Showing papers on "Energy (signal processing) published in 1984"

A saddle-point solution in the Weinberg-Salam theory

Abstract: We give a close approximation to a static, but unstable, solution of the classical field equations of the Weinberg-Salam theory, where the weak mixing angle ${\ensuremath{\Theta}}_{w}$ is considered to be small. Its energy increases from \ensuremath{\sim}8 TeV to \ensuremath{\sim}14 TeV as the Higgs coupling $\ensuremath{\lambda}$ runs from $0 \mathrm{to} \ensuremath{\infty}$. Furthermore, it has a large magnetic dipole moment and its baryonic (and leptonic) charge is \textonehalf{}. The possible physical relevance of this solution is discussed.

Electron energy levels in GaAs- Ga 1 − x Al x As heterojunctions

Abstract: Calculated results for energy levels of electrons in GaAs-${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}$ As heterojunctions are presented and their sensitivity to various parameters\char22{}including acceptor doping level in the GaAs, heterojunction barrier height, effective-mass and dielectric-constant discontinuities, interface grading, and ambient temperature\char22{}is examined.

Excitations from a filled Landau level in the two-dimensional electron gas

Abstract: We consider an interacting two-dimensional electron system, with a uniform positive background, in a strong perpendicular magnetic field at zero temperature, under conditions where an integral number of Landau levels are filled and the Coulomb energy $\frac{{e}^{2}}{\ensuremath{\epsilon}{l}_{0}}$ is smaller than the cyclotron energy $\ensuremath{\hbar}{\ensuremath{\omega}}_{c}$. The elementary neutral excitations may be described alternatively as magnetoplasma modes, or as magnetic excitons---i.e., a bound state of a hole in a filled Landau level and one electron in an otherwise empty level---and they are characterized by a conserved wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$. The dispersion relations may be calculated exactly, to first order in $\frac{\frac{({e}^{2}}{\ensuremath{\epsilon}{l}_{0})}}{\ensuremath{\hbar}{\ensuremath{\omega}}_{c}}$, for the lowest magnetoplasmon band, which comes in to the cyclotron frequency at $k=0$. We also calculate the spin-wave dispersion relations for the case where one spin state of a Landau level is completely occupied, and we discuss qualitatively the exciton spectrum for a partially filled Landau level, under the conditions of the fractional quantized Hall effect.

Energy levels of Wannier excitons in G a A s − Ga 1 − x Al x As quantum-well structures

Abstract: Energy levels of Wannier excitons in a quantum-well structure consisting of a single slab of GaAs sandwiched between two semi-infinite layers of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ are calculated with the use of a variational approach. Owing to lowering of symmetry along the axis of growth of this quantum-well structure and the presence of energy-band discontinuities at the interfaces, the degeneracy of the valence band of GaAs is removed, leading to two exciton systems, namely, the heavy-hole exciton and the light-hole exciton. The values of the binding energies of the ground state and of a few lowlying excited states of these two exciton systems are calculated as a function of the size of the GaAs quantum well for several values of the heights of the potential barriers and their behavior is discussed. The results thus obtained are also compared with the available experimental data. The reliability of the various approximations made in this calculation is discussed.

A comparison of constant energy, constant temperature and constant pressure ensembles in molecular dynamics simulations of atomic liquids

Abstract: Dynamic and static properties of the LJ fluid at a density and temperature close to the triple point are determined and compared for molecular dynamics computer simulations using (N, V, E), (N, V, T), (N, P, H) and (N, P, T) ensembles. As expected, the mean values of thermodynamic properties for all the different ensembles show good agreement. For the velocity autocorrelation function and the time dependence of the mean square displacement it is shown that the constant temperature and/or constant pressure algorithms used produce results which are identical, within statistical accuracy, to those obtained using the constant energy ensemble. The equations of motion are presented in a readily implementable form.

Earthquake Energy Absorption in SDOF Structures

Abstract: The energy absorption in, and the inelastic behavior of, simple structures during strong earthquake excitation are investigated. The purposes of the investigation are to evaluate the performance of simple structures during various types of ground motion, and to attempt to identify more succinctly, than at present, the factors that influence structural deformation and damage. The study focuses on the amount of energy imparted to a structure, the amount of energy dissipated by inelastic deformations and damping, the displacement ductility, and the number of yield excursions and reversals experienced during the excitation. Based on the amount of energy imparted to structures, a possible effective motion criterion is defined.

Monte Carlo Calculation of Quantum Systems. II. Higher Order Correction

Abstract: The higher order correction term is obtained for the Monte Carlo calculation of the path integral. The correction is expressed only by a modification of the potential term: \(V(\textbf{\itshape r}_{1}, \cdots , \textbf{\itshape r}_{N}) \rightarrow V+(\hbar ^{2}/24m)(\beta /M)^{2} \sum _{i=1}^{N} (\partial V/\partial \textbf{\itshape r}_{i})^{2}\), where r i 's are coordinates of particles, N is number of particles with mass m , β is ( k T ) -1 and M is number of partitions. By this method one can reduce the computation time remarkably. A rapid convergence of energy is obtained for the case of harmonic oscillator.

Density-functional exchange-correlation potentials and orbital eigenvalues for light atoms

Abstract: Using accurate correlated wave functions calculated earlier by Bunge and by Larsson, we have constructed the Hohenberg-Kohn-Sham density functionals and exchange-correlation (ground-state) potentials and have obtained orbital energy eigenvalues for a number of light atoms by in principle exact numerical algorithms. While the uppermost occupied density-functional eigenvalue always gives an exact excitation energy as has been shown earlier, we find that eigenvalues for deeper shells lie above the corresponding excitation energy. We have compared our essentially exact density-functional (DF) results with those obtained in the local-density (LD) approximation. We find that the LD theory approximates the exchange-correlation energy rather well, but that it gives larger errors in the exchange-correlation potential and in the DF orbital eigenvalues. In all cases we have found that the LD error in the orbital eigenvalue is larger than the difference between the true DF eigenvalue and the corresponding exact excitation energy. Possible implications of these results for solid-state work are briefly discussed.

Crystal-field excitations in CeB 6 studied by Raman and neutron spectroscopy

Abstract: The ${\ensuremath{\Gamma}}_{8}\ensuremath{-}{\ensuremath{\Gamma}}_{7}$ crystalline-electric-field (CEF) transition of ${\mathrm{CeB}}_{6}$ has been identified near 530 K (46 meV, 372 ${\mathrm{cm}}^{\ensuremath{-}1}$) with use of inelastic neutron and polarized Raman scattering. From the anomalous temperature behavior of the transition energy observed in Raman scattering we deduce a ${\ensuremath{\Gamma}}_{8}$ ground state split by 20 ${\mathrm{cm}}^{\ensuremath{-}1}$ (30 K). The novel CEF level scheme yields a consistent and unified interpretation of so far seemingly unrelated thermal, elastic, and magnetic data.

Liquid-solid transition and the fractional quantum-Hall effect

Abstract: The critical Landau-level filling factor ${\ensuremath{ u}}_{c}$ for the transition from Laughlin's liquid state to a Wigner crystal is determined by comparing the energies of these states. The Wigner-crystal energy is substantially improved over the Hartree-Fock result by using a variational wave function which includes particle correlations. The liquid-state energy is obtained from the Monte Carlo calculation of Levesque, Weis, and Mac-Donald. We find ${\ensuremath{ u}}_{c}$ to be slightly larger than ? which is consistent with the experimental observation by Mendez and co-workers that the fractional quantum-Hall effect does not occur for $\ensuremath{ u}l~\frac{1}{7}$. The improvement in the crystal energy by correlation is essential to this agreement since without correlations, ${\ensuremath{ u}}_{c}\ensuremath{\sim}\frac{1}{10}$. In addition the crystal correlation energy explains the very low temperatures required to see the $\ensuremath{ u}=\frac{1}{5}$ liquid state.

Dielectric properties of TiC x , TiN x , VC x , and VN x from 1.5 to 40 eV determined by electron-energy-loss spectroscopy

Abstract: The optical properties of ${\mathrm{TiC}}_{x}$, ${\mathrm{TiN}}_{x}$, ${\mathrm{VC}}_{x}$, and ${\mathrm{VN}}_{x}$ in the energy range 1.5 to 40 eV were determined with the use of electron-energy-loss spectroscopy. The optical joint density of states deduced from a Kramers-Kronig analysis were compared with joint densities of states calculated from self-consistent Gaussian-LCAO band-structure data. Good agreement between theory and experiment was obtained. Characteristic differences among the spectra of the particular compounds are discussed in terms of different electronic properties deduced from band-structure calculations. The influence of nonmetal vacancies on the optical properties was investigated.

Classification and supermultiplet structure of doubly excited states

Abstract: Based upon the analysis of electron correlations in hyperspherical coordinates for arbitrary $L$, $S$, and $\ensuremath{\pi}$ states, a classification scheme for all doubly excited states of two-electron atoms is presented. A new set of internal correlation quantum numbers, $K$, $T$, and $A$, are introduced. Here ($K$,$T$) describe angular correlations and $A=\ifmmode\pm\else\textpm\fi{}1,0$ describes radial correlations. These quantum numbers are used to label the first-order wave functions which are approximated as $\ensuremath{\Psi}={F}_{\ensuremath{\mu}}^{n}(R){\ensuremath{\Phi}}_{\ensuremath{\mu}}(R;\ensuremath{\Omega})$ in terms of hyperspherical coordinates. The channel index $\ensuremath{\mu}$ is $\ensuremath{\mu}\ensuremath{\equiv}|{(K,T)}_{N}^{A}^{2S+1}L^{\ensuremath{\pi}}〉$, where $N$ is the dissociation limit of the channel. Rules for constructing the correlation diagram for channel potential ${U}_{\ensuremath{\mu}}(R)$ and the labeling of each channel are discussed. By comparing the rotation-averaged surface charge densities, it is shown that channels which have identical ${(K,T)}_{N}^{+,\ensuremath{-}}$ have isomorphic correlation patterns, irrespective of the overall $L$, $S$, and $\ensuremath{\pi}$. Such isomorphism is shown to be the underlying origin of the supermultiplet structure of intrashell doubly excited states. In fact, it is shown that such supermultiplet structures actually extend to all states which have $A=+1 or \ensuremath{-}1$. A new Grotrian diagram for energy levels grouped according to ${(K,T)}^{+}$ and ${(K,T)}^{\ensuremath{-}}$ displays rotor-like structure. Such diagrams can easily reveal missing or misclassified levels. It is also shown that all $A=0$ states are similar to singly excited states where for a given ${(K,T)}_{N}^{0}$, the triplet state always has a slightly lower energy than the singlet state. Approximate selection rules for photoabsorption and for $e$-H and $e$-${\mathrm{He}}^{+}$ scatterings are discussed.

Exchange and correlation potentials for electron-ion systems at finite temperatures

Abstract: We have calculated the exchange and correlation corrections to the free energy (${F}_{\mathrm{xc}}$) and the chemical potential (${\ensuremath{\mu}}_{\mathrm{xc}}$) of an electron system at arbitrary degeneracies and temperatures. These are needed for density-functional, average-atom-type calculations of properties of plasmas and liquid metals. The problem of inverting the parametric relation between ${\ensuremath{\mu}}_{\mathrm{xc}}$ and the density correction ${n}_{\mathrm{xc}}$ is considered. We present easy-to-use analytic fits to ${F}_{\mathrm{xc}}$ and ${\ensuremath{\mu}}_{\mathrm{xc}}$ as functions of the temperature and density. We also consider the effect of a lineraly responding ion background on the electron chemical potential as well as the ion chemical potential, as these are needed in calculations of ionelectron systems at arbitrary degeneracies and densities.

Coupling-Constant Metamorphosis and Duality between Integrable Hamiltonian Systems

Abstract: We introduce a noncanonical ("new-time") transformation which exchanges the roles of a coupling constant and the energy in Hamiltonian systems while preserving integrability. In this way we can construct new integrable systems and, for example, explain the observed duality between the H\'enon-Heiles and Holt models. It is shown that the transformation can sometimes connect weak- and full-Painlev\'e Hamiltonians. We also discuss quantum integrability and find the origin of the deformation $\ensuremath{-}\frac{5}{72}{\ensuremath{\hbar}}^{2}{x}^{\ensuremath{-}2}$.

Measurement of Cell Size in Hydrocarbon-air Mixtures and Predictions of Critical Tube Diameter, Critical Initiation Energy and Detonability Limits

Abstract: Experimental measurements of the detonation cell s ize in mixtures of Hg, C 2 H 2 > ^4, ^5, 38> d ^4^10 with air over a range of fuel concentrations have been carried out in three cylindrical tubes of diameters 5, 15, and 30 cm. The cell s ize has been determined from the signatures on smoked aluminum foi ls placed inside the tube as wel l as from the frequency of the pressure f luctuations recorded by piezoelectric transducers. Based on the cell size data obtained, estimates of the critical tube diameter using the empirical law of Soloukhin and Mitrofanov (dc 1 3 X ) have been found to be in agreement wi th experimental data from direct measurement of the critical tube i tself. Hence, the important empirical law dc 1 3X is thus verif ied. Estimates of the critical charge weight from the cell s ize data using the surface energy theory proposed by Lee have been found to agree reasonably wel l with the experimental resul ts of Elsworth. Based on the criteria for stable propagation in tubes (d* A / T T ) and in two-dimensional channels (W* 3 X ) , detonability limits can also be predicted from a knowledge of the cell s ize X. Based on W e s t b r o o k ' s kinetic calcu lat ions, it is found that the cell s ize data are directly proportional to the induction time of the oxidat ion process which confirms qual i tat ively Shche l k i n ' s model. However, on a quantitat ive bas is , Shche l k i n ' s model predicts cell s ize an order-of-magnitude larger than the present experimental data. Presented at the 9th ICODERS, Poit iers, France, July 3-8, 1983. Copyright 1984 by the American Institute of Aeronautics and Astronaut ics, Inc. All rights reserved. ^Associate Professor, Department of Mechanical Engineering. ^-Senior Research Scient ist , Department of Mechanical Engineering. ^Professor, Department of Mechanical Engineering. §Undergraduate Student, Department of Mechanical Engineering.

Temperature Dependence of the Exchange Splitting of Fe by Spin-Resolved Photoemission Spectroscopy with Synchrotron Radiation

Abstract: Temperature-induced changes in the electronic structure of Fe(100) have been investigated by spin- and angle-resolved photoemission for temperatures between room temperature and the Curie temperature ${T}_{\mathrm{C}}$. States nearly stationary in energy (${\ensuremath{\Gamma}}_{25}^{\ensuremath{'}\ensuremath{\uparrow}}$, ${\ensuremath{\Gamma}}_{12}^{\ensuremath{\uparrow}}$) have been observed for photon energy $h\ensuremath{ u}=60$ eV. However, from a strong increase in minority-spin intensity for $h\ensuremath{ u}=31 \mathrm{and} 21$ eV, a downwards shift of the ${\ensuremath{\Delta}}_{5}^{\ensuremath{\downarrow}}$ band is inferred to occur upon heating towards ${T}_{\mathrm{C}}$ for large $k$ vectors.

Observation of a surface magnetic phase transition on Cr(100)

Abstract: We have observed a surface magnetic phase transition on Cr(100) using angle-resolved photoelectron spectroscopy. The temperature dependence of a surface resonance indicates a transition temperature of 780\ifmmode\pm\else\textpm\fi{}50 K. This is consistent with theoretical predictions of a ferromagnetic Cr(100) surface. The room-temperature ferromagnetic surface exchange potential is estimated to be 0.8\ifmmode\pm\else\textpm\fi{}0.1 eV, with an associated surface magnetic moment of (2.4\ifmmode\pm\else\textpm\fi{}0.3)${\ensuremath{\mu}}_{B}$.

Exponential Band Tails in Random Systems

Abstract: We present a simple derivation of the exponential band tails universally observed in three-dimensional disordered materials. The physical picture employed is that of Halperin and Lax in which states are localized by long-wavelength potential fluctuations. When the effect of the small-scale fluctuations of the potential are included as well, via the scaling arguments of Thouless, there results an exponential dependence of the density of states on energy below an energy ${E}_{1}$. The magnitudes and dependences of ${E}_{1}$ and the width of the tail on disorder agree with the experiment.

Electronic Structure of Small Iron Clusters

Abstract: We report calculations of energy levels and the charge and spin densities for small free (not embedded) clusters of iron atoms. Our method uses spin-density-functional theory in the local approximation. Single-particle functions are expanded in a basis set of symmetrized linear combinations of Gaussian orbitals. Matrix elements of the electrostatic potential are computed with the aid of a fit to the electron density, again with the use of symmetrized combinations of Gaussian orbitals. The matrix elements of the exchange-correlation potential are evaluated by direct numerical integration using a grid developed for this purpose. The clusters considered are ${\mathrm{Fe}}_{7}$, ${\mathrm{Fe}}_{9}$, and ${\mathrm{Fe}}_{15}$. Our results are compared with those obtained by other calculational procedures. The ionization potential of the ${\mathrm{Fe}}_{9}$ cluster is determined by a transition-state calculation and is compared with experiment.

Pulsed-laser-stimulated field ion emission from metal and semiconductor surfaces: A time-of-flight study of the formation of atomic, molecular, and cluster ions

Abstract: In field ion emission, ions are formed beyond a critical distance from the emitter surface. This distance is imposed by the condition that the energy level of the tunneling atomic electron must line up with a vacant electronic state in the solid. As a result the field-emitted ions exhibit a critical energy deficit in the energy distribution. The critical energy deficit is related to the binding energy and the ionization energy of the emitted atoms, and the work function or the electron affinity of the emitter surface. A time-of-flight measurement of the ion energy distribution and the critical energy deficit of various types of ions from both metal and semiconductor surfaces has been made using the pulsed-laser atom probe. From the result mechanisms of the formation of various types of ions in pulsed-laser-stimulated field ion emission, including novel molecular and cluster ions such as ${\mathrm{D}}_{3}^{+}$, ${\mathrm{N}}_{2}$${\mathrm{H}}^{+}$, ${\mathrm{ArH}}^{+}$, ${\mathrm{RhHe}}^{2+}$, ${\mathrm{PtHe}}^{2+}$, ${\mathrm{PtHe}}_{2}^{2+}$,${\mathrm{Si}}_{1\mathrm{t}\mathrm{o}\phantom{\rule{0ex}{0ex}}11}^{2+}$, ${\mathrm{Si}}_{1\phantom{\rule{0ex}{0ex}}\mathrm{t}\mathrm{o}\phantom{\rule{0ex}{0ex}}6}^{+}$, etc., are investigated. It is found that photoexcitation plays an important role in pulsed-laser field evaporation of silicon. Pulsed-laser field evaporation of Si can be sustained almost indefinitely by a field-gradient- and temperature-pulse-induced surface diffusion of Si atoms from the emitter shank. The most abundant Si-cluster ions are ${\mathrm{Si}}_{4}^{2+}$, ${\mathrm{Si}}_{5}^{2+}$, and ${\mathrm{Si}}_{6}^{2+}$. These clusters and ${\mathrm{Si}}_{13}^{4+}$ are the only small, symmetrically structured atomic units one can remove from a Si crystal. The numbers 4, 5, 6, and 13 are the magic numbers in Si-cluster formation. The critical number is found to be 3 for 2+ ions. The higher ionization energies of heavy metal atoms, data of which are not readily available, can also be derived from a measurement of the critical energy deficit of field-evaporated ions.

Nonadiabatic Transitions in Level Crossing with Energy Fluctuation.I.Analytical Investigations

Abstract: A simple stochastic model is proposed describing the nonadiabatic transitions in level crossing with energy fluctuation The model is an extension of Zener's model having a diagonal energy term fluctuating as a Markoffian Gaussian process The transition rate P ∞ defined in terms of the diabatic basis is calculated exactly by the formal perturbation expansion with respect to the off-diagonal coupling in the two limiting cases: In the slow fluctuation limit, P ∞ coincides with the Landau-Zener formula, \(P_{\infty}=P_{\text{LZ}} \equiv 1 - \exp (-2 \pi J^{2}/\hbar |v|)\), where J is the off-diagonal coupling constant and v is the velocity of the change of the mean energy difference In the strong damping limit, which is a limiting case of large fluctuation amplitude in the rapid fluctuation limit, P ∞ is given by the formula, \(P_{\infty}=P_{\text{SD}} \equiv \{1 - \exp (-4 \pi J^{2}/\hbar |v|)\}/2\)

Cosmic-ray muon spectrum up to 20 TeV at 89°zenith angle

Abstract: The 800-ton cosmic-ray spectrograph (MUTRON) has been used to measure the sea-level energy spectrum of cosmic-ray muons arriving from 86\ifmmode^\circ\else\textdegree\fi{} to 90\ifmmode^\circ\else\textdegree\fi{} zenith angles in the momentum region of 100-20 000 GeV/c. The measured muon energy spectrum can be interpreted by using a cosmic-ray primary spectrum of (1.80 ${\mathrm{cm}}^{\ensuremath{-}2}$${\mathrm{s}}^{\ensuremath{-}1}$${\mathrm{sr}}^{\ensuremath{-}1}$${\mathrm{GeV}}^{\ensuremath{-}1}$)${E}^{\ensuremath{-}2.70}\mathrm{dE}$ ($E$ in GeV) and a scaling model incorporating an increasing interaction cross section for meson production in hadron-hadron interaction. The muon charge ratio at energies up to 15 TeV in the same zenith-angle range has been measured. It shows a small enhancement with increasing energy. By combining both results we may conclude that the cosmic-ray primary particle composition stays the same up to about 100 TeV as that obtained by direct measurements in the energy range below 1 TeV.

Coupled-cluster approach to electron correlation in one dimension. II. Cyclic polyene model in localized basis

Abstract: The many-electron correlation problem for one-dimensional metalliclike systems with Born--von K\'arm\'an boundary conditions, represented by the Pariser-Parr-Pople and the Hubbard Hamiltonian cyclic polyene models, ${\mathrm{C}}_{\mathrm{N}}$${\mathrm{H}}_{\mathrm{N}}$, N=2n=4\ensuremath{ u}+2, \ensuremath{ u}=1,2,..., is studied using the coupled-cluster approach in the localized Wannier basis representation. Various truncation schemes for the pair clusters are examined. It is shown that already the intracell pair-cluster approximation, which can be handled analytically and yields the same expression for the correlation energy as the variational approach of Ukrainskii, provides a reasonable approximation in the entire range of the coupling constant. Using all doubly excited clusters composed of locally excited particle-hole pairs, one obtains the exact correlation energy in the fully correlated limit assuming that the coupled-pair equations are corrected for the connected quadruply excited cluster contributions. This is achieved by using the recently developed approximate coupled-pair approach with corrections for the quadruply excited clusters (ACPQ), which is almost identical, except for a numerical factor of certain diagrams, with the standard approximate coupled-pair approach. The ACPQ approach removes the singularities which otherwise plague the standard coupled-pair approach and yields very good correlation energies in the entire range of the coupling constant even when the ${n}^{3}$ doubly excited pair clusters are truncated to only n+10 locally and quasilocally excited pair clusters in the localized Wannier basis representation.

Fragment velocities, energies, and masses from fast neutron induced fission of U 235

Abstract: We report on a complete ($2E,2v$) experiment for fast neutron induced fission on $^{235}\mathrm{U}$. The energy dependence of fragment properties so far known only for thermal neutron induced fission is studied. Experimental problems as well as difficulties in data analysis are considered in detail in order to obtain clean and unbiased results. In particular, a self-consistent determination of the fragment total kinetic energy (${E}_{K,\mathrm{tot}}$) was achieved by comparing the results obtained via the respective velocities and pulse heights. We find systematic discrepancies of 2 MeV if ${E}_{K,\mathrm{tot}}$ is determined from the observed pulse heights with the calibration scheme of Schmitt et al. Therefore, refined calibration constants were deduced by comparison with accurate radiochemical mass yields. Measurements were performed at neutron energies of 0.50 and 5.55 MeV. Our results include mean values of fragment properties before and after neutron evaporation, e.g., of fragment velocities and masses, total kinetic energies, and the respective variances. We also show the distributions of fragment mass, of ${E}_{K,\mathrm{tot}}$, and of the variance of ${E}_{K,\mathrm{tot}}$. In addition, the number of prompt fission neutrons $\ensuremath{ u}$ is given as a function of fragment mass. Our resolution of 2.1 mass units reveals fine structure not only in the fragment mass distribution but also in ${E}_{K,\mathrm{tot}}({A}^{*})$ and $\ensuremath{ u}({A}^{*})$. For the lower neutron energy of 0.50 MeV the present results compare reasonably well with similar measurements performed with thermal neutrons. Apparently the 0.5 MeV increase in saddle point excitation does not alter the results significantly. The improved accuracy of this measurement is demonstrated by comparison of our neutron emission data with direct measurements of fission neutrons. At the higher neutron energy of 5.55 MeV we observe the expected decrease of shell and pairing effects which indicates an increase in nuclear temperature. These results are in qualitative agreement with the model of Wilkins, Chasman, and Steinberg. However, a striking discrepancy exists for the number of fission neutrons, where we find that the increase in the total number of fission neutrons is totally accounted for by heavy fragments alone.

Dynamic defect reactions induced by multiphonon nonradiative recombination of injected carriers at deep levels in semiconductors

Abstract: Violent lattice vibrations, induced by nonradiative capture of a free carrier by a deep-level defect in semiconductors, enhance greatly defect reactions such as movement of the defect itself or production of a new one, through reduction of the thermal activation energy (TAE). A theory of this phenomenon is presented. When capture takes place at a critical value ${\ensuremath{\Delta}}_{P}$ of a configuration coordinate ${Q}_{P}$, the total energy of the induced vibrations is larger than ${E}_{P}$ of the minimum lattice energy obtained under ${Q}_{P}={\ensuremath{\Delta}}_{P}$. A defect reaction with TAE of ${E}_{A}$ in thermal equilibrium takes place when another configuration coordinate ${Q}_{R}$ exceeds a critical value ${\ensuremath{\Delta}}_{R}$. Both ${Q}_{P}$ and ${Q}_{R}$ are a linear combination of many normal-mode coordinates in general. Energy flow from ${Q}_{P}$ to ${Q}_{R}$ occurs through the direction cosine $g$ between them in the phonon space, and $g$ is nonvanishing when there exist normal-mode components common between them. Under the condition that ${Q}_{P}$ started from ${\ensuremath{\Delta}}_{P}$ at time zero while ${Q}_{R}$ reaches ${\ensuremath{\Delta}}_{R}$ thereafter, we determine the minimum lattice energy written as ${E}_{P}+{E}_{H}$. Energy ${E}_{H}$ is smaller than ${E}_{A}$ when $g\ensuremath{ e}0$ and gives the TAE of the quantum yield of the defect reaction occurring subsequently after carrier capture. We find that ${E}_{H}={E}_{A}\ensuremath{-}{E}_{P}$ for ${E}_{P}l{g}^{2}{E}_{A}$, ${E}_{H}=\frac{{[{({E}_{A})}^{\frac{1}{2}}\ensuremath{-}|g|{({E}_{P})}^{\frac{1}{2}}]}^{2}}{(1\ensuremath{-}{g}^{2})}$ for ${g}^{2}{E}_{A}l{E}_{P}l\frac{{E}_{A}}{{g}^{2}}$, and ${E}_{H}=0$ for ${E}_{P}g\frac{{E}_{A}}{{g}^{2}}$. The TAE of the defect reaction observed is given by ${E}_{H}$ plus the TAE of carrier capture, which is shown to explain experimental data quite well.

Continuum x rays produced by light-ion--atom collisions

Abstract: Radiative processes in light-ion collisions are all-inclusively formulated and discussed. Production cross sections of the radiations are calculated in terms of the second Born approximation. The cross sections of atomic bremsstrahlung production and radiative ionization have been estimated and compared with experimental results of continuum x rays from an A1 target bombarded with few-MeV protons. It is found that atomic bremsstrahlung is the most predominant component in the region of x-ray energy $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\ge}{T}_{m}$, where ${T}_{m}$ is the maximum energy transferred from a projectile to a free electron at rest, and the disagreement between the theory and experiment, which had previously been seen in this energy region, was clearly resolved.