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.     

Water-sediment thermal interaction in a lagoon exchanging water with the sea (winter case)

Summary In this paper the thermal behaviour of both a shallow and well-mixed layer of water and its underlying sediment (e.g. a lagoon) is described. The water-sediment thermal interaction is analytically obtained by assuming a sinusoidal trend for the water depth. During the first half-period, the water leaves the lagoon at temperatureT(t) and in the second half-period the water comes back but at a constant temperatureT ^*. In this study theoretical results in a case without the solar flux will be shown (i.e. the case correponds to a typical winter case). Paper presented at the IX Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 8–10, 1992, Rome.     

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.     

On the radiative cooling of a surface during dew formation

Summary From the energy balance of a radiant surface with zero mass and perfectly insulated from below, the equilibrium temperature is calculated as a function of both sensible and latent heat transfer resistance. Under the hypothesis of Robinson,r _L=r _H is the air in contact with the surface is saturated, the equilibrium temperature is estimated as a function of air temperature, dew temperature and wind speed. This result could be useful to calculate night-time temperature of passive solar collectors or the yield of dew collectors. Paper presented at the 1^o Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 19–22, 1984, Rome.     

Spatial projection techniques in general relativity. Remarks about the construction of the spatial tensor analysis

The Eastbrook Whalquist definition of a spatial tensor analysis is compared with Massa's. It is shown that the second approach includes the first one as a special one. Lavoro eseguito nell'ambito dell'attività del Gruppo Nazionale della Fisica Matematica del C.N.R.Lavoro eseguito nell'ambito dell'attività del Gruppo Nazionale della Fisica Matematica del C.N.R.     

The phase separation line in the two-dimensional Ising model

We prove that, at low temperature, the line of separation between the two pure phases shows large fluctuations in shape. This implies the translation invariance of the correlation functions associated with some non translation invariant boundary conditions and should be a peculiarity of the dimensionality of the model.This work has been partially supported by the Consiglio Nazionale delle Ricerche (Gruppo Nazionale per l'Analisi Funzionale): CNR(GNAFA).     

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.     

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.     

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.).     

On the baroclinic instability of nonplanar flows

Summary Different ocean models with one or two layers having constant static stability and supporting constant-shear flows, whose directions are allowed to change with depth, are examined in the frame-work of the linear nonzonal baroclinic stability theory and in the absence of the β-effect. The analysis is reduced to solving a simple Sturm-Liouville boundary value problem in one dimension. A fairly general dispersion relation is found which correctly reproduces several special cases analysed by other authors. This relation shows a fair variety of possible behaviours for stability curves of two-layer models. The results show that the presence of a nonplanar shear-flow may have profound consequences on the stability character of the stationary geostrophic flow. In fact, it appears that the stability properties are strongly dependent on the propagation angle of the disturbance so that wave numbers which appear stable in the usual zonal theory may result unstable on such a nonzonal flow andvice versa. Paper presented at the 1^o Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 19–22, 1984, Rome.     

Positron bound systems in benzene and in naphthalene

Summary Positron decay in liquid naphthalene and benzene was investigated through measurements of lifetimes and magnetic quenching. The obtained results when compared with those already known in solid phase suggest that positrons become bound in two different systems. The first has a lifetime τ₃≈1 ns which is unaffected by the melting, while the second (τ₄≈3 ns) arises above melting and behaves as a relaxed positronium. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) del Ministero della Pubblica Istruzione, and by GNSM (Gruppo Nazionale di Struttura della Materia) del CNR.     

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     

Vortices and localization in Euler flows

We study the time evolution of a non-viscous incompressible two-dimensional fluid when the initial vorticity is concentrated inN small disjoint regions of diameter . We prove that the time evolved vorticity is also concentrated inN regions of diameterd, vanishing as 0. As a consequence we give a rigorous proof of the validity of the point vortex system. The same problem is examined in the context of the vortex-wave system.Research partially supported by MURST, (Ministero dell'Università e della Ricerca Scientifica e Tecnologica), CNR (Consiglio Nazionale delle Ricerche-Gruppo Nazionale per la Fisica Matematica) and CNR contract n.92.00544.01     

Ozone measurements in atmosphere and correlations with7Be in an Italian alpine valley

Summary Since 1990 our Institute has been conducting⁷Be measurements in the atmosphere in order to use it as a tracer for air coming from the upper layers of the atmosphere and for the stratospheric ozone. In this paper we present the results on⁷Be measurements and ozone concentrations obtained with a one-year monitoring campaign carried out in Sondrio, an alpine twon in Northern Italy. For a few interesting events correlations between beryllium and ozone is observed.⁷Be reveals itself as a good marker which reaches ground level during particularly rare events such as stratospheric intrusions. Paper presented at the IX Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 8–10, 1992, Rome.     

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.     

Comparison of measured and calculated high-resolution spectra of far-infra-red stratospheric emission

Summary Far-infra-red stratospheric emission spectra obtained from a balloon-borne platform at 38 km altitude with a spectral resolution of 0.0033 cm⁻¹ are compared with calculated spectra obtained with a radiative-transfer model. The overall agreement between the model and the measurements shows that a good understanding of this spectral interval has been obtained. Differences larger than the measurement error are observed in a few cases. The possible causes of these differences and the work that is needed in order to improve the data base of the model are discussed. Paper presented at the 1^o Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 19–22, 1984, Rome.     

Remote sensing of oils on the sea surface with an airborne infra-red scanner

Summary The ability of an infra-red scanner to detect oil spills over the sea surface is discussed on the basis of the first results of the Archimedes experiment 1983. During this experiment an infra-red scanner was flown over two oil spills and a real-time detection was achieved. The IR images were also recorded and processed in order to improve the film-thickness evaluation. An infra-red visible package was then developed which allows a real-time remote sensing of oil spills, eventually from small aircrafts. Paper presented at the 1^o Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, Rome, June 19–22, 1984.     

Upwelling induced by periodic wind stress

Summary In this paper we exhibit analytical solutions for the upwelling and the coastal currents induced by periodic wind stress. We present solutions for an infinite deep ocean and for a shallow ocean. There is upwelling only when the period of the forcing is longer than the inertial period,i.e. f>ω. When the period of the forcing is shorter than the inertial period,i.e. f<ω, there is not upwelling, but propagating waves are excited. Paper presented at the IV Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 22–24, 1987, Rome     

“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.     

Measurement and analysis of scattered radiation by means of a variable f.o.v. transmissometer

Summary A variable field-of-view transmissometer has been used in order to distinguish the contribution of forward-scattered radiation to the power transmitted through a diffusing medium. Measurements were made in fog and in the laboratory, where the medium was a suspension of polystyrene spheres in water. The analysis of scattered radiation gave information on the diffusing medium. In laboratory measurements it was possible to separate the contributions of different orders of scattering. Paper presented at the 1^o Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 19–22, 1984, Rome.