studies of the evolution of the hot Giant Dipole Resonance (GDR) properties as a function of excitation energy are reviewed. The discussion will mainly focus on the A ∼ 100-120 mass region where a large amount of data concerning the width and the strength evolution with excitation energy are available. Models proposed to interpret the main features and trends of the experimental results will be presented and compared to the available data in order to extract a coherent scenario on the limits of the development of the collective motion in nuclei at high excitation energy. Experimental results on the GDR built in hot nuclei in the mass region A ∼ 60-70 will be also shown, allowing to investigate the mass dependence of the main GDR features. The comparison between limiting excitation energies for the collective motion and critical excitation energies extracted from caloric curve studies will suggest a possible link between the disappearance of collective motion and the liquid-gas phase transition.

Quantum phase transition for the Dicke model with the dipole–dipole interactions

In this paper, we reveal a second-order phase transition for the Dicke model with the dipole–dipole interaction between the atoms. By means of Holstein–Primakoff transformation, the energy spectra for both the normal and superradiant phases are explicitly obtained and therefore the scaling behaviour near the critical transition point can be given clearly. It is also shown that the dipole–dipole interactions between the atoms deeply affect the critical transition point, the ground-state and excitation energies and the scaled atomic inversion in the ground state.

Simulation of electron‐excited X‐ray spectra with NIST–NIH Desktop Spectrum Analyzer (DTSA)

AbstractDesktop Spectrum Analyzer (DTSA) is a comprehensive software platform for the collection, simulation, processing, qualitative, and quantitative analysis of electron‐excited X‐ray spectra. Spectrum simulation is based upon solving the rigorous expression for inner shell ionization, integrated from the incident beam energy to the critical excitation energy, taking into account energy loss of the primary electrons and the loss of ionization power due to electron backscattering. The user can choose from a wide range of cross sections for K‐, L‐ and M‐shell ionization, as well as for the X‐ray continuum. Both energy‐dispersive and wavelength‐dispersive spectrometers can be simulated. The simulated spectrum can be inspected at each stage of its generation, propagation through the specimen, propagation through the detector windows and structure, and after application of the detector‐broadening function. The final spectrum is calculated on an absolute dose and spectrometer efficiency basis, permitting comparison to experimentally measured spectra. Correspondence for K‐shell intensity within ±20% relative to experimental values can be achieved over a wide range of photon energy. Appropriate counting statistics can be applied to the simulated spectra, enabling the user to test the accuracy of peak fitting with multiple linear least squares or sequential simplex nonlinear procedures. Published in 2005 by John Wiley & Sons, Ltd.

Estimating Method for Electron Beam Accelerating Voltage Used in Energy‐Dispersive X‐Ray Microanalysis: Application in Failure Analysis of Wafer Fabrication

Energy‐dispersive x‐ray microanalysis (EDX) techniques have been widely used for elemental analysis of thin film layers in wafer fabrication. However, during EDX analysis of thin film samples, sometimes it is difficult for us to identify elemental information of the surface layer from the underneath layers. Therefore, it is necessary for us to estimate the electron beam accelerating voltage used, and the penetration depth of the electron beam under this accelerating voltage before performing EDX analysis. In this particle, we will introduce a simple method/formula (V i = k E i, where the value of k is a constant, the unit of V i is in kV, and E i is in keV) to estimate the electron beam accelerating voltage, which should be used for determining the element of interest, and discuss the penetration depth of the electron beam under different beam accelerating voltages. The actual beam accelerating voltage used for a characteristic x‐ray line in EDX should be more than the theoretical value. In general, it is about 1.5–2.0 times that of the theoretical critical excitation energy. In this study, we have determined the over‐voltages of the characterization x‐ray lines of Al Ka, Si Ka, Ti Ka, W Ma, and W La, using TiN and TiW thin films and microchip Al bondpad (with Al/TiW/Ti metallization) samples. We have determined an experimental value of the over‐voltage constant, k, which was about 1.42, and it is near to the low side of 1.5–2.0 times. Moreover, we will also discuss an application case in failure analysis of wafer fabrication on nonstick bondpads.

Caloric curves and critical behavior in nuclei

Data from a number of different experimental measurements have been used to construct caloric curves for five different regions of nuclear mass. These curves are qualitatively similar and exhibit plateaus at the higher excitation energies. The limiting temperatures represented by the plateaus decrease with increasing nuclear mass and are in very good agreement with results of recent calculations employing either a chiral symmetry model or the Gogny interaction. This agreement strongly favors a soft equation of state. Evidence is presented that critical excitation energies and critical temperatures for nuclei can be determined over a large mass range when the mass variations inherent in many caloric curve measurements are taken into account.

Magnetic field effect for two electrons in a two-dimensional random potential

We study the problem of two particles with Coulomb repulsion in a two-dimensional disordered potential in the presence of a magnetic field. For the regime when without interaction all states are well localized, it is shown that above a critical excitation energy electron pairs become delocalized by interaction. The transition between the localized and delocalized regimes is similar to the metal-insulator transition at the mobility edge in the three-dimensional Anderson model with broken time reversal symmetry.

Measures for spectral quality in low-voltage X-ray microanalysis.

Characteristic x-ray production with energetic electrons depends strongly on the overvoltage, the ratio of the incident beam energy to the critical excitation energy for the atomic species of interest. Low-voltage x-ray microanalysis (beam energy < or = 5 keV) is especially susceptible to artifacts due to sample charging because the overvoltage is low and even slight charging can strongly affect peak intensities. The Duane-Hunt bremsstrahlung limit is a good diagnostic to detect sample charging. Dynamic charging effects, however, can influence spectra despite an apparently satisfactory Duane-Hunt limit. Dynamic charging effects must be examined by time series experiments, or through use of dynamic energy windows continuously measuring count rates placed across the spectrum. When charging is a problem, conductive surface coatings can eliminate the effects. When pristine surfaces must be examined without coating, the use of a conductive grid can control charging so that useful x-ray spectra can be obtained.

Physical Considerations in Low Energy Biological Analysis

Abstract Biological scanning electron microscopy is increasingly performed at low beam energies in order to improve image contrast, reduce charging artifacts, and minimize beam induced damage to the sample. It is natural then to wish to also use these same low accelerating voltage for X-ray microanalysis using an energy dispersive spectrometer (EDS) but a variety of fundamental physical effects affect the performance that can be achieved. An X-ray photon can only be emitted when the incident beam Eo energy exceeds the critical excitation energy Ecrit for that line. As the beam energy is reduced the number of elements that can be excited falls and it is becomes necessary to use L- and M-lines rather than the K-lines accessible at higher energies. At 5keV, the upper limit of ‘low voltage’ microscopy, K-lines can be excited from elements up to calcium, L-lines can be detected up to cesium, and the rest of the periodic table is available using M-lines. The entire periodic table is therefore, in principle, available at an overvoltage U&amp;gt;2, where U = E0/Ecrit But at lower energies the number of accessible excitations falls and some elements cannot, with current technology, then be analyzed and for general purposes an incident energy lower than about 3keV is probably too limiting to be useful.

Excitation energy as a basic variable to control nuclear disassembly

Thermodynamical features of Xe system is investigated as functions of temperature and freeze-out density in the frame of lattice gas model. The calculation shows different temperature dependence of physical observables at different freeze-out density. In this case, the critical temperature when the phase transition takes place depends on the freeze-out density. However, a unique critical excitation energy reveals regardless of freeze-out density when the excitation energy is used as a variable insteading of temperature. Moreover, the different behavior of other physical observables with temperature due to different $\rho_f$ vanishes when excitation energy replaces temperature. It indicates that the excitation energy can be seen as a more basic quantity to control nuclear disassembly.

The efficiency of X-ray production at low energies.

The absolute efficiency of X-ray production has been determined for the K-lines of Al, Si and Cu; for the L-lines of Fe, Co, Cu, Ge and As; and for the M-lines of Hf, Ir, Pt, Au and Bi, using overvoltage ratios in the range 1-10. These emissions, with the exception of the Cu K, have critical excitation energies below 2.6 keV and are therefore typical of the lines used for X-ray microanalysis at low beam energies. For K-lines it is found that the experimental results are in good agreement with a Bethe, or a Casnati, crosssection model. For the L- and M-lines a Monte Carlo technique has been used to find an effective X-ray generation cross-section for each of the elements. The functional forms of these cross-sections are found to be in general agreement with proposed theoretical models.

The Average Properties of the Dense Molecular Gas in Galaxies

We have observed the HCN J = 3 → 2 and J = 1 → 0 emission from several nearby starburst and normal galaxies. These lines have large critical densities (nH2 > 106 cm-3) and excitation energies (Eu > 25 K). Thus, they probe the warm and dense molecular gas where massive stars typically form. The average cloud densities of these galaxies, as estimated from the ratio of the HCN J = 3 → 2 and 1 → 0 integrated intensities, is correlated with their star formation efficiency. Therefore, the average densities of the molecular clouds in starburst nuclei are higher than those of more quiescent galaxies. Further, the starburst galaxies NGC 253 and M82 have a much higher fraction of molecular mass at high density (nH2 > 104 cm-3) than the normal galaxies IC 342 and the Milky Way. These results imply that the clouds in starburst nuclei form stars more efficiently than those in normal galaxies.

Monte Carlo modeling of secondary x‐ray fluorescence across phase boundaries in electron probe microanalysis

AbstractSecondary fluorescence induced by photoelectric absorption of x‐rays generated by an electron beam can occur when the characteristic x‐ray energy of material “A” exceeds the critical excitation energy of material “B.” An expression is developed to calculate secondary fluorescence across a planar boundary from a discrete source placed at any (X, Y, Z) coordinates relative to the boundary. The expression can be incorporated into a Monte Carlo electron trajectory simulation which calculates the discrete distribution of primary x‐ray generation.

Submillimeter HCN and HCO(+) emission from galaxies

view Abstract Citations (53) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Submillimeter HCN and HCO + Emission from Galaxies Jackson, James M. ; Paglione, Timothy A. D. ; Carlstrom, John E. ; Rieu, Nguyen-Q. Abstract We report the first detection of extragalactic HCN and HCO^+^ J = 4 - 3 emission at λ = 850 microns. Because their critical densities (n~10^7^ cm^-3^) and excitation energies (E_u_ ~ 45 K) are large, these lines trace dense, warm gas. We have detected HCN J = 4 - 3 toward NGC 253, NGC 4945, and IC 342 and report a sensitive upper limit toward M82. We have also detected HCO^+^ J = 4 - 3 in both NGC 253 and M82. The HCN J = 4 - 3/J = 1 - 0 intensity ratio, which is larger for higher gas densities, is at least 12 times larger in NGC 253 than that in M82. Single-component gas excitation model calculations for NGC 253 show that, averaged over the central 300 pc, the molecular gas is warm (T ~> 50 K) and dense (n ~ 5 x 10^5^ cm^-3^). For M82, the lower HCN J = 4 - 3/J = 1 - 0 line ratio requires an average gas density at least 10 times smaller. In both M82 and NGC 253, the J = 4 - 3/J = 1 - 0 line ratios peak at the nucleus. Consequently, molecular gas is densest in the central 300 pc. The submillimeter HCN and HCO^+^ lines reveal large differences in the dense gas properties, despite similar CO and far-infrared luminosities. Publication: The Astrophysical Journal Pub Date: January 1995 DOI: 10.1086/175113 Bibcode: 1995ApJ...438..695J Keywords: Interstellar Matter; Molecular Clouds; Molecular Gases; Star Formation; Starburst Galaxies; Carbon Monoxide; Gas Density; Molecular Excitation; Stellar Physics; Astrophysics; GALAXIES: ISM; ISM: MOLECULES; GALAXIES: INDIVIDUAL ALPHANUMERIC: IC 342; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 253; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 4945 full text sources ADS | data products SIMBAD (6) NED (6)

Limiting excitation energies in fusion evaporation reactions

The mass dependence of the critical excitation energy above which a hot nucleus does not decay any more by particle evaporation, has been studied in the mass range 75≦A≦130. The experimental values of the critical excitation energy have been obtained by means of a new integral method based on the analysis of the widths of the evaporation residue mass distributions as a function of the residue velocity. The obtained mass dependence appears to be stronger than expected by various models and a predicted dependence on theZ/A ratio of the decaying nucleus seems to be confirmed.