A multianalytical approach to investigate stone biodeterioration at a UNESCO world heritage site: the volcanic rock-hewn churches of Lalibela, Northern Ethiopia

A multianalytical approach combining Optical Microscopy (OM), Backscattered Variable Pressure Scanning Electron Microscopy + Energy Dispersive X-ray Spectroscopy (VP-BSEM + EDS), Powder X-ray Diffractometry (PXRD), Raman Spectroscopy, and Microbiological techniques has been applied to characterize decay products and processes occurring at the surface of two rock-hewn churches (Bete Gyorgis and Bete Amanuel) at the UNESCO’s World Heritage site of Lalibela, Northern Ethiopia. The two churches were carved into volcanic scoria deposits of basaltic composition. In their geological history, the Lalibela volcanic rocks underwent late to post-magmatic hydrothermal alteration together with partial laterization and are therefore characterized by a decay-prone highly vesicular microtexture with late stage to post-magmatic precipitation of secondary mineral phases (calcite–zeolite–smectite). The main objective of the study was to gain a better insight into the weathering products and mechanisms affecting the surface of the stone monuments and to assess the relative contribution of natural “geological” weathering processes versus biological/salt attack in stone decay at this unique heritage site. Results indicate that while the main cause of bulk rock deterioration and structural failure could be related to the stone inherited “geological” features, biological attack by micro- (bacteria) and/or macro- (lichens) organisms is currently responsible for severe stone surface physical and chemical weathering leading to significant weakening of the stone texture and to material loss at the surface of the churches walls. A prompt and careful removal of the biological patinas with the correct biocidal treatment is therefore recommended.     

Homogenization and concentrated capacity in reticular almost disconnected structuresHomogénéisation et capacité concentrée dans les structures réticulaires presque déconnectées

We compute the homogenized-concentrated limit for a pair of non-linearly coupled diffusion equations in a perforated cylindric domain with coaxial cylindric holes periodically distributed along its axis. This problem arises from visual transduction. To cite this article: D. Andreucci et al., C. R. Acad. Sci. Paris, Ser. I 335 (2002) 329–332.     

Study of J/Ψ three-body decays involving baryons and a pseudoscalar meson in the diquark model

The J/Ψ decay into a baryon pair and a pseudoscalar meson is computed, for some channels, in lowest order in perturbative QCD, modeling the baryon with a quark-diquark system. We use a set of parameters that has been proposed by some authors in order to fit the proton magnetic form factor G _M ^P, the angular distribution of protons in the processes γγ → pp?, γp → γp and the width of η_c → γγ.     

Forbush decreases: Origin and development in the interplanetary space

Summary A complete catalogue of type-IV radiobursts and Forbush decreases (high-latitude neutron monitor data) occurred in the period 1957–1969 is presented. Solar flares are found to produce Forbush decreases only provided that they are accompanied by type-IV radioemission developed in frequency from micrometric to metric wavelengths; the Forbush-decrease amplitude seems to be related to the energy of the radioemission. The observed amplitudes, Sun-Earth transit speeds and recovery times of Forbush decreases are found to be tied to the heliolongitude of the parent solar flares; a detailed analysis of these effects shows that the Forbush-decrease modulated region is essentially a wide 100°÷200° spiral conelike region which extends along the inter-planetary-magnetic-field lines from the Sun to the radially advancing front; therefore, this region co-rotates with the Sun and expands radially. The cosmic-ray depression inside the modulated region is related to the average Sun-Earth orbit transit speed and recovers with a time constant τ≈11 days. Moreover, the time behaviour and the amplitude distribution of Forbush decreases not associated with type-IV solar flares indicate that these events can be produced by type-IV solar flares occurred in the Sun's backside.

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Riassunto Si presenta un catalogo completo delle radioemissioni di tipo IV e delle diminuzioni di Forbush (registrate da monitori di neutroni ad alta latitudine) avvenuti nel periodo 1957–1969. Si trova che i brillamenti solari producono diminuzioni di Forbush solo se sono accompagnati da radioemissione di tipo IV che si sviluppa in frequenza dalle onde centimetriche alle metriche; l'ampiezza delle diminuzioni di Forbush sembra essere connessa all'energia della radioemissione. Si trova che le ampiezze, le velocità di transito Sole-Terra e i tempi di recupero delle diminuzioni di Forbush osservate a terra sono legati alle eliolongitudini dei brillamenti solari che le hanno prodotte; lo studio dettagliato di questi effetti mostra che la regione modulata durante una diminuzione di Forbush ha essenzialmente la forma di un cono distorto definito dalle linee di forza a spirale del campo magnetico interplanetario che dal Sole vanno al fronte avanzante radialmente, quindi questa regione coruota col Sole mentre si espande radialmente. La depressione dell'intensità dei raggi cosmici dentro la regione modulata è correlata con la velocità media del fronte dal Sole all'orbita della Terra e recupera con una costante di tempo τ≈11 giorni. Inoltre, gli andamenti temporali e la distribuzione delle ampiezze delle diminuzioni di Forbush non associate con tipi IV indicano che questi eventi possono essere prodotti da tipi IV avvenuti nell'emisfero non visibile del Sole.

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Резюме Предлагается полный каталог радиовспышек типа IV и убываний Форбуша (на основе высокоширотных данных с нейтронных мониторов), имевших место в период с 1957 г. по 1969 г. Обнаружено, что солнечные вспышки образуют убывания Форбуша только при условии, что они сопровождаются радиоизлучением типа IV в области длин волн от микрометра до метра. Амплитуда убывания Форбуша, по-видимому, связана с энергией радиоизлучения. Обнаружено, что наблюденные амплитуды, скорости прохождения от Солнца до Земли и времена восстановления убываний Форбуша связаны с гелиодолготой исходных солнечных вспышек. Подробный анализ этих эффектов показывает, что модулированная область убывания Форбуша представляет, по существу, широкую 100°÷200° спиральную конусоподобную область, которая простирается вдоль линий межпланетного магнитного поля от Солнца до радиально расширяющегося фронта. Депрессия космических лучей внутри модулированной области связана со скоростью прохождения от Солнца до Земли и восстанавливается с временной постоянной τ=11 дней. Кроме того, временное поведение и распределение амплитуд убываний Форбуша, не связанных с солнечными вспышками типа IV, указывает на то, что эти события могут быть связаны с солнечными вспышками типа IV, кторые произошли на обратной стороне Солнца.

     

Detection of solar neutrons by ground-based neutron monitors

Summary The analysis of the Rome NM-64 data, performed on the occasion of 8 solar flares accompanied by gamma-bursts and 10 solar flares accompanied by large X-ray fluxes, shows that the solar energetic neutron events could be detected by ground-based stations particularly when they are located at mountain altitude and middle-low latitudes. Suggestions for improving the neutron monitor efficiency for detecting solar neutron events are given. Paper presented at the 2^o Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May-2 June 1984.     

Critical indices for the spherical model from conformal covariant self consistency conditions

Critical indices for the spherical model are computed in arbitrary dimension space. Well-known exact results are recovered in three-dimensional space. The key tools used are conformal covariant self consistency conditions.     

A Monte Carlo simulation with an “improved” action for the O(3) non-linear sigma model

We present the results of a Monte Carlo simulation for a lattice action improved à la Symanzik. The results seem to give a faster approach of the lattice physics to its continuum limit.     

Hadron spectroscopy in lattice QCD

A Monte Carlo computation of meson and hadron spectroscopy within lattice QCD is made. We give a detailed discussion of the statistical and systematic errors of the results and analyze the present limitations of our approach. The results are in agreement with the observed spectrum. We also estimate the values of up, down and strange quark masses.     

Switching behavior of current filaments inp-germanium connected in parallel

Impact ionization breakdown in extrinsic germanium at low temperatures as a phase transition of a non-conducting to a conducting state is associated with a region of negative differential resistance, typically resulting in a current-voltage characteristic of multiple hystereses and switching between different conducting states. As an experimental model, we investigate the parallel combination of two devices each with a perfectlyS-shaped characteristic. The behavior of this simple combination can serve for understanding the complexity of more general cases. Further, we observed an interesting switching behavior between both devices which cannot be understood as an instability of the individual devices or in terms of a local stability analysis of the coupled system.     

A linear hyperbolic system and an optimal control problem

We prove a theorem of existence, uniqueness, and continuous dependence for a linear hyperbolic system with Darboux-type conditions under assumptions on the coefficients, which are in a sense the most general possible. Moreover, an application of this result to an optimal control problem is given.