Half-lifeof 60Fe nuclei
1 March 2015
A. Wallner (Australian National University) et al. took a new measurement of the half-life of neutron-excess 60Fe nuclei. The results of the two previous experimental measurements of this quantity differ about two times, presumably because of inaccuracies in the determination of initial number of 60Fe nuclei in samples. The 60Fe isotope for the new measurements was obtained at the Paul Scherrer Institute (Switzerland). Then γ-emission upon the decays of daughter 60Co nuclei in 60Fe → 60Co → 60Ni chains was being registered over four years at Vienna University of Technology. The number of 60Co in a sample increases with time, and the half-life of 60Fe was determined by the increase of γ-radiation flux. The initial 60Fe isotope fraction relative to the number of 55Fe in the same samplewas measured by the accelerator mass-spectroscopy method, which diminished systematic inaccuracy. The measured half-life is close to the result of one of the two preceding experiments, and the value specified over all the data makes up (2.60 ± 0.05)×106 years. In nature, 60Fe isotope is accrued in massive star cores, is synthesized under supernova bursts and under irradiation of meteorites (before they fall onto the Earth) by cosmic rays. This isotope of cosmic origin is present in sedimentary rock at sea bottom. With the help of the specified half-life value one will be able to investigate in more detail the chemical evolution of galactic and solar-system matter and to ascertain the contribution of each of the sources.
Source: Phys. Rev. Lett. 114 041101 (2015)
Analogue of the Mobius strip in optical radiation polarization
1 March 2015
P. Banzer (The Max Planck Institute for the Science of Light, Germany) and his colleagues were the first to obtain in experiment the light field in which the polarization vector is twisted like a Mobius strip. The existence of a light field with topologically complicated polarization was predicted theoretically by I. Freund (the Bar-Ilan University, Israel) in 2005. Such a field was obtained using so-called q-plates, i.e., devices based on liquid crystals that induce wave polarization variation in a space-variable fashion. On passing through a q-plate the light acquired the orbital angular momentum, and a light field occurred at the output with circular polarization at the center of the beam cross section and linear polarization with varying direction on the periphery. After the beam focusing by the microscope objective the longitudinal polarization component occurred outside the focal plane, and 3D polarization distribution appeared in the form of a Mobius strip with three or five half-turns. This field structure was revealed from light scattering by a gold nanoparticle moved in the microscope focal plane (the nanotomography method). Light fields with Mobius-strip type polarization can find application in the creation of unique micro- and nano-devices and in the production of new types of metamaterials.
Source: Science 347 964 (2015)
Magnetic field generation due to Weibel instability
1 March 2015
In the Rochester University (USA), an experiment was performed on Omega Laser Facility demonstrating magnetic field generation in collisionless plasma counter streams as a result of filamentation instability (Weibel instability). Plasma counter streams were generated by high-power laser heating and evaporation of substance of two plastic discs. The observations were carried out using an intense proton flux created in nuclear fusion reactions upon laser heating and implosion of a D-3He-filled container. This proton beam “rayed” the area of mutual penetration of plasma counter streams, and the magnetic field distribution was detected from the declination of proton trajectories. This distribution showed a clearly pronounced filamentation corresponding to Weibel instability with an exponential field strengthening, as well as additional magnetic fields occurring by the Biermann battery mechanism upon disc evaporation. Weibel instability resulted in a convergence of several percent of plasma stream energy into magnetic field energy even in the absence of seed fields. Analogous magnetic field generation processes can take place in shock waves from supernovae or from sources of cosmic gamma-ray bursts.
Source: Nature Physics 11 173 (2015)
Gamma-ray halo around Andromeda Nebula
1 March 2015
Gas halos that exist around galaxies are observed by X-ray gas emission and other effects. In particular, the gas halo around galaxy M31 was registered by the characteristic features in the UV spectrum of quasars on the line of sight and by the distortion of the relic radiation spectrum passing through the M31 halo. If sufficiently strong magnetic fields surround the galaxies, then extensive cosmic-ray (fast charged-particle) halos confined by the magnetic field must also exist. And in the interaction of galactic cosmic rays with rarefied gas, gamma rays must be generatedin the halo. M.S. Pshirkov (SAI MSU, INR RAS, and Pushchino Radio Astronomy Observatory, Astro Space Center of Lebedev Physical Institute), V.V. Vasilyev (Max Planck Institute for Astronomy), and K.A. Postnov (Sternberg Astronomical Institute MSU) were the first to detect such gamma-ray haloaround galaxy M31 on the basis of cosmic gamma-ray Fermi LAT data. The Fermi LAT data obtained over 5.5 years were used. Extended gamma-ray emission with total luminosity of (8.4 ± 4.6)×1038 erg s-1 and the spectral index Γ=1.52 ± 0.21 was revealed in the energy range of 0.3-100 GeV within 3° (40 kpc) from the center of M31 at 4.4 σ confidence level. The existence of gamma-ray halo proves that 10-100 nGs must span around M31 galaxy up to distances of about 40 kpc.
Source: arXiv:1501.03460 [astro-ph.GA]
Relic gravitational waves
1 March 2015
A possible detection by BICEP2 telescope of the contribution to polarization of relic radiation from the gravitational waves generated at the cosmological inflation stage was reported in 2014. However, cosmic dust clouds could account for a similar polarization pattern. The new joint analysis of the data from telescopes BICEP2, Keck Array, and Planck showed that the excess polarization signal revealed in 2014 can actually be exhaustively explained by photon scattering by dust. BICEP2 performed observations at a frequency of 150 GHz, while Planck span the same sky area at a frequency of 353 GHz at which the whole polarization signal is due to dust. The clear correlation of signals at different frequencies indicated that the BICEP2 signal must result from the same dust, and the restriction was obtained for the tensor-to-scalar mode ratio r<0.12 at a confidence level of 95 %. Therefore, the report on the discovery of primary B mode of polarization and relic gravitational waves was untimely. More exact cosmic dust data are needed for their detection against the background.
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.
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