Direct recording of solar pp-neutrino
1 October 2014
Solar neutrinos born in the p + p → 2H + e+ + νe fusion reaction were first recorded directly in Borexino experiment being carried out in the Gran Sasso National Laboratory (Italy). In 99.76 % of cases this reaction initiates the proton-proton cycle in which more than 99% of the entire solar energy is released. In the region of low pp-neutrino energies the background signals are strong, which makes their observation a fairly complicated task. To be screened from the background produced by cosmic rays the Borexino detector was placed in the tunnel under the mountain, and to lower the background from the beta-decays an ultra-pure scintillator is used in Borexino which is based on products of the ancient oil mined from deep nests retaining few radioactive 14C atoms. Photomultipliers register the radiation generated in the scintillator by the electrons recoiled when neutrinos are scattered by them. The measured pp-neutrino flux of (6.6 ± 0.7)×1010 cm-2 s-1agrees well with the value of 5.98×(1 ± 0.006)×1010 cm-2 s-1 predicted in the standard model of the Sun, and the absence of pp-neutrinos is excluded at the level of 10 σ. Comparison of solar radiation in the optical range and its neutrino luminosity proves stability of the Sun over ≈ 105 years in which photons are emitted from the solar core.
Source: Nature 512 383 (2014)
1 October 2014
L.F. Gomez (University of Southern California, USA) et al. used the X-ray diffraction method to study the quantum vortices in superfluid 100-1000 nm helium drops consisting of ≈ 108-1011 atoms. The nanodrops were formed upon helium scattering from nozzle to vacuum. During several milliseconds of the drop flight through the detector they were exposed to femtosecond pulses of a high-power free-electron X-ray laser. X-ray photos showed that ≈ 40 % of nanodrops had the shape of a flattened ellipsoid of revolution and rotated with frequencies up to ≈ 106-107 rev/s. Helium gives low contrast in X-ray photos, and therefore xenon atoms were admixed to it for they scatter X rays well. The Bragg diffraction pattern showed the presence in nanodrops of superfluid vortices arranged in a trigonal lattice. The vortex concentration in nanodrops was up to ≈ 105 times higher than had been observed before in macroscopic helium volumes. For ≈ 1 % of wheel-shaped nanodrops the Bragg pattern was absent. The vortices in these nanodrops are possibly distributed chaotically because of disbalance or quantum turbulence.
Source: Science 345 906 (2014)
Reconnection of vortex rings in 4He
1 October 2014
P.M. Walmsley (University of Manchester, Great Britain) et al. studied the collision of chaotically oriented quantum vortex rings (VR) in superfluid 4He at a temperature of nearly 0.05 K. Upon collision of two VR they could experience reconnection to form a single VR with a larger or smaller radius. The observations were conducted using time-of-flight spectroscopy in a small vessel filled with liquid 4He. Near the injector, the VR received charge from a tungsten needle, and the electric current was measured which was carried by the VR from injector to collector over the time from the beginning of injection. The produced VR had similar prescribed radii of several micrometers. The VR velocity and, accordingly, the time within which they reached the collector depended on their radii, namely, large VR moved slower. The maximum current was reached at the instant of initial VR arrival. The signal at longer times corresponded to the arrival of larger VR produced upon reconnection, and the discrete signals at shorter times testified to the formation of smaller VR and even two successive mergers of VR with other VR. The experimental data are in good quantitative agreement with the theoretical calculations of VR reconnection.
Source: Phys. Rev. Lett. 113 125302 (2014)
Glass structure variation under pressure
1 October 2014
T. Edwards (University of California, Davis, USA) with colleagues observed the instant of transformation of borosilicate (with 11B isotope) glass from BO3He to BO4 configuration in the course of its compression to a pressure of 2 GPa. Earlier these configurations were observed separately before and after compression, and the very instant of transition has not been examined. The glass sample under compression was observed by nuclear magnetic resonance spectroscopy. The spectrum was continuously measured depending on pressure at room temperature. The spectroscopic data obtained are interpreted as follows. In the course of compression the flat BO3complex was elastically deformed into a trihedral pyramid owing to the boron atom escape from the plane in which three oxygen atoms reside. Given this, the boron atom approached the fourth oxygen atom to form tetrahedral BO4 complexes.
Source: Science 345 1027 (2014)
Lithium production in primary nucleosynthesis
1 October 2014
The 2H(α,γ)6Li lithium nucleosynthesis reaction cross section for energies that had existed in the epoch of primary nucleosynthesis in the Universe when it was only several minutes of age was first measured in the LUNA experiment (Gran Sasso, Italy). In conditions of low background an alpha-particle beam hit a deuterium target, and γ-photons from 2H(α,γ)6Li reaction were recorded, which allowed its cross section to be measured. According to the standard theory of primary nucleosynthesis with the cross section specified in the LUNA experiment, the relative 6Li/7Li content in the primary gas makes up (1.5 ± 0.3) × 10-5, whereas in old stars (whose chemical composition was thought to be as close to the primary one) this value is ≈ 5 × 10-2. Thus, the new LUNA data confirm the conclusion that for the yet unknown reason the lithium content in the Universe must have changed considerably already after the primary nucleosynthesis. The hypotheses were put forward that 6Li is produced in stellar flares or in another yet unknown process.
Source: : Phys. Rev. Lett. 113 042501 (2014)
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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