R. S. Brusa

Doppler-broadening measurements of positron annihilation with high-momentum electrons in pure elements

Doppler-broadening measurements of the electron–positron annihilation line in twenty-seven single-element samples are presented. A coincidence technique has been used to suppress the background and to evidence the contribution of positron annihilation with core electrons. Systematic dependences on the atomic number of the target material are found in ratio curves obtained dividing the measured spectra by the spectrum of a reference material. The positron lifetime technique has been used to detect the presence of positron traps in all the samples. The change in the highmomentum part of the annihilation line due to positron trapping is illustrated. The measured data are in a good qualitative agreement with recent theoretical calculation and constitute the most complete measurement series, up to now, to establish a future data-base for positron annihilation spectroscopy. 2002 Elsevier Science B.V. All rights reserved.

Exploring the WEP with a pulsed cold beam of antihydrogen

The AEGIS experiment, currently being set up at the Antiproton Decelerator at CERN, has the objective of studying the free fall of antimatter in the Earth?s gravitational field by means of a pulsed cold atomic beam of antihydrogen atoms. Both duration of free fall and vertical displacement of the horizontally emitted atoms will be measured, allowing a first test of the WEP with antimatter.

HELIUM-IMPLANTED SILICON : A STUDY OF BUBBLE PRECURSORS

The interaction of helium atoms with the radiation damage imparted to (100) silicon single crystal by He+ implantation at 5×1015 cm−2, 20 keV, and liquid–nitrogen temperature is investigated by means of various complementary techniques during and after thermal treatments. Thermal programmed desorption was used to study the dissociation kinetics of helium from the defects and to plan suitable heat treatments for the other techniques. The helium profiles were determined by 8 MeV 15N2+ elastic recoil detection, quantitative data on damage were obtained by channeling Rutherford backscattering spectrometry, double crystal x-ray diffraction, and positron annihilation spectroscopy. Isothermal treatments at 250 °C produce first helium redistribution and trapping in vacancy-like defects, rather than helium desorption from traps. The process is thermally activated with an effective activation energy, dispersed in a band from 1.1 to about 1.7 eV. For higher temperature treatments (2 h at 500 °C) the traps are almost...

Porosity in low dielectric constant SiOCH films depth profiled by positron annihilation spectroscopy

The 3γ annihilation of orthopositronium and the Doppler broadening of the positron annihilation line have been measured by implanting low energy positrons in low dielectric constant (low-k) SiOCH films. The evolution and stability of film porosity with thermal treatments in the 400–900 °C temperature range has been studied. The films have been produced by plasma enhanced chemical vapor deposition and after annealing in N2 atmospheres at 480 °C have been treated in N2+He plasma. The minimum free volume of the pores in the as-produced samples has been estimated to correspond to that of a sphere with radius r=0.6 nm. The treatment in the N2 plasma was found to seal the pores up to 45 nm depth. Both the composition of the films (as obtained by Rutherford backscattering spectroscopy and elastic recoil detection analysis) and the chemical environment of the pores probed by positrons were found to be very stable up to 600 °C thermal treatment. Above such a temperature a reduction of the hydrogen content accompan...

Absolute total cross sections for electron scattering on CH4 molecules in the 1-4000 eV energy range

Absolute total cross sections for electron-methane scattering have been measured in the energy range from 0.9 to 4000eV in two separate experiments. The present results, the first available above 500 eV, are in good agreement with other recent experimental and theoretical data. The law energy scattering is dominated by a shape resonance; a broad hump has been detected centred around 100eV. A short critical review of previous measurements is given.

Structural characterization of amorphous hydrogenated carbon and carbon nitride films deposited by plasma-enhanced CVD

Abstract Microstructural investigations of a-C:H and a-CN x :H films obtained by plasma-enhanced CVD were performed by means of Raman spectroscopy and positron annihilation spectroscopy (Doppler-broadening technique). Thermal gas evolution analysis has been used to gain insights about the void distribution. The effects of deposition parameters (self-bias voltage and nitrogen partial pressure in the plasma) on the film microstructure were studied. The incorporation of increasing amounts of nitrogen originates an increase in void density as well as a progressive graphitization of a-C:H films. Raman scattering from a-C:H films deposited at self-bias voltages higher than −800 V reveals a more graphitic structure of these films with respect to those deposited at lower bias.

Deuterium storage in nanocrystalline magnesium thin films

Nanocrystalline magnesium deuteride thin films with the β-MgD2 structure were prepared by vacuum evaporation of hexagonal magnesium (h-Mg) samples and thermal annealing in 0.15 MPa D2 atmosphere at 373 K. Thermal desorption spectroscopy analysis indicated that the rate-limiting step in the deuterium desorption was given by the thermal decomposition of the deuteride phase. The activation energy Δg of the β-MgD2→h-Mg+D2 reaction scaled from 1.13±0.03 eV in 650-nm-thick films to 1.01±0.02 eV in 75-nm-thick films most likely as consequence of different stress and defect level. Positron annihilation spectroscopy analysis of the thin-film samples submitted to deuterium absorption and desorption cycles reveal the presence of a high concentration of void-like defects in the h-Mg layers after the very first decomposition of the β-MgD2 phase, the presence of these open volume defects reduces the D2 absorption capacity of the h-Mg thin film.

Positron annihilation study of vacancy-like defects related to oxygen precipitates in Czochralski-type Si

We report the direct measurement of vacancy-like defects related to oxygen in the oxygen precipitation process in Czochralski Si. The vacancy-like defects were detected by measuring the positron lifetime and narrowing of the positron–electron annihilation momentum distribution. Oxygen atoms surrounding the vacancy-like defects were detected by analyzing the high-momentum part of the positron–electron momentum distribution measured by a Doppler broadening coincidence technique. It was found that the majority of the defects associated with oxygen have an effective open volume smaller than that of a silicon monovacancy.

Total cross sections for electron scattering on NO2, OCS, SO2 at intermediate energies

Total cross sections for electron scattering on SO2, NO2, OCS have been measured in the energy range from 90 to 4000 eV. Our data extrapolated to low energies match other experiments on OCS and NO2; for SO2 discrepancies amount to as much as 20%. The experimental values for these gases and for CO2 (the latter measured previously in our laboratory) have hen fitted with a Born-like formula. Two parameters are sufficient to reproduce the SO2, NO2 and CO2 cross sections from 20 to 4000 eV within the experimental errors. A double Yukawa potential with four parameters is needed to reproduce the OCS cross sections over the same energy interval.

A moiré deflectometer for antimatter

The precise measurement of forces is one way to obtain deep insight into the fundamental interactions present in nature. In the context of neutral antimatter, the gravitational interaction is of high interest, potentially revealing new forces that violate the weak equivalence principle. Here we report on a successful extension of a tool from atom optics—the moiré deflectometer—for a measurement of the acceleration of slow antiprotons. The setup consists of two identical transmission gratings and a spatially resolving emulsion detector for antiproton annihilations. Absolute referencing of the observed antimatter pattern with a photon pattern experiencing no deflection allows the direct inference of forces present. The concept is also straightforwardly applicable to antihydrogen measurements as pursued by the AEgIS collaboration. The combination of these very different techniques from high energy and atomic physics opens a very promising route to the direct detection of the gravitational acceleration of neutral antimatter.