Positroniumlike system in quartz

Summary The ortho-state of the Ps-like system, which in crystalline quartz lives in Bloch states, has been detected in the time annihilation spectrum. Its lifetime and abundance are τ₃=0.49ns,I ₃=0.13. The magnetic quenching of this system has been measured and compared with that of Ps-like system living in other molecular media. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) of Ministero della Pubblica Istruzione and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

Exchange collisions between positronium and Cu2+ ions

Summary The fate and properties of the ortho-Ps in aqueous solutions of CuSO₄ and CuCl₂ have been investigated through lifetime measurements and through a magnetic-quenching experiment. The obtained results are interpreted in terms of exchange collisions between Ps and unpaired electron. An upper limit is given for the chemical-quenching rate. Our measurements have shown that the presence of paramagnetic centres determines a strong shift of the levels of the Ps hyperfine structure. This work was entirely supported by CISM (Centro Interuniversitario di Struttura della Materia) of the Ministero della Publica Istruzione and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

Contact density and hyperfine splitting of positronium in naphthalene

The contact density |Ψ(0)|² and the hyperfine splitting of positronium in naphthalene have been determined by means of a magnetic quenching experiment. Results show that the perturbing effect of the medium on the hyperfine splitting is not interpretable as a change of the contact density alone, since on passing from vacuum to the medium, the contact density and the splitting do not scale together. The first is reduced by a factor of 0.25, the second by a factor of 0.067. A simple phenomenological model is presented. This work was entirely supported by CISM (Centro Interuniversitario di Struttura della Materia) of the Ministero della Pubblica Istruzione and by GNSM (Gruppo Nazionale di Struttura della Materia) of the Consiglio Nazionale delle Ricerche.     

Speculations on the absorption and emission properties of the F-centre in alkali halides

Summary A review of acquisitions and of unresolved problems concerning F-centres in alkali halides is presented. The main conclusions area) the temperature dependence of the bands is mainly due to the thermal expansion for the absorption process and to electron-phonon interaction for the emission process: for the latter half-width and Huang-Rhys factor values are related to the transverse optical frequency ones;b) the lifetime τ_R of the relaxed excited state is not intrinsic at low temperature because of the interaction between F-and α-centres always present in real samples;c) even though the absolute quantum yield decreases when F-centre concentration increased, the relative quantum yield η_r is unaffected by change on F-centre concentration, strongly evidencing a nonhomogenous distribution of F-centres in real samples;d) a temperature-dependent, fine structure in τ_R (an η_r) values is observed but only when temperature control of the samples is better than a few hundredth of a degree: this effect has been interpreted in terms of interactions which involve F-, α-and F′-centres. Work jointly supported by the Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

Positron spin relaxation in naphthalene

Summary The remaining spin polarizationP of positrons was measured in naphthalene against longitudinal magnetic-field strength. The measuring method was based on the relative yield of Ps in triplet and singlet state. In a zero fieldP is reduced to 50% of the initial polarization and approaches 100% upon increasing the applied field to larger values. This effect, which in teflon is quite absent, appears to be determined by the specific behaviour of positrons in naphthalene. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) of Ministero Pubblica Istruzione and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

Lattice reordering in Pb implanted Ge crystals

Channeling effect and sheet resistivity techniques have been used to investigate the damage induced by 40 keV Pb implantation in Ge crystals. At 450° the reordering of the Ge lattice occurs simultaneously to the out-diffusion of the implanted Pb atoms while the recovery of the sheet resistivity occurs at higher temperatures. 90% of the implanted atoms still retained in the crystal are located in substitutional sites. Work supported in part by Centro Siciliano di Fisica Nucleare e di Structura della Materia and by Gruppo Nazionale di Struttura della Materia del Consiglio Nazionale delle Ricerche.     

Positron spin relaxation in solids

Summary The positron spin relaxation in a longitudinal magnetic field was investigated in some solids where Ps formation occurs. The experimental results indicate that this relaxation manifests itself in those organic compounds where the positron can excite low-lying triplet states. The values of positron slowing-down time, obtained by adopting a schematic model, range from 13 to 28 ps. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) of Ministero della Pubblica Istruzione, and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

Analysis of a qualitative criterion for forecasting off-centre displacement of impurity ions in ionic crystals

Summary With the aid of the Sanderson model for nonmolecular structure a qualitative criterion is derived for forecasting off-centre configurations of monovalent impurity ions (Li⁺, F⁻, Cl⁻, Na⁺, Ag⁺, Cu⁺) in alkali halide crystals. The same criterion is checked for Mn²⁺ impurity ions in some oxides. We also present a critical review of the criteria introduced in the past, as well as of the experimental results up to now known in the literature. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction. Work jointly supported by the Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

A thermoelastic method to determine the thermal diffusivity

We present a method to measure the thermal diffusivity of metals. A homogeneous small heat source is fed into a cyclindrical sample of titanium by thermoelastic compression. This heating process is followed by a thermal relaxation controlled by the thermal diffusivity. The resulting temperature variation is detected by a miniature temperature sensor mounted on the lateral surface of the sample. The value so obtained for the thermal diffusivity of titanium is (6.6±0.2)·10⁻⁶ m²/s. Gruppo Nazionale Struttura della Materia del Consiglio Nazionale delle Ricerche     

“Laser snow” effect in CS2 vapour induced by krypton laser

We report on the laser snow effect observed in CS₂ vapour illuminated by uv lines of a krypton laser. We discuss the effect of diffusion and convection processes on the threshold value of the CS₂ vapour pressure. We discuss also the kinetics of the formation process related to the proposed model of chemical reactions induced by laser light.Work partially supported by the Gruppo Nazionale di Struttura della Materia del CNR and by the INFN.     

The differential method for the study of small variations of positron lifetime spectra

A new method for the study of small perturbations of positron annihilation characteristics is proposed. The method is based on the direct comparison of two lifetime spectra, leading to the determination of the mean life variation and of the difference spectrum. The combined knowledge of and of a shape parameter of the difference spectrum is often sufficient for the complete quantitative and qualitative characterization of the observed spectral modifications. Specific examples are discussed.Work jointly supported by the Gruppo Nazionale di Struttura della Materia del Consiglio Nazionale delle Ricerche, and by the Centre di Ricerche FIAT, Orbassano (Torino)In partial fulfilment of the requirements for the degree of Dottore in Ingegneria Nucleare at the Politecnico di Milano     

On the possibility of a forecast for off-centre configuration of Ag+ and Cu+ in alkali halide crystals

Summary The simple cut criterion based on the accurate determination of the radii of the ions in alkali halides and previously introduced by the authors for forecasting off-centre configuration of Li⁺ and F⁻ has been extended to heavy ions (Ag⁺ and Cu⁺). It has been found that this criterion is valid for the Ag⁺ ion, whereas for Cu⁺ gives a less precise forecast because of the lack in knowledge of the effective partial charge on Cu⁺ ion. It has been evidenced that the critical value of the ratior ₊ ^* /r ₊ between impurity and host ion radius which allows off-centre configuration is dependent on the impurity ion mass. Work jointly supported by the Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

On some measurements of the propagation properties of thermal waves in the Nb-H and Ta-H systems

Summary By means of an experimental technique based on a nonsteady-state method,i.e. on the propagation of thermal waves, we have measured at room temperature for oscillation frequencies between 6 and 30 mHz the thermal diffusivity, the lateral thermal-loss coefficient and the ?fractional heat loss? in Nb and Ta wires as a function of hydrogen doping. The appearance of hydride formation and precipitation notably changes the behaviour of these quantities. From these measurements it was also possible to calculate the propagation velocity and attenuation coefficient of thermal waves. It was found that the increase of the hydrogen concentration in the solid solution produces a gradual decrease in the velocity and increase in the attenuation coefficient until the solubility limit is reached. For larger hydrogen concentrations, the velocity showed a tendency to increase towards the value of the pure metal, while the attenuation coefficient decreased below the value of the pure metal. These results as a whole appear quite promising for studying the properties of hydrogenated systems. This work was supported by the Gruppo Nazionale di Struttura della Materia of the Consiglio Nazionale delle Ricerche and by the Centro Interuniversitario di Struttura della Materia of the M.P.I., Roma (Italia).     

Temperature gradients effects in low-power laser annealing of ion implantedα-Ge

The influence of temperature gradients in laser annealing of-Ge, obtained below the melting threshold, was tested by irradiating the samples with a linear fringe pattern from a pulsed ruby laser. The peak temperature reached on the specimen surface was calculated to be well below the melting threshold of the material. Temperature gradients are observed to enhance the crystallization process in the material. Spontaneous periodic structures, or ripples, having a period equal to the laser wavelength, are frequently observed in areas corresponding to minima of the fringe pattern.Gruppo Nazionale di Struttura della Materia     

The proton-air cross-section at 1016 eV

Summary A study of the Čerenkov light lateral distribution function for cosmic-ray showers with energies of about 10¹⁶ eV has been used to determine the distribution of depths of shower maximum at these energies. This distribution has an exponential tail due to fluctuations in the depths of early interactions of the proton primary component. The form of this tail may be used to infer an inelastic proton-air cross-section of (590±70) mb at ≈10¹⁶ eV.

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Riassunto Si usa uno studio della distribuzione laterale della luce di Čerenkov per sciami cosmici con energie di circa 10¹⁶ eV per determinare la distribuzione di profondità del massimo degli sciami a queste energie. Questa distribuzione ha una coda esponenziale dovuta a fluttuazioni nelle profondità delle prime interazioni della componente protonica delle primarie. La forma di questa coda si può usare per inferire una sezione d'urto anelastica protone-aria di (590±70) mb a ≈10¹⁶ eV.

     

A simple criterion for forecasting off-centre configuration of monovalent impurity ions in alkali halide crystals

Summary With the aid of the Sanderson model for nonmolecular structures a simple criterion has been derived for forecasting off-centre configuration of monovalent impurity ions in alkali halide crystals based on the effect that the Sanderson partial charges on atoms have on their radii. Work jointly supported by Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

Two-photon transitions between discretenS states in atomic He

Summary The two-photon transition probabilities from the ground state 1¹ S and from the metastable states 2¹ S and 2³ S to the lowest excited states are computed in He. A configuration interaction approach is used, and a detailed comparison between the ?length? and the ?velocity? gauges leads to an estimate of 1% accuracy. Correlation effects are found to account for about 20% of the transition probabilities. The life-time of the 2¹ S state is ≅19.6 ms. The two-photon transition probabilities display a dramatic dependence on frequency, characterized by resonances and transparencies. Based on work supported in part by the Gruppo Nazionale di Struttura della Materia of the C.N.R. and by the Frascati Synchrotron Radiation Contract (P.U.L.S.).     

The influence of thes-f interaction on the conduction band behaviour in ferromagnetic semiconductors. A theory of the red shift effect

The mechanism of the thermomechanical instability of an elastically slowly strained solid is explored, by a normal mode analysis of temperature and strain fluctuations. The strain dependence of the Grüneisen parameter is found to play a major role, since it controls the rates of damping of the lowest wavevector acoustic excitations pumped by the imposed strain. For -titanium the instability is found to occur at a critical strain comparable with that of the elastic limit. The instability can be envisaged in terms of a fast growing deformation, presumably associated with an acoustic emission.Gruppo Nazionale di Struttura della Materia del Consiglio Nazionale delle Ricerche, UR7     

A GMSA calculation of the radial distribution functions of molten RbCl

Summary The generalized mean spherical approximation (GMSA) for a fluid of charged hard spheres of equal diameters is used for calculating the radial distribution functions of molten RbCl. The results are compared to neutron diffraction data and a good overall agreement is found between the experimental and the theoreticalg _ij (r). Further application of this same approach to the case of ionic melts with ionic species of different diameters is discussed. Work supported in part by Comitato Regionale Ricerche Nucleari and Struttura della Materia (CRRNSM), Sicily, Italy.     

Tensorial piezomagnetic coefficients for single-domain titanomagnetite crystals at saturation magnetization

Summary The piezomagnetic and magnetostrictive behaviour of titanomagnetite single-domain crystals is described in a general tensorial formalism with respect to a reference initial configuration in which crystals possess a saturation remanence magnetization and are subjected to a prestress field. The equivalence between the present linearized theory and previous semi-empirical formulae is proved. Numerical values are given for the piezomagnetic coefficients in terms of the magnetostrictive constants Δ₁₀₀ and Δ₁₁₁ which have been experimentally determined for titanomagnetite single crystals.

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Riassunto Si propone una descrizione generale tensoriale del piezomagnetismo e della magnetostrizione per cristalli di titanomagnetite a dominio singolo rispetto ad una configurazione iniziale di riferimento in cui i cristalli posseggono una magnetizzazione rimanente di saturazione e sono soggetti ad un campo di sforzo iniziale. Si dimostra l'equivalenza della presente teoria linearizzata e formule semiempiriche proposte precedentemente. Si forniscono valori numerici dei coefficienti piezomagnetici desunti dalle misure sperimentali delle costanti di magnetostrizione Δ₁₀₀ e Δ₁₁₁ effettuate su cristalli singoli di titanomagnetite.