Determination of isotopic ratios of uranium samples using passive gamma spectroscopy with multiple detectors

Uranium samples of various enrichments have been passively counted on the University of Texas detector gamma–gamma coincidence system. By observing gamma rays emitted from ²³⁵U and its daughters compared to gamma rays emitted by ²³⁸U daughters and comparing the data to standards of known enrichments, a technique has been developed to take a uranium sample of unknown enrichment and passively count it to determine its uranium isotopic concentration. Because the gamma rays from ²³⁵U are generally in the low-energy regime, there is a strong susceptibility to background interferences, especially from the Compton background produced from higher energy gamma rays. Other interferences, such as those from the decay series of uranium also exist for ²³⁵U gamma rays. In this light, we have collected data using list-mode to produce two-dimensional gamma–gamma coincidence spectra, which allows us to gate the low-energy gamma rays from ²³⁵U with gamma rays that are in coincidence. In doing this, much of the low energy interferences are reduced, and one can analyze the ²³⁵U gamma rays with high precision. Because of the high density of uranium, self-shielding has significant effects especially in the low-energy regime. To correct for this attenuation the detector system has been modeled by MCNP and self-shielding factors have been calculated across the energy spectrum. A big advantage to this method is the capability of performing this analysis with small (<1 g) samples in a non-destructive and relatively inexpensive manner. If necessary, this analysis can be performed within 24 h if an urgent nuclear forensics scenario arises.     

A review of recent developments in self-attenuation correction leading to a simple dual energy lump correction model for use in the quantitative gamma assay of plutonium

The application of quantitative high-resolution gamma-ray spectrometry for the non-destructive assay of plutonium bearing items, such as waste drums, is complicated by self-attenuation if the plutonium is present as lumps. By definition, lumps are small compared to the bulk matrix and so are not accounted for in the gross matrix correction yet can exert a significant influence on the assay result. Compared to a calibration using dilute standards, self-attenuation results in an under-reporting of the mass of plutonium present. The availability of representative standards is unrealistic for diverse waste streams and so a means to detect and compensate for the presence of lumps is needed. An experimental approach that can in principle generate an item specific correction factor is to exploit the differential attenuation between a set of gamma-lines of known relative emission intensity. In the case of routine measurements of drummed Pu wastes the choice of lines is often limited, the most appropriate often being those at 129 keV and 414 keV from ²³⁹Pu. This paper discusses the problems and potential of exploiting this pairing in a simple dual energy approach to the long standing and challenging problem of self-attenuation.     

Determination of atomic parameters from powder diffraction intensity ratios

The basis for a systematic use of intensity ratios is given in order to determine positional and occupational parameters. Given a series of N powder diffraction intensities, the matrix of intensity ratios is defined. Expressing them as vectors, it is shown that they form a subset of $\text{1}\text{2}\text{(N}{}^2\text{? N)}$ elements in a vectorial space with dimension N ? 1. Since any intensity ratio may be obtained from the product or division of other ratios, their vectorial representation is used to define a criterion for independence for a set of ratios, transforming the product in a sum of the corresponding vectors. Any set of N ? 1 independent ratios may be used in the determination of atomic parameters. In order to find the most suitable set, it is proposed to assign an index to each intensity ratio, which depends upon the difference between observed and calculated values and their derivatives with respect to the parameters to be determined. Then, an independent set of intensity ratios is chosen among those with higher indices, for each point of a convenient grid in the space of the parameters, finding the best set at each point. Finally, the best absolute set is defined as that with the minimum value of an agreement indicator between observed and calculated values. An illustrative example of the procedure indicated above is shown, and the results are compared with those obtained by other powder integrated intensity methods. In this work, comparison with the Rietveld profile refinement method is not considered.     

A method for self-attenuation and sample-height correction for counting efficiency of HPGe using Marinelli beaker geometry

A procedure for self-attenuation and sample height correction in HPGe gamma spectrometry efficiency has been presented. An MCNP model of an HPGe detector was used to calculate the full energy peak efficiency (FEPE) for a group of different samples with different heights in Marinelli beaker geometry. A proper function has been fitted to the simulation results to obtain the correction function. The function has been used to calculate the FEPE of a spiked soil sample in different sample heights by considering the experimentally known FEPE of another standard solution source. A good agreement between the experiments and calculations have been shown.     

Gamma-ray spectrometry for the self-attenuation correction factor of the sand samples from Antalya in Turkey

In this paper, we attempt to determine the self-attenuation correction factor for 37 different sand samples collected from Antalya region of Turkey with densities changing from 2.205 to 2.679 g  \(\hbox {cm}^{-3}.\) Transmission method has been used in order to obtain self-attenuation correction factor in comparison with the air and ultrapure water samples for each case. Self-attenuation correction factor versus energy fit curve is obtained. While the self-attenuation correction factor has large values at low energies, it becomes smaller at high energies and tends to become constant thereafter.     

Determination of K shell X-ray intensity ratios for some heavy elements

The K shell X-ray intensity ratios K_α2/K_α1, K_β1,3/K_α1 and K_β2/K_α1 for 21 elements with 65⩽Z⩽92 have been measured using an incident photon energy of 123.6 keV. The X-rays have been measured with a Si(Li) semiconductor detector. K_β and K_α X-rays have been analyzed into the components K_β1,3 and K_β2 and K_α1 and K_α2, respectively, using a computer program. The experimental results were compared with the theoretical values of Scofield and available experimental results. All X-ray intensity ratios values have been plotted versus atomic number.     

Determination of ultraviolet radiation intensity using polysulphone film

The use of a polysulphone film as a physical standard for measuring light dosage in accelerated weathering tests is described. Tables are given of equivalent dose at 305 nm and correction factors for temperature of exposure. Examples of application are outlined showing effect of lamp and filter age on UV intensity and comparison of different light sources.     

A convenient and practical procedure to derive correction factors to estimate atomic ratios by XPS

In quantitative XPS, the preparation of reference samples is difficult or impossible, and the correction factor method has been employed. Therefore, a convenient and practical procedure to get correction factors to estimate atomic ratios is proposed, in which it is assumed that the XPS intensity distribution corresponds to the distribution of the photoionization cross section. This procedure can be applied to some instruments and various samples without the previous collection, preparation of reference sample groups, and determination of sensitivity factors for each element.     

A convenient and practical procedure to derive correction factors to estimate atomic ratios by XPS

In quantitative XPS, the preparation of reference samples is difficult or impossible, and the correction factor method has been employed. Therefore, a convenient and practical procedure to get correction factors to estimate atomic ratios is proposed, in which it is assumed that the XPS intensity distribution corresponds to the distribution of the photoionization cross section. This procedure can be applied to some instruments and various samples without the previous collection, preparation of reference sample groups, and determination of sensitivity factors for each element. Received: 8 October 1997 / Revised: 23 February 1998 / Accepted: 28 February 1998     

Determination of attenuation coefficient for self-absorption correction in routine gamma ray spectrometry of environmental bulk sample

A simple method to determine -ray attenuation coefficients using Ba-133 -rays has been developed and applied to self-absorption correction in routine -ray spectrometry for environmental samples composed of unknown matrix elements. Experimental values of the mass attenuation coefficient obtained by the method agree well with calculated values for samples of known elemental composition which was determined by means of chemical analysis.     

Determination of aliphatic amines using fluorescence intensity of 4-methyl umbelliferone

Effect of aliphatic amines and their concentration on fluorescence intensity of 4-methyl umbelliferone (4-MU) one of coumarin derivative were investigated. This compound has blue light emission under UV lamp in methanolic solution. Spectral investigation of 4-MU solution containing amine compounds showed lower intensity at 350-410 nm (quenching region) and higher intensity at 430-500 nm (enhancing region) as compared to pure methanolic solution of 4-MU. Fluorescence intensity at isoemitting point was independent from concentration of amines. The variation of fluorescence intensity could be used for determination of aliphatic amines in both quenching and enhancing region. Linear ranges for determination of amines in the quenching region were obtained from Stern-Volmer diagram of 4-MU. Determination of amines at fluorescence enhancing region using inverse fluorescence intensity against inverse amine concentration (bireciprocal plot) were investigated and related equations was also proposed.     

Determination of self shielding factors and gamma attenuation effects for tree ring samples

Determination of tree ring chemistry using Neutron Activation Analysis (NAA) is part of an ongoing research between Penn State University (PSU) and Cornell University, The Malcolm and Carolyn Wiener Laboratory for Aegean and Near Eastern Dendrochronology. Tree-ring chemistry yields valuable data for environmental event signatures. These signatures are a complex function of elemental concentration. To be certain about concentration of signature elements, it is necessary to perform the measurements and corrections with the lowest error and maximum accuracy possible. Accurate and precise values of energy dependent neutron flux at dry irradiation tubes and detector efficiency for tree ring sample are calculated for Penn State Breazeale Reactor (PSBR). For the calculation of energy dependent and self shielding corrected neutron flux, detailed model of the TRIGA Mark III reactor at PSU with updated fuel compositions was prepared using the MCNP utility for reactor evolution (MURE) libraries. Dry irradiation tube, sample holder and sample were also included in the model. The thermal flux self-shielding correction factors due to the sample holder and sample for were calculated and verified with previously published values. The Geant-4 model of the gamma spectroscopy system, developed at Radiation Science and Engineering Center (RSEC), was improved and absolute detector efficiency for tree-ring samples was calculated.     

Seasonal effects on the intensity of natural background gamma radiation spectrum

The natural background gamma (NBG) radiation spectra were accumulated indoors, using a 7.6 cm×7.6 cm NaI(Tl) detector, for a period of 18 months. Six prime peaks of different origins were chosen to study the seasonal effects on the intensity of NBG radiation spectrum in Baghdad. The energy range covered by this work stretched from 0.2 to 7.0 MeV. The standard deviation (due to seasonal effects) of the corrected net area of prime peaks varied from 1.7% to 12.1%. This confirms the seasonal effects on some peaks. A quantitative measurement of the peaks is given in terms of relative intensity.     

Effect of the crystal structure on the X-ray intensity ratios of 3d metals

The linear muffin-tin-orbital method is used to calculate the d electron populations of 3d metals in different crystal structures. It is found that valence electronic structure and hence the Kβ/Kα X-ray intensity ratio (R) of a 3d metal changes with the crystal structure. It is concluded that R of a 3d metal depends on the energy band structure rather than the chemical environment.     

Measuring equilibrium bicarbonate concentrations directly in cellular mitochondria and in human serum using europium/terbium emission intensity ratios

A series of Eu and Tb complexes of four different chiral ligands incorporating an azaxanthone sensitiser has been evaluated as probes for the bicarbonate anion. Their binding affinities were assessed at ambient pH with bicarbonate, lactate, citrate, phosphate and serum albumin. Binding was signalled by modulation of circularly polarised luminescence and apparent affinity constants were measured by examining changes in emission intensity ratios. Competition experiments show that with these species and ATP present at normal physiological values, bicarbonate can be determined selectively over the concentration range 10 to 35?mM. Bicarbonate levels are also reported by using a mixture of Eu and Tb complexes of a common ligand, examining the ratio of red/green emitted light. These methods have been adapted for the determination of bicarbonate in human serum and used for the assessment of mitochondrial levels of bicarbonate in several different cell types with confocal microscopy.     

Determination of plutonium isotope ratios at very low levels by ICP-MS using on-line electrochemically modulated separations

Electrochemically modulated separations (EMS) are shown to be a rapid and selective means of extracting and concentrating Pu from complex solutions prior to isotopic analysis by inductively coupled plasma mass spectrometry (ICP‐MS). This separation is performed in a flow injection mode, on‐line with the ICP‐MS. A three‐electrode, flow‐by electrochemical cell is used to accumulate Pu at an anodized glassy carbon electrode by redox conversion of Pu(III) to Pu (IV&VI). The entire process takes place in 2% (v/v) (0.46 M) HNO₃. No redox chemicals or acid concentration changes are required. Plutonium accumulation and release is redox dependent and controlled by the applied cell potential. Large transient volumetric concentration enhancements can be achieved. Based on more negative U(IV) potentials relative to Pu(IV), separation of Pu from uranium is efficient, thereby eliminating uranium hydride interferences. EMS‐ICP‐MS isotope ratio measurement performance will be presented for femtogram to attogram level plutonium isotope injections.     

Effect of dilution and correction of the count rate data in gamma scintillation counting

Serious errors could be introduced into experimental results due to the observed non-additive nature of the count rate data recorded by gamma scintillation counters, particularly when the samples are subjected to excessive dilution. Simple procedures for correcting the experimental results are suggested.