Molecular effects in the energy loss of alpha particles in gaseous media

Previous measurements in gaseous hydrocarbon and carbon-oxygen compounds in the range 0.3 ≦ E_α ≦ 2.0 MeV revealed that the Bragg additive rule failed to give molecular stopping cross sections ε_α from the measured atomic solid carbon stopping cross section ε(C) and diatomic ε_expt(H₂) or ε_expt(o₂). For example, when hydrogen was present in the compound, the molecular data could be fitted either by using ε(C) and an ε'(H) which was higher than $\text{1}\text{2}\text{ε}{}_{\mathrm{expt}}\text{(H}{}_2\text{)}$ by as much as 21%, or by using $\text{1}\text{2}\text{ε}{}_{\mathrm{expt}}\text{(H}{}_2\text{)}$ and another ε'(C) which was greater than ε(C) by as much as 22%. A similar difficulty has also been encountered in interpreting the stopping cross sections recently measured for C-H-F and C-H-O compounds. ε(C), ε_expt(H₂) and ε_expt(O₂) can be used to fit (CH₃)₂O, but it is necessary to use ε'(C) and ε_expt(H₂) for C₂H₂F₂. Nevertheless, a closer examination of the stopping cross sections for eight hydrocarbon compounds reveals a correlation between the stopping cross sections of atomic hydrogen and carbon which are required to satisfy the Bragg rule, and the type of bond associated with the atomic carbon in the molecule. The stopping cross section of a carbon atom is greater when the atom is bound by a triple bond than a single bond, whereas that of a hydrogen atom is less in a triple-bond compound than in a single-bond compound. Double-bonded compounds yield a carbon stopping cross section between the single-bond and triple-bond values at lower energies and closer to the triple-bond values at higher energies.     

Optical absorption and photoconductovity in amorphous indium selenide thin films

Optical absorption coefficient and photoconductivity measurements were performed on amorphous indium selenide thin films produced by vacuum evaporation. The dependence of the absorption coefficient α on the photon energy ħω at the edge of the absorption band is well described by the relation $\text{α}\text{h}\text{̵}\text{ω = B(}\text{h}\text{̵}\text{ω - E}{}_{\mathrm{<mtext>opt</mtext>}}\text{)}{}^2$ where B is a quality factor and E_opt is the optical band gap. The steady state photoconductivity as a function of the temperature and light intensity is tentatively interpreted in terms of a simple kinetic model proposed by Weiser et al.     

Temperature dependence of the surface diffusion distance of bismuth atoms adsorbed on mica,carbon and silicon monoxide surfaces

The mean distance of surface diffusion of bismuth adatoms on mica, carbon and silicon monoxide surfaces has been determined at different temperatures by measurement of the instantaneous sticking coefficient and the nucleus density.The surface diffusion distance has been found to increase with decreasing temperature in accordance with the formula

$\text{X=}\text{1}\text{2}\text{a}{}_0\text{v}{}_{\mathrm{0d}}\text{v}{}_{\mathrm{0a}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{exp}\text{E}{}_{\mathrm{a}}\text{? E}{}_{\mathrm{d}}\text{2RT}$

at temperatures above 413, 373 and 383 K for mica, carbon and silicon monoxide respectively. Here X is one-half of the diffusion distance, E_a is the adsorption energy, E_d the activation energy for surface diffusion, a₀ the diffusion jump distance and v_0a and v_0d the vibrational frequencies associated with re-evaporation and with surface diffusion respectively. Below these temperatures it has been found that the temperature dependence of the diffusion distance deviates from the above formula; this can be explained by the presence of residual gas molecules adsorbed on the surfaces.From the temperature dependence of the diffusion distance, the respective values of the pre-exponential term $\text{a}{}_0\text{(}\text{v}{}_{\mathrm{0d}}\text{v}{}_{\mathrm{0a}}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and the difference of energies E_a?E_d have been estimated as 7.6 Å and 5.8 kcal mol^?1 for mica, 17 Å and 3.2 kcal mol^?1 for carbon and 58 Å and 1.3 kcal mol^?1 for silicon monoxide.     

The Erlangen multidetector neutron facility

The Erlangen multidetector neutron facility is used in investigations of nuclear reactions and scattering processes with polarized neutrons. The dc neutron beam is produced by the $\text{D}\text{(}\text{d}\text{,}\text{n}\text{)}{}^3\text{He}$ reaction in the energy range of E_n = 9–14 MeV. Behind an iron collimator embedded in a concrete shielding, angular distributions are simultaneously measured with 22 scintillation detectors. The overall amplification of the electronics is stabilized by a small readjustment of the photomultiplier voltage of each detector on the high voltage side. The n-γ discrimination of all detectors is controlled on-line and displayed via a personal computer.     

On the interpretation of tunnelling into an amorphous semiconductor

Several recent measurements of the tunnel conductance G(V) in metal-insulator-amorphous semiconductor film junctions have shown, at V = 0, the $\text{G(0) ∝}\text{exp}\text{(-BT}{}^{\mathrm{<mtext>-</mtext><mtext>1</mtext><mtext>4</mtext>}}\text{)}$ temperature dependence characteristic of the variable-range phonon-assisted tunnelling model of Mott. It is suggested, however, that observation of this temperature dependence for a bias $\text{V》kT}\text{e}$ indicates failure to achieve tunnel injection and internal equilibrium in the amorphous film. At equilibrium in the low temperature case, effective electron-phonon coupling can lead to the proportionality $\text{G(V)∝}\text{∫}\text{0}\text{eV}\text{?(E)}\text{d}\text{E}$ where ?(E) is the energy density of states, rather than to G(V) ∝ ?(eV) as is commonly assumed.     

Effect of mechanical properties of material on rate of fatigue crack propagation

Many experimental and analytical equations on a rate of a fatigue crack propagation have been proposed. However, it seems that they can not fully express its complex behavior. There are still many problems remaining to be solved in order to clarify its mechanism. One of them is to clarify the relation between the rate of the crack propagation and the mechanical properties of material. In this paper, the rate of the crack propagation is analysed to clarify this problem. This analysis is based on the observation results of the fatigue crack propagation behavior previously by the authors. The analytical result is compared with the experimental one to make sure that they agree with each other. The conclusion obtained is; the rate of fatigue crack propagation is expressed by using the stress intensity factors as

$\text{dl}\text{dN}\text{= {}\text{c}\text{[Y}{}^2\text{F}{}^{\mathrm{a}}\text{E}{}^{\mathrm{a(1?n)}}\text{]}\text{} (K}{}_{\max }\text{)}{}^2\text{(K}{}_{\mathrm{a}}\text{)}{}^{\mathrm{a(2?n)}}$

. where C is a constant; E, Young's modulus; F, plastic coefficient; Y, yield stress; K_max and K_a, maximum and amplitude of the stress intensity factor, and α and n, exponents of the Manson-Coffin's law and work-hardening.     

An analysis of reported fatigue crack growth rate data with special reference to Ti-6A1-4V

A review of the published literature on fatigue crack growth suggests that power-law growth in Ti-6A1-4V is sensitive to microstnictural changes which result in variations in the fatigue mechanism. Microstnictures which promote secondary cracking along α/β interfaces display slow growth rates while microstructures which promote dimpled rupture display fast growth rates. Examples of similar effects are found in other alloy systems.Typically, the power-law growth are found in other alloy systems. It is also suggested that the power-law regime begins at $\text{ΔK ～- 13 MNm}{}^{\mathrm{<mtext>?</mtext><mtext>case3</mtext><mtext>2</mtext>}}$, coinciding with the lower limit of striation formation on the fracture surface. The upper limit occurs at about $\text{K}{}_{\max }\text{= 1/2K}{}_{\mathrm{c}}$. At higher growth rates, the Forman equation appears to be adequate.The normalized stress intensity factor, $\text{ΔK/E}$, required to produce a given growth rate in Ti-6A1-4V is on the order of that for other Ti-base alloys, ferritic steels, martensitic steels and aluminum alloys. Austenitic steels, which deform by planar slip are much more resistant to crack growth over much of the stress intensity range normally encountered.     

A model for fatigue crack growth in a threshold region

The prediction of fatigue crack growth at very low ΔK values, and in particular for the threshold region, is important in design and in many engineering applications. A simple model for cyclic crack propagation in ductile materials is discussed and the expression

$\text{da}\text{dN}\text{=}\text{2}{}^{\mathrm{1+n}}\text{(1?2v)(ΔK}{}^2{}_{\mathrm{eff}}\text{?ΔK}{}^2{}_{\mathrm{c,eff}}\text{)}\text{4(1+n)π σ}{}^{\mathrm{1?n}}{}_{\mathrm{yc}}\text{E}{}^{\mathrm{1+n}}\text{?}{}^{\mathrm{1+n}}{}_{\mathrm{f}}$

developed. Here, n is the cyclic strain hardening exponent, σ_yc is cyclic yield, and ε_f is the true fracture strain. The model is successfully used in the analysis of fatigue data BS 4360-50D steel.     

Surface scattering effects in CdS thin films

A method, based upon the careful investigation of the dependence of the Hall coefficient on thickness, to determine the value of the mean surface scattering length λ is presented. Hall data presented for slowly deposited cadmium sulphide thin films indicate the relationship between λ and the carrier concentration. Data are also presented to exhibit the critical dependence of λ upon substrate temperature and deposition rate. The temperature dependence of the parameter λ/μ_b indicates the predicted $\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ relationship.     

Some optical and electrical properties of the As2Se3−xTex system

Some optical and electrical properties of the As₂Se_3?xTe_x system (0 ≤ x ≤ 0.15) have been studied. The value of the optical gap decreases from 1.74 eV to 1.59 eV. The slope of the absorption edge changes, so that value of E_o from the relation α ～ exp ($\text{h}\text{?}\text{ω}\text{E}{}_{\mathrm{o}}$) exhibits a maximum value in neighbourhood of composition As₂Se_2.9Te_0.1. The d.c. and a.c. conductivities have been studied. The value of the activation energy E_el ～ 1.8 eV is almost unchanged up to x = 0.15. The a.c. conductivity fulfils the relation $\text{б(ω) ～ ω}{}^{\mathrm{<mtext>s</mtext>}}$ where s ? 1 and 1.1. at frequencies f < 1 kHz and f > 1 kHz respectively.     

Fiber-optic lightguides in scintillator telescopes for charged particle detection

The use of flexible fiber-optic bundles as lightguides for thin scintillators in a telescope is described. The telescopes will be used in the pion induced fission experiment $\text{π}{}^{\mathrm{+}}{}^6\text{Li}\text{→ 2}{}^3\text{He}$. The fiber-optic lightguide detector was tested, under identical condi against a conventional lightguide-detector arrangement and for α-particles of 5.5 MeV. An efficiency > 99% was obtained with a relative transmission efficiency of ～ 45% compared to the conventional lightguide arrangement.     

Grain boundaries and their effect on thermonigration in pure lead at low diffusion temperatures

Thermomigration in pure lead has been observed by a material flow technique over the temperature range 595-475 K. In all circumstances the flow of material was from hot to cold. At high diffusion temperatures this flow could be described, in both single crystals and polycrystals, by a reduced heat of transport $\text{Q}{}^1\text{? h}{}_{\mathrm{<mtext>f</mtext>}}\text{= 0.106±0.004}\text{eV}$. In the temperature range 515-475 K thermomigration rates in polycrystals were found to exceed those in single crystals maintained under the same conditions. The observed increases in the polycrystal flow rates varied inversely with grain diameter and were consistent with a model of grain boundary thermomigration with an effective grain boundary heat of transport ΔE′ = 0.25 eV.While the atomic flow in grain boundaries is more sensitive to temperature gradients than is that in the volume, it is suggested that the bias in atomic movement due to the asymmetry of scattering interactions is relatively smaller in grain boundaries.     

Effect of texture and grain structure on electromigration in Al-0.5%Cu thin films

The electromigration resistance of Al-0.5%Cu meander lines was found to increase with increasing grain size s and degree of {111} preferred orientation and with decreasing spread σ in the grain size distribution. This dependence on microstructure can be expressed in terms of the empirical quantity $\text{(}\text{s}\text{σ}{}^2\text{)}\text{log}\text{(}\text{I}{}_{111}\text{I}{}_{200}\text{)}{}^3$ which correlates well with the electromigration lifetime of films obtained by different deposition techniques.     

Nouveaux conducteurs ioniques caracterises par une structure a tunnels entrecroises

The ionic conductivity of pyrochlores A_1+α(Ta_1+αW_1?α)O₆ was investigated for A = Na and T1. The thallium compounds are rather good conductors (0.34 ≤ ΔE ≤ 0.40 eV and $\text{5 10}{}^{\mathrm{?8}}\text{(Θ}\text{cm}\text{)}{}^{\mathrm{?1}}\text{≤}{}^{\mathrm{\sigma }}\text{25°}\text{C}\text{≤ 5.5 10}{}^{\mathrm{?5}}\text{(Θ}\text{cm}\text{)}{}^{\mathrm{?1}}$); the sodium oxides are poor conductors (0.76 eV ≤ ΔE ≤ 1.48 eV and $\text{10}{}^{\mathrm{?7}}\text{(Θ}\text{cm}\text{)}{}^{\mathrm{?1}}\text{≤}{}^{\mathrm{\sigma }}\text{500}\text{K}\text{≤ 10}{}^{\mathrm{?5}}\text{(Θ}\text{cm}\text{)}{}^{\mathrm{?1}}$). The differences between these two classes of pyrochlores are explained in terms of structure. New non-stoechiometric oxides T1_12+x(M_30+xW_3?x)O₉₀, with M = Ta, Nb, and 0 ≤ x ≤ 3, were isolated. They are, like pyrochlores, characterized by an intersecting tunnel structure, which is an intergrowth of pyrochlore and A₂M₇O₁₈ structures. These oxides show ionic conduction properties which are very close to those of pyrochlores: the tantalum oxides are better conductors (0.30 eV ≤ ΔE ≤ 0.37 eV ; $\text{3.6 10}{}^{\mathrm{?7}}\text{(Θ}\text{cm}\text{)}{}^{\mathrm{?1}}\text{≤}{}^{\mathrm{\sigma }}\text{25°}\text{C}\text{≤ 1.4 10}{}^{\mathrm{?6}}\text{(Θ}\text{cm}\text{)}{}^{\mathrm{?1}}$) than the niobium oxides (0.36 eV ≤ ΔE ≤ 0.42 eV ; $\text{10}{}^{\mathrm{?7}}\text{(Θ}\text{cm}\text{)}{}^{\mathrm{?1}}\text{≤}{}^{\mathrm{\sigma }}\text{25°}\text{C}\text{≤ 3.8 10}{}^{\mathrm{?7}}\text{(Θ}\text{cm}\text{)}{}^{\mathrm{?1}}$). The evolution ofionic conduction properties of all these compounds is discussed.     

4.6 Some optical properties of thin Cd3As2 films

The transmission of vacuum deposited Cd₃As₂ films has been measured in the spectral range 0.6–15 ωm. The absorption coefficient α has been calculated from the data for crystalline films obtained at substrate temperature T_s = 440 K and also for amorphous films obtained on substrates at T_s = 300?393 K. The position of the low-energy absorption edge in the spectral range 0.1–0.3 eV is discussed and the value Δ_g^opt = 0.06 eV for indirect transion Γ → Γ₁₅ is obtained for crystalline films. For amorphous films, a shift of the minimum absorption edge to $\text{Δ}\text{E}{}_{\mathrm{g}}{}^{\mathrm{opt}}\text{? 0.4?0.5}\text{eV}$ was observed. For higher energies two direct transitions have been established: the first at E₁ = 0.9 eV in the photon energy range 0.9–1.2 eV.and the second at E₂ = 1.2?1.3 eV in the photon energy range 1.3?1.6 eV. The last value is in good agreement with the value calculated by Lin Chung and with reflectivity data; it corresponds to the transition Γ₁₅ → X₁ in the hypothetical band model of Lin Chung. Differences in the reflective index values for crystalline and amorphous Cd₃ As₂ are also discussed.     

Prior-to-failure extension of flaws under monotonic and pulsating loadings

An equation governing the prior to failure crack propagation is proposed. For a rate-sensitive solid containing two-dimensional crack and subject to the tensile mode of fracture the differential equations are integrated numerically for the loads increasing monotonically in time. The resulting integral curves gs = σ(l) and l= l(t), i.e. load vs crack length and length vs time, indicate that the growth of cracks in the subcritical range is strongly rate dependent.The fatigue growth, viewed as a sequence of slow growth periods, is simulated on EAI 380 analogue computer. The fourth power law proposed by Paris is confirmed only within certain range of high-cycle fatigue propagation and for a rate-insensitive solid. Otherwise, that is for a more pronounced rate dependency induced by viscosity of a solid and/or in the proximity of the final instability point the growth is markedly enhanced. For sufficiently small ratios of the applied stress intensity range ΔK to the toughness K_c, the suggested fatigue growth law consists of two terms, i.e.

$\text{d}\text{l}\text{d}\text{n}\text{=}\text{l}{}_1\text{12}\text{4}\text{ΔK}\text{K}{}_{\mathrm{c}}{}^4\text{+Cf}{}^{\mathrm{?1}}\text{ΔK}\text{K}{}_{\mathrm{c}}{}^2\text{, l}{}_1\text{=}\text{πK}{}^2{}_{\mathrm{c}}\text{8Y}{}^2$

First term is the familiar Paris expression while the second one accounts for the rate-dependent contribution; f denotes frequency and Y is the yield strength. Rate-sensitivity C is defined by eq. (1.13).     

Fracture of thin sections containing surface cracks

Surface-cracked specimens of several thicknesses of 7075-T651 and 7075-T6 aluminum were tested in uniaxial tension. For thicknesses t less than 0.25 in., the gross fracture stress σ_f of 7075-T651 Al was empirically related to flaw size by the following expression:

$\text{δ}{}_{\mathrm{f}}\text{σ}{}_{\mathrm{ult}}\text{= 1 + S(}\text{a}\text{φ}{}^2\text{.t}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}$

where σ_ult is the ultimate strength, a the crack depth, φ a function of crack shape, and S a proportionality constant equal to ?1.7 in.^{$\text{?1}\text{2}$}. For 0.25-in. thick 7075-T651 aluminum, σ_f was found to obey this relationship only when $\text{a}\text{φ}{}^2$ is less than 0.065 in.; for larger flaws, such that $\text{0.065 <}\text{a}\text{φ}{}^2\text{< 0.11, σ}{}_{\mathrm{f}}$ is better predicted by Irwin's surface-crack equation with an apparent K_IC value of $\text{32.2}\text{ksi-in.}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$.Fracture data for thin sections of 2014-T6 and 2014-T651 Al tested at ?423°F are analyzed in terms of the empirical relationship above and are found to be in good agreement. For these alloys, S has a value of ?2.6 in.^{$\text{?1}\text{2}$}.Applicability of the empirical relationship and Irwin's surface-crack analysis to the fracture of thin sections is discussed in terms of crack size, section thickness, and plastic zone size.     

Fully engineered high performance UHF SQUID magnetometer

A miniaturised UHF (430 MHz) ac-biased SQUID magnetometer operating at 4.2 K has been designed in which the first stage amplification is provided by a liquid helium cooled GaAs FET amplifier. The magnetometer, which uses in situ varactor tuning for the SQUID tank circuit and FETAmp has a signal energy sensitivity (figure of merit) ? 1.3 × 10^?3JHz^?1 ($\text{ 1 × 10}{}^{\mathrm{?5}}\text{Φ}{}_0\text{Hz}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$ flux sensitivity) in flux lock. The system operates in full flux lock over a frequency range from dc to 5 MHz.