共查询到20条相似文献,搜索用时 15 毫秒
1.
We have searched for an almost stable, charged particle produced in 400 GeV proton-nucleus collisions. A total of 5 × 10 10 light secondary particles were sampled in a secondary beam of 70 GeV/ c momentum. If a 4.5 to 6.0 GeV mass particle is produced with a cross section comparable with the production cross section of the upsilon then this experiment places an upper limit on the lifetime of such a particle of about 5 × 10 ?8 s. 相似文献
2.
Membrane filtration has been demonstrated to be effective for the removal of liquid-borne nanoparticles (NPs). Such technique
can be applied to purify and disinfect drinking water as well as remove NPs in highly pure chemicals used in the industries.
This study aims to study the filtration process of a model membrane filter, the Nuclepore filter. Experiments were carried
out using standard filtration tools and the nanoparticle tracking analysis (NTA) technique was used to measure particle (50–500 nm)
concentration upstream and downstream of the filter to determine the filtration efficiency. The NTA technique has been calibrated
using 150-nm polystyrene latex particles to determine its accuracy of particle concentration measurement. Measurements were
found reliable within a certain concentration limit (about 10 8–10 10 particles/cm 3), which is dependent on the camera settings during the measurement. Experimental results are comparable with previously published
data obtained using the aerosolization method, validating the capability of the NTA technique. The capillary tube model modified
from that developed for aerosol filtration was found to be useful to represent the experimental results, when a sticking coefficient
of 0.15 is incorporated. This suggests that only 15% of the particle collisions with the filter results in successful attachment.
The small sticking coefficient found can be explained by the unfavorable surface interactions between the particles and the
filter medium. 相似文献
3.
The performance of a Po-210 radioactive ionizer for aerosol charge neutralization and bipolar charging has been evaluated. Monodisperse neutral and singly charged aerosols have been passed through the ionizer and their approach to the steady-state Boltzmann charge measured by measuring the fraction of particles that are electrically charged. The result shows that an Nt of approximately 2.5 × 10 5 cm ?3 s is needed for the development of a steady-state charge fraction equal to that given by the Boltzmann' s law where N is the ion concentration and t is the residence time of the aerosol in the neutralizer. In addition, the performance of the ionizer for particle charge neutralization in a vibrating-orifice aerosol generator has been evaluated and the result shows that the source is less effective for this application and the residual charge on the particles is considerably higher than that given by the Boltzmann's law. 相似文献
4.
Abstract The temporal fluctuations in the intensity of light scattered by a moving layer of emulsions and suspensions containing Brownian particles are investigated experimentally, and a comparison is made with light scattered by a translating phase screen. The intensity fluctuations of the scattered light are detected through an imaging system, which collects the light emanating only from a limited volume in the medium. The effect of translational motion of the particle layer on the decay rate of the autocorrelation function of intensity fluctuations depends on the illuminating form of a laser beam and on the point spread function of the imaging system. The Brownian motion of the particles causes the scattered light to fluctuate more rapidly than that arising from the translating phase screen. In the multiple-scattering regime, the influence of this diffusional motion increases with an increase of the particle concentration in the layer. 相似文献
5.
Dynamic light scattering (DLS) techniques for studying sizes and shapes of nanoparticles in liquids are reviewed. In photon correlation spectroscopy (PCS), the time fluctuations in the intensity of light scattered by the particle dispersion are monitored. For dilute dispersions of spherical nanoparticles, the decay rate of the time autocorrelation function of these intensity fluctuations is used to directly measure the particle translational diffusion coefficient, which is in turn related to the particle hydrodynamic radius. For a spherical particle, the hydrodynamic radius is essentially the same as the geometric particle radius (including any possible solvation layers). PCS is one of the most commonly used methods for measuring radii of submicron size particles in liquid dispersions. Depolarized Fabry-Perot interferometry (FPI) is a less common dynamic light scattering technique that is applicable to optically anisotropic nanoparticles. In FPI the frequency broadening of laser light scattered by the particles is analyzed. This broadening is proportional to the particle rotational diffusion coefficient, which is in turn related to the particle dimensions. The translational diffusion coefficient measured by PCS and the rotational diffusion coefficient measured by depolarized FPI may be combined to obtain the dimensions of non-spherical particles. DLS studies of liquid dispersions of nanometer-sized oligonucleotides in a water-based buffer are used as examples. 相似文献
6.
The paper presents a signal processing system used for nitrogen dioxide detection employing cavity enhanced absorption spectroscopy.
In this system, the absorbing gas concentration is determined by the measurement of a decay time of a light pulse trapped
in a cavity.
The setup includes a resonance optical cavity, which was equipped with spherical and high reflectance mirrors, the pulsed
diode laser (414 nm) and electronic signal processing system. In order to ensure registration of low-level signals and accurate
decay time measurements, special preamplifier and digital signal processing circuit were developed.
Theoretical analyses of main parameters of optical cavity and signal processing system were presented and especially signal-to-noise
ratio was taken into consideration. Furthermore, investigation of S/N signal processing system and influence of preamplifier
feedback resistance on the useful signal distortion were described.
The aim of the experiment was to study potential application of cavity enhanced absorption spectroscopy for construction of
fully optoelectronic NO 2 sensor which could replace, e.g., commonly used chemical detectors. Thanks to the developed signal processing system, detection
limit of NO 2 sensor reaches the value of 0.2 ppb (absorption coefficient equivalent = 2.8 × 10 −9 cm −1). 相似文献
7.
针对光子相关光谱颗粒粒径测量法不能在线测量高浓度超细颗粒的问题,通过分析溶液浓度对光子相关光谱测量法的影响,设计了一种后向散射光路,提出了后向光子相关光谱测量法.实验采用50 nm、100 nm以及500 nm三种标准乳胶球颗粒,在不同的浓度下分别用两种方法进行了测试.结果证明,后向光子相关光谱测量法能有效抑制多重散射的影响,适用于高浓度超细颗粒粒径的在线测量. 相似文献
8.
The attachment of radioactive atoms and ions to spherical aerosol particles has been studied theoretically. For uncharged atoms the deposition is considered to be solely governed by thermal diffusion. With calculations based on the “limiting-sphere”-method of Arendt-Kallman it is found, that the attached activity is proportional to Φ 2 for aerosol particle diameters Φ smaller than 0.1 μm, and proportional to Φ for Φ greater than 1 μm. For charged ions the diffusion process is modified by the influence of electrostatic forces between the diffusing ions and the aerosol particles. In the frequently occurring case of a symmetrically bipolar charged aerosol this influence can be expressed by a function G p(Φ), which depends on the diameter Φ and the effective number p of elementary charges on the aerosol particle. For an aerosol particle diameter Φ greater than 0.1 μm the attached activity is proportional to Φ 1.1, and for Φ smaller than 0.01 μm it is proportional to Φ 1.55. The effects of neglecting various terms in the calculation are considered. The distribution of natural radioactivity on atmospheric aerosols has been calculated for various particle size distributions according to Junge. The calculation shows that about 90% of the total natural activity should be attached to particles smaller than Φ=0.5 μm, and about 35% to particles smaller than Φ=0.1 μm. The time T 1/2, in which the concentration of the radioactive particles decreases to half the initial value, depends on the concentration of the aerosol particles and on their size distribution. For 10 4 aerosol particles per cm 3 and the size distributions mentioned, T 1/2 varies between 15 and 30 seconds for radioactive ions. For radioactive atoms T 1/2 is greater than it is for ions in the range of aerosol particle diameters below Φ=0.25 μm, and smaller if Φ greater than 0.25 μm. 相似文献
9.
The recently obtained results on e +e ? → hadrons and multigamma production in pp collisions provide some support for a previously made argument that heavy, highly charged subnucleons may exist, and a search for such particles in cosmic rays has been made. In 620 hours no candidates were detected, and an upper limit for the unaccompanied particle flux of 7 · 10 ?10 cm ?2 sec ?1 sr ?1 at sea level under 600 g/cm 2 concrete is obtained with 90% confidence. 相似文献
10.
Screening of excitonic states by a system of 2D electrons (or holes) in GaAs/AlGaAs single quantum wells is studied. With increasing concentration of 2D charge carriers, a threshold-type disappearance of excitonic states is observed in both luminescence and reflectance spectra. The higher the quality of the 2D system, the lower the corresponding threshold concentration. In the best systems, the collapse of excitonic states occurs at unexpectedly low electron densities n e =5×10 9 cm ?2, which correspond to the mean dimensionless distance between the particles r s =8. This value far exceeds the threshold values observed for 3D systems ( r s ≈2), as well as the values obtained for quantum wells in previous studies. The problem of measuring the concentration of low-density 2D charge carriers in photoexcitation conditions is solved by applying the method of optical detection of the dimensional magnetoplasma resonance. This method provides reliable measurements of the density of a 2D system to the values about 10 9 cm ?2. 相似文献
11.
Dynamic light scattering signals from particles, exhibit fractal characteristics. This feature can be used to determine the particle size. The use of the fractal dimension, as a quantitative method to analyze the properties of dynamic light scattering signals from submicron particles, is presented. The analysis is performed directly on the time‐resolved scattered intensity, and the Box Dimensions of light scattering signals of particles with diameters 100, 200, 500 and 1000 nm. The experimental results show that the fractal dimensions of light scattering signals correlate well with particle size. In the submicron size range, the smaller the particles, the larger their fractal dimensions. Compared with the PCS technique, only several hundreds of samples are required in the fractal method. Therefore, the data processing is easily accomplished. However, this method only provides the mean particle size, but not the particle size distribution. 相似文献
12.
The attachment of the decay products of thorium emanation to aerosol particles has been studied. The dependence of the attached activity on the particle size was determined for spherical particles with radii R ranging from 0·04 to 0·6 microns. The particles used were homogeneous dioctylphthalate droplets and polystyrene micro-spheres. It is found that the attached activity is proportional to R 2/(1+hR). This dependence can be derived theoretically by considering the deposition to be solely governed by the diffusion process (not by electrostatic forces) and assuming a quasi-stationary density distribution for the diffusing atoms. The constant h is uniquely determined by the average gaskinetic velocity and the diffusion constant of the diffusing atoms. For the decay products of thorium and radium emanation (atomic weight ≈210) h ≈ 7 · 10 4 cm ?1. The derived equation holds for a wide range of particle sizes: For the particles with radii larger than about 10 ?4 cm this means that the attachment is proportional to the radius; for particle radii below about 10 ?6 cm it is proportional to the surface (R 2) of the particles. It is also possible to derive an expression for the time-dependence of the attachment process from the theoretical considerations. The rate at which the average concentration of the radioactive atoms decreases is proportional to exp ?t/τ where τ=1+hR/πR 2 N¯ v ( ¯ v=average gaskinetic velocity of the diffusing atoms; N=aerosol concentration). 相似文献
13.
The total light scattering method for particle size measurement has a series of outstanding features, e.g. very simple instrumental arrangement, no serious requirements for its optical, photodetecting and electronic systems and its ease of use. Nevertheless, some problems still remain unsolved which should be dealt with to improve its further applications. An improvement has been made in this work. Comprehensive theoretical studies showed that this method is applicable to particle size analysis in the range 0.1–10 μm, i.e. for those particles whose diameter lies between the lower limit of the diffraction method and the upper limit of photon correlation spectroscopic (PCS) technique. Experimental studies with different monodisperse polystyrene latex spheres as standard reference material give good agreement with their nominal values. 相似文献
14.
The influence of annealing on the concentration profiles of boron implanted into silicon with does of 10 14 ions/cm 2 up to 10 16 ions/cm 2 and an energy of 70 keV was studied. The concentration profiles were measured with Secondary Ion Mass Spectrometry (SIMS).
The broadening of the concentration profiles during annealing can be described as a superposition of effects resulting from
a relatively immobile and a mobile boron fraction. The properties of the immobile boron fraction were studied by measuring
the influence of a boron implantation on the distribution of a homogeneous boron background dope. From these experiments it
was concluded that the immobile boron fraction consists of boron precipitates. The properties of the mobile fraction were
studied from concentration profiles that were obtained after annealing during different periods at the same temperature. It
was found that during the initial stage of the annealing process a fast broadening of the profile occurs; this was assumed
to be due to an interstitial type boron diffusion. After prolonged annealing the much slower substitutional type diffusion
prevails, due to trapping of the interstitial boron atoms by vacancies.
The reliability of the SIMS method, as applied to profile measurements, was checked for the high boron doses used in this
investigation. Excessive boron precipitates, obtained after annealing of a high dose, such as 10 16 ions/cm 2 at about 1000°C, appear to give some increase of the ion yield. 相似文献
15.
The dispersed composition of water samples prepared by the multiple-dilution technique has been investigated opto-physically using two laser diagnostics methods: dynamic light scattering and laser phase microscopy. We studied suspensions of antibodies (Ab) to the human interferon-gamma (IFNγ) in water with an initial concentration of 0.125 mg ml?1 (5×1014 mol cm?3), which were subjected to 12 successive 100-multiple volume dilutions (C12) in water. Individual antibodies and particles of about 300 nm in size, which can be considered as aggregates of individual antibodies, have been found in the initial suspensions. It turned out that some mesoparticles with a concentration on the order of 103 cm?3 exist also in suspensions subjected to C12 dilution. These particles have a refractive index close to typical values of protein refractive indices. A possible explanation of the origin of particles with sizes on the order of several hundreds of nanometers, revealed in highly diluted suspensions, is proposed. On the one hand, some of these particles may be aggregates of antibodies from the initial suspension, which remained (due to the flotation effect) in the bulk of the liquid after dilutions. On the other hand, the appearance of solid contaminant particles at the same mesoscopic scale (i.e., on the order of several hundreds of nanometers) cannot be excluded in the dilution process. 相似文献
16.
With the method of differential optical absorption spectroscopy (DOAS), average concentrations of aerosol particles along light path were measured with a flashlight source in Chiba area during the period of one month. The optical thickness at 550 nm is compared with the concentration of ground-measured suspended particulate matter (SPM). Good correlations are found between the DOAS and SPM data, leading to the determination of the aerosol mass extinction efficiency (MEE) to be possible in the lower troposphere. The average MEE value is about 7.6m^2.g^-1 , and the parameter exhibits a good correlation with the particle size as determined from the wavelength dependence of the DOAS signal intensity. 相似文献
17.
Transmission fluctuation spectrometry with temporal correlation (TFS‐TC) is a new method for particle analysis. When a narrow light beam irradiates on a particle dispersion flow, the variation of the number of particles in the small measuring zone will cause the transmitted light to fluctuate, which includes the complete information on both particle size distribution (PSD) and particle concentration. The method may be used for real‐time, inline/online applications due to its simplicity of measuring principle and experimental setup. Until recently, the theory has been limited to low particle concentrations. In this work, an experimental study of the TFS‐TC measurement is presented for a very wide range of the particle concentrations. By introducing an empirical correction including the high concentration effects and considering the effect from rheological conditions in the inversion algorithm, the particle size distribution and particle concentration are reconstructed, resulting in the coverage of a broad range of particle size and concentration. 相似文献
18.
Using the electroreflectance method space charge layers on crystals of different conductivities have been identified. The space charge layers were formed by adsorption of oxygen or atomic hydrogen. The limit of sensitivity required the irradiation with 5×10 13 photons/cm 2×sec of band gap energy. After exposure to atomic hydrogen all samples showed accumulation layers. With a partial pressure of oxygen above 350 mm Hg crystals of high conductivity ( σ=47 ohm ?1 cm ?1) exhibit depletion layers, which change into accumulation layers, if the partial pressure is reduced below the limit. Crystals of a lower conductivity ( σ=10 ?3–10 ?1 ohm ?1 cm ?1) show accumulation layers up to the highest applied oxygen pressure of 760 mm Hg. The phenomena are attributed to a dynamical equilibrium between adsorption and photo-desorption of oxygen. This equilibrium depends on oxygen pressure and free carrier concentration. By comparing a calculated curve with the experimental results the value of 3.31 ev is obtained for the energy gap, light polarized perpendicular to the c-acis. 相似文献
19.
Within the past 20 years, particle size analysis with laser diffraction (LD) has been subject to rapid development, extending the size range stepwise from 1–200 μm to about 0.1–3500 μm. The limits of LD are discussed in terms of light sources, the influence of the beam diameter, special Fourier optics and a new detector design. It is shown that the size range is not only restricted by the wavelength of the laser and the transmission limits of the medium. Its extension is mainly related to improvements in the measurement of the angular intensity distribution. Influences from stability and flow dominate on the coarse side of the measuring range. On the fine side, the spatial extension of aerosols and the resulting demand for extended working distances can be covered only in a parallel laser beam. Extended Fourier optics in combination with an adapatable beam expansion technique and a detector with virtual borders between semicircular elements overcome the existing limits and extend the size range to a lower limit of about 0.05 μm and an upper limit above 10 mm. The sensititivity limit of LD is approaching that of single particle counting techniques. For medical spray and inhaler applications, a 0.1% optical concentration can be converted to particle size distributions even for time-resolved analyses with sample intervals of a few milliseconds. The reproducibility of the sensor, with a standard deviation typically much less than 0.5%, is no longer the limiting factor. The reproducibility of the results is mainly dominated by the reproducibility of sampling, sample splitting, dispersion and the contamination of the optical path. The latter can be improved by the control of flow, especially for in-line and inhaler applications. 相似文献
20.
We report on the first dark-matter (DM) search results from PandaX-I, a low threshold dual-phase xenon experiment operating at the China JinPing Underground Laboratory. In the 37-kg liquid xenon target with 17.4 live-days of exposure, no DM particle candidate event was found. This result sets a stringent limit for low-mass DM particles and disfavors the interpretation of previously-reported positive experimental results. The minimum upper limit, 3.7 × 10 ?44 cm 2, for the spin-independent isoscalar DM-particle-nucleon scattering cross section is obtained at a DM-particle mass of 49GeV/c 2 at 90% confidence level. 相似文献
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