Application of positron annihilation as a nondestructive method of checking the quality of materials

Methodological features of an investigation of the defect structure of technically important materials by using positron annihilation are discussed. It is shown that the important annihilation parameters characterizing the positron-sensitive defects are the mean positron lifetime , the width of the annihilation photon correlation curves (APCC) at half the height and the APCC form parameter f, and the linear Doppler broadening parameter of the annihilation -line S.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 40–43, May, 1982.     

Positron annihilation in free volume elements of polymer structures

Free volume effects are important for positronium formation, pickoff annihilation and chemical reactions. In order to make the method of positron annihilation an effective technique for quantitative studies of molecular solids, for example polymers, the relation between the intensity of the longlived component of lifetime distribution of positron annihilation and the number of defects (elementary free volumes) was considered. The analysis assumes selective trapping of positrons and positronium in the defects of ordered and disordered (crystalline and amorphous) sites, respectively, the sizes of nonhomogeneities are assumed to be lower than positron diffusion length. The results on positronium annihilation in porous poly(phenylene oxide) allow one to estimate nontrapped positronium diffusion coefficient which is equal to . The relation between the positronium lifetime and effective size of free volume for large pores (effective radius 1 nm) is considered. Experimental results were obtained using the CONTIN program; some comments on its application for calculations of size distribution of elementary free volumes in polymers are discussed.     

Kinetics of positron annihilation in ionic crystals

A kinetic scheme of decay of positron and positronium states for the general case — a real ionic crystal with point lattice defects and a developed surface — is examined. The properties of positron and positronium states in the volume of the crystal, positron and positronium color centers, and surface positron and positronium states, are analyzed. It is shown that all the available experimental data qualitatively confirm the conclusion of an annihilation mechanism based on the postulated kinetic scheme of positron annihilation in real ionic crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 50–55, September, 1977.     

Compact efficient intracavity optical parametric oscillator with a passively Q-switched Nd : YVO<Subscript>4</Subscript>/Cr<Superscript>4+</Superscript> : YAG laser in a hemispherical cavity

A compact eye-safe optical parametric oscillator (OPO) using a noncritically phase-matched KTP crystal intracavity pumped by a passively Q-switched Nd:YVO₄ laser is experimentally demonstrated. To enhance the performance of passive Q-switching, a Cr⁴⁺:YAG saturable absorber crystal is coated as an OPO output coupler in a nearly hemispherical cavity. With an incident pump power of 2.5 W, the compact intracavity OPO cavity, operating at 62.5 kHz, produces average powers at 1573 nm up to 255 mW and peak powers higher than 1 kW. PACS 42.60.Gd; 42.65.Yj; 42.55.Xi     

X-ray “tails” on powder patterns of annealed Au-Pt and Fe-Ni alloys

The paper describes tails and side-bands observed on the diffraction patterns of samples after diffusion heating. The origin of tails is explained by the presence of vacancy clusters. Side-bands on X-ray patterns of an Au-Pt sample are interpreted in terms of a pre-precipitate structure or of a metastable phase in the region rich in gold.Thanks for valuable advice are due to RNDr. M. ernohorský CSc., whose suggestions gave rise to this paper.     

Experimental determination of the thermo-optic coefficient (dn /dT ) and the effective stimulated emission cross-section (σ e ) of an a -axis cut 1.-at. % doped Nd:GdVO4 crystal at 1.06 μm wavelength

In this paper, we report the experimental determination of the thermo-optical coefficient (dn/dT) and the effective stimulated emission cross-section (_e) at 1064 nm for an a-axis cut 1.3-at.% doped Nd:GdVO₄ crystal in a monolithic laser configuration. The pump-power-induced thermal lensing effect in the monolithic laser was used to determine these parameters. While measuring the dn/dT parameter, we also took into consideration the effect of pump-power-induced thermal expansion of the crystal. The dn/dT values obtained were 2.64-6 and 4.87×10^-6 K^-1, respectively, for directions parallel and perpendicular to the c-axis of the crystal. We show that the neglect of pump-power-induced thermal expansion of the crystal can overestimate the value of dn/dT by 30–50%. With the measured variation of the focal length of the thermal lens as a function of the absorbed pump power, we also computed the overlap integrals at the threshold pump power. These overlap integral values were used to estimate the product of the effective stimulated emission cross-section (_e) and the excited state lifetime () of the Nd:GdVO₄ crystal, which was found to be 1.476×10^-22 cm²s. With the reported values of for a 1.3-at.% doped Nd:GdVO₄ crystal, we estimated the value of _e to be in the range 14.76×10^-19 to 16.4×10^-19 cm². The value of the effective stimulated emission cross-section measured in this way was found to be around two times higher in magnitude than earlier reported values measured by spectroscopic methods. PACS 42.60.Lh; 42.60.Da; 42.60.By; 42.55.Rz     

Phase transformations in Cu-Sn alloys containing Ni,Al, Ge,and In

After quenching from 700C and aging, Cu-Sn alloys containing Ni and Al were found to have a precipitation of-solid solution, with a DO₃ type of ordering, of metastable ,, and phases. It was established that the phase has an 4 type of ordering while the phase has the Cu₃Ti type. Observations revealed that Cu-Sn-Al alloys contained two phases, and, with an identical structure but with different ratios of electron concentration per atom.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 34–37, April, 1981.     

Paramagnetic shift of μ+ in MnO and its time dependence

Conclusion The paramagnetic shift K of ⁺ in MnO is measured from 140K to 420K. K is negative at 14OK, increases rapidly with increasing temperature, but does not show a linear relation to the susceptibility . At T = 170K and 230K, the value of K depends greatly on the fitting time-range of the spectrum. These features are explained by a site change of ⁺ caused by its diffusion and trapping in the crystal.     

Formation of F-centers in Kcl-Rbcl solid solutions with Sr2+ and OH− impurities

The kinetics of F-center formation are studied in crystals of KCl-RbCl. solid solutions, both in the pure state and with Sr²⁺ and OH^– impurities. The crystals were irradiated by electrons at 1.2 MeV at room temperature. The initial stage of coloration in crystals that have been purified of divalent cation impurities is the same as that of pure crystals; only when Sr²⁺ is introduced is this stage sharply accelerated. The efficiency of F-center formation in the second stage of coloration is significantly smaller in purified crystals than in pure crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 7, pp. 38–41, July, 1971.     

Three-Mode Entangled State Representation of Continuum Variables and Optical Four-Wave Mixing

We establish a new three-mode entangled state representation , of continuum variables, which make up a complete set. Using optical four-wave mixing and a beam splitter transform we can prepare , . Based on , a new number-difference--operational-phase uncertainty relation is established and the corresponding squeezing dynamics is discussed.     

Doppler-broadening measurements of microvoids at the Au/GaAs interface

Recent positron lifetime studies made on the Au/GaAs interface with an applied electric field returning a significant fraction of bulk implanted positrons to the interface have revealed the presence of microvoids( 1 nm diameter) at the interface. In this work an attempt has been made to study these microvoids by observing the Doppler broadening on the annihilation radiation coming from them. This is done both by observing theS-parameter as a function of applied bias and by applying the generalized least-squares method to the deconvolution of the annihilation radiation lineshape. The general conclusion is that the Doppler-broadened data are consistent with the majority of positrons trapping into microvoids, probably associated with grain boundaries. The data suggest that these open volume defects are more associated with the Au film rather than the Au-Ga alloyed interfacial region.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

The theory of the spinning impact of imperfectly elastic bodies

The first theoretical solution is given of the spinning impact of an imperfectly elastic sphere on a rigid, imperfectly rough plane. The method of solution is based on the general static theory of the impact of rough bodies, elaborated by one of the authors in 1952 [5]. The problem leads to a non-linear differential equation, which can be solved only by an approximate numerical method. The results of the theory are in good agreement with the experiments carried out in 1947 [4].

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In conclusion the authors thank Dr. Vaclav Goldbach, lecturer at our department, for valuable help in making the experimental apparatus and in carrying out the difficult measurements by the method described in section 3 of this paper. Our thanks also go to O. Brha for help in the graphical solution and drawing of the figures.     

Spectral properties of random Schrödinger operators with unbounded potentials

We investigate spectral properties of random Schrödinger operators H = - + _n()(1 + |n|) acting onl ²(Z ^d), where _n are independent random variables uniformly distributed on [0, 1].Research partially supported by a Sloan Doctoral Dissertation Fellowship and NSERC under grant OGP-0007901Research partially supported by NSF grant DMS-9101716     

Dynamics of Josephson Junction under “Green” Noise

The stochastic dynamics of the Josephson junction is studied when external noise is nonwhite and high-pass filtered (green noise). In this case the diffusion coefficient of the Fokker-Planck equation is zero. Therefore the phase locking between the wave functions of the Cooper pairs is described and interpreted by the Krylov-Bogoljubov averaging method. The maximum (critical) Josephson current is redefined. Nonlinear diffusion and the lifetime of the phase locking of wave functions are considered. Background illumination, zero-point fluctuations and filtered white noise are used as examples of green noise. It is shown that the effect of green noise on the superconducting state of the junction is much less destructive than that of white one.     

A model of Piron's preparation-question structures in Ludwig's selection structures

We give a model of the basic Jauch-Piron (JP) approach to quantum physics, i.e., of preparation-question structure (with four basic axioms and without axioms C, P, A), in terms of Ludwig's selection structure; in the latter structure the primitive notion of individual sample of a physical entity is formally described (without making reference to any probability concept). Once we interpret Piron's concept of question in Ludwig's context of a selection structure, we find that there is no difficulty in formalizing notions such as performable together questions; moreover, results such as = or ()= can be formally proved. We develop the theory along the lines of the JP approach; the set of JP propositions is derived and it turns out to be a complete lattice, as happens in Piron's theory, but with a different physical interpretation of the lattice operations. Finally, we study some connections between the standard Ludwig foundation and our approach.     

Structural study of 12 MeV electron irradiated polypropylene by X-ray diffraction

12 MeV electron beams with different doses of 4, 8, 16 and 32 Mrad from LINAC designed by Nanjing University have been used to irradiate polypropylene films and their structures studied by X-ray diffraction afterwards. We have found that the electron irradiation does not change the total crystallinity of polypropylene but greatly inflences its crystal structure with increase of phase and decrease of phase as irradiation dose increases (within 16 Mrad). However, it is unexpected that the structure of irradiated sample with 32 Mrad are similar to that of the control sample. This behavior has also been observed in positron annihilation lifetime measurements.     

Spatially Periodic Solutions in Relativistic Isentropic Gas Dynamics

We consider the initial value problem, with periodic initial data, for the Euler equations in relativistic isentropic gas dynamics, for ideal polytropic gases which obey a constitutive equation, relating pressure p and density , p=², with 1, 0<<c, where c is the speed of light. Global existence of periodic entropy solutions for initial data sufficiently close to a constant state follows from a celebrated result of Glimm and Lax (1970). We prove that given any periodic initial data of locally bounded total variation, satisfying the physical restrictions ||v₀||<c, where v is the gas velocity, there exists a globally defined spatially periodic entropy solution for the Cauchy problem, if 1<₀, for some ₀>1, depending on the initial bounds. The solution decays in L_loc¹ to its mean value as t.     

The positron beam at the stuttgart pelletron accelerator and its applications toβ +γ positron lifetime measurements

A slow-positron source has been installed in the therminal of an electrostatic 6.5 MeV accelerator and provides a monoenergetic positron beam in the few-MeV range. It will be used to operate a fast positron lifetime spectrometer based on ⁺ coincidences. The properties of the beam, the expected performance of the spectrometer, its advantages over conventional lifetime measurements, a number of intended applications, as well as recent positron-electron scattering experiments and plans for positron channelling and channelling-radiation studies are outlined.This paper is based on a talk given by W.B. at the Int. Symp. Production of Low-Energy Positrons with Accelerators and Applications, held at Justus-Liebig-Universität Giessen, FRG, September 25–27, 1986     

The propagation of excitation energy in perturbed crystals

The process of the propagation of excitation energy in perturbed dielectric crystals with a weak bond between the atoms having zero permanent dipole moment is studied. It is shown that on certain assumptions this process of propagation can be regarded as the motion of a Frenkel exciton in the electrostatic field of the defect. Frenkel's exciton can be characterized in this case as a neutral polarizable particle having induced dipole moment equal to the change in the induced dipole moment of the crystal during the excitation of one of its atoms.

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СВЕТОСИЛА ЭЛЕКТРОНН О-ОПТИЧЕСКИХ СИСТЕМ

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