1,060 research outputs found
Time of Flight system to investigate positronium cooling
No full textA positronium Time of Flight (TOF) apparatus, conceived to work with continuous positron beams of intensity up to ∼10 6 positrons/s, was developed. The geometry of the TOF chamber and the acquisition chain are described in detail. The performances of the set up were preliminary tested with a laboratory positron beam of ∼5 × 10 3 positrons/s by measuring the Time of Flight of Ps emitted from oxidized nanochannels produced in a Si single crystal. A TOF spectrum of ∼10 4 events was collected in 9 days with a time resolution of 8 ns. The analysis of the TOF spectrum is discussed. This apparatus is going to be assembled at the intense positron source NEPOMUC at FRM-II reactor, where the measurement time of each spectrum will be reduced to less than 2 h, making this TOF system appropriate to investigate positronium emission after cooling in porous materials held at cryogenic temperature. © The Author(s) 2012
Positronium cooling at cryogenic temperature for advanced experiments
No full textNew Ps spectroscopy measurements, formation of antihydrogen for antimatter-matter comparison experiments, production of Ps beams require the efficient production of cooled positronium in vacuum. At present the most efficient positron-positronium converters are silica based ordered or disordered porous materials, in which formed Ps decreases its kinetic energy by collisional cooling. Recently new positron-positronium converters based on oxidized nanochannels in silicon were found to be very promising because of the tunability of the nanochannel size, which allows to overcome the limits imposed to the Ps cooling by the quantum confinement. With these converters, Ps with temperatures as low as 150 K was detected in vacuum by a TOF apparatus. The Ps formation, quantum confinement, collisional cooling and emission into vacuum from nanochanneled silicon will be discussed in light of recent results. © Published under licence by IOP Publishing Ltd
Collisional cooled Ps emitted into vacuum from silica-based porous materials: Experiment to measure the Ps cooling time
No full textIn recent experiments on positronium time of flight (Ps-TOF) we have studied emission of cooled and thermalized Ps into vacuum from oxidized nanochannels synthetized in silicon. Ps cools down through collisions with the walls of the channels before exiting into vacuum. An important unknown parameter in the Ps-TOF measurements is the permanence time in the medium, i.e. the Ps cooling time before emission into vacuum. In this paper we describe an experiment that allows us to estimate the cooling time of Ps by analyzing the Ps-TOF spectra of cool Ps at three different distances from the sample
CO2 Laser irradiation of GeO2 planar waveguide fabricated by rf-sputtering
No full textGeO2 transparent glass ceramic planar waveguides were fabricated by a RF-sputtering technique and then irradiated by a pulsed CO2 laser. The effects of CO2 laser processing on the optical and structural properties of the waveguides were evaluated by different techniques including m-line, micro-Raman spectroscopy, atomic force microscopy, and positron annihilation spectroscopy. After laser annealing, an increase of the refractive index of approximately 0.04 at 1.5 μm and a decrease of the attenuation coefficient from 0.9 to 0.5 db/cm at 1.5 μm was observed. Raman spectroscopy and microscopy results put in evidence that the system embeds GeO2 nanocrystals and their phase varies with the irradiation time. Moreover, positron annihilation spectroscopy was used to study the depth profiling of the as prepared and laser annealed samples. The obtained results yielded information on the structural changes produced after the irradiation process inside the waveguiding films of approximately 1 μm thickness. In addition, a density value of the amorphous GeO2 samples was evaluated.Fil: Chiasera, A.. Consiglio Nazionale delle Ricerche. Istituto di Fotonica e Nanotecnologie; ItaliaFil: Macchi, Carlos Eugenio. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Instituto de Física de Materiales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Tandil. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires; ArgentinaFil: Mariazzi, C.. Università di Trento. Dipartimento di Fisica; ItaliaFil: Valligatla, S.. Consiglio Nazionale delle Ricerche. Istituto di Fotonica e Nanotecnologie; Italia. University of Hyderabad. School of Physics; India. Università di Trento. Dipartimento di Fisica; ItaliaFil: Lunelli, L.. Bruno Kessler Foundation; Italia. National Research Council Institute of Biophysics; ItaliaFil: Pederzolli, C.. Bruno Kessler Foundation; ItaliaFil: Rao, D. N.. University of Hyderabad. School of Physics; IndiaFil: Somoza, Alberto Horacio. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Instituto de Física de Materiales; Argentina. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Tandil. Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires; ArgentinaFil: Brusa, R. S.. Università di Trento. Dipartimento di Fisica; ItaliaFil: Ferrari, M.. Consiglio Nazionale delle Ricerche. Istituto di Fotonica e Nanotecnologie; Itali
Radiation damage characterization in implanted silica
No full textThe radiation damage produced in silica glass implanted with Ar+ and Au+ ions at different implantation energies and fluences was depth profiled by positron annihilation spectroscopy. The modification of the sub-nanovoids structure of the glass was investigated by measuring the change in the Doppler broadening of the 511keV positron annihilation line induced by the quenching of positronium, and the increasing of positron annihilation with oxygen defects. Two distinct defected regions were pointed out. The first region was found to be confined below the ion projected range Rp, and the second extended deep into the sample, up to two to three times the Rp plus the range straggling Rp. The type, origin and concentration of the defects are discussed
Single-crystal silicon coimplanted by helium and hydrogen: Evolution of decorated vacancy like defects with thermal treatments
No full textSi p-type (100) samples were coimplanted at room temperature with He+ ions at 30 keV with a dose of 1×1016 ions/cm2 and successively with H+ ions at 24 keV with a dose of 1×1016 ions/cm2. A series of samples was thermally treated for 2 h from 100 to 900 °C at 100 °C steps to study the evolution of pointlike and extended defects by two complementary techniques: positron Doppler broadening spectroscopy and transmission electron microscopy. Depth profiling the samples with a positron beam led to the identification of five different traps and the evolution of their profile distributions with thermal treatments. The positron traps were identified as decorated vacancy clusters of different sizes. Their decoration by implanted ions and in some case by oxygen was probed by coincidence Doppler broadening spectroscopy. Up to 300 °C annealing temperature positrons probe three distributions of different decorated defects covering regions of the sample down to 400–450 nm. Starting from 300 °C annealing temperature no defects were revealed by positrons in the region next to the peak of the implanted ions distributions positioned around 280 nm, where extended defects are expected; this indicates complete filling of the defects by H and He. From 300 to 600 °C decorated vacancy clusters of different sizes appear progressively in the region below 280 nm, with a distribution moving deeper into the sample. Comparison with previous measurements on He-implanted samples points out the chemical action of H. Hydrogen atoms interact with the previous damage by He, producing more stabilized vacancylike defects distributed through the damage region of the sample. Electron microscopy shows the transformation of the extended defects from platelets to blisters and cavities
Positron bunching system for producing positronium clouds into vacuum
No full textA magnetic transport line, a magnetic field terminator and a positron buncher were designed and built to focus low-energy positron pulses from a Surko-type accumulator on a porous target. The 25 electrodes buncher, which produces a parabolic potential, was designed to implant 5 ns positron bunches with a spot of 3 mm into a target held at cryogenic temperature. These pulses will be used to obtain cooled Ps clouds into vacuum for laser excitation in spectroscopy experiments. By using high-voltage fast switches and a proper mu-metal shield the requirement to form Ps in a free (magnetic and electric) field region was satisfied compatibly with the request of injecting positrons at energies of 5-9 keV. The optical design, the electrical circuitry of the buncher and the construction solutions of the whole apparatus will be presented and explained. © Published under licence by IOP Publishing Ltd
Optimization of a multi-ring detector for Ps time of flight measurements
No full textWe have designed a multi-ring detector (MRD) based on Bismuth Germanate (BGO) crystals, coupled to Silicon PhotoMultipliers (SiPM) for measuring the Ps time of flight (TOF). The set-up geometry was optimized by Monte Carlo simulations to take into account at different Ps velocities: (i) the background noise due to backscattered positrons, (ii) the crosstalk between adjacent detectors, (iii) the lifetime of Ps decay. Three parameters were defined to evaluate the different configurations and a figure of merit was obtained. This allows the choice of the best set up configuration for measuring Ps emitted with a particular energy range, optimizing the signal to noise ratio and keeping the acquisition time acceptable
Single-crystal silicon coimplanted by helium and hydrogen: Evolution of decorated vacancylke defects with thermal treatments
No full textSi p-type 100 samples were coimplanted at room temperature with He+ ions at 30 keV with a dose of 1
1016 ions/cm2 and successively with H+ ions at 24 keV with a dose of 11016 ions/cm2. A series of
samples was thermally treated for 2 h from 100 to 900 °C at 100 °C steps to study the evolution of pointlike
and extended defects by two complementary techniques: positron Doppler broadening spectroscopy and transmission
electron microscopy. Depth profiling the samples with a positron beam led to the identification of five
different traps and the evolution of their profile distributions with thermal treatments. The positron traps were
identified as decorated vacancy clusters of different sizes. Their decoration by implanted ions and in some case
by oxygen was probed by coincidence Doppler broadening spectroscopy. Up to 300 °C annealing temperature
positrons probe three distributions of different decorated defects covering regions of the sample down to
400–450 nm. Starting from 300 °C annealing temperature no defects were revealed by positrons in the region
next to the peak of the implanted ions distributions positioned around 280 nm, where extended defects are
expected; this indicates complete filling of the defects by H and He. From 300 to 600 °C decorated vacancy
clusters of different sizes appear progressively in the region below 280 nm, with a distribution moving deeper
into the sample. Comparison with previous measurements on He-implanted samples points out the chemical
action of H. Hydrogen atoms interact with the previous damage by He, producing more stabilized vacancylike
defects distributed through the damage region of the sample. Electron microscopy shows the transformation of
the extended defects from platelets to blisters and cavities
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