Total spectrum of photon emission by an ultra-relativistic positron channeling in a periodically bent crystal

We present the results of numerical calculations of the channelling and undulator radiation generated by an ultra-relativistic positron channelling along a crystal plane, which is periodically bent. The bending might be due either to the propagation of a transverse acoustic wave through the crystal, or due to the static strain as it occurs in superlattices. The periodically bent crystal serves as an undulator. We investigate the dependence of the intensities of both the ordinary channelling and the undulator radiations on the parameters of the periodically bent channel with simultaneous account for the dechannelling effect of the positrons. We demonstrate that there is a range of parameters in which the undulator radiation dominates over the channelling one and the characteristic frequencies of both types of radiation are well separated. This result is important, because the undulator radiation can be used to create a tunable source of X-ray and gamma-radiation.Comment: published in J. Phys. G: Nucl. Part. Phys. 26 (2000) L87-L95, http://www.iop.org ; 12 pages, 4 figures, LaTe

Stable propagation of a modulated positron beam in a bent crystal channel

The propagation of a modulated positron beam in a planar crystal channel is investigated. It is demonstrated that the beam preserves its modulation at sufficiently large penetration depths which opens the prospect of using a crystalline undulator as a coherent source of hard x-rays. This finding is a crucial milestone in developing a new type of lasers radiating in the hard x-ray and gamma-ray range.Comment: 11 pages, 4 figures, iopar

The influence of the dechanneling process on the photon emission by an ultra-relativistc positron channeling in a periodically bent crystal

We investigate, both analytically and numerically, the influence of the dechanneling process on the parameters of undulator radiation generated by ultra-relativistic positron channelling along a crystal plane, which is periodically bent. The bending might be due either to the propagation of a transverse acoustic wave through the crystal, or due to the static strain as it occurs in superlattices. In either case the periodically bent crystal serves as an undulator which allows to generate X-ray and gamma-radiation. We propose the scheme for accurate quantitative treatment of the radiation in presence of the dechanneling. The scheme includes (i) the analytic expression for spectral-angular distribution which contains, as a parameter, the dechanneling length, (ii) the simulation procedure of the dechanneling process of a positron in periodically bent crystals. Using these we calculate the dechanneling lengths of 5 GeV positrons channeling in Si, Ge and W crystals, and the spectral-angular and spectral distributions of the undulator over broad ranges of the photons. The calculations are performed for various parameters of the channel bending.Comment: published in J. Phys. G: Nucl. Part. Phys. 27 (2001) 95-125, http://www.iop.or

Channeling of Positrons through Periodically Bent Crystals: on Feasibility of Crystalline Undulator and Gamma-Laser

The electromagnetic radiation generated by ultra-relativistic positrons channelling in a crystalline undulator is discussed. The crystalline undulator is a crystal whose planes are bent periodically with the amplitude much larger than the interplanar spacing. Various conditions and criteria to be fulfilled for the crystalline undulator operation are established. Different methods of the crystal bending are described. We present the results of numeric calculations of spectral distributions of the spontaneous radiation emitted in the crystalline undulator and discuss the possibility to create the stimulated emission in such a system in analogy with the free electron laser. A careful literature survey covering the formulation of all essential ideas in this field is given. Our investigation shows that the proposed mechanism provides an efficient source for high energy photons, which is worth to study experimentally.Comment: 52 pages, MikTeX, 14 figure

The physical properties of AM CVn stars: new insights from Gaia DR2

AM CVn binaries are hydrogen deficient compact binaries with an orbital period in the 5-65 min range and are predicted to be strong sources of persistent gravitational wave radiation. Using Gaia Data Release 2, we present the parallaxes and proper motions of 41 out of the 56 known systems. Compared to the parallax determined using the HST Fine Guidance Sensor we find that the archetype star, AM CVn, is significantly closer than previously thought. This resolves the high luminosity and mass accretion rate which models had difficulty in explaining. Using Pan-STARRS1 data we determine the absolute magnitude of the AM CVn stars. There is some evidence that donor stars have a higher mass and radius than expected for white dwarfs or that the donors are not white dwarfs. Using the distances to the known AM CVn stars we find strong evidence that a large population of AM CVn stars have still to be discovered. As this value sets the background to the gravitational wave signal of LISA, this is of wide interest. We determine the mass transfer rate for 15 AM CVn stars and find that the majority have a rate significantly greater than expected from standard models. This is further evidence that the donor star has a greater size than expected.Comment: Accepted by A&A in main journa

Hybridization-related correction to the jellium model for fullerenes

We introduce a new type of correction for a more accurate description of fullerenes within the spherically symmetric jellium model. This correction represents a pseudopotential which originates from the comparison between an accurate ab initio calculation and the jellium model calculation. It is shown that such a correction to the jellium model allows one to account, at least partly, for the sp2-hybridization of carbon atomic orbitals. Therefore, it may be considered as a more physically meaningful correction as compared with a structureless square-well pseudopotential which has been widely used earlier.Comment: 16 pages, 10 figure

Reatores Aeróbicos para o Biotratamento de Águas Poluídas e Efluentes Sintéticos Contendo Ácido 3-Clorobenzóico

The degradation of 3-chlorobenzoic acid in polluted waters and synthetic effluents by a previously isolated indigenous strain of Pseudomonas putida was studied. Batch biodegradation assays were performed using a 2 L microfermentor at 28 °C with agitation. To simulate polluted water, 100 mg.L–1 of 3-chlorobenzoic acid were added to surface river water. Continuous‑flow assays were performed in an aerobic up-flow fixed-bed reactor constructed from PVC employing hollow PVC cylinders as support material. Synthetic wastewater was prepared by dissolving 3-chlorobenzoic acid in non-sterile groundwater. Biodegradation was evaluated by spectrophotometry, chloride release, gas chromatography and microbial growth. In batch experiments the indigenous strain of Pseudomonas putida degrades 100 mg.L–1 of 3-chlorobenzoic acid in 28 hours with a removal efficiency of 92.2 and 87.2%, expressed as compound and chemical oxygen demand removal, respectively. In the continuous-flow reactor the removal of an average influent concentration of 98.6 mg.L–1 reached 91.7% of compound and 88.9% of COD removal. The process efficiency remained approximately constant despite changes in the influent flow, compound concentration and temperature. The absence of metabolites was determined by gas chromatography performed at the end of the batch process and at the effluent of the continuous reactor. The ability of the isolated indigenous strain to degrade 3-chlorobenzoic acid in both batch and continuous reactors represents a promising feature to improve the treatment of effluents.Foi estudada a degradação do ácido 3-clorobenzóico em águas poluídas e efluentes sintéticos por uma cepa nativa previamente isolada de Pseudomonas putida. Ensaios de biodegradação de lotes foram realizados usando um microfermentador de 2 L a 28 °C sob agitação. Para simular água poluída, 100 mg.L–1 de ácido 3-clorobenzóico foram adicionados a água superficial de rio. Foram realizados ensaios de fluxo contínuo num reator aeróbico de fluxo ascendente de leito fixo construído em PVC, utilizando cilindros de PVC como material de suporte. Água sintética residuária foi preparada dissolvendo ácido 3-clorobenzóico em água subterrânea não estéril. A biodegradação foi avaliada por espectrofotometria, liberação de cloreto, cromatografia gasosa e crescimento microbiano. Nos experimentos em lote, a cepa nativa de Pseudomonas putida degrada 100 mg.L–1 de ácido 3-clorobenzóico em 28 horas com uma eficiência de remoção de 92,2 e 87,2%, expressada como remoção de composto e demanda de oxigênio químico, respectivamente. No reator de fluxo contínuo, a remoção de uma concentração média de afluente de 98,6 mg.L–1 atingiu 91,7% do composto e 88,9% da remoção de COD. A eficiência do processo permaneceu aproximadamente constante apesar das mudanças de fluxo, concentração do composto e temperatura. A ausência de metabólitos foi determinada por cromatografia gasosa realizada no final do processo em lote e no efluente do reator continuo. A capacidade da cepa nativa para degradar o ácido 3-clorobenzóico em ambos os lotes e reatores contínuos representa uma função promissora para melhorar o tratamento de efluentes.Fil: Gallego, A.. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Sanidad Nutrición Bromatología y Toxicología; ArgentinaFil: Rossen, Ariana Altair. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Sanidad Nutrición Bromatología y Toxicología; ArgentinaFil: Gemini, V.. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Sanidad Nutrición Bromatología y Toxicología; Argentina. Instituto Nacional de Tecnología Industrial; ArgentinaFil: Fortunato, M. S.. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Sanidad Nutrición Bromatología y Toxicología; ArgentinaFil: Rossi, S.. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Sanidad Nutrición Bromatología y Toxicología; ArgentinaFil: Planes, E.. Instituto Nacional de Tecnología Industrial; ArgentinaFil: Korol, S.. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Sanidad Nutrición Bromatología y Toxicología; Argentin