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Issue 12, 2024
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Extracts from the Internet


Detecting geoneutrinos

Antineutrinos from radioactive decays inside the Earth have been registered for the first time by the Kamioka Liquid scintillator antineutrino detector (KamLAND) in Japan. The detector consists of a 13-meter diameter balloon filled with 1000 tonnes of liquid scintillator. The photons registered in the scintillator are emitted by positrons produced in the reaction opposite beta-decay, with an energy threshold of 1.8MeV. Geoneutrinos are mainly produced in the decays of 238U, 235U, and 232Th, but only those from 238U and 232Th have energies above the threshold. Over the two years of the experiment, the geoneutrino detection rate has been about one per month. The main difficulty encountered in previous experiments - one of separating the signal from the background, mainly due to atomic reactors - prevented them from yielding more than the upper limit on the geoneutrino generation rate. Nor, due to uncertainties in the Earth's interior model, were the theoretical predictions accurate enough. Given the heat release effect of the radioactive decays, observing geoneutrinos may teach scientists more about the heating history of the Earth following its formation - thus providing the first practical application of neutrino studies to geophysics. Source: Nature 436 499 (2005)

Superthin crystals

Researchers at Manchester University in the UK and the Institute for Microelectronics Technology in Chernogolovka, Russia, have developed a method for producing 2D crystals just one atomic layer thick. The method, which applies to practically any crystal provided its bulk interlayer coupling is weak, cleaves off individual layers by rubbing crystal edges onto another surface. In this way 2D crystals of, for example, boron nitride, graphite, and of various complex oxides were obtained. Rubbing crystal edges against a surface produced layers varying in thickness and shape, and samples of most interest - single-layer ones - were selected using optical, electron-beam, and atomic-force microscopes. Further study of the layers so obtained showed that they remain stable and retain their structures and properties for several weeks under usual conditions (i. e., at room temperature in air). The new technique has the capacity to test the theoretical models of 2D crystals and can find applications in microelectronics. Source: cond-mat/0508070

The microwaveguide

The diffraction limit restricts the cross section of an optical waveguide to l/2n, where l is the wavelength of the light in vacuum. Recently, experiments have been performed in which light was converted to surface plasmons before being transmitted through the waveguide. Because the value of n is effectively very large for plasmons, the waveguide cross section could be greatly reduced compared to that for photons. However, the waveguides which were used in these experiments - namely, microscopic slits in photonic crystals, metal tapes, and chains of metal nanoparticles - had some disadvantages, such as the complexity of fabrication and wave energy loss. Now, I.I.Smolyaninov, Yu.J.Hung, and C.C.Davis of the University of Maryland have developed a much more efficient, dielectric waveguide for plasmons. The team used a lithography technique to pattern a metal film with an array of microscopic regions of a dielectric material, with a period of 500nm in two directions. This 2D array served to transform the light wave into polaritons, which were focused by a parabolic mirror into a narrow beam capable of propagating along a curve-shaped waveguide. In this way, due to the large value of n near the plasmon resonance, waveguide thicknesses of as low as tenths of a nanometer can be obtained. Compared to other types of waveguides, the frequency transmitted through a dielectric waveguide can be greatly reduced - resulting in much lower signal energy loss. The Maryland experiments suggest that using dielectric waveguides can lead to at least an order of magnitude smaller optoelectronic devices. Source: Phys. Rev. Lett. 95 074801 (2005)

Photonic crystal accelerator

E.Smirnova and her colleagues at the Massachusetts Institute of Technology have demonstrated that materials with a photonic crystal structure can be used to improve the quality of accelerator electron beams and to additionally accelerate beam electrons. The photonic crystal developed by the team - a triangular array of metal rods with one rod absent at the center - transmitted electromagnetic waves at a frequency of 17.0GHz. When an accelerator beam of 16.5-MeV electrons was directed along the array axis and then subjected to microwave pulses, a beam energy increase of 1.4 MeV was obtained, the acceleration gradient being as large as 35MeV per meter. By amplifying and transmitting only the fundamental mode of the beam, the photonic crystal markedly suppressed instabilities arising from to the generation of higher-order harmonics. Source: Proc. Natl. Acad. Sci. 102 10451 (2005)

Plasma jets

Many astrophysical objects - for example, quasars and young stars - involve ejecting narrow plasma jets in their activity. While the formation of the jets is believed to be mainly controlled by powerful magnetic fields, jet acceleration and collimation mechanisms are not yet firmly established. Now an experiment by P.M.Bellan and his colleagues at MIT has produced such jets on a small scale. The authors believe that the jets they observed are generally similar to and can serve as models of astrophysical jets. The setup they used consisted of a metal disk (cathode) and a flat metal ring (anode) around it, which modelled a central cosmic object (for example, a black hole) and an accretion disc, respectively. The disk and the ring were provided with a number of nozzles along their common radii, which were used to inject plasma into the device. The potential difference applied between the disk and the ring gave rise to an electric current in the plasma, and an external magnet created a poloidal magnetic field in the system. The injected plasma created arches between the pairs of nozzles, which merged together, followed by the collimation of plasma into a narrow jet along the axis of the disk. The jet existed for a few tens of microseconds before being destroyed by instabilities. Source: Phys. Rev. Lett. 95 045002 (2005)

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The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) — the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics.

Permanent editor is Yu.N. Eroshenko.

It is compiled from a multitude of Internet sources.
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