Selective Production of Rydberg-Stark States of Positronium

Rydberg positronium (Ps) atoms have been prepared in selected Stark states via two-step (1s→2p→nd/ns) optical excitation. Two methods have been used to achieve Stark-state selection: a field ionization filter that transmits the outermost states with positive Stark shifts, and state-selected photoexcitation in a strong electric field. The former is demonstrated for n=17 and 18 while the latter is performed for n=11 in a homogeneous electric field of 1.9  kV/cm. The observed spectral intensities and their dependence on the polarization of the laser radiation are in agreement with calculations that include the perturbations of the intermediate n=2 manifold. Our results pave the way for the generation of Rydberg Ps atoms with large electric dipole moments that are required for the realization of schemes to control their motion using inhomogeneous electric fields, an essential feature of some proposed Ps free-fall measurements requiring focused beams of long-lived atoms

Production of 2(3) S-1 positronium atoms by single-photon excitation in an electric field

We report experiments in which positronium (Ps) atoms are produced in the 2 3 S1 level by single-photon excitation from the ground state. To accomplish this, Stark-mixed n = 2 states were optically excited in electric fields. By adiabatically switching off the electric field after laser excitation, some of the mixed states evolved into pure 2 3 S1 levels, whose presence was detected via the time dependence of their annihilation γ radiation. The observed ≈4% production efficiency relative to that of Rydberg Ps states is consistent with a Monte Carlo simulation that takes into account the rate at which the electric fields were switched off

Single-shot positron annihilation lifetime spectroscopy with LYSO scintillators

We have evaluated the application of a lutetium yttrium oxyorthosilicate (LYSO) based detector to single-shot positron annihilation lifetime spectroscopy. We compare this detector directly with a similarly configured PbWO4 scintillator, which is the usual choice for such measurements. We find that the signal to noise ratio obtained using LYSO is around three times higher than that obtained using PbWO4 for measurements of Ps excited to longer-lived (Rydberg) levels, or when they are ionized soon after production. This is due to the much higher light output for LYSO (75% and 1% of NaI for LYSO and PbWO4 respectively). We conclude that LYSO is an ideal scintillator for single-shot measurements of positronium production and excitation performed using a low-intensity pulsed positron beam

Electrostatically Guided Rydberg Positronium

We report experiments in which positronium (Ps) atoms were guided using inhomogeneous electric fields. Ps atoms in Rydberg-Stark states with principal quantum number n=10 and electric dipole moments up to 610 D were prepared via two-color two-photon optical excitation in the presence of a 670  V cm−1 electric field. The Ps atoms were created at the entrance of a 0.4 m long electrostatic quadrupole guide, and were detected at the end of the guide via annihilation gamma radiation. When the lasers were tuned to excite low-field-seeking Stark states, a fivefold increase in the number of atoms reaching the end of the guide was observed, whereas no signal was detected when high-field-seeking states were produced. The data are consistent with the calculated geometrical guide acceptance

Positronium production in cryogenic environments

We report measurements of positronium (Ps) formation following positron irradiation of mesoporous SiO2 films and Ge(100) single crystals at temperatures ranging from 12–700 K. As both of these materials generate Ps atoms via nonthermal processes, they are able to function as positron-positronium converters at cryogenic temperatures. Our data show that such Ps formation is possibly provided the targets are not compromised by adsorption of residual gas. In the case of SiO2 films, we observe a strong reduction in the Ps formation efficiency following irradiation with UV laser light (λ=243.01 nm) below 250 K, in accordance with previous observations of radiation-induced surface paramagnetic centers. Conversely, Ps emission from Ge is enhanced by irradiation with visible laser light (λ=532 nm) via a photoemission process that persists at cryogenic temperatures. Both mesoporous SiO2 films and Ge crystals were found to produce Ps efficiently in cryogenic environments. Accordingly, these materials are likely to prove useful in several areas of research, including Ps mediated antihydrogen formation conducted in the cold bore of a superconducting magnet, the production of Rydberg Ps for experiments in which the effects of black-body radiation must be minimized, and the utilization of mesoporous structures that have been modified to produce cold Ps atoms

Positronium decay from n=2 states in electric and magnetic fields

We report measurements and the results of calculations demonstrating that the annihilation dynamics of positronium (Ps) atoms can be controlled by Stark and Zeeman mixing of optically excited states. In the experiments a trap-based pulsed positron beam was employed to generate a dilute Ps gas with a density of ∼107 cm−3 using a porous silica target. These atoms were excited via 13S1→23PJ transitions in parallel electric and magnetic fields using a nanosecond pulsed dye laser, and Ps annihilation was measured using single-shot lifetime spectroscopy. The composition of the excited n=2 sublevels was controlled by varying the polarization of the excitation laser radiation and the strength of the electric and magnetic fields in the excitation region. The overall decay rates of the excited states can vary by a large amount, owing to the enormous differences between the annihilation and florescence lifetimes of the accessible field-free states. The energy-level structure, spectral intensities, and florescence and annihilation lifetimes in the presence of the fields were determined from the eigenvalues and eigenvectors of the complete n=2 Hamiltonian matrix in an |nSℓJMJ⟩ basis. Using these data as the input to a Monte Carlo model yielded calculated values which could be compared with experimentally measured quantities; qualitative agreement with the measurements was found. Varying the electric field in the presence of a weak parallel magnetic field provides control over the amount of level mixing that occurs, making it possible to increase or decrease the Ps lifetime. Field-controlled Ps decay can be used as an ionization-free detection method. Conversely, increasing the excited-state lifetime can potentially be exploited to optimize multistep excitation processes using mixed intermediate states. This will be useful either in minimizing losses through intermediate-state decay during excitation or by making it possible to separate excitation laser pulses in time. In addition, the adiabatic extraction of appropriate eigenstates from the electric field in which they are excited can, in principle, be used to prepare pure 23S1 atoms. The availability of atoms in these states produced via single-photon excitation will facilitate high-resolution microwave spectroscopy of the Ps n=2 fine structure

Measurement of Rydberg positronium fluorescence lifetimes

We report measurements of the fluorescence lifetimes of positronium (Ps) atoms with principal quantum numbers n=10–19. Ps atoms in Rydberg-Stark states were produced via a two-color two-step 1S3→2P3→nS3/nD3 excitation scheme and subsequently detected after traveling 1.2 m. The measured time-of-flight distributions were used to determine the mean lifetimes of the Rydberg levels, yielding values ranging from 3μs to 26μs. Our data are in accord with the expected radiative lifetimes of Rydberg-Stark states of Ps

Estimulación del sistema mediado por interferón tipo I de lenguado senegalés (Solea senegalensis) en respuesta a infecciones por nodavirus

El lenguado senegalés es susceptible a la infección por el Virus de la Necrosis Nerviosa Viral (VNNV). Los betanodavirus se clasifican en cuatro genotipos, siendo los genotiposSJNNV, RGNNV y virus recombinantes RGNNV-SJNNV los que causan mortalidad en lenguado. En condiciones experimentales es el recombinante el que provoca mayor mortalidad. Las diferencias en la tasa de mortalidad pueden indicar variaciones en la interacción entre los distintos genotipos y el sistema inmune de lenguado. El sistema del interferón tipo I es un componente esencial de la respuesta inmune frente a infecciones virales, induciendo la expresión de genes que codifican proteínas antivíricas, tales como la Mx, la ISG15 y la PKR. El objetivo del presente trabajo ha sido cuantificar la transcripción de Mx, ISG15 y PKR en respuesta a infecciones por SJNNV, RGNNV y un recombinante RG-SJ.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Challenges for standardization of Clostridium difficile typing methods

Typing of Clostridium difficile facilitates understanding of the epidemiology of the infection. Some evaluations have shown that certain strain types (for example, ribotype 027) are more virulent than others and are associated with worse clinical outcomes. Although restriction endonuclease analysis (REA) and pulsed-field gel electrophoresis have been widely used in the past, PCR ribotyping is the current method of choice for typing of C. difficile. However, global standardization of ribotyping results is urgently needed. Whole-genome sequencing of C. difficile has the potential to provide even greater epidemiologic information than ribotyping

El sistema del interferón tipo I protege a juveniles de lenguado senegalés (Solea senegalensis) frente a la infección por VHSV

El lenguado senegalés (Solea senegalensis) es susceptible a la infección por el Virus de la Septicemia Hemorrágica Viral (VHSV) bajo condiciones experimentales. El objetivo de este trabajo es determinar el papel del sistema del interferón tipo I de lenguado frente a las infecciones por VHSV. Para llevar a cabo este objetivo se realizó un ensayo en el que el sistema del IFN I de juveniles de lenguado se estimuló con poli I:C. Trascurridas 24 h estos mismos animales se inocularon con una dosis letal de un aislado de VHSV patógeno para lenguado. Los controles utilizados fueron: (I) animales inoculados con VHSV sin previa estimulación con poli I:C, (II) animales inoculados sólo con medio L15 y (III) animales inoculados únicamente con poli I:C.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech