Silicon nanochannel plates (NCPs) are efficient positron to positronium converters and are employed as sources of abundant cold positronium. Characterization of the emitted positronium energy ...spectrum is crucial to assess the NCP performance and support their further optimization. We present here a Time-of-Flight (ToF) system optimized to measure the energy spectrum of positronium produced by NCP converters held at cryogenic temperatures. The ToF-apparatus was tested by implanting 7keV positrons into NCPs at room temperature before performing ToF experiments using positron implantation energies of 7 and 11keV into NCPs held at 20K. At these conditions we succeeded in determining the energy spectrum of the positronium escaping from the nanochannels into the vacuum down to an energy of 10meV.
Pulsed production of antihydrogen Amsler, Claude; Antonello, Massimiliano; Belov, Alexander ...
Communications physics,
01/2021, Letnik:
4, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
Antihydrogen atoms with K or sub-K temperature are a powerful tool to precisely probe the validity of fundamental physics laws and the design of highly sensitive experiments needs ...antihydrogen with controllable and well defined conditions. We present here experimental results on the production of antihydrogen in a pulsed mode in which the time when 90% of the atoms are produced is known with an uncertainty of ~250 ns. The pulsed source is generated by the charge-exchange reaction between Rydberg positronium atoms—produced via the injection of a pulsed positron beam into a nanochanneled Si target, and excited by laser pulses—and antiprotons, trapped, cooled and manipulated in electromagnetic traps. The pulsed production enables the control of the antihydrogen temperature, the tunability of the Rydberg states, their de-excitation by pulsed lasers and the manipulation through electric field gradients. The production of pulsed antihydrogen is a major landmark in the AE
$$\bar{g}$$
ḡ
IS experiment to perform direct measurements of the validity of the Weak Equivalence Principle for antimatter.
In 2015 May, the Southern hemisphere of comet 67P/Churyumov–Gerasimenko became visible by the OSIRIS cameras on-board the Rosetta spacecraft. The resolution was high enough to carry out a detailed ...analysis of the surface morphology, which is quite different from the Northern hemisphere. Previous works show that fine particle deposits are the most extensive geological unit in the Northern hemisphere. In contrast, the Southern hemisphere is dominated by outcropping consolidated terrain. In this work, we provide geomorphological maps of the Southern hemisphere with the distinction of both geological units and linear features. The geomorphological maps described in this study allow us to gain a better understanding of the processes shaping the comet nucleus and the distribution of primary structures such as fractures and strata.
We describe a multi-step “rotating wall” compression of a mixed cold antiproton–electron non-neutral plasma in a 4.46 T Penning–Malmberg trap developed in the context of the AEḡIS experiment at ...CERN. Such traps are routinely used for the preparation of cold antiprotons suitable for antihydrogen production. A tenfold antiproton radius compression has been achieved, with a minimum antiproton radius of only 0.17 mm. We describe the experimental conditions necessary to perform such a compression: minimizing the tails of the electron density distribution is paramount to ensure that the antiproton density distribution follows that of the electrons. Such electron density tails are remnants of rotating wall compression and in many cases can remain unnoticed. We observe that the compression dynamics for a pure electron plasma behaves the same way as that of a mixed antiproton and electron plasma. Thanks to this optimized compression method and the high single shot antiproton catching efficiency, we observe for the first time cold and dense non-neutral antiproton plasmas with particle densities
n
≥ 10
13
m
−3
, which pave the way for an efficient pulsed antihydrogen production in AEḡIS.
Graphical abstract
The potential of Terrestrial Laser Scanner imaging (TLS) as a tool to map chert, an amorphous variety of silica diffused in sedimentary rocks, is here discussed together with an original method for ...its automatic detection. Reflectance measurements in the VIS-NIR band (400–2500nm) show that chert displays low reflectance in the IR wavelengths that are operated by several commercial TLS. To develop and test a recognition method an outcrop of limestone with chert nodules was scanned with an IR (1541nm) TLS. The intensity information, after proper distance correction, was coupled with geometric and intensity descriptors for training Support Vector Machines (SVM) to separate vegetation from rock and limestone from chert. Results, cross inspected in the field and with reference pictures, demonstrate that TLS data can be efficiently exploited to map chert when the monochromatic information of the intensity is integrated with feature descriptors and SVM classifiers.
In this work, we analyze the ∼103 km-diameter Melkart impact crater on the surface of Ganymede, which is a ∼ 103 km impact crater located at the boundary between the two main geological units of the ...satellite, the light and dark terrains. Such crater is covered both by Galileo SSI and NIMS datasets allowing inference of any possible correlation between its geology and composition, as well as to investigate the evolution of the substrate units. We provide a geomorphological characterization of the crater by creating a high-resolution geological map, investigating the DTM and performing a strain analysis to infer its deformation. We find that the Melkart dome underwent strike-slip deformation with right-lateral kinematics. Moreover, this crater is a low strain end member with subtle deformation, which has been formed in an area of gradual transition between light and dark terrains. The results obtained through band depth and crystallinity maps of the study area do not reflect the evolutionary history of the crater, instead, they are representative of the different geological terrains affecting the subsurface of Melkart. Such result implies that the possible compositional differences between the crater's units might have been obliterated through time. Our analysis supports the interpretation that the material constituting the substrate of the crater might be heterogeneous suggesting that: (i) the transition between the two geological units might be gradual or ii) the dark material located over the study area only represents a thin layer draping the icy crust.
•Analysis of a crater located at the boundary of Ganymede light and dark terrains.•High resolution geological map of Melkart impact crater on the surface of Ganymede.•Melkart is a low strain end member with subtle deformation from strain analysis.•The crater substrate material is heterogeneous from spectral analysis.
Abstract
The study of dust, the most abundant material in cometary nuclei, is pivotal in understanding the original materials forming the Solar system. Measuring the coma phase function provides a ...tool to investigate the nature of cometary dust. Rosetta/OSIRIS sampled the coma phase function of comet 67P/Churyumov–Gerasimenko, covering a large phase angle range in a small amount of time. Twelve series were acquired in the period from 2015 March to 2016 February for this scientific purpose. These data allowed, after stray light removal, measuring the phase function shape, its reddening, and phase reddening while varying heliocentric and nucleocentric distances. Despite small dissimilarities within different series, we found a constant overall shape. The reflectance has a u-shape with minimum at intermediate phase angles, reaching similar values at the smallest and largest phase angle sampled. The comparison with cometary phase functions in literature indicates OSIRIS curves being consistent with the ones found in many other single comets. The dust has a negligible phase reddening at α < 90°, indicating a coma dominated by single scattering. We measured a reddening of 11–14 %/100 nm between 376 and 744 nm. No trend with heliocentric or nucleocentric distance was found, indicating the coma doesn’t change its spectrum with time. These results are consistent with single coma grains and close-nucleus coma photometric results. Comparison with nucleus photometry indicates a different backscattering phase function shape and similar reddening values only at α < 30°. At larger phase angles, the nucleus becomes significantly redder than the coma.
PDF
