Temperature dependence of recoilless fraction in tungsten

The Mössbauer recoilless fractions of¹⁸²W,¹⁸³W,¹⁸⁴W, and¹⁸⁶W have been calculated by using Born-von Karman model of lattice vibrations. There is a good agreement between the experimental and calculated results.     

Temperature dependence of the work function of hafnium islands on tungsten

Two-dimensional islands of hafnium near {100{ faces of tungsten and their terraces were examined by field electron microscopy. The two-dimensional sublimation energy of the islands was found to be 6.1±0.5 eV. The field electron emission from the islands grows with temperature, the effect being considerably greater than theoretically expected. The negative temperature coefficient of ϕ, α_T, for the islands was estimated at (4–6)×10⁻⁴ eV grad⁻¹ or more, whereas for pure tungsten α_T was found to be 2×10⁻⁵ eV grad⁻¹.     

Time dependence of the source recoilless fraction for a cobalt-57 in rhodium source

The effect of source resonance self absorption (SRSA) has been measured for⁵⁷Co in a rhodium matrix by determining the ratio of the intensity of the 14.4 keV gamma transition to that of the associated 22 keV rhodium x-ray, directly counting both intensities from a stationary source. A ratio, ρ, is defined asf _s/f _so, wheref _s is the recoil-free fraction with the SRSA effect present, andf _so is in the absence of SRSA effects [1]. The value of ρ is to within an additive constant, proportional to the gamma over the X-ray intensities. In the absence of SRSA it is expected that both the 14.4 keV and the 22 keV decay rate would be identical to that of the⁵⁷Co parent, and ρ would be unity and time independent. But because of SRSA a fraction of the 14.4 keV gammas are reabsorbed by⁵⁷Fe, which is increasing as a result of⁵⁷Co decay, and hence, ρ decreases with time. By direct measurement our results confirm that the effect of SRSA can be appreciable even over short time intervals, and we find that in many cases SRSA may have a pronounced effect on Mössbauer experiments, affecting both the line width andf-value, and contrary to general practice in the field, needs to be evaluated in high intensity source experiments.     

Energy dependence of electron-induced radiation damage in tungsten

Abstract

The energy dependence of low dose damage production in commercial and high purity polycrystalline tungsten wires was studied near 350 K with 1.6 to 2.4 MeV electrons. From resistivity measurements at 291 K the threshold energy for the onset of observable damage was determined as 50 × 2 eV. An ‘effective’ threshold of 52 ±2 eV was also determined by directly fitting the energy dependence of the damage rates to theoretical displacement cross sections calculated from step-function displacement probabilities. A decrease of two orders of magnitude in impurity content reduced damage rates by about a factor of two but did not affect threshold. These results combined with current defect recovery models for tungsten, low temperature threshold data, and computer-calculated bcc damage theory suggest: (1) Observed damage consisted of equal concentrations of vacancies and impurity-trapped Stage I free interstitials. (2) Across Stage II (100 K to 600 K) onset threshold should be within 50 ±2 eV. (3) Minimum recoil energy required for free interstitial production near 0 K is 53 ± 5 eV. (4) Threshold has little dependence on crystal direction. An empirical method is presented for predicting threshold energies in the bcc transition metals by assuming the directional dependence of threshold is directly proportional to that of Young's modulus. By the use of one universal proportionality constant (1.2 × 10^?11 eV.cm²/dyne), thresholds for a number of metals and directions are calculated and shown to have significantly better agreement with experiment than the best available theoretical estimates.     

Irradiance dependence of photofield emission from tungsten

Measurements of the irradiance dependence of photofield emission from tungsten have been extended to 1.0 GW m⁻². The photoyield (the photocurrent per unit irradiance) from the (111) crystal plane is found to be almost independent of irradiance. The (211) and (310) crystal planes are found to exhibit small nonlinearities which are attributed to changes in the work function of the emitting surface caused by laser heating. Even at the highest irradiance, no measurable current of photoelectrons arising from multiphoton excitation is observed.     

Strain-rate dependence of the brittle-to-ductile transition temperature in tungsten

We have investigated the strain-rate dependence of the brittle-to-ductile transition (BDT) temperature in pre-cracked tungsten single-crystals and polycrystals. There is an unambiguous Arrhenius relationship over four decades of strain rate, giving an activation energy for the process controlling the BDT of 1.05?eV. This is equal to the activation energy for double-kink formation on screw dislocations, suggesting that their motion controls the brittle–ductile transition.     

Temperature dependence in sputtering

We have obtained the result on the temperature dependence in sputtering of silver which is contrary to the current believes by taking steps to eliminate spurious effects due to changes of residual gas pressure and target temperature, and by determining the relevant energy range for thermal sputtering. The result i3 thought to be due basically to the thermal spike effect, and agreement is shown with a postulated thermal spike model, in which the instantaneous variation of spike temperature against thermal diffusivity is treated on the basis of Carslaw's solution of the normal heat conduction equation. The thermal diffusivity is shown to be proportionate to the thermal conductivity subject to a constant metal density in the temperature range 300–500K and a heat capacity that obeys the Dulong-Petit law which holds good for most metals at high temperatures (> θ_D). The thermal conductivity versus preheat target temperature curve matches the Makinson electronic thermal conductivity curve for metals.     

Temperature dependence of viscosity

A correction to the simple exponential temperature dependence of viscosity arises because of a difference between the real and ideal conditions for a viscous fluid flow. This correction is similar to those to pressure and volume in the real gas law. As a result, the expression for viscosity takes the form

Linear intensity dependence of photo-induced field emission from tungsten

As a first step in an investigation of the polarization dependence of photo-induced field emission from tungsten, the dependence of the photocurrent on the intensity of illumination has been studied. The beam from a CW krypton ion laser operating at selected wavelengths in the visible region of the spectrum was focussed onto the field emitter, and the emission current in a narrow energy range (chosen to minimize tip heating effects) was measured as a function of intensity. The apparatus was designed to ensure that the position of the focal spot and the intensity of illumination could be adjusted without mutual influence. The photocurrent from each crystal plane was found to be linearly proportional to the luminous intensity over the range of intensity investigated (0–35 MW m^?2). It is concluded that in tungsten there exists a low intensity regime where the external current in photo-induced field emission is caused predominantly by one-photon excitations.     

Temperature dependence of muonium in hydrocarbons

The diamagnetic and muonium fractions and their decay constants were measured for cyclohexane, neopentane andn-hexane for the temperature region covering liquid, plastic and non-plastic crystals. The muon and muonium parameters were almost the same for liquids and plastic crystals, but they showed a marked change in non-plastic crystals; a rapid decay of Mu precession, relaxation of the diamagnetic muon component and an increase in the lost polarization have been found to be characteristic of non-plastic crystals.     

Field emission fluctuation phenomena of potassium microcrystals on tungsten: II. Temperature dependence and simulation of noise properties

The temperature dependence of the flicker noise of potassium microcrystals investigated in Part I is described in terms of spectral densities, exponents of spectral density functions, knee frequencies, and noise activation energies. The results of a simulation procedure of the spectral properties and their temperature dependence by different Lorentzian functions are presented and discussed together with some autocorrelation functions obtained by Wiener-Khintchine transformation of the simulated spectra. The observed noise is thought to be due to fluctuations in shape of the crystal in the built-up state, changes in cluster size and premelting phenomena.     

Temperature dependence of morphology of supermolecular structures growing in the thin layer melt of fractions of poly(ethylene oxide)

Poly(ethylene oxide) fractions with molecular weights in the range from 1.55 × 10³ to 1.5 × 10⁵ were studied. The optical microscopic observations of the morphology of supermolecular structures grown isothermally from the melt were used to determine the dependance of structural-morphological changes on temperature. Two morphological transitions were established: (1) a high temperature transition at 49 [Ptilde] 3° C from hedritic structures to positive spherulitoids and (2) a low temperature transition at 38 [Ptilde] 2° C from positive spherulitoids to negative spherulites. It is supposed that the transition from hedrites to positive spherulitoids proceeds through a transition from b-axis-oriented lamellae with tilted chains to [401] lamellae with chains perpendicular to the lamellar surface.     

Temperature dependence of coupling constants

We study the temperature-dependence of coupling constants at the one-loop level for massive ?⁴ theory and massive scalar electrodynamics (SED). It is found that the scalar coupling constant λ for m² > 0 decreases with temperature leading to a phase transition to a non-interacting phase. In a model with m² < 0, λ increases as 1n T. The gauge coupling constant of SED increases uniformly with temperature.     

Temperature dependence of short-range-order parameters

The short-range-order parameters are calculated as a function of the temperature, in a linear approximation. The calculation is made in two versions: taking into account the ordering energy in the first three and the first eight coordination spheres. The temperature T varies in the range 1.11T_C to 3.0T_C. It is shown that up to temperatures of order 3T_C the spectrum of short-range-order parameters is determined by the total set of nonzero ordering energies.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 48–52, April, 1978.     

Temperature dependence of infrared spectra

A simple form of cell for the study of infrared absorption spectra at low and high temperatures is described. Preliminary results of a study of the temperature-phase dependence of fluorobenzene and m-toluic acid spectra are reported.     

Temperature dependence of liquid volume

It is shown that the temperature dependence of the liquid volume is well described by the equation $ V = A + BT + CT^2 + V_e \exp ( - E/RT), $ V = A + BT + CT^2 + V_e \exp ( - E/RT), where A, B, C, V _e , and E are constants. This equation reflects two processes owing to which the liquid volume increases with temperature, namely, anharmonic oscillations of molecules and formation of “holes.”     

Temperature dependence of liquid viscosity

Least squares programs were used to evaluate the correlation between recent experimental results and theoretical, semi-theoretical, or empirical relations between liquid viscosity and temperature. It was found that none of these describes the experimental dependence in the whole range of temperature. A new dependence based on the free volume concept and cell-hole liquid theory is proposed. The theory permits linearization of the viscosity-temperature data in the range (T_i, T_i+1), where the T_i's are the liquid-liquid transition temperatures. It was demonstrated that these transitions, both in small molecular and in polymeric liquids, occur in discrete steps: T_i = a_iT_g, where T_g is the glass transition temperature and the a_i's are numerical parameters. Not all the transitions T_i were apparent in all liquids. Transition T₂ = 1.26T_g was observed for most polymeric liquids. On the basis of thermal analysis it was demonstrated that the T_i's coincide with the temperatures at which small changes in the apparent specific heat were detected. These temperatures can be assigned to the maximum rate of crystallization and melting temperatures of the metastable and stable crystalline forms.     

Temperature dependence in reflection Kikuchi-like patterns

A remarkable temperature effect has been found in Kikuchi-like patterns obtained with 30 keV-electrons from single crystals of low Debye temperature (e.g. lead).     

Temperature dependence of superradiance intensity

Using the idea of exchange interaction in a system of two-level atoms participating in a superradiance process, we derive from first principles the superradiance Hamiltonian of such a system, which is found to be analogous to the Heisenberg Hamiltonian. We consistently calculate the coupling constant of the interaction that leads to the emergence of a superradiance state in the system. We also predict the existence of isospin excitations in the superradiance state, whose presence reduces the intensity of the corresponding superradiance pulse. Finally, we calculate the temperature dependence of the intensity of the superradiance pulse and find it be analogous to the Bloch T ^3/2-law for spin systems. Zh. éksp. Teor. Fiz. 116, 1148–1160 (October 1999)