Ultraviolet characterization of integrating spheres

We have studied the performance of polytetrafluoroethylene integrating spheres in the ultraviolet (UV) region with wavelengths as short as 200 nm. Two techniques were used for this study; first, the spectral throughput of an integrating sphere irradiated by a deuterium lamp was analyzed by a monochromator. Second, a UV laser beam was directed into an integrating sphere, and spectrally dispersed laser induced fluorescence was studied. Significant absorption and fluorescence features were observed in the UV region and attributed to the contamination in the integrating sphere. We demonstrate that integrating spheres are easily contaminated by environmental pollutants such as polycyclic aromatic hydrocarbons emitted from engine exhaust. Baking of the contaminated integrating sphere can reverse some but not all of the effects caused by contaminants. The implications for using integrating spheres for UV measurement are discussed.     

On the fluorescence from integrating spheres

Our recent study of the performance of integrating spheres shows prominent UV induced fluorescence features that are associated with contamination of the diffusing wall material by hydrocarbons. Because of multiple reflections of the radiation inside the integrating sphere, fluorescence is induced multiple times with each reflection of the incident radiation by the wall. Here, we report a simple theory on the fluorescence of integrating spheres developed from first principles. The results indicate a strong dependence of fluorescence on the reflectance of the diffusing material at both the excitation and fluorescence wavelengths as well as the geometry of the integrating sphere. Because of multiple reflection of the exciting radiation, a gain of more than an order of magnitude in fluorescence is possible compared with direct and single irradiation of a flat piece of the diffusing/fluorescing material. Applications of such fluorescence analysis for integrating spheres are discussed.     

Computer modeling of integrating spheres

I present a Monte Carlo model for predicting the performance of integrating spheres as a function of incident flux direction. The model was developed specifically to aid in the design of integrating spheres used as cosine collectors but is of general applicability. I discuss a method of generating uncorrelated random numbers. The probability density functions associated with uniform irradiance over a circular entrance port and Lambertian reflectors or emitters are presented. A comparison of the model with analytic equations predicting performance for an unbaffled integrating sphere is included. The average of the data generated by the model agrees with the analytic solution for sphere throughput to better than 0.25% (? = 8.3%).     

Markov models of integrating spheres for hyperspectral imaging

Theoretical models of the signal detected by a CCD camera during hyperspectral imaging with an integrating sphere are derived using Markov chains with absorbing states. The models provide analytical expressions that describe the real reflectance of the sample as a function of the detected signal at each pixel of the image. Validation of the models was done by using reflectance standards and tissue phantoms. The models provide accurate analytical solutions for samples and spheres that are near-Lambertian reflectors.     

Method of integral cross sections in heat conduction problems

We substantiate estimates of the upper and lower bounds of the effective thermal conductivity of piecewise homogeneous bodies. A numerical scheme for calculating the temperature field has been developed and implemented, and a comparison between the results of calculations by different schemes has been carried out.Odessa Polytechnic Institute. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 2, pp. 322–329, March–April, 1995.     

Modeling of gas absorption cross sections by use of principal-component-analysis model parameters

Monitoring the amount of gaseous species in the atmosphere and exhaust gases by remote infrared spectroscopic methods calls for the use of a compilation of spectral data, which can be used to match spectra measured in a practical application. Model spectra are based on time-consuming line-by-line calculations of absorption cross sections in databases by use of temperature as input combined with path length and partial and total pressure. It is demonstrated that principal component analysis (PCA) can be used to compress the spectrum of absorption cross sections, which depend strongly on temperature, into a reduced representation of score values and loading vectors. The temperature range from 300 to 1000 K is studied. This range is divided into two subranges (300-650 K and 650-1000K), and separate PCA models are constructed for each. The relationship between the scores and the temperature values is highly nonlinear. It is shown, however, that because the score-temperature relationships are smooth and continuous, they can be modeled by polynomials of varying degrees. The accuracy of the data compression method is validated with line-by-line-calculated absorption data of carbon monoxide and water vapor. Relative deviations between the absorption cross sections reconstructed from the PCA model parameters and the line-by-line-calculated values are found to be smaller than 0.15% for cross sections exceeding 1.27 x 10(-21) cm(-1) atm(-1) (CO) and 0.20% for cross sections exceeding 4.03 x 10(-21) cm(-1) atm(-1) (H2O). The computing time is reduced by a factor of 10(4).     

Dielectronic recombination cross sections for fusion plasmas

The almost simultaneous appearance of calculated and measured dielectronic recombination cross sections for singly and few-times charged target ions has produced an interesting situation. The large disparity between theory and experiment has provoked speculation about a possible “electric field effect”. The implications of this idea are examined, in the context of distorted-wave theory, and using the isolated-resonance-approximation.     

Evaluation of correction factors for transmittance measurements in single-beam integrating spheres

An integrating sphere for transmittance measurements at normal and oblique angles of incidence has been constructed. The sphere is a single-beam instrument that uses a small-area silicon diode as the detector. The entry port is only 0.37% of the total wall area and has an oblong shape to permit measurements at high angles of incidence for scattering samples. A small beam size has been made possible by using a low-noise preamplifier system for the detector circuit. The oblong port shape and a small beam size make it possible to perform simulated double-beam measurements at near-normal incidence. Modified correction factors for the sample reflectance have been derived. Special attention has been paid to the separation into a diffuse and a specular component of the transmitted light. Results have been compared with the results of measurements on a double-beam instrument, and the correction factors for specular and diffuse samples have been experimentally verified. The importance of using the right correctionfactors for different types of samples has been evaluated together with the influence of the sphere parameters.     

Absolute cross sections for the electron-impact formation of cytosine anions

The process of negative ion formation by electron impact on cytosine (a nitrous base of nucleic acids) has been studied in crossed molecular and electron beams for electron energies in the interval from 0.4 to 5.0 eV. Using a specially developed method, the molecular beam intensity was determined and the energy dependence of the absolute cross section Q for the formation of cytosine anions was studied. The maximum ionization cross section σ = 4.2 × 10^?18 cm² was observed for an electron energy of 1.5 eV.     

Predictions and measurements of scattering and absorption over broad wavelength ranges in tissue phantoms

Predictions from Mie theory regarding the wavelength dependence of scattering in tissue from the near UV to the near IR are discussed and compared with experiments on tissue phantoms. For large fiber separations it is shown that rapid, simultaneous measurements of the elastic scatter signal for several fiber separations can yield the absorption coefficient and reduced scattering coefficient. With this information, the size of the scattering particles can be estimated, and this is done for Intralipid. Measurements made at smaller source detector separations support Mie theory calculations, demonstrating that the sensitivity of elastic scatter measurements to morphological features, such as scatterer size, is enhanced when the distance between the source and detector fibers is small.     

Spectrally resolved absolute fluorescence cross sections for bacillus spores

Absolute fluorescence cross sections for Bacillus subtilis and B. cereus bacterial spores as both aqueous suspensions and aerosols were measured at a number of excitation wavelengths between 228 and 303 nm. The fluorescence was spectrally resolved at each excitation wavelength. We found that the optimum excitation wavelength for spore fluorescence is between 270 and 280 nm. The fluorescence cross section for aqueous suspensions is four times larger than for dry aerosols when measured under similar conditions. Measurements on wet aerosols showed an increase in fluorescence cross section over dry aerosols, indicating an enhancement of the fluorescence when the bacterial spores are wet. Mie scattering cross sections at 90 degrees to the direction of the incident radiation and extinction cross sections as a function of wavelength for B. subtilis suspensions and fluorescence cross sections for tryptophan are also reported.     

Coupled multigroup cross sections for hydrogen interactions in plasmas

Using analytical fits to the experimental cross sections for H₃ H₂, and H₂⁺ interactions in plasmas, developed by Gryzinski, Riviere, Jones, and Freeman, we obtain coupled multigroup cross sections and rate coefficients for hydrogen transport applications. Multigroup cross sections and rate coefficients, for specified energy group boundaries, plasma particle and temperature profiles, and cylindrical plasma confinement radius, are generated against a spatially dependent, local Maxwellian scattering background. Cross sections are formatted for direct use in production multigroup S_n, Monte Carlo, or specific transport applications. Ten coupled hydrogen reactions are included and resulting cross sections for ionization, scattering, and production can be coupled or decoupled. Reactions treated include H, H₂ ionization by electrons and protons, H, H₂ charge exchange, and H₂, H₂⁺ dissociative mechanisms. We detail the formalism used to compute effective cross sections and rates and give practicle results for two fusion reactors.     

Excited state absorption cross sections of I13/2 of Er in ZBLAN

Optical transition intensity parameters of Er³⁺ in ZBLAN were carefully calculated, and the obtained results were compared with those reported by others. The emission cross sections of ⁴S_3/2 → ⁴I_13/2 and ⁴I_11/2 → ⁴I_13/2 transitions were confirmed according to the Fuchtbauer–Ladenburg (FL) formula. The excited state absorption cross sections for ⁴I_13/2 → ⁴S_3/2 and ⁴I_13/2 → ⁴I_11/2 transitions were derived by using the reciprocity relationship in the framework of McCumber theory. The laser gain properties of ⁴S_3/2 → ⁴I_13/2 and ⁴I_11/2 → ⁴I_13/2 transitions were discussed.     

The measurement of neutron cross sections for contrast-enhancing penetrant fluids

The purpose of this experiment was to evaluate penetrant fluids of high neutron cross section. These fluids are useful in neutron radiographic investigations for contrast enhancement Saturated solutions of the gadolinium salts and one chelate were formed by using one of several solvents, and the resulting fluid’s macroscopic neutron cross sections were measured experimentally. The results of this experiment provided a quantitative measure of the mean and standard deviation of the macroscopic neutron cross section for the penetrant fluids. Inaddition, a confidence interval was generated for each fluid. The measured neutron cross section for a control fluid was in close agreement with published values.     

Design of an integrating cavity absorption meter

The design of integrating cavity absorption meters of general geometry is analyzed for cases in which the incident illumination of the cavity is spatially uniform and isotropic, such as the meter of Fry et al. [Appl. Opt. 31, 2055 (1992)]. The analysis by Kirk [Appl. Opt. 34, 4397 (1995)] for the probability of photon survival in a spherical meter is extended to general geometries. An estimate of the effect of the shape of the cavity on the estimated absorption coefficient is given.     

Measurement of quenching cross sections for laser-induced fluorescence of atomic arsenic

The measurement of collisional quenching cross sections for the (4p)(2) (5s)(1) (4)P(1/2) state of atomic arsenic is reported. The arsenic (4)P(1/2) state was prepared by excitation from the (4p)(3) (4)S(3/2) ground state with 197.2-nm laser radiation. The fluorescence signal from the (4p)(2) (5s)(1) (4)P(1/2) ? (4p)(3) (2)D(1/2) transition was monitored at 249.3 nm. Quenching cross sections were obtained for hydrogen, methane, nitrogen, carbon monoxide, and ethylene.     

Semi-empirical inelastic cross sections for electron transport in liquid water

Electron inelastic cross sections for water in the liquid phase are important for developing Monte Carlo codes that simulate the full degradation of any radiation beam in biological matter. The limited experimental information for condensed targets and the complexity of the background theory has led to largely heuristic semi-empirical models. The present work makes use of the dielectric formalism under the first Born approximation to develop inelastic cross sections for low-energy electron transport in liquid water. A Drude model was used to describe the energy-loss distribution at the dipole limit on the basis of optical data, while the impulse approximation and an empirical generalised-oscillator-strength provided the extension to finite momentum transfer. Born corrections established earlier for water vapour were applied at low impact energies. Core-electron transitions were treated by a binary model with exchange terms. Sum-rules were satisfied to within 1-2% while an 1-value of about 80 eV was obtained. A comparison with other studies is provided.     

Scattering and absorption cross sections of light diffusing materials retrieved from reflectance and transmittance spectra of collimated radiation

Four-flux radiative transfer models have been extensively used to describe reflectance and transmittance (R&T) spectra of light scattering and absorbing (S&A) media. Solutions to the differential equations corresponding to the collimated fluxes are obtained by subsequent application of boundary conditions. Explicit expressions for the collimated R&T of light are reported, when considering a light S&A medium contained between two glass slides, an experimental arrangement which is appropriate for liquid suspensions and viscous matrices containing solid particles. A spectral simulated annealing method is applied to retrieve, from measured R&T spectra of collimated light under normal incident radiation, the scattering and absorption coefficients of the composite medium. First, the accuracy of the method is established by applying it to synthetic collimated R&T data. Secondly, we apply the method to experimental data and use it to determine the S&A coefficients of a layer of TiO₂ particles dispersed in a PVP/water matrix.     

Instrumentation for measuring fluorescence cross sections from airborne microsized particles

An experimental instrument for measuring a laser-induced fluorescence spectrum from a single aerosol particle is described. As a demonstration of instrument capabilities, the results of monodisperse 4.7 microm sodium chloride particles doped with fluorescent riboflavin, produced with an inkjet aerosol generator, are presented. The fluorescence of the aerosol particles is excited in the wide range from 210 to 419 nm using a pulsed, tunable optical parametric oscillator laser. The maximum of the fluorescence emission of separately measured particles is detected at 560 nm. The dependence of the fluorescence on the excitation wavelength is studied and fluorescence cross sections are estimated. Agreement between the measured fluorescence data and the literature data for riboflavin is observed.     

Modelling interaction cross sections for intermediate and low energy ions

When charged particles slow in tissue they undergo electron capture and loss processes that can have profound effects on subsequent interaction cross sections. Although a large amount of data exists for the interaction of bare charged particles with atoms and molecules, few experiments have been reported for these 'dressed' particles. Projectile electrons contribute to an impact-parameter-dependent screening of the projectile charge that precludes straightforward scaling of energy loss cross sections from those of bare charged particles. The objective of this work is to develop an analytical model for the energy-loss-dependent effects of screening on differential ionisation cross sections that can be used in track structure calculations for high LET ions. As a first step a model of differential ionisation cross sections for bare ions has been combined with a simple screening model to explore cross sections for intermediate and low energy dressed ions in collisions with atomic and molecular gas targets. The model is described briefly and preliminary results compared to measured ejected electron energy spectra.