342 research outputs found
First and second order optimality conditions for optimal control problems of state constrained integral equations
This paper deals with optimal control problems of integral equations, with
initial-final and running state constraints. The order of a running state
constraint is defined in the setting of integral dynamics, and we work here
with constraints of arbitrary high orders. First and second-order necessary
conditions of optimality are obtained, as well as second-order sufficient
conditions
Polarized triplet production by circularly polarized photons
A process of the pair production by a circularly polarized photon in the
field of unpolarized atomic electron has been considered in the
Weizaecker-Williams approximation. The degree of longitudinal polarization of
positron and electron has been calculated. An exclusive cross-section as well
as a spectral distribution are obtained. We estimate the accuracy of our
calculations at the level of a few percent. We show the identity of the
positron polarization for considered process and for process of pair production
in the screened Coulomb field of nucleus.Comment: 9 pages, 3 picture
Cross-relaxation and phonon bottleneck effects on magnetization dynamics in LiYF4:Ho3+
Frequency and dc magnetic field dependences of dynamic susceptibility in
diluted paramagnets LiYF:Ho have been measured at liquid helium
temperatures in the ac and dc magnetic fields parallel to the symmetry axis of
a tetragonal crystal lattice. Experimental data are analyzed in the framework
of microscopic theory of relaxation rates in the manifold of 24
electron-nuclear sublevels of the lowest non-Kramers doublet and the first
excited singlet in the Ho ground multiplet split by the crystal
field of S symmetry. The one-phonon transition probabilities were computed
using electron-phonon coupling constants calculated in the framework of
exchange charge model and were checked by optical piezospectroscopic
measurements. The specific features observed in field dependences of the in-
and out-of-phase susceptibilities (humps and dips, respectively) at the
crossings (anti-crossings) of the electron-nuclear sublevels are well
reproduced by simulations when the phonon bottleneck effect and the cross-spin
relaxation are taken into account
Dissymmetrization of crystals: Theory and experiment
The distribution of Cu2+ impurities in ZnSeO4·6H2O and Na2Cd(SO4)2·2H2O crystals was investigated by electron paramagnetic resonance. The tangential selectivity by sectors in pyramid growth was revealed. Dissymmetrization, i.e., decrease of point group symmetry, is explained by tangential selectivity occupying impurities and formation defects. An analysis by group theory is presented to explain dissymmetrization of crystals. The connection between the distribution of point defects in a crystal and the growth process is discussed. © 1980 Springer-Verlag
Possible Method for Measuring the Proton Form Factors in Processes with and without Proton Spin Flip
The ratio of the squares of the electric and magnetic proton form factors is
shown to be proportional to the ratio of the cross sections for the elastic
scattering of an unpolarized electron on a partially polarized proton with and
without proton spin flip. The initial proton at rest should be polarized along
the direction of the motion of the final proton. Similar results are valid for
both radiative scattering and the photoproduction of pairs on a proton in
the Bethe--Heitler kinematics. When the initial proton is fully polarized in
the direction of the motion of the final proton, the cross section for the process, as well as for the and processes, without (with) proton spin flip is expressed only in terms of
the square of the electric (magnetic) proton form factor. Such an experiment on
the measurement of the cross sections without and with proton spin flip would
make it possible to acquire new independent data on the behavior of
and , which are necessary for resolving the
contradictions appearing after the experiment of the JLab collaboration on the
measurement of the proton form factors with the method of polarization transfer
from the initial electron to the final proton.Comment: 7 pages, revtex
Electron paramagnetic study of fe3+ and gd3+ in na2zn(S04)24h20
The epr of Fe3- and Gd3- in single crystals of Na2Zn(S04)24H20 was studied at liquid nitrogen temperature using a Q band spectrometer. For each ion, two spectra superimposed in the (010) plane and along b were observed. The spectra were described by the most general spin Hamiltonian, the constants of which were obtained for both ions. Analysing the pseudo-symmetry of the fourth-order term of the spin Hamiltonian, it is found that Fe3- only modifies the orientation of the local structure slightly and that the charge defect is probably balanced by a Na- vacancy. It is shown that the local structure around Gd3- is very different from the one around Fe3-. © 1986 The Institute of Physics
One-dimensional Model of a Gamma Klystron
A new scheme for amplification of coherent gamma rays is proposed. The key
elements are crystalline undulators - single crystals with periodically bent
crystallographic planes exposed to a high energy beam of charged particles
undergoing channeling inside the crystals. The scheme consists of two such
crystals separated by a vacuum gap. The beam passes the crystals successively.
The particles perform undulator motion inside the crystals following the
periodic shape of the crystallographic planes. Gamma rays passing the crystals
parallel to the beam get amplified due to interaction with the particles inside
the crystals. The term `gamma klystron' is proposed for the scheme because its
operational principles are similar to those of the optical klystron. A more
simple one-crystal scheme is considered as well for the sake of comparison. It
is shown that the gamma ray amplification in the klystron scheme can be reached
at considerably lower particle densities than in the one-crystal scheme,
provided that the gap between the crystals is sufficiently large.Comment: RevTeX4, 22 pages, 4 figure
Local environment of Gd3+ in MgF2
The electron paramagnetic resonance (EPR) of Gd3+ in MgF2 reveals that Gd3+ has two different environments in the lattice. One of them has D2h symmetry, the EPR spectrum is characterized by a large zero field splitting [B2 0= 968.10-4 cm-1; B2 2= 357.10-4 cm-1] and the fourth order term of the spin Hamiltonian is axial; it is assumed that one Gd3+ substitutes two Mg2+. The other Gd3+ center has only monoclinic symmetry. From the analysis of the fourth order term of the spin Hamiltonian of the corresponding spectrum it is seen that one Gd3+ substitutes one Mg2+ and that this substitution produces a large local distortion of the lattice. © 1990 Springer-Verlag
EPR of Gd3+ in Na2Cd(SO4) 22H2O, comparison with previous results obtained for Fe3+
An EPR experiment on Gd3+ in a single crystal of Na 2Cd(SO4)22H2O (CdK) was carried out at Q band frequency and nitrogen temperature. Two spectra related by symmetry were observed. All spin Hamiltonian constants have been calculated. The pseudosymmetries of the fourth-order term of the spin Hamiltonian are compared to those of Fe3+. It is observed that the substitution of Gd 3+ for Cd2+ induced a very important local distortion of the host lattice, which is discussed. © 1985 American Institute of Physics
Study of the environment of Fe3+ in Na2Cd(SO 4)2 2H2O by EPR
An EPR experiment on Fe3+ in a single crystal of Na 2Cd(SO4)2 2H2O was carried out at Q band frequency and nitrogen temperature. The angular dependencies of the spectra were studied in the three crystallographic planes. In order to have a very precise orientation of the crystal, the experiment was done with a twinned crystal (superposition of two spectra in all three planes). The zero-field splitting of Fe3+ is of the same order of magnitude as the quantum hv, so that many ΔM=2 transitions were observed. All spin-Hamiltonian parameters (20) were calculated using observed ΔM=1 and ΔM=2 transitions for 30 different orientations of the magnetic field. The pseudosymmetries of the fourth-order term of the spin Hamiltonian of Fe 3+ are compared to those we calculated using the superposition model. It is seen that Fe3+ is substituted for Cd2+ and that the orientation of the ocatahedral environment of Fe3+ does not differ much in orientation from that of Cd2+. Differences between the distortions of the Cd-O6 and Fe-O6 octahedra are discussed. © 1985 American Institute of Physics
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