Values Of Τ2 Research Articles

Positron annihilation measurements in high-energy alpha-irradiated n-type Gallium Arsenide

Positron annihilation lifetime spectroscopy and Doppler broadening annihilation line-shape measurements have been carried out in 40-MeV alpha-irradiated n-type GaAs. After irradiation, the sample has been subjected to an isochronal annealing over temperature region of 25–800 °C with an annealing time of 30 min at each set temperature. After each annealing, the positron measurements are taken at room temperature. Formation of radiation-induced defects and their recovery with annealing temperature are investigated. The lifetime spectra of the irradiated sample have been fitted with two lifetimes. The average positron lifetime τavg = 244 ps at room temperature after irradiation indicates the presence of defects, and the value of τ2 (262 ps) at room temperature suggests that the probable defects are mono-vacancies. Two distinct annealing stages in τavg at 400–600 °C and at 650–800 °C are observed. The variations in line-shape parameter (S) and defect-specific parameter (R) during annealing in the temperature region 25–800 °C resemble the behaviour of τavg indicating the migration of vacancies, formation of vacancy clusters and the disappearance of defects between 400 and 800 °C.

Positron annihilation measurements in high energy alpha irradiated undoped indium antimonide

Positron annihilation lifetime spectroscopy and Doppler broadening annihilation line-shape measurements were carried out in 40 MeV alpha-irradiated undoped InSb. After irradiation the sample was subjected to an isochronal annealing over temperature region of 25°C–400°C with annealing time of 30 min at each set temperature. After each annealing the positron measurements were carried out at room temperature. Formation of radiation induced defects and their recovery with annealing temperature were investigated. A three component positron lifetime analysis was undertaken to observe the trapping of positrons in the sample after irradiation and during annealing. The average positron lifetime value τ avg = 313 ps at room temperature after irradiation indicated the presence of defects and the high value of τ2 at room temperature suggested that the probable defects were divacancies. A two stage recovery of defects was observed during post irradiation isochronal annealing over the temperature region 25°C–400°C. The variations in line-shape parameter (S) and defect specific parameter (R) during annealing in the temperature region 25°C–400°C resembled the behavior of τ avg indicating the migration of vacancies, formation of vacancy clusters and the disappearance of defects between 300°C to 400°C.

Positron annihilation investigation of BaSrFBr:Eu by X-ray irradiation

The mechanical property of the BaSrFBr:Eu phosphor layer of X-ray image plates was investigated by using resolution (LP/mm) and coincidence Doppler broadening (CDB) positron annihilation as well as positron annihilation lifetime (PAL). The image plate samples of BaSrFBr:Eu phosphors in this experiment were irradiated by using hospital X-rays. The LP/mm values of the irradiated BaSrFBr:Eu image plates varied from 3.35 to 1.25 for up to 20,000 exposures. CDB positron annihilation and lifetime spectroscopy were used to analyze defect structures in the phosphor layer. Even when the LP/mm values were greatly decreased due to exposures, the S parameter and the lifetime (τ1, τ2) values were almost constant with increasing number of exposures. A positive relationship existed between the SEM images and positron annihilation spectroscopy (PAS). According to the SEM images and the positron annihilation spectroscopy (PAS) results, measurements of the defects with PAS indicate that the image-plate phosphor can be safely used for hospital X-rays in the course of diagnostic radiography at an average rate of 20,000 times for one year.

Modelling the impact of injection time on the bolus shapes in PET-MRI AIF Conversion

With the introduction of combined PET/MRI systems, AIF conversion can be made under certain circumstances (see [1]). We propose a model that allows modification of the injection parameters in the AIF fit to account for differences caused by different injection durations [2]. Brain 18F-Choline PET and DSC-MRI data were obtained using Siemens mMR. The MR contrast agent was injected with a rate of 4ml/sec and the PET tracer was injected manually. Perfusion Mismatch Analyzer [3] was used to extract the MRI-AIF. Carotid arteries were segmented on a post contrast MPRAGE image. PET frames were registered onto this MPRAGE image using rigid registration and partial volume correction was done using the iterative Yang method [4]. The AIFs were fitted using a convolution of a ‘double Butterworth’ function, representing the injection, with a tri-exponential function representing the elimination [Eq. 1]. The bolus shape can be adjusted by changing Δτ (τ2 - τ1). This was tested with a population based MRI AIF [5], as well as with clinical data. 1 where For the population based input function, Figure ​Figure11 shows that when Δτ was increased, lower and wider peaks were seen, and with decreased Δτ, higher but narrower peaks were observed. Figure ​Figure22 shows that the function fits both clinical PET and MRI AIFs well. Values of τ1 and τ2 were changed to modify the MRI-AIF and Figure ​Figure33 shows the modified MRI-AIF together with the original fitted PET-AIF, normalized to their peaks. Two AIFs have similar peak shapes but start to differ at the elimination phase as Gd-DOTA and 18F-Choline have different tissue uptake rates. Figure 1 Simulated MRI-AIFs using Parker’s population-based input function refitted with the developed function. AIF shapes with different injection durations, Δτ is shown. Figure 2 The double Butterworth convolution function used to fit (a) DSC-MRI data and (b) 18F-Choline PET data together with a plot where the timescale of PET-AIF was limited to MRI-AIF’s to show different bolus widths. Figure 3 The MRI-AIF with modified τ1 and τ2 values plotted together with the PET-AIF. The MRI-AIF peak is scaled to PET-AIF’s peak. This enables conversion of the early part of the AIFs from one modality to another even if different injection protocols are used.

Interstitial Solution Carbon Concentration and Defects of Ti + Nb ULC-BH Steel by Internal Friction and Positron Annihilation Methods

The interstitial solution carbon concentration and defects in continuously annealed Ti+Nb bearing ULC-BH (ultra-low carbon bake hardening) steel samples are investigated by multi-functional internal friction apparatus, positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening spectroscopy (CDBS). The relationship of internal friction peaks, interstitial solution carbon concentrations and movable dislocations in the samples under different conditions is analyzed. A correlation of lifetime component τ1 values with interstitial solution carbon concentrations in the samples for different continuous annealing processes is established, while a correlation of lifetime component τ2 values with multi-vacancies, vacancy clusters, microvoids and other types of defects for various continuous annealing processes is also demonstrated. Furthermore, the average lifetime results illustrate the overall defect densities for various continuous annealing processes. The CDBS analysis reflects the chemical surroundings of the defects at the annihilation sites and reveals that the peak heights of the ratio curves relate to the total number of defects such as interstitial carbon atoms, dislocations, vacancies and other types of defects. The results show that internal friction, PALS and CDBS are effective techniques to identify and characterize the interstitial solution carbon concentration, multi-vacancies, vacancy clusters, microvoids and other types of microscopic defects in annealed Ti+Nb bearing ULC-BH steel.

Solid State Assemblies and Photophysical Characteristics of Linear and Bent-Core π-Conjugated Oligophenylenevinylenes

New classes of luminescent linear, bent-core, and star-shaped oligophenylenevinylenes (OPVs) having 1,4-para and 1,3-meta rigid aromatic cores were designed and developed. 3-Pentadecylphenol, a renewable resource molecule, was chosen as the flexible unit at the longitudinal or middle position of the OPV aromatic core for solid state ordering. Depending upon the nature of the π-core, the OPVs exhibited either mosaic-type liquid crystalline textures or spherulitic crystalline solids. The enthalpies of melting transitions revealed that the bent-core OPV structure showed enhanced solid state packing compared to linear or star-shaped OPVs. Small and wide-angle X-ray diffraction analysis confirmed layered-like assemblies in OPV molecules. Photophysical experiments such as excitation, emission, and time-resolved fluorescence decay dynamics were carried out to trace the molecular self-organization of OPV chromophores. Time correlated single photon counting technique (TCSPC) luminescent decay profiles and decay lifetimes (τ1 and τ2 values) revealed that the OPV chromophores showed faster exciton decay in the tightly packed bent-core structure. The weakly packed star-shaped OPV showed enhanced excited state luminescence stability up to 10 ns. A direct correlation between the OPV chemical structure, solid state ordering, and photophysical characteristics was established.

Refined Distances Between Paramagnetic Centers of a Multi‐Copper Nitrite Reductase Determined by Pulsed EPR (iDEER) Spectroscopy

Refined Distances Between Paramagnetic Centers of a Multi‐Copper Nitrite Reductase Determined by Pulsed EPR (<i>i</i>DEER) Spectroscopy

Investigation of positron annihilation in Fe52Co34Hf7B6Cu1 amorphous alloy treated by intermediate frequency magnetic pulse

The effect of intermediate frequency magnetic pulse treatments on Fe52Co34Hf7B6Cu1 amorphous alloy prepared by melt-spun technique is investigated. The microstructure and structural defects of the treated specimens are investigated by TEM, Mössbauer spectroscopy and position annihilation lifetime spectra. The results show that the treated specimens by intermediate frequency magnetic pulse are partially crystallized, the content of the crystallization phase increases with the increase of the magnetic pulse frequency, and the crystalline volume fraction is 33.1% at a frequency of 2000 Hz. For the as-quenched amorphous alloy, the annihilation lifetime τ1 is 150.5 ps in the monovacancy-like free volume, and the relative intensity I1 is 77.7%. The annihilation lifetime τ2 is 349.7 ps in the microvoid, and the relative intensity I2 is 22.3%. With the increase of the magnetic pulse frequency, the values of τ1 and τ2 of the treated specimens decrease, I1 increases, I2 and τ decrease compared with the as-quenched amorphous alloy.

Positron annihilation study of irradiated China domestic A508-3 steels

China domestic A508-3 steels are irradiated by protons with energy of 240 keV under the damage doses of 0.26 dpa, 0.50 dpa and 1.00 dpa, respectively. The results from positron lifetime spectrum analysis and microharness test show that τ1, τ2 lifetimes of A508-3 steels increase with increasing radiation dose. The variation of short lifetime value of τ1 from 120 ps to 151 ps means that monovacancies and divacancies emerge in the steels after proton radiation. The variation of long lifetime value of τ2 from 226 ps to 262 ps means that micro voids containing several vacancies are produced by radiation damage. The average diameter of voids under the three damage level is 0.41nm, 0.45nm, and 0.45nm, respectively. Radiation hardening is due to the improvement of dislocation motion resistance which is caused by radiation defects such as vacancies, voids and possible copper enriched clusters.

Investigation on annealed Fe43Co43Hf7B6Cu1 amorphous alloy by positron annihilation spectroscopy

Amorphous Fe43Co43Hf7B6Cu1 alloy prepared by melt-spun technique were annealed at temperature 200 ℃, 300 ℃, 400 ℃ and 500 ℃ for 30 min. The structure and structural defects in annealed specimens were investigated by positron annihilation lifetime spectra, X-ray diffraction (XRD) and Mssbauer spectroscopy (MS), etc. . The results show that in the as-quenched amorphous alloy, more than 85% of the positrons are localized at vacancy-sized free volumes in the amorphous based phase, annihilation lifetime τ1=158.4 ps, and the other 11.9% of positrons are trapped by microvoids, with lifetime τ2=397 ps. After annealing at temperature 200 ℃, 300 ℃ and 400 ℃ for 30 min, the atomic short range diffusion will give rise to the migration and annihilation of vacancy-like defects. The major component τ1 will be reduced continuously. The value of τ1 will decrease to 149 ps at 400 ℃, their relative intensities I1 decreases and reaches 80.8%. Meanwhile the annihilation lifetime of positrons in the microvoids τ2 will change from 353 ps to 364 ps, their relative intensities I2 are increased to 18.0%. When annealed at temperature 500 ℃ for 30 min, new trapping centers of positrons are produced owing to the crystallization of amorphous phase, I2 increases drastically to 26.7%.

Electric field enhancement of electron emission rates from Z1/2 centers in 4H-SiC

Z 1 / 2 defect centers were produced by irradiating 4H-SiC bulk samples with 1 MeV electrons at room temperature. The emission rate dependence on the electric field in the depletion region was measured using deep level transient spectroscopy and double-correlation deep level transient spectroscopy. It is found that the Z1/2 defect level shows a strong electric field dependence with activation energy decreasing from Ec−0.72 eV at zero field to Ec−0.47 eV at 6.91×105 V/cm. The phonon assisted tunneling model of Karpus and Perel [Sov. Phys. JETP 64, 1376 (1986)] completely describes the experimental data. This model describes the dependence of the emission rate on electric field F as en(F)=eno exp(F2/Fc2), where Fc is the characteristic field that depends on the phonon assisted tunneling time τ2. The values of Fc and τ2 were determined and the analysis of the data leads to the suggestion that Z1/2 may be a substitutional point defect.

Positronium in a cavity in a high-Tc superconductor

The behavior of positronium localized in a microvoid in a high-temperature superconductor (HTSC) environment is examined. The interaction of such a positronium with conduction electrons is treated as arising from the Ps ↔ e+ + e− process, which is mathematically described here in terms of the well-known Anderson model for a magnetic impurity in a normal metal. In this model, the interaction underlying this process is due to the positronium state hybridization with the states of conduction elections and the state of the positronium remaining within the void. Similarly to the case of a magnetic impurity, the density of states of an interacting positronium exhibits a 'Kondo resonance' if the Fermi liquid surrounding the void is in the normal state. Based on experimental data on the lifetime τ2 of void-trapped positrons, it is concluded that the hybridization interaction is much stronger than the intra-atomic relativistic electron–positron interaction in the Ps atom. The model used to describe the interacting Ps atom provides a relation between the experimental values of τ2(T) and the properties of the electronic structure of the metal. Experimental results for ceramic HTSC samples of (Bi,Pb)2Sr2Ca2Cu3O7 and (Bi,Pb)-2223 are interpreted, which show a sharp decrease in τ2(T) at T = Tc, where Tc is the superconducting transition temperature. Using the adopted model, some conclusions are drawn as to how the properties of the pseudogap correlate with the experimental τ2(T) dependence observed in (Bi,Pb)-2223 for T > Tc.

Viscoelastic properties of lungs and thoracic wall of anesthetized mechanically ventilated piglets

ObjectiveTo investigate the viscoelastic properties of lungs and thoracic wall in piglets. Study designProspective experimental study. AnimalsSix piglets weighting 30 kg. MethodsAnimals were tracheotomized, anesthetized and mechanically ventilated under controlled conditions. After control measurements of the mechanical properties of the lung of the pigs had been taken, acute lung injury (ALI) was induced by saline lavage. Lung and thoracic wall tissue resistance (ΔR), which reflects viscoelastic properties and/or time constant inequalities, were determined by using a rapid airway occlusion technique during constant flow inflation ( V˙), at constant tidal volume. V˙ was varied from 0.1–0.2 to 1.2 L second−1 on a single breath. Multiple data sets of ΔR of lung (ΔRL) and thoracic wall (ΔRw) to inspiratory time (TI = VT/ V˙) were fitted to a model whose prediction equation was ΔR = R2[1 −exp(−TI/τ2)], where R2 and τ2 are the ‘viscoelastic’ resistance and time constant, respectively. Subscripts L and W are used to represent lung and theracic wall, respectively (R2L, R2W, τ2L, τ2W). Two more sets of physiological measurements were then taken – the first under zero end-expiratory pressure (ZEEP) and the second under a positive end-expiratory pressure (PEEP) of 10 cmH2O. ResultsData of ΔR adequately fitted to the prediction equation in all instances. In control, R2,L was 15.3 (10.7–22.6) cmH2O L−1 second−1 (median, interquartile range), τ2,L 3.3 (1.9–5.5) seconds, R2,w 6.5 (2.2–10.3) cmH2O L−1 second−1 and τ2,w 2.9 (1.1–4.3) seconds. In ALI, R2,L significantly increased to 129.6 (105.9–171.3) cmH2O L−1 second−1 on ZEEP but not significantly decreased to 48.9 (17.8–109.6) cmH2O L−1 second−1 with PEEP. The corresponding values of τ2,L were 7.1 (5.1–11.6) and 4.4 (3.1–5.5) seconds. The values pertaining to thoracic wall did not change significantly among conditions. Conclusions and Clinical relevanceViscoelastic properties of the lung and thoracic wall in piglets can be described by a viscoelastic model. Values of parameters of this model were markedly increased in ALI and decreased with PEEP.

Steady-state analysis of a bistable system subject to a coloured multiplicative noise and a white additive noise with coloured cross-correlated noises

The steady-state properties of a bistable system are investigated when both the multiplicative noise and the coupling between additive and multiplicative noises are coloured with different values of noise correlation times τ1 and τ2. After introducing a dimensionless parameter R(R = α/D, D is the intensity of the multiplicative noise and α is the intensity of the additive noise), and performing the numerical computations, we find the following points: (1) For the case of R>1, λ (the intensity of correlation between additive and multiplicative noises), τ1 and τ2 can induce the stationary probability distribution (SPD) transition from bimodal to unimodal in structure, but for the cases of R≤1, the bimodal structure is preserved; (2) α can also induce the SPD transition from bimodal to unimodal in structure; (3) the bimodal structure of the SPD exhibits a symmetrical structure as D increases.

Moisture sorption/desorption behavior of various manmade cellulosic fibers

AbstractThe kinetics of dynamic water vapor sorption and desorption on viscose, modal, cotton, wool, down, and polyester fibers and lyocell knit fabrics were investigated according to the parallel exponential kinetics (PEK) model. The total equilibrium moisture regain (Minf(total)) in all the materials decreased with increasing temperature. However, the partial equilibrium fast sorption, determined by PEK simulation at 60% relative humidity (RH) and 36°C, was larger than that at 20°C, whereas the partial equilibrium slow sorption was smaller. The characteristic times in fast sorption (τ1) and in slow sorption (τ2) for lyocell were reduced when the conditions were changed from 60% RH and 20°C to 36°C, whereas those for the other fibers increased. Lyocell exhibited the highest Minf(total) value and the lowest τ1 and τ2 values, and this suggested high equilibrium moisture content and fast moisture uptake/release, that is, high moisture accessibility for lyocell. The relationships between the moisture regain, hysteresis, water retention capacity, and Brunauer–Emmett–Teller surface volume in the materials were also examined. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1621–1625, 2005

Localization of the N-terminal Domain in Light-harvesting Chlorophyll a/b Protein by EPR Measurements

The conformational distribution of the N-terminal domain of the major light-harvesting chlorophyll a/b protein (LHCIIb) has been characterized by electron-electron double resonance yielding distances between spin labels placed in various domains of the protein. Distance distributions involving residue 3 near the N terminus turned out to be bimodal, revealing that this domain, which is involved in regulatory functions such as balancing the energy flow through photosystems (PS) I and II, exists in at least two conformational states. Models of the conformational sub-ensembles were generated on the basis of experimental distance restraints from measurements on LHCIIb monomers and then checked for consistency with the experimental distance distribution between residues 3 in trimers. Only models where residue 3 is located above the core of the protein and extends into the aqueous phase on the stromal side fit the trimer data. In the other state, which consequently is populated only in monomers, the N-terminal domain extends sideways from the protein core. The two conformational states may correspond to two functional states of LHCIIb, namely trimeric LHCIIb associated with PSII in stacked thylakoid membranes and presumably monomeric LHCIIb associated with PSI in nonstacked thylakoids. The switch between these two is known to be triggered by phosphorylation of Thr-6. A similar phosphorylation-induced conformational change of the N-terminal domain has been observed by others in bovine annexin IV which, due to the conformational switch, also loses its membrane-aggregating property.

Fast Transient Fluorescence Technique for Studying Homopolymer Mobility in a Swelling Gel

Summary: The mobility of pyrene end-capped polystyrene (Py-PSt) trapped in swollen polystyrene gels (PSt) was investigated. PSt gels were prepared by free radical crosslinking copolymerization (FCC) of styrene (St) and ethylene glycol dimethacrylate (EGDM). The pyrene end-capped polystyrene was produced by atom transfer radical polymerization (ATRP). After drying, these disc-shaped PSt gels were left in a toluene solution of Py-PSt of various molecular weights. During this process, Py-PSt chains were trapped in the gel. These swollen gels were re-dried in air and then immersed in pure toluene solution for monitoring the mobility of Py-PSt chains in and out of the gel. These reswelling experiments were performed at room temperature in real time by monitoring pyrene lifetimes outside (τ1) and inside (τ2) of the PSt gel by using in-situ fast transient fluorescence (FTRF) measurements. It was observed that τ2 values decrease as swelling proceeds; however, τ1 values stay constant during swelling. The Li-Tanaka equation was employed to produce the swelling parameters. The swelling time constant, τc was found to increase as the crosslinker density of the gels and the molecular weight, of Py-PSt chains were increased. It was observed that the collective diffusion coefficient, Dc decreased by increasing the molecular weight by obeying Dc ≈ M−1 law. Position of the PSt gel in the fluorescence quartz cell before (a) and after (b) swelling. I0 and Ip are the excitation and the emission intensities.

Spontaneous Ignition of n-Alkane Droplets with Various Volatility

Ignition delays of a cool flame (τ1) and a hot flame (τt) were measured experimentally for single n-decane, n-dodecane, n-tetradecane and n-hexadecane droplets, which have similar volatilities to common commercial hydrocarbon fuels, in hot air. Droplet diameter was 0.7 mm. Ambient pressure was 0.3 and 1.0 MPa. Ambient temperature was 550 K to 1000 K. Results show the similarity of the examined fuels in terms of reactivity of the low- and high-temperature reactions. Values of τ1 and τt were longer for fuels with lower volatility. Values of τ2 (=τt−τ1) were similar for examined fuels. Both the ignition limits of cool and hot flames were similar between fuels, but were slightly lower for less-volatile fuels.

Effect of tetra ionic substitution on the dielectric properties of Cu-ferrite

X-ray diffraction (XRD), of the two systems of mixed ferrites Cu1+xTixFe2–2xO4 (where x = 0.0, 0.1, 0.2, 0.3, and 0.4); indicates that the samples of x = 0 (CuFe2O4) and x = 0.1 of the Cu–Ti system were formed in tetragonal structure and all other samples of the two system were formed in the cubic system. The ac conductivity , dielectric constant e′, dielectric loss e″ and the loss tangent tan δ were determined against frequency at room temperature for Cu–Ge and Cu–Ti ferrites. The measurements of and tan δ were performed over a wide range of frequency and temperature. The Maxwell–Wagner model was applied to analyze the dielectric properties of the investigated systems, according to which the dielectric parameters such as the relaxation time τ. A value of τ1 = 5 × 10–7 s was found for Cu–Ge ferrite and τ2 = 1.85 × 10–6 s for Cu–Ti ferrites. The hopping rate (g) was found to be 2 × 106 s–1 and 5.4 × 105 s–1 for the two systems Cu–Ge and Cu–Ti ferrites, respectively. The conduction of the two-ferrite systems was discussed on the basis of the hopping mechanism. The activation energy for conduction was calculated and found in the range of 0.27–0.39 eV for Cu–Ge ferrite and 0.21–0.30 eV for Cu–Ti ferrite. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of curing time and sulfur content on the dielectric relaxation of styrene butadiene rubber

AbstractDielectric investigations were carried out on styrene–butadiene rubber cured by an increase in either the sulfur content or curing time in a wide range of frequencies from 100 Hz up to 10 MHz and at room temperature (≈25°C). After subtraction, the losses due to the Maxwell–Wagner effect, the dielectric data showed deviation from the Debye formula but were found to be fitted in the frequency domain by the Havriliak–Nagami function, which is a combination of Cole–Cole and Cole–Davidson functions. This function describes the relaxation mechanisms of the aggregates caused by the movement of the main chain that were expected to be formed by the addition of the different ingredients to the investigated rubber. These mechanisms are also discussed in terms of the distribution function of Fröhlich between two limiting values of relaxation times τ1 and τ2. The relaxation times obtained from both functions increased with an increase in either sulfur content or curing time. The data obtained are discussed in terms of the crosslinking density determined from the Flory–Rehner relation. The effect of thermal aging on such systems was also studied, and the data obtained were compared with those before aging. This investigation led us to conclude that to get an end product that is highly resistant to thermal aging, the system should be cured for a time greater than 30 min with the addition of more than 5 phr sulfur. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1613–1621, 2003