arXiv subjects

Gianangelo Bracco

Publications and source records attributed to Gianangelo Bracco.

4 recordsLinked to original sources

arXiv2023

Reflection imaging with a helium zone plate microscope

Neutral helium atom microscopy is a novel microscopy technique that offers strictly surface-sensitive, non-destructive imaging. Several experiments have been published in recent years where images are obtained by scanning a helium beam spot across a surface and recording the variation in scattered intensity at a fixed total scattering angle $θ_{sd}$ and fixed incident angle $θ_{i}$ relative to the overall surface normal. These experiments used a spot obtained by collimating the beam (referred to as helium pinhole microscopy). Alternatively, a beam spot can be created by focusing the beam with an atom optical element. However up till now imaging with a focused helium beam (referred to as helium zone plate microscopy) has only been demonstrated in transmission. Here we present the first reflection images obtained with a focused helium beam. Images are obtained with a spot size (FWHM) down to 4.7 $μ$m $\pm$ 0.5 $μ$m, and we demonstrate focusing down to a spot size of about 1 $μ$m. Furthermore, we present the first experiments measuring the scattering distribution from a focused helium beam spot. The experiments are done by varying the incoming beam angle $θ_{i}$ while keeping the beam-detector angle $θ_{sd}$ and the point where the beam spot hits the surface fixed - in essence, a microscopy scale realization of a standard helium atom scattering experiment. Our experiments are done using an electron bombardment detector with adjustable signal accumulation, developed particularly for helium microscopy.

physics.ins-det↗

arXiv2022

Neutral Helium Microscopy (SHeM): A Review

Neutral helium atom microscopy, also referred to as scanning helium microscopy and commonly abbreviated SHeM or NAM (neutral atom microscopy), is a novel imaging technique that uses a beam of neutral helium atoms as an imaging probe. The technique offers a number of advantages such as the very low energy of the incident probing atoms (less than 0.1 eV), unsurpassed surface sensitivity (no penetration into the sample bulk), a charge neutral, inert probe and a high depth of field. This opens up for a range of interesting applications such as: imaging of fragile and/or non-conducting samples without damage, inspection of 2D materials and nano-coatings, with the possibility to test properties such as grain boundaries or roughness on the Angström scale (the wavelength of the incident helium atoms) and imaging of samples with high aspect ratios, with the potential to obtain true to scale height information of 3D surface topography with nanometer resolution: nano stereo microscopy. However, for a full exploitation of the technique, a range of experimental and theoretical issues still needs to be resolved. In this paper we review the research in the field. We do this by following the trajectory of the helium atoms step by step through the microscope: from the initial acceleration in the supersonic expansion used to generate the probing beam over the atom optical elements used to shape the beam, followed by interaction of the helium atoms with the sample (contrast properties) to the final detection and post-processing. We also review recent advances in scanning helium microscope design including a discussion of imaging with other atoms and molecules than helium.

physics.atm-clus↗

arXiv2021

Commissioning and improvements of the instrumentation and launch of the scientific exploitation of OARPAF, the Regional Astronomical Observatory of the Antola Park

The \oarpaf telescope is an $80\rm cm$-diameter optical telescope installed in the Antola Mount Regional Reserve, in Northern Italy. This work presents the results of the characterization of the site, as well as developments and interventions that have been implemented, with the goal of exploiting the facility for scientific and educational purposes. During the characterization of the site, an average background brightness of $22.40 \, m_{AB}$ ($B$ filter) -- $21.14 \, m_{AB}$ ($I$) per arcsecond squared, and a $1.5$--$3.0"$ seeing, have been measured. An estimate of the magnitude zero points for photometry is also reported. The material under commissioning includes 3 CCD detectors for which we provide the linearity range, gain, and dark current; a 31 orders échelle Northern Italy. This work presents the results of the characterization of the site, as well as developments andspectrograph with $R\sim 8500$--$15000$, and a dispersion of $n = 1.39\times 10^{-6}\rm px^{-1}λ+ 1.45\times 10^{-4}\rm nm/px$, where $λ$ is expressed in $\rm nm$. The scientific and outreach potential of the facility is proven in different science cases, such as exoplanetary transits and active galactic nuclei variability. The determination of time delays of gravitationally lensed quasars, the microlensing phenomenon and the tracking and the study of asteroids are also discussed as prospective science cases.

astro-ph.IM↗

arXiv2018

Centre line intensity of a supersonic helium beam

Supersonic helium beams are used in a wide range of applications, for example surface scattering experiments and, most recently, microscopy. The high ionization potential of neutral helium atoms makes it difficult to build efficient detectors. Therefore, it is important to develop beam sources with a high centre line intensity. Several approaches for predicting the centre line intensity exist, with the quitting surface model incorporating the largest amount of physical dependencies. However, until now only a limited amount of experimental data has been available. Here we present a study where we compare the quitting surface model with an extensive set of experimental data. In the quitting surface model the source is described as a sphere from where the particles leave in a molecular flow determined by Maxwell-Boltzmann statistics. We use numerical solutions of the Boltzmann equation to determine its properties. The centre-line intensity is then calculated using an analytical integral. This integral can be reduced to two cases, one which assumes a continuously expanding beam until the skimmer aperture, and another which assumes a quitting surface placed before the aperture. We compare the two cases to experimental data with a nozzle diameter of 10 micron, skimmer diameters ranging from 4 micron to 390 micron, a source pressure range from 2 to 190 bar, and nozzle-skimmer distances between 17.3 mm and 5.3 mm. To support the two analytical approaches, we have also performed equivalent ray tracing simulations. We conclude that the quitting surface model predicts the centre line intensity well for skimmers with a diameter larger than 120 micron when using a beam expanding until the skimmer aperture. For the case of smaller skimmers the trend is correct, but the absolute agreement not as good. We propose several explanations for this, and test the ones that can be implemented analytically.

physics.atm-clus↗