共查询到20条相似文献,搜索用时 31 毫秒
1.
We present results for the helicity amplitudes of the lowest-lying hyperon resonances Y *, computed within the framework of the Bonn Constituent-Quark model, which is based on the Bethe-Salpeter approach. The seven
parameters entering the model were fitted to the best-known baryon masses. Accordingly, the results for the helicity amplitudes
are genuine predictions. Some hyperon resonances are seen to couple more strongly to a virtual photon with finite Q 2 than to a real photon. Other Y *'s, such as the S01(1670) Λ-resonance or the S11(1620) Σ-resonance, couple very strongly to real photons. We present a qualitative argument for predicting the behaviour of
the helicity asymmetries of baryon resonances at high Q 2.-1 相似文献
2.
Abstract The simplest model for the electronic properties of small metal particles is an ideal Fermi gas confined to a finite volume. When the confining region of size L has a regular shape such as a sphere or a cube, there are two distinct scales of energy which characterize the spectrum of eigenvalues near the Fermi energy EF ≡ ? 2 k 2 f/2 m. The inner scale δ ~ EF /( kFL) 2 is the mean spacing between successive energy levels, while the outer energy scale Δ ~ EF /( kFL) describes clustering of several levels, or shell structure. Consequences for the behaviour of thermodynamic properties are investigated. There are three regimes of temperature T: normal metallic ( T > Δ), shell-metallic (δ < T < Δ) and semiconductor-like ( T < δ). Finally, if the shape of a hard-walled container is allowed to vary so as to minimize the energy, it is argued that the optimal shape fluctuates between spherical and distorted as L is changed. 相似文献
3.
An activation technique was used to investigate relative yields of fission products from the reaction 232Th(n, f ) for neutron energies between 1.3 and 1.8MeV covering the region around the first hyperdeformed resonances. Intensities
of characteristic γ-ray transitions were analyzed to search for changes in the mass distribution for neutron energies corresponding to the resonances
and below the resonances. Relative increases in yield between 7 and 23% are observed for A ≈ 100 and 132 in the resonance region around 1.6MeV. It is proposed that the yield enhancement of daughter nuclei of the
preformed fragments 132Sn and 101Zr arises from cold fission of a di-cluster configuration. The experimental results support theoretical predictions for the
existence of hyperdeformed octupole shapes based on the di-nuclear configuration 132Sn + 101Zr. 相似文献
4.
Based on electron transport theory,the performance of k x and k r filtered thermoelectric refrigerators with two resonances are studied in this paper.The performance characteristic curves between the cooling rate and the coefficient of performance are plotted by numerical calculation.It is shown that the maximum cooling rate of the thermoelectric refrigerator with two resonances increases but the maximum coefficient of performance decreases compared with those with one resonance.No matter which resonance mechanism is used(k x or k r filtered),the cooling rate and the performance coefficient of the k r filtered refrigerator are much better than those of the k x filtered one. 相似文献
5.
The 18O(p, α) 15N reaction has been investigated in the energy range Ep = 72–935 keV. The three known resonances above Ep = 620 keV have been confirmed and four new resonances have been found below Ep = 340 keV. All observed resonances correspond to known compound states in 19F. Information on resonance energies, total widths and ωγ values is reported. The low-energy resonances are superimposed on a non-resonant reaction yield, which varies smoothly with beam energy and which exhibits pronounced α-particle angular distributions asymmetric around 90°. The explanation of these data requires either interferring amplitudes of broad resonances with differing parities or a direct (p, α) reaction mechanism. The investigated energy range corresponds to the important temperature range of T = (0.05–2.5) × 10 9 K. The energy averaged astrophysical reaction rates are compared with predictions. 相似文献
6.
We study the effective actions S
(k) obtained by k iterations of a renormalization transformation of the U(1) Higgs model in d=2 or 3 spacetime dimensions. We identify a quadratic approximation S
Q
(k)
to S
(k) which we call mean field theory, and which will serve as the starting point for a convergent expansion of the Green's functions, uniformly in the lattice spacing. Here we show how the approximations S
Q
(k)
arise and how to handle gauge fixing, necessary for the analysis of the continuum limit. We also establish stability bounds on S
Q
(k)
, uniformly in k. This is an essential step toward proving the existence of a gap in the mass spectrum and exponential decay of gauge invariant correlations.Dedicated to the memory of Kurt SymanzikSupported in part by the National Science Foundation under Grant PHY 82-03669 相似文献
7.
The thermal conductivity of nitrogen is determined in a conductivity column instrument in the temperature range of 338 to 2518 K with an estimated uncertainty of about ± 1·5 per cent. The experimental data points are correlated by a cubic polynomial in temperature, viz. k( T)/(mW m -1 K -1) = 12·18 + 0·05224( T/K) - 0·6482 × 10 -6( T/K) 2 - 0·2765 × 10 -9( T/K) 3. These conductivity values determined from heat transfer data taken in the continuum regime are found to be in fair agreement with the values obtained from similar data referring to low pressure range. The present results are compared with the conductivity determinations of other workers and with the predictions of various theories developed for polyatomic gases. It is pointed out that a reliable calculation of thermal conductivity over an extended temperature range is impossible at the present time due to the absence of a large variety of experimental molecular data needed for such an effort. Average values of the vibrational energy diffusion coefficient, D vib, are computed from the present k( T) data. 相似文献
8.
The yield curve of the reaction 56Fe( p,γ) 57Co has been measured over the energy range E p =1,300–1,900 keV and the decay of nine resonances has been investigated. For twelve of the resonances the strengths have been determined. The angular distributions of the gamma rays have been recorded for resonances at E p =1,599, 1,623, 1,643 and 1,649 keV, giving spin-parity assignments J π=3/2 ? for all four resonances. The resonances at E p =1,623, 1,643 and 1,649 keV have been identified as the split analogue resonances of the 367 ( J π=3/2 ?) states in 57Fe. The M1 transition strengths to the corresponding antianalogue states have been measured and compared with theoretical predictions. 相似文献
9.
The neutron strength distributions of the three high-spin 1 k
17/2, 2 h
11/2 and 1j 13/2 states of 209Pb have been obtained within the formalism of the core-polarisation effect where the effect of interaction of the neutron
shell-model states of 209Pb with the collective vibrational states (originating also from the giant resonances) have been taken into consideration.
The theoretical results have been discussed in the light of works on 1 k
17/2, 2 h1 11/2 and 1j 13/2 neutron orbitals of 209Pb. The shell-model energies of the neutron states have been obtained by Skyrme-Hartree-Fock method 相似文献
10.
Quantum transport properties of electrons in simple magnetic-barrier (MB) structures and in finite MB superlattices are investigated
in detail. It is shown that there exists a transition of transmission resonances, i.e., from incomplete transmission resonances in simple MB structures consisting of unidentical blocks, to complete transmission
resonances in comparatively complex MB structures (, n is the number of barriers). In simple unidentical block arrangements in double- and triple-MB structures we can also obtain
complete transmission by properly adjusting parameters of the building blocks according to k y-value (k y is the wave vector in y direction). Strong suppression of the transmission and of the conductance is found in MB superlattices which are periodic
arrangements of two different blocks. The resonance splitting effect in finite MB superlattices is examined. It is confirmed
that the rule ( i.e., for n-barrier tunneling the splitting would be ( n-1)-fold) obtained in periodic electric superlattices can be extended to periodically arranged MB superlattices of identical
blocks through which electrons with tunnel, and it is no longer proper for electrons with k
y
<0 to tunnel.
Received: 18 August 1997 / Revised: 20 September 1997 / Accepted: 13 October 1997 相似文献
11.
Charm production in pp collisions is considered in the framework of perturbative QCD. The values of two parameters, the charm quark mass m
c
and the QCD scale μ 2, are determined from the comparison of the theoretical calculations with experimental data. The RHIC data on charm and beauty
production are compared with the k
T
-factorization approach predictions and with standard NLO QCD. The calculated results underestimate the STAR Collaboration
data. The role of possible nuclear effects is discussed. The predictions for LHC energies are also given. 相似文献
12.
The reactions of O‐(4‐methylphenyl) S‐(4‐nitrophenyl), O‐(4‐chlorophenyl) (4‐nitrophenyl), O‐(4‐chlorophenyl) S‐phenyl, and O‐(4‐methylphenyl) S‐phenyl dithiocarbonates ( 1 , 2 , 3 , and 4 , respectively) with a series of secondary alicyclic (SA) amines are subjected to a kinetic investigation in 44 wt% ethanol‐water, at 25.0 °C and an ionic strength of 0.2 M. The reactions are followed spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients ( kobs) are found. For some of the reactions, plots of kobs vs. free amine concentration at constant pH are linear but others are nonlinear upwards. This kinetic behavior is in accordance with a stepwise mechanism with two tetrahedral intermediates, one zwitterionic (T ±) and the other anionic (T ?). In some cases, there is a kinetically significant proton transfer from T ± to an amine to yield T ?. Values of the rate micro constants k1 (amine attack to form T ±), k?1 (its back step), k2 (nucleofuge expulsion from T ±), and k3 (proton transfer from T ± to the amine) are determined for some reactions. The Brønsted plots for k1 are linear with slopes β1 = 0.2–0.4 in accordance with the slope values found when T ± formation is the rate‐determining step. The sensitivity of log k1 and log k?1 to the p Ka of the amine, leaving and non‐leaving groups are determined by a multiparametric equation. For the reactions of 1 – 4 with 1‐formylpiperazine and those of 3 and 4 with morpholine the k2 and k3 steps are rate determining. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
13.
Absorption cross sections of argon for argon resonance radiation have been measured by several techniques. The apparent cross sections are small (0·1 to 1·6 × 10 -18 cm 2) for resonance absorption and the values depend on the technique used for measurement. These observations are interpreted in terms of extensive reversal and broadening in the source. The excitation and quenching of resonance fluorescence was studied to provide information about the rates of the processes The rate constants were estimated relative to k r, the rate constant for radiation. Radiation imprisonment leads to a reduction of k r from its natural value and observations of the decay of resonance fluorescence suggest that k r ~ 1·5 × 10 5 s -1 in our system at [Ar] = 2 × 10 17 atom cm -3. Combining this value with the relative values for the quenching rate constants gives k 1 < 1·5 × 10 -13, k 1′(M = N 2) ~ 6 × 10 -12, k 1′(M = NO) ~ 4 × 10 -10, in units of cm 3 s -1 molecule -1. 相似文献
14.
In employing the concept of the isotropy of space, the coefficients D of P-even, T-odd asymmetry in the angular distributions of prescission alpha particles emitted as third particles in the ternary fission of nonoriented target nuclei that is induced by polarized cold neutrons are expressed in the lowest orders of perturbation theory in the neutron polarization vector in terms of two P-even scalar coefficients D3 and D5 associated with, respectively, triple and quintuple correlations depending on the unit vectors kα, kLF, and σn, which determine the features of the coefficients being studied. On the basis of the above representation, the experimental values of the coefficients D3 and D5 are determined by using the experimental values of D and the angular distributions of alpha particles emitted in the analogous reaction induced by unpolarized neutrons. The resulting coefficients D3 and D5 are compared with the analogous coefficients found by means of the classical method of trajectory calculations and by means of quantum-mechanical fission theory relying on the concept that it is the rotational mechanism that is responsible for the appearance of the asymmetries being studied. It turns out that the classical method, which disregards the interference between fission amplitudes for different S-wave neutron resonances, leads to an irremovable contradiction between the calculated coefficient D3 and the analogous experimental coefficient for the 233U target nucleus. In the case of employing the quantum-mechanical approach, it is concluded that three-body calculations of the Coriolis interaction—perturbed amplitude of the angular distributions of alpha particles moving in the Coulomb fields of fission fragments are required for the 233U target nucleus. 相似文献
15.
Abstract Vinyl benzoate was polymerized by γ-radiation from a cobalt-60 source and the kinetics of polymerization were studied at several temperatures. The results showed that the rate of polymerization was proportional to I0.66 , where I is the radiation dose rate. The net activation energy for the polymerization reaction, ( Ep -1/2 Et ), was found to be 3.62 kcal, where Ep and Et are the activation energies for the propagation and termination stages of the reaction. The radical yield measured by the loss of DPPH in the solution after irradiation was G=5.0, while the G (Radical) effective in initiating polymerization was 0.94. The ratio of the rate constants, k2 p/kt = 5.8 × 10 ?4 at 60° and 1.59 × 10 ?4 at 25° The rate of polymerization was higher than that of styrene but lower than that of vinyl acetate under comparable conditions. 相似文献
16.
Materials which undergo structural phase transitions with bilinear coupling between the spontaneous strain ( e sp i ) and the thermodynamic order parameter (Q) of the type λ i e sp i Q show characteristic fluctuation pattern at T > T c. Snapshots of such pattern show tweedlike structures with butterfly shaped structure factors. Some experimental fluctuation patterns are reviewed and compared with the theoretical predictions. At T ≤ Tc , characteristic microstructures often include structural twinning. The relevant energy expressions for order/disorder systems are recalled and their contribution to thermodynamic fluctuations are described. It is anticipated that some of the physical mechanisms may play a role also in improper ferroelastics with coupling of the type λ i e sp i Q 2 k where Qk is a component of a degenerate order parameter Q = {Qk}. 相似文献
17.
Abstract Inverse secondary kinetic isotope effects are determined for the dimerization of all‐ cis‐cyclononatetraenyl radical, 1, to its corresponding dimer, all‐ cis‐9,9′‐bicyclonona‐1,3,5,7‐tetraene, 2, (step 1, k H/ k D=0.5), and cyclization of the latter to 9,9′‐bisbicyclo[4.3.0]cyclonona‐2,4,7‐triene, 3 (step 2, k H/ k D=0.75). These results are obtained by comparison of 1H‐ and 2D‐NMR spectra of 3 and employment of a simple statistical method for acquiring kinetic data. This new strategy appears superior to conventional methods in being fast, simple, and less expensive. 相似文献
18.
The resonances of atomic Lu have been investigated by photoelectron and photoion spectroscopy using monochromatized synchrotron
radiation in the vacuum ultraviolet energy region. The total photoion yield has been compared to calculations in which the
extended Fano theory (Mies formalism) and the Hartree-Fock method were applied. The resonance structure is dominated by the
spin-orbit splitting of the 5 p core hole. In the photoion yield spectra of singly and doubly charged ions a high fraction of Lu 2+ ions has been found in the region of the 5 p
-1
( 2
P
1/2
) nd resonances. Photoelectron spectra, recorded in this resonance region, have been investigated with respect to deexcitation
channels connected with Lu 2+ ions. The 5 p
-1
( 2
P
1/2
) nd resonances predominantly autoionize by spin-flip into states, which decay in the second step into Lu 2+ final ionic states.
Received: 2 September 1998 / Accepted: 17 September 1998 相似文献
19.
The collinear atom-diatom collision system provides one of the simplest instances of chaotic or irregular scattering. Classically,
irregular scattering is manifest in the sensitive dependence of post-collision variables on initial conditions, and quantally,
in the appearance of a dense spectrum of dynamical resonances. We examine the influence of kinematic factors on such dynamical
resonances in collinear (He, H
2
+
) collisions by computing the transition state spectra for collinear (He, HD +) and (He, DH +) collisions using the time-dependent quantum mechanical approach. The nearest neighbor spacing distribution P( s) and the spectral rigidity Δ 3( L) for these resonances suggest that the dynamics is predominantly irregular for collinear (He, HD +) and predominantly regular for collinear (He, DH +). These findings are reinforced by a significantly larger “correlation hole” in ensemble averaged survival probability ≪ P( t)≫ values for collinear (He, HD +) than for collinear (He,DH +). In addition we have also examined measures of classical chaos through the dependence of the final vibrational action, n
f, on the initial vibrational phase φ
i of the diatom, and Poincaré surfaces-of-section. They show that (He, HD +) collisions are partly chaotic over the entire energy range (0–2.78 eV) while (He, DH +) collisions, in contrast, are highly regular at collision energies below the classical threshold for reaction. Above the
threshold, the scattering remains regular for initial vibrational states v=0 and 1 of DH +. 相似文献
|