Self-absorption Correction Research Articles

Monolayer-Protected Clusters with Fluorescent Dansyl Ligands

Monolayer-protected Au cluster (MPC) molecules with mixed monolayers of alkanethiolate and ω-carboxylalkanethiolate ligands were functionalized with the fluorescent label dansyl cadaverine. The emission intensity per MPC-attached dansyl group varies with (a) the number of dansyl sites per MPC (i.e., loading, 4 to 24), (b) with the linker chainlength between Au core and dansyl site, and (c) with chainlengths of alkanethiolates in the surrounding MPC monolayer coating. Emission intensity per MPC-attached dansyl group increases with loading and with the length of core−dansyl linker chains when the surrounding alkanethiolate monolayers have chain lengths equal to or greater than the linker. These results are interpreted in terms of distance-dependent energy transfer from the excited dansyl label to the metal-like MPC core and various steric constraints on thermal fluctuations of linker chains that influence the distance of approach of the label to the core. Correction for self-absorption by the highly colored...

Direct gamma-X spectrometry measurement of in environmental samples using experimental self-absorption corrections

Direct gamma-X spectrometry is a suitable technique for quantifying low-level 129I activity, down to a few becquerels per kilogram, in environmental samples. Direct gamma-X spectrometry is a non-destructive technique that is rapid, which can be used for measuring several radioisotopes (multielementary) and that is easy to implement. A description is given of an experimental method for determining the self-absorption correction in the low-energy range, below 25 keV, to quantify 129 Ii n di !erent environmental matrices. The method is based on the measurement of the mass energyattenuation coe$cient k . (E) of the sample and the standard used to calibrate the measuring system. The k . (E) data are introduced in a simplied formulation of the correction factor that is validated for several container-detector sets commonly used in low-level activity environmental measurement. The method has also been tested on 125I, 129I and 137Cs, using di!erent energy transitions. For marine algae species with high iodine concentrations (’2]10~3 as a fraction of mass), an original method of determining the 129I/505!-I ratio as against the k . (E) determination in the vicinity of the iodine K-absorption edge energy is described. ( 1999 Elsevier Science B.V. All rights reserved.

Quantitation of Sulfur Oxyanions in Industrial Metal Refining Solutions with Dispersive and Fourier Transform Raman Spectroscopy

Fourier transform (FT)-Raman and dispersive, visible excitation Raman spectra are reported for standard solutions and highly colored industrial metal refining solutions containing sulfur oxyanions, primarily sulfate and sulfamate. Good-quality spectra are obtained for both standard and industrial solutions. However, quantitation of both sulfur oxyanion concentrations is complicated by self-absorption effects. In the FT-Raman spectra, self-absorption of the incident laser beam yields an exponential correlation between Raman peak intensity, directly proportional to the apparent sulfate concentration, and the actual sulfate concentration. An internal standard, dispersive Raman spectroscopy, and excitation at 647 nm were used in an attempt to apply a self-absorption correction for absorption of the incident laser beam by ratioing the relative Raman intensities of the standard and species of interest. This approach improves the accuracy for sulfate and sulfamate determination to ±6.1% and ±3.4%, respectively. This study represents the most detailed characterization presented so far of the errors expected in quantitative Raman spectroscopy of industrial solutions. These results suggest that Raman spectroscopy is a uniquely powerful technique for the qualitative and quantitative characterization of industrial metal refining solutions.

Site-Specific X-Ray Absorption of Twin-Free (105) YBa2Cu3O7?? Films

O 1s and Cu 2p near-edge X-ray absorption fine structure (NEXAFS) spectra from highly oriented YBa2Cu3O7—δ (YBCO) films are compared to established spectra from detwinned single crystals. The (105) orientation of the films enables nearly twin-free growth and allows to investigate with NEXAFS the unoccupied electronic structure for all three polarizations E ‖ a, E ‖ b, and E ‖ c directly. While the Cu 2p spectra are very similar to those of the single crystals, the O 1s spectra of the films exhibit differences which can be traced to a substrate contribution. The YBCO film spectrum can be extracted from the data by an extended self-absorption correction.

Combination of emission spectroscopy and fast imagery to characterize high-voltage circuit breakers

To investigate the break phenomenon in high-voltage circuit breakers, a novel experimental approach combining spectroscopy and imagery for measuring electron density, temperature and arc geometry is presented. Images of the arc were taken using a narrow band optical filter, and spectra of the entire arc and in the same spectral region were recorded simultaneously. We report tests both on this method and on the available literature data and the results obtained on two different circuit breaker models. For the high-current phase (I = 2 kA to 10 kA) we chose to study the well isolated 624-641 nm fluorine lines radiating at elevated temperatures, expected to be in the range 15 000-20 000 K. With electronic densities higher than , the Stark effect, as the predominant line broadening mechanism, yields a direct relation between plasma conditions and the measured spectrum. Corrections for eventual self-absorption were applied with the arc geometry deduced from the corresponding images. The results thus obtained compare favourably with and show less scatter than those of the conventional Boltzmann diagram method. They also agree with numerical simulations. During the extinction phase, copper lines between 510 nm and 522 nm radiating at lower temperatures were selected for similar measurements. The images reveal that either a filament-like arc or well isolated hot gas pockets are present during extinction. The measured temperature was still high even a few hundred microseconds before ultimate extinction. This result is fully compatible with computations of the cooling process. The hot gas pockets could be used as a tracer for flow velocity measurements.

Use of the 1001 keV peak of 234mPa daughter of 238U in measurement of uranium concentration by HPGe gamma-ray spectrometry

For the direct gamma-ray spectrometric measurements of uranium concentrations in the samples, the use of 1001 keV peak of 234mPa, second daughter of 238U is emphasized. This “clean” peak is well resolved by HPGe detectors and gives accurate indication of uranium concentration in the samples without any self-absorption correction. The 1001 keV peak of 234mPa in the 238U chain is selected because it does not include any contribution from any other gamma emissions and does not have any interference with other peaks in the high-energy region even if the samples contain high amounts of thorium. The activity of 234mPa is determined by calibrating a HpGe detector with uranium standards. The measurement of 1001 keV gamma-ray emission from 234mPa by high-resolution gamma-ray spectrometry provides the basis for a reliable determination of 238U in the samples. The results obtained from the 63.3 keV peak of 234Th and those of the 1001 keV peak from 234mPa in the 238U chain for the uranium standards are compared with the certified values of the same samples. The results obtained from the measurements of 1001 keV peak from 234mPa agreed to within 2–5% with the certified activity values of 238U in the samples with uranium content ranging from 0.014 to 1.02 wt%. The results indicate that the uranium concentrations in the samples can be determined to within 5% error by about 14 h counting in a HpGe detector system.

Standardization and half-life measurement of 55Fe

Methods for measurement of the specific activity and the KX-ray emission rate per 1 g of a 55Fe solution are described. The emission rate was determined by a pressurized proportional 4 π-counter with an uncertainty of 0.3% and absorption and self-absorption corrections were determined experimentally for each source. The specific activity of the 55Fe solution was determined with an uncertainty of 0.7% by the 4 π KX- γ-coincidence tracing method with 54Mn as the tracer. 55Fe to 54Mn efficiency ratio equations were derived. The 55Fe half-life was measured in the pressurized proportional 4 π-counter over a period of ∼2.5 half-lives. The result is 995±3 days.

Development of total uranium analytical method by L X-ray fluorescence

This paper describes development of an L X-ray fluorescence technique to perform total uranium analysis using an internal excitation source which is added directly to the sample. The method has been demonstrated with synthetic U samples in the limited concentration range 1–15 g/l, and provides the advantage of simplicity, involving no mechanical parts which would normally be found in an external excitation source. Total uranium is determined by counting L X-rays fluoresced by a microCurie level spike of 109Cd added directly to the sample and without shielding the excitation source from the detector. A method for correction of sample self-absorption is included in the analysis.

Experimental studies of self-absorption and backscattering in alpha-particle sources

Several theories have been developed to explain why the counting efficiency of a 2π geometry alpha-particle detector is not 50%. These theories include backscattering and self-absorption as the causes of this discrepancy. Experimental measurements were made in order to confirm their predictions. In general, there was very good agreement between theoretical predictions and experimental results for the effects due to backscattering. When the thickness of the sample is considerable, self-absorption must be carefully taken into consideration to correctly measure the true activity concentration of the solution used to prepare the source. Experimental measurements were made and a method to perform the self-absorption corrections is proposed. These corrections are essential in calculating the activity of sources containing long half-life nuclides. The experimental results give reliability to methods characterizing alpha-particle sources without the use of a primary standard for the calibration of the experimental setup.

Correction of Self Absorption on XAS Measurements in Fluorescence Mode

Self absorption and influence of neighbouring absorption edges make it necessary to correct data of XAS measurements in fluorescence mode. To simulate the fluorescence process and to calculate the absorption coefficients from experimental data, informations about stoichiometric composition and fluoresnee yields are needed. Some examples show the necessity of correction of fluorescence spectra to get true absorption coefficients and the advantage of the method: In ZnSe the Se absorption coefficient can be calculated from the Zn L absorption spectrum. In the same system the intensity breakdown of Zn L fluorescence absorption spectra due to Se absorption above the Se L-edges can be removed.

Photon-absorbed fractions for cylindrical geometry: a TLD photon-absorbed fraction model.

The purpose of this study is to develop a mathematical model and calculate photon-absorbed fractions in a homogeneous nonradioactive cylinder placed inside off-center and outside a cylindrical homogeneous distribution of activity. In the second case, both the radioactive cylinder and the nonradioactive one are placed in a tissue-equivalent nonradioactive medium. The values of the photon-absorbed fractions are investigated for various geometrical configurations using water as the material filling the cylinders and the medium in between and an isotope commonly used in Nuclear Medicine, 99mTc. The calculations for off-center cylinders allows for modeling inhomogeneous distributions of activity within a tumor by placing several "cold" cylinders of various sizes in a radioactive finite cylinder. This three-dimensional model calculates photon-absorbed fractions for inhomogeneous activity distributions that can be used in quantitative nuclear medicine for self-absorption correction, thus introducing a more realistic correction than the one-dimensional corrections. These calculations are also used to model the response of a cylindrical TLD (thermoluminiscent dosimeter) placed inside a homogeneous radioactive cylinder and outside the homogeneous radioactive cylinder, in an absorbing nonradioactive surrounding medium. The purpose of these calculations is to evaluate the photon-absorbed fraction in the TLD as an instrument of measuring the time-integrated activity of a homogeneous radioactive source versus an inhomogeneous one. The dependence of the TLD-absorbed fraction on the position of the TLD with respect to the radioactive cylinder is investigated.

A practical approach for self-absorption correction in210Pb gamma-spectrometric dating

Self-absorption correction of low-energy (<100 keV) γ-radiation is a much debated subject when applying low-energy γ-rays for dating purposes. Some investigations have hitherto focused on the possibility of using either irradiating of the sample and subsequent absortiometry or introducing physical correction. In the present paper an automatic procedure was elaborated using the occurrences of other low-energy γ-lines in the sample, those of234Th. Theoretical considerations are described on the basis of the mass absorption coefficients as a function of energy and atomic number. These considerations are verified by experiments with rock forming oxides and carbonates mixed with Standard Reference Material. Finally, an example is given for the application of the self-absorption correction approach.

Experimental Determination of the Self-Absorption Correction Factor

The self-absorption correction factor for molybdenum (Mo) have been measured with a photon excited x-ray spectrometer. A comparison is made of the experimental results with the calculated values obtained by using the theoretical x-ray fluorescence intensity. The experimental results are in good agreement with the theoretical values.

Recent developments in data acquisition and processing on the Oxford scanning proton microprobe

A new data acquisition system has been developed for the Oxford SPM facility. This is based on a single PC-compatible computer with a custom designed interface unit and permits the recording of several channels of pulsed detector data together with x and y scanning voltages, beam charge and deadtime. In addition, the system will also control the beam position allowing the generation of irregular scan areas, line scans at any angle and the selection of point analyses using the mouse. The hardware and software of the system are described and some examples of data collected using the system are presented. One problem of quantitative microanalysis using focused beam PIXE is the fact that the local matrix composition and secondary electron emission characteristics may change on a scale comparable with the beam size across the sample. This leads to uncertainties in the measured charge (especially at the low currents found in microbeams) and local self-absorption correction which can introduce significant inaccuracy into the analysis. This problem has been largely solved at Oxford by the simultaneous use of RBS analysis. The shape of the RBS spectrum yields the local matrix composition and its area gives a measure of the total charge, so that the dependence on precise charge measurement is much reduced. This “ Q factor” technique is described in detail and it is shown that once the system has been calibrated routine standardless accuracy of the order of 5–10% can be obtained.

Self-absorption effect in fluorescence XAS of polarization dependent white lines

The application of fluorescence yield (FY) in the soft energy domain, and the problem of self-absorption corrections in the presence of polarization dependent white lines are discussed here. A comparison of X-ray FY and total electron yield (EY) X-ray absorption spectra at the Bi 2Sr 2CaCu 2O 8 CuL 3 edge is presented.

Performance and promise of the Compton suppression well counter

Determination of gamma-ray activities in small environmental samples poses a challenge to measurement technologies: to achieve the lowest possible detection limits, an instrument must combine high efficiency with superior peak-to-background characteristics. These requirements are simultaneously met by a Compton-suppression well counter (CSWC) of intrinsic germanium. The Brookhaven CSWC has a 5-cm deep, 1-cm diameter well and active volume of ∼ 125 cm 3. It is positioned in the 3.3-in through-hole of 10 × 12 in NaI(Tl) crystal. This CSWC was first assembled in the mid 1980s, and used in several research studies: double β − decay of 48Ca, the radioactivity of 180mTa, and high-sensitivity neutron activation analysis of iridium in rocks and minerals. More recently it has been employed in environmental radioactivity research on the transport of the members of the uranium and thorium series in natural soils. The counter typically measures samples of ∼ 3 g of soil, has a peak-to-Compton ratio of ∼ 300:1, and a sensitivity for 235U of ∼ 50 ng. It can detect gamma rays of less than 30 keV, hence can be used to measure environmentally significant nuclides like 210Pb, 230Th and 241Am in small soil samples or soil fractions. Because of the simple geometry self-absorption corrections are straightforward. The paper will discuss the performance of this device, and the projected performance of an advanced model which will incorporate a number of changes materially increasing the sensitivity.

Development of self-absorption coefficients for the determination of gamma-emitting radionuclides in environmental and mixed waste samples

In gamma spectrometry the use of theoretical self-absorption coefficients (SACs) can eliminate the need to determine the SAC empirically by counting a known source through each sample. Models have been developed to estimate self-absorption corrections for the determination of non-specified gamma-emitting radionuclides in environmental and mixed waste samples of unknown, widely varying compositions.

The determination of the oxidation state of iron by the electron microprobe

A method is described which allows the determination of Fe 2+ /Fe 3+ ratios with the electron microprobe by analyzing the FeLα and FeLβ X-ray emission spectra. We use the ratios of intensities measured on the high energy flank of the Lα and the low energy flank of the Lβ peak. The sensitivity achieved with this «flank method» is about 3-4 times higher than with the conventional peak shift method or peak intensity ratio method. The method was calibrated with the iron oxides wustite, magnetite and hematite and was then applied to a synthetic spinel solid solution series magnetite-hercynite (Fe 3 O 4 -FeAl 2 O 4 ) and to several natural olivines (Mg 2 SiO 4 -Fe 2 SiO 4 solid solutions). The results obtained by the flank method vary not only with Fe 3+ , but also with total iron content. A quantitative method for determining the oxidation state of Fe must therefore not only be based on a method with a high resolution but must also include corrections for self-absorption and matrix effects. Our method can be applied, if the bulk composition and crystal structure of the samples are known and working curves are established with similar materials

Measurement of Uranium Series Radionuclides in Rock and Groundwater at the Koongarra ore Deposit, Australia, by Gamma Spectrometry

AbstractGamma spectrometry without any self-absorption correction was developed to measure low energy gamma rays emitted by uranium and actinium series radionuclides in rock samples and groundwater residues collected at the Koongarra ore deposit, Australia. Thin samples were prepared to minimize the self-absorption by uranium in the samples. The present method gave standard deviations of 0.9 to 18% for the measurements of concentrations of uranium and actinium series radionuclides. The concentrations of238U,230Th and235U measured by gamma spectrometry were compared with those by alpha spectrometry that requires a complicated chemical separation procedure. The results obtained by both methods were in fairly good agreement, and it was found that the gamma spectrometry is applicable to rock and groundwater samples having uranium contents up to 8.1% (103Bq/g) and 3 Bq/1 of238U, respectively. The detection limits were calculated to be of the order of 10−2Bq/g for rock samples and 10−2Bq/1 for groundwater samples. The concentrations of uranium and actinium series radionuclides can be determined precisely in these samples using gamma spectrometry without any self-absorption correction.

Activity determination of 192Ir solid sources by ionization chamber measurements using calculated corrections for self-absorption

A re-entrant-well ionization chamber calibrated in terms of activity for standard solution sources and a spherical ionization chamber calibrated in terms of air kerma rate were used to determine independently the activity of a typical 192Ir brachytherapy source. Corrections for absorption and attenuation of photons inside the source and the capsule were calculated with Monte Carlo methods. The results of both methods agree within 1%. It is concluded that the specification of brachytherapy sources in terms of activity and reference air kerma rate yield consistent results if the real geometry and composition of the source are taken into account.