Positive muons in bismuth: determination of their sites,detection of a large local lattice contraction and giant associated electric field gradients,evidence for short and long range diffusion

Using a high purity Bi single crystal the temperature and orientation dependence of the zero and transverse field muon spin relaxation rate has been studied in detail. The results imply that the ⁺ occupies one of the two possible interstitial sites in the distorted rhombohedral crystal structure of Bi below 10 K and the other site above 80 K. At both sites the nearest neighbor Bi atoms are found to be shifted towards the ⁺ by 10% of their nominal distance, implying a large local lattice contraction. In concomitance extremely strong electric field gradients are manifest at the nn Bi nuclei. An almost temperature independent reduced relaxation rate in the temperature range from 20 K to 60 K is interpreted in terms of short range diffusion along a limited chain of alternating types of sites. Above 100 K both long range and short range diffusion are indicated.     

Frequency and efficiency measurements on FIR laser lines close to the N+ 3 P 2−3 P 1 transition

FIR laser lines close to the N^{+ 3} P ₂–³ P ₁ transition at 122 m can be used as a local oscillator for heterodyne detectors. The frequencies of some candidate laser lines from CH₂F₂,¹³CH₃OH, CD₃OD, and CD₃OH have been measured and the output power of the most interesting lines has been optimized.     

The influence of substrate material and annealing procedure on the properties of superconducting thin films

Thin layers YBa₂Cu₃O_7–x. are deposited by a laser ablation technique using a pulsed excimer laser operating at 308 nm. The influence of the substrate material and the annealing procedure on the superconducting behaviour of the 123 film and the reactions between the film and the substrate are studied by resistance, X-ray patterns and TEM measurements. The best results are obtained for deposition on (100) SrTiO₃ substrates. The resistance of the 1 m thick film shows a metallic behaviour, an onset in superconductivity at a temperature of 90 K, and has zero resistance at 86 K. The 123 material has a preferential oriented c-axis perpendicular to the surface plane.     

Synthesis of 24-epiteasterone

Starting from ergosterol, the synthesis of the brassinosteroid 24-epiteasterone has been achieved by the use of a new scheme for introducing a 3-hydroxy-6-keto group as the result of the Birch reduction of the corresponding 5-hydroxy-⁷-3,6-dione.Institute of Bioorganic Chemistry, Belorussian Academy of Sciences, Minsk. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 224–227, March–April, 1992.     

Harary-Read numbers for catafusenes: Complete classification according to symmetry

The classical Harary-Read numbers for catafusenes (catacondensed simply connected polyhexes) are reproduced without using generating functions. A complete (mathematical) solution is given for the distribution of these numbers over the different symmetry groups to which the catafusenes belong.     

Study of the dehydration of Portland Cement by Mössbauer spectrometry

Egyptian Portland Cement in the form of one inch cube was hydrated at different times of hydration. Nine cubes of each period of hydration were heated for five minutes 200, 300, 400 up to 1000°C then were quenched in air. The compressive strength was measured for these samples and related to unheated ones. These cubes were ground and measured by Mössbauer spectrometry to correlate the effect of dehydration of cement pastes on the states of iron, with the decrease of compressive strength. It was observed that starting from 400°C the central doublet characteristic of the hydration process decreased as the dehydration temperature was increased. At 1000°C the dehydration process was complete, the central doublet disappeared and the compressive strength vanished. The hydration process was found to be reversible. The application of Mössbauer spectrometry to estimate the degree of fire in concrete building was demonstrated.     

Otpically illuminated dielectric interfaces as high-speed far-infrared modulators

A new device for the gigahertz modulation of far-infrared radiation is analytically and numerically analyzed. It consists of a thin layer of a high-mobility, direct-bandgap semiconductor, such as GaAs, in which a high-density electron-hole plasma is rapidly created and destroyed, thereby rapidly changing the free-carrier reflectivity of the active layer. Illumination by a high-power, near-infrared laser diode array generates the plasma through intrinsic photoconduction. It is shown that this device acis primarily as an amplitude modulator, and that its efficiency increases sharply with increasing far-IR frequency, in contrast to a Schottky diode, which acts primarily as a phase modulator, and whose efficiency falls off sharply with far-IR frequency. The breakeven frequency lies at about 1.5 THz, depending slightly on the assumed device parameters. The relative advantage of the new device increases rapidly with increasing far-infrared frequency. At an operating frequency of 2.5 THz (119 m), for example, a 1 GHz modulation bandwidth may be achieved with a single-sideband conversion loss of only-21 db, versus a Schottky's loss of-39 db, assuming a laser diode power of 1 W, which is readily available from recently developed laser diode arrays.     

Radiative decays of ρ and ω mesons

The results of the measurements of radiative decays of ρ and ω mesons with the Neutral Detector at thee ⁺ e ^? collider VEPP-2M are presented. The branching ratio of the decay ω→π ⁰γ was measured with higher than in previous experiments accuracy: $${\rm B}(\omega \to \pi ^0 \gamma ) = 0.0888 \pm 0.0062$$ . The ρ⁰→π ⁰ γ branching ratio was measured for the first time: $$B(\rho ^0 \to \pi ^0 \gamma ) = (7.9 \pm 2.0) \cdot 10^{ - 4} $$ . The decays ρ, ω→ηγ were studied. Their branching ratios with the assumption of constructive ρ?ω interference are: $$\begin{gathered} B(\omega \to \eta \gamma ) = (7.3 \pm 2.9) \cdot 10^{ - 4} , \hfill \\ B(\rho \to \eta \gamma ) = (4.0 \pm 1.1) \cdot 10^{ - 4} \hfill \\ \end{gathered} $$ . The branching ratios of ρ, ω→ηγ and ω→e ⁺ e ^? decays were also measured: $$\begin{gathered} B(\omega \to \pi ^ + \pi ^ - \pi ^0 ) = 0.8942 \pm 0.0062, \hfill \\ B(\omega \to e^ + e^ - ) = (7.14 \pm 0.36) \cdot 10^{ - 5} \hfill \\ \end{gathered} $$ . The upper limit for the ω→π ⁰ π ⁰ γ branching ratio was placed: B(ω→π ⁰ π ⁰ γ)<4·10^?4 at 90% confidence level.     

Backward particle production in neutrino neon interactions

Backward proton and pion production is studied in ν and \(\bar v\) charged current interactions in neon. The results are compared with other experiments and theory. The complete backward proton data is compatible with protons produced by reinteractions in the nucleus. However in events with only one proton, muon variables appear correlated to those for the backward proton, as expected by the two-nucleon correlation model.