Showing papers by "Technical University of Denmark published in 1972"

Class of constructive asymptotically good algebraic codes

Abstract: For any rate R, 0 , a sequence of specific (n,k) binary codes with rate R_n > R and minimum distance d is constructed such that \begin{equation} \lim_{n \rightarrow \infty} \inf \frac{d}{n} \geq (1 - r ^{-1} R)H^{-1} (1 - r)> 0 \end{equation} (and hence the codes are asymptotically good), where r is the maximum of \frac{1}{2} and the solution of \begin{equation} R = \frac{r^2}{1 + \log_2 [1 - H^{-1}(1 - r)]}. \end{equation} The codes are extensions of the Reed-Solomon codes over GF(2^m) With a simple algebraic description of the added digits. Alternatively, the codes are the concatenation of a Reed-Solomon outer code of length N = 2^m - 1 with N distinct inner codes, namely all the codes in Wozeneraft's ensemble of randomly shifted codes. A decoding procedure is given that corrects all errors guaranteed correctable by the asymptotic lower bound on d . This procedure can be carried out by a simple decoder which performs approximately n^2 \log n computations.

Effect of Voids on Angular Correlation of Positron Annihilation Photons in Molybdenum

Abstract: POSITRON annihilation investigations of defects in crystals have shown that for sufficiently high defect concentrations (typically above about 10−6) all positrons become trapped in the defects before annihilation, thus changing the characteristics of the annihilation process. For example, trapping of positrons may result in the increase in the positron lifetime, a narrowing of the 2-γ angular correlation distribution, and a reduction in the Doppler broadening of the annihilation line. Vacancies in metals1,2, deformation effects in metals3, and defects (F-centres or cation vacancies) in ionic crystals4 have all now been studied by this technique. The trapping of positronium (Ps) in defects in quartz5 and ice6 has also been investigated. Here we report the study of another crystal defect, the void, a small vacancy of diameter few tens of Angstroms. In practice voids may contain gas atoms and may be disturbed either in a random fashion or arranged in a macro lattice7.

The Band Structure of Silver and Optical Interband Transitions

Abstract: The band structure of silver has been calculated using the relativistic augmented plane wave method. Relativistic effects must be included if a quantitative interpretation of optical experiments is made. The L′2 → L1 gap is 1 eV smaller in the relativistic calculation than it is in a non-relativistic band structure obtained from the same potential. This reduction of the L-gap is in the present case sufficient to lead to an extra interband edge below the main edge at 3.98 eV. The optical function e2(ω) and the energy distributions of photoemitted electrons have been calculated.

Variable-length codes and the Fano metric

Abstract: It is shown that the metric proposed originally by Fano for sequential decoding is precisely the required statistic for minimum-error-probability decoding of variable-length codes. The analysis shows further that the "natural" choice of bias in the metric is the code rate and gives insight into why the Fano metric has proved to be the best practical choice in sequential decoding. The recently devised Jelinek-Zigangirov "stack algorithm" is shown to be a natural consequence of this interpretation of the Fano metric. Finally, it is shown that the elimination of the bias in the "truncated" portion of the code tree gives a slight reduction in average computation at the sacrifice of increased error probability.

The effects of sample imperfections on optical spectra

Abstract: The results of the theory of light reflection from rough surfaces have been extended to cover the case of light transmission through a sample with rough surfaces, as well as reflection and transmission for inhomogeneous samples. A concept of “Inhomogeneity Induced Roughness” of the constant phase plane has been introduced to describe the specular reflection from inhomogeneous media. The drastic influence of the multiple scattering effect on the optical spectra has been demonstrated for a simple model. The generalized formulae for reflection and transmission of light for imperfect samples are given. Finally, several peculiar experimental observations concerning exciton spectra in II–VI compounds, light scattering at the plasma frequency in metals, electroreflection, and other effects are discussed. Die Theorie der Lichtreflexion an rauhen Oberflachen wird fur den Fall der Lichttransmission durch eine Probe mit rauhen Oberflachen sowie der Reflexion und Transmission von nicht homogenen Proben erweitert. Der Begriff einer „Rauhheit durch Inhomogenitat” der konstanten Phasenflache wird eingefuhrt, um die Spiegelung an nichthomogenen Medien zu beschreiben. Der drastische Einflus der Vielfachstreuung auf die optischen Spektren wird an einem einfachen Modell erlautert. Es wird eine verallgemeinerte Formel fur die Reflexion und Transmission von Licht an unvollkommenen Proben angegeben. Gestutzt auf diese Vorstellungen wird eine Erklarung vorgeschlagen, um verschiedene, spezielle, experimentelle Beobachtungen auf dem Gebiet der Exzitonspektren von II–VI-Verbindungen, Lichtstreuung bei der Plasmafrequenz in Metallen, Elektroreflexion und anderer Effekte zu erklaren.

Positron Lifetimes in Pure and Doped Ice and in Water

Abstract: Positron lifetime spectra were measured in mono‐ and polycrystalline light ice, polycrystalline heavy ice, doped light ice, as well as in light and heavy water. All spectra were resolved into three components. At temperatures between −196° and −100°C the lifetimes and relative intensities are virtually constant, being 0.12 nsec and 20%, 0.44 nsec and 28%, 0.68 nsec and 52%, and attributed to annihilation of para‐Ps, free positrons, and ortho‐Ps, respectively. Above −100°C the longest lifetime increases with temperature to 1.1 nsec at 0°C. Its relative intensity is unchanged, while the parameters for the other components show a complex behavior. The spectra for mono‐ and polycrystalline light ice and for polycrystalline heavy ice are identical. For water long lifetime components attributed to ortho‐Ps are 1.86 nsec, 27% for H2O and 2.01 nsec, 22% for D2O. Theoretical explanations are suggested. Fast frozen solutions of HF with concentrations larger than approximately 10−5 mole fraction have a lifetime comp...

Correlation of Void Size and Positron Annihilation Characteristics in Neutron-irradiated Molybdenum

Abstract: Correlation of Void Size and Positron Annihilation Characteristics in Neutron-irradiated Molybdenum

Tensile properties of metastable stainless steels

Abstract: Tensile properties of two similar stainless steels, types 301 and 304, were investigated for different strain-rates, grain sizes and testing temperatures, and the bulk concentration of martensite formed during deformation was determined by X-ray diffraction.

High‐Resolution Inelastic Neutron Scattering from Some Aromatic Molecular Polycrystals for Study of Crystal Excitations and Anharmonic Effects

Abstract: Inelastic neutron scattering spectra have been obtained at high resolution from a number of aromatic polycrystals, mainly halogenated benzenes, in the range 4 cm−1‐1600 cm−1, at room temperature and below. Since the spectra observed reflect only nuclear motion (in contrast to optical spectroscopy), analysis reveals direct information on the harmonicity. In para‐dichlorobenzene the angular dependence of the intensity of incoherent scattering, both elastic and inelastic, the relation between the total inelastic and total elastic intensities, and the temperature dependence of these spectra (and those for other substances) are all consistent with a weakly anharmonic model of the dynamics (phonons which interact with one another to give a temperature dependent lifetime and frequency). Assuming that multiphonon effects are negligible, this weakly anharmonic model is valid in p–C6H4Cl2 up to room temperature, 30°K below the melting point. At low temperatures (100°K) the phonon lifetimes are long enough to allow ...

Shoaling of cnoidal waves

Abstract: An equation is derived which governs the propagation of a cnoidal wave train over a gently sloping bottom. The equation is solved numerically, the solution being tabulated in terms of fH (Eq. 47) as a function of Ei = (Etr/pg) 1/3/gT2 and hi = h/gT2. Results are compared with sinusoidal wave theory. Two numerical examples are included.

On forward and backward precession of rotors

Abstract: A study of the excitation of backward and forward precessional motion of rotors is presented. It is shown that if the rotor is supported in bearings with rotational symmetry, a rotating unbalance of the shaft cannot excite backward precessions, but if the bearings have even the slightest anisotropy, backward synchronous precessions are excited just as in the case of forward precessional motions. However, internal damping forces have a marked effect in reducing the amplitudes of the backward precessions.

The determination of work of compression of protein monolayers at the air-water interface

Abstract: In the present study, monolayers of various proteins were investigated at the air-water interface. The work of compression,W c (Helmholtz free energy) has been determined from the surface pressure-area compression isotherms. A linear relationship was found betweenW c and the amount of protein present at the surface. Further, it is shown that this relation holds both for completely unfolded (Bovine-serum-albumin, Ovalbumin) and for incompletely unfolded (Transferrin, Myoglobin) proteins.β-Lipoprotein isotherms also gave a similar dependence. It is further shown that the amount of protein lost into the subphase can be determined from a plot ofW c versus protein added at the interface. The results are discussed in relation to the constitution of protein molecules at the surface.

Short-Time Creep Transition for Hardened Cement Paste

Abstract: Short-time creep recovery tests in two-point bending were performed on hardened cement paste beams water-cured at #100° and 25°C. The experimental variables were pore structure, equilibrium moisture content, and temperature. The recovery curves were analyzed using linear viscoelastic concepts, which revealed a transition in the time range 0.2 to 10s which is both linear with stress and reproducible. A lognormal distribution of retardation times characterizes the transition. It is tentatively proposed that internal redistribution of capillary water is responsible for the transition. Experimental observations which support this view are: (1) The size of the transition increases with increasing amounts of capillary water for saturated pastes; (2) the transition strength decreases greatly when the capillary water is removed; (3) the apparent activation energy of the transition (3.8 kcal/mol) agrees well with that for viscous flow of water.

Membrane osmometry of aqueous micellar systems

Abstract: The number average micellar molecular weights,M n , were determined with the application of membrane osmometry, for various nonionic and a cationic surfactant, in aqueous solutions These results show that these micelles are monodisperse, as also reported by other investigators The dependence ofM n on temperature was also studied It was found that both the nonionic and the cationic micelles showed an abrupt change in logM versus temperature plots In the case of nonionics, the abrupt change was observed at approximately 30 °C lower than the cloud-point