Properties of nanostructured diamond-silicon carbide composites sintered by high pressure infiltration technique

A high-pressure silicon infiltration technique was applied to sinter diamond–SiC composites with different diamond crystal sizes. Composite samples were sintered at pressure 8 GPa and temperature 2170 K. The structure of composites was studied by evaluating x-ray diffraction peak profiles using Fourier coefficients of ab initio theoretical size and strain profiles. The composite samples have pronounced nanocrystalline structure: the volume-weighted mean crystallite size is 41–106 nm for the diamond phase and 17–37 nm for the SiC phase. The decrease of diamond crystal size leads to increased dislocation density in the diamond phase, lowers average crystallite sizes in both phases, decreases composite hardness, and improves fracture toughness

Are Antiprotons Forever?

Up to one million antiprotons from a single LEAR spill have been captured in a large Penning trap. Surprisingly, when the antiprotons are cooled to energies significantly below 1 eV, the annihilation rate falls below background. Thus, very long storage times for antiprotons have been demonstrated in the trap, even at the compromised vacuum conditions imposed by the experimental set up. The significance for future ultra-low energy experiments, including portable antiproton traps, is discussed.Comment: 12 pages, latex; 4 figures, uufiled. Slightly expanded discussion of expected energy dependence of annihilation cross section and rate, and of estimates of trap pressure, plus minor text improvement

A method to measure the resonance transitions between the gravitationally bound quantum states of neutrons in the GRANIT spectrometer

We present a method to measure the resonance transitions between the gravitationally bound quantum states of neutrons in the GRANIT spectrometer. The purpose of GRANIT is to improve the accuracy of measurement of the quantum states parameters by several orders of magnitude, taking advantage of long storage of Ultracold neutrons at specula trajectories. The transitions could be excited using a periodic spatial variation of a magnetic field gradient. If the frequency of such a perturbation (in the frame of a moving neutron) coincides with a resonance frequency defined by the energy difference of two quantum states, the transition probability will sharply increase. The GRANIT experiment is motivated by searches for short-range interactions (in particular spin-dependent interactions), by studying the interaction of a quantum system with a gravitational field, by searches for extensions of the Standard model, by the unique possibility to check the equivalence principle for an object in a quantum state and by studying various quantum optics phenomena

Study of the neutron quantum states in the Earth's gravitational field

We studied the neutron quantum states in the potential well formed by the Earth's gravitational field and a horizontal mirror. The estimated characteristic sizes of the neutron wave functions in two lowest quantum states correspond to their expectations with an accuracy of ~25%. A spatial density distribution in a standing neutron wave above a mirror was measured for a set of a few lowest quantum states. A position-sensitive neutron detector with an extra high spatial resolution of 1-2 mu m was developed and tested for this particular task. Although this experiment was not designed or optimized to search for an additional short-range force, nevertheless it allowed us to slightly improve the published boundary in the nanometer range of characteristic distances. We studied systematical uncertainties in the chosen "flow-through" method as well as the feasibility to improve further the accuracy in this experimen

Investigation of the neutron quantum states in the Earth's gravitational field

We studied the neutron quantum states in the potential well formed by the Earth's gravitational field and a horizontal mirror. The estimated characteristic sizes of the neutron wave functions in two lowest quantum states correspond to their expectations with an accuracy of approximate to 25 %. The spatial density distribution in a standing neutron wave above a mirror was measured for a set of a few lowest quantum states. A position-sensitive neutron detector with an extra high spatial resolution of 1 mu m to 2 mu m was developed and tested for this particular task. Although this experiment was not designed or optimized to search for an additional short-range force, nevertheless it allowed us to slightly improve the published boundary in the nanometer range of characteristic distances. We studied systematical uncertainties in the chosen "flow-through" method as well as the feasibility to improve further the accuracy in this experiment

Measurement of quantum states of neutrons in the Earth's gravitational field

The lowest stationary quantum state of neutrons in the Earth's gravitational field is identified in the measurement of neutron transmission between a horizontal mirror on the bottom and an absorber on top. Such an assembly is not transparent for neutrons if the absorber height is smaller than the "height" of the lowest quantum state

The tommotiid <i>Camenella reticulosa</i> from the early cambrian of South Australia: morphology, scleritome reconstruction, and phylogeny

The tommotiid Camenella reticulosa is redescribed based on new collections of well preserved sclerites from the Arrowie Basin (Flinders Ranges), South Australia, revealing new information concerning morphology and microstructure. The acutely pyramidal mitral sclerite is described for the first time and the sellate sclerite is shown to be coiled through up to 1.5 whorls. Based on Camenella, a model is proposed by which tommotiid sclerites are composed of alternating dense phosphatic, and presumably originally organic−rich, laminae. Camenella is morphologically most similar to Lapworthella, Kennardia, and Dailyatia, and these taxa are interpreted to represent a monophyletic clade, here termed the "camenellans", within the Tommotiida. Potential reconstructions of the scleritome of Camenella are discussed and although a tubular scleritome construction was recently demonstrated for the tommotiids Eccentrotheca and Paterimitra, a bilaterally symmetrical scleritome model with the sclerites arranged symmetrically on the dorsal surface of a vagrant animal can not be ruled out