Computer analysis of complex gamma-ray spectra from semiconductor detectors

The problem of gamma-ray spectrum analysis is treated by combination of peak-searching, choosing the fitting intervals and fitting the line shape by a shape function. The performance of the program is examined by analysis of a test spectrum,^177mLu.     

A Geant4 Monte Carlo method for synthesizing radioxenon beta-gamma coincidence spectra

This paper describes a Monte Carlo (MC) simulation method for calculating radioxenon beta-gamma coincidence spectral information. These spectral components include detector response simulations by Geant4 modeling, detector energy and resolution calibrations using the histograms of detector response, beta-gamma coincidence efficiency values and spectral interference ratios. The work presented in this paper demonstrates the feasibility of using the spectral information to create beta-gamma coincidence spectra at various radioxenon activity concentrations. The analysis of these synthetic spectra by XECON software shows an excellent correlation between the analysed radioxenon activity concentration and number of MC samplings.     

Spectrum shift fitting technique for atomic emission spectrometry

The use of the principle of additivity for the atomic emission spectra registered by a multi-element solid-state detector (SSD) requires to take into account the possible non-controlled spectrum shift. The method of solution for the problem, based on the construction of parabolic model of signal change on pixels under the spectrum shift with respect to detector, is suggested. The algorithm of the model construction depends neither on the spectral line shape nor on the spectrometer apparatus function. The method can be applied with equal success both for the short spectral regions (10–20 pixels) and for the long regions (10 000 and more pixels). The values of relative spectrum shifts can lie in a continuous range from 0.05 to 100 and more pixels. The advantages of this method are shown on the examples of subtraction of spectra in the process of registration on diffraction spectrograph PGS-2 (Carl Zeiss, Jena) with the help of a linear solid state detector (MAES-10, VMK-Optoelectronika, Russia, Novosibirsk).     

MicroSAMPO — Personal computer based advanced gamma spectrum analysis system

The widely used SAMPO Ge(Li) and HPGe gamma spectrum analysis program has been adapted to IBM Personal Computers in a thoroughly revised version MicroSAMPO. The program is intended for peak search, peak fitting, nuclide identification and activity calculations. The use of calibrated peak shape functions for peak area determination makes it possible to resolve complex multiplets with strongly overlapping peaks. Menu-driven user interface, colour graphics diplays, semi-automatic calibrations and interactive options have been designed to make the program more user-friendly. It is well suited for both spectroscopic research and routine analysis.     

AutoCal: a method for automatic energy calibration of β–γ systems based on a 137Cs spectrum

A method was developed and implemented that calculates energy versus channel calibrations. This method utilizes a ¹³⁷Cs spectrum acquired in β–γ coincidence. Both the γ singles and the β–γ coincident spectrum are utilized. The γ singles spectrum is utilized to calculate the γ detector energy versus resolution function. The Compton line in the β–γ coincidence is then used to calculate the β detector energy versus resolution function. Currently a linear regression (y = mx + b) is utilized to fit the data. However, higher order polynomials may easily be implemented if desired.     

Simplification of gamma-ray spectral data by using Fourier transform

A method is proposed to represent gamma-ray response spectra by Fourier series for the purpose of compressing spectral data. The usefulness of the method was confirmed by applying it to a spectral library of a NaI detector. In the method, a response spectrum as a wave is described by superposition of sine (cosine) waves with low frequencies, whose coefficient parameters can be obtained by a Fast Fourier Transform program. The relation between the number of parameters and the fitting error is discussed, and as the result, it is shown that the number of parameters can be reduced to about a half. The merits and features are presented in practical application of the method to the analysis of gamma-ray spectra.     

Generation of response functions of a NaI detector by using an interpolation technic

A computer method is developed for generating response functions of a NaI detector to monoenergetic gamma-rays. The method is based on an interpolation between measured response curves by a detector. The computer programs are constructed for Heath's response spectral library. The principle of the basic mathematics used for interpolation, which was reported previously by the author, et al., is that response curves can be decomposed into a linear combination of intrinsic-component patterns, and thereby the interpolation of curves is reduced to a simple interpolation of weighting coefficients needed to combine the component patterns. This technique has some advantages of data compression, reduction in computation time, and stability of the solution, in comparison with the usual functional fitting method. The processing method of segmentation of a spectrum is devised to generate useful and precise response curves. A spectral curve, obtained for each gamma-ray source, is divided into some regions defined by the physical processes, such as the photopeak area, the Compton continuum area, the backscatter peak area, and so on. Each segment curve then is processed separately for interpolation. Lastly the estimated curves to the respective areas are connected on one channel scale. The generation programs are explained briefly. It is shown that the generated curve represents the overall shape of a response spectrum including not only its photopeak but also the corresponding Compton area, with a sufficient accuracy.     

High‐resolution peak analysis in TOF SIMS data

High mass resolution time‐of‐flight secondary ion mass spectrometry (TOF SIMS) can provide a wealth of chemical information about a sample, but the analysis of such data is complicated by detector dead‐time effects that lead to systematic shifts in peak shapes, positions, and intensities. We introduce a new maximum‐likelihood analysis that incorporates the detector behavior in the likelihood function, such that a parametric spectrum model can be fit directly to as‐measured data. In numerical testing, this approach is shown to be the most precise and lowest‐bias option when compared with both weighted and unweighted least‐squares fitting of data corrected for dead‐time effects. Unweighted least‐squares analysis is the next best, while weighted least‐squares suffers from significant bias when the number of pulses used is small. We also provide best‐case estimates of the achievable precision in fitting TOF SIMS peak positions and intensities and investigate the biases introduced by ignoring background intensity and by fitting to just the intense part of a peak. We apply the maximum‐likelihood method to fit two experimental data sets: a positive‐ion spectrum from a multilayer MoS₂ sample and a positive‐ion spectrum from a TiZrNi bulk metallic glass sample. The precision of extracted isotope masses and relative abundances obtained is close to the best‐case predictions from the numerical simulations despite the use of inexact peak shape functions and other approximations. Implications for instrument calibration, incorporation of prior information about the sample, and extension of this approach to the analysis of imaging data are also discussed.     

Monte Carlo generated spectra for QA/QC of automated NAA routine

A quality check for an automated system of analyzing large sets of neutron activated samples has been developed. Activated samples are counted with an HPGe detector, in conjunction with an automated sample changer and spectral analysis tools, controlled by the Canberra GENIE 2K and REXX software. After each sample is acquired and analyzed, a Microsoft Visual Basic program imports the results into a template Microsoft Excel file where the final concentrations, uncertainties, and detection limits are determined. Standard reference materials are included in each set of 40 samples as a standard quality assurance/quality control (QA/QC) test. A select group of sample spectra are also visually reviewed to check the peak fitting routines. A reference spectrum was generated in MCNP 4c2 using an F8, pulse-height, tally with a detector model of the actual detector used in counting. The detector model matches the detector resolution, energy calibration, and counting geometry. The generated spectrum also contained a radioisotope matrix that was similar to what was expected in the samples. This spectrum can then be put through the automated system and analyzed along with the other samples. The automated results are then compared to expected results for QA/QC assurance.     

Quality assurance challenges in X-ray emission based analyses, the advantage of digital signal processing.

There is a large scatter in the results of X-ray analysis with solid-state detectors suggesting methodological origin. In order to improve the methodology, detector response functions have been investigated by many researchers and analysts. This was necessary as the departure of the response function of some detector-signal processing electronics from the normally assumed Gaussian line shape can exceed 100% in area. Several detector models have been proposed to improve understanding and establish a firm basis for quantitative work. After reviewing some contradictory results, we describe a signal processor that offers quality assurance, by producing two spectra for each measurement. One is the normal spectrum of accepted events, while the second spectrum contains all of the rejected events. For each measurement, therefore, all X-ray events are recorded, enabling quality control. In addition to this improvement, the digital signal processor of Cambridge Scientific, Canada, delivers a high throughput rate, excellent resolution, decreased low energy tailing and a line shape justified by the physics of the detector. Comparative measurements are presented to demonstrate the improved rejection of background from gamma rays as well as a significant improvement in pile-up recognition. The rejected events spectrum gives insight into the origin of the response function, which suggests that the flat plateau of the frequently used Hypermet function, normally attributed to detector dead layers, originates from pile-up with the low energy noise events. A detailed analysis demonstrates how the relative intensities of the X-ray lines can change in a varying noise environment, thus potentially explaining the unacceptable large scatter in the experimental data currently found in the literature. The comparison of the accepted and rejected events adds the possibility of monitoring the electronic efficiency of signal recognition that has generally been ignored in this field.     

Experimental comparisons of statistical uncertainties in analysis of nuclear spectra with the Cambio software application

Cambio is a software application well-suited for analysis of high-resolution nuclear spectral data in which the critical information resides in well-masked peaks or in which the data are sparse and good statistics cannot be obtained. The statistical uncertainty in the results is reported in terms of standard deviations. In the practical use of any analysis application, it is important to have a good understanding of the accuracy of the reported statistical uncertainties in the results. The authors report the results of an experiment that provides a quantitative measure of the accuracy of the reported uncertainties in analyses using Cambio. Ten calibration sources ranging in activity from 25 to 450 kBq were placed 20.5 m from a bare, 100 % relative-efficient high-purity germanium detector. Peaks in these sources occur from 20 to 2,800 keV. Data were accumulated for 128 h to create a parent spectrum from which 1024 8-hour statistically-equivalent daughter spectra were created by random sampling of the parent spectrum data. The parent spectrum was analyzed by fitting parameters which included an underlying continuum, a background of natural peaks adjusted by detector efficiency and material attenuation, and a foreground of the ten sources adjusted by detector efficiency only. Each of the daughter spectra were then analyzed starting with the parameters of the fit to the parent spectrum and then minimizing Chi-square with respect to changes in these parameters. The standard deviations of the estimated activities of the sources from all 1024 spectra were then calculated and compared to the reported statistical uncertainty in the activities and peak areas of the sources.     

Determination of phosphorus, sulfur and the halogens using high-temperature molecular absorption spectrometry in flames and furnaces—A review

The literature about the investigation of molecular spectra of phosphorus, sulfur and the halogens in flames and furnaces, and the use of these spectra for the determination of these non-metals has been reviewed. Most of the investigations were carried out using conventional atomic absorption spectrometers, and there were in essence two different approaches. In the first one, dual-channel spectrometers with a hydrogen or deuterium lamp were used, applying the two-line method for background correction; in the second one, a line source was used that emitted an atomic line, which overlapped with the molecular spectrum. The first approach had the advantage that any spectral interval could be accessed, but it was susceptible to spectral interference; the second one had the advantage that the conventional background correction systems could be used to minimize spectral interferences, but had the problem that an atomic line had to be found, which was overlapping sufficiently well with the maximum of the molecular absorption spectrum. More recently a variety of molecular absorption spectra were investigated using a low-resolution polychromator with a CCD array detector, but no attempt was made to use this approach for quantitative determination of non-metals. The recent introduction and commercial availability of high-resolution continuum source atomic absorption spectrometers is offering completely new possibilities for molecular absorption spectrometry and its use for the determination of non-metals. The use of a high-intensity continuum source together with a high-resolution spectrometer and a CCD array detector makes possible selecting the optimum wavelength for the determination and to exclude most spectral interferences.     

A maximum information utilization approach in X-ray fluorescence analysis

X-ray fluorescence data bases have significant contradictions, and inconsistencies. We have identified that the main source of the contradictions, after the human factors, is rooted in the signal processing approaches. We have developed signal processors to overcome many of the problems by maximizing the information available to the analyst. These non-paralyzable, fully digital signal processors have yielded improved resolution, line shape, tailing and pile up recognition. The signal processors account for and register all events, sorting them into two spectra, one spectrum for the desirable or accepted events, and one spectrum for the rejected events. The information contained in the rejected spectrum is mandatory to have control over the measurement and to make a proper accounting and allocation of the events. It has established the basis for the application of the fundamental parameter method approach. A fundamental parameter program was also developed. The primary X-ray line shape (Lorentzian) is convoluted with a system line shape (Gaussian) and corrected for the sample material absorption, X-ray absorbers and detector efficiency. The peaks also can have, a lower and upper energy side tailing, including the physical interaction based long range functions. It also employs a peak and continuum pile up and can handle layered samples of up to five layers. The application of a fundamental parameter method demands the proper equipment characterization. We have also developed an inverse fundamental parameter method software package for equipment characterisation. The program calculates the excitation function at the sample position and the detector efficiency, supplying an internally consistent system.     

Accurate and precise spectrochemical analysis of Bi-Pb-Sr-Ca-Cu-O high-temperature superconductor materials

For quality tests of ceramic Bi-Pb-Sr-Ca-Cu-O superconductor materials (precursors and final products) accurate stoichiometric determinations of the metallic major components are necessary. Three methods were developed during the last decade in our group. Their properties and the results obtained are compared. The first classical analytical procedure requires to much manpower. The second combined chemical-spectrometric procedure is a routine method today, but also unsuitable for rapid tests. A new fully spectrometric procedure was recently developed using a simultaneously working Echelle spectrometer with CID detector, autosampler and a special self-made data evaluation software. The basis of this method are multiline measurements of each analyte element, a new method of spectral line selection for main component precision determinations, multi-component calibrations, and frequent external standardizations. For this method the sum of the confidence intervals of all element determinations was less than 1%, and no systematic error was detected.     

Evaluating ‘goodness-of-fit’ for linear instrument calibrations through the origin

Instrument calibrations for environmental analyses frequently entail fitting straight lines forced through the origin, where either the correlation coefficient, Pearson's r, or the percent relative standard deviation, %RSD, for a set of response factors is used to measure the ‘goodness-of-fit’. However, these two approaches do not produce comparable linear calibrations. To do this, a weighted regression line needs to be calculated. A weighted regression coefficient is subsequently defined to evaluate the ‘goodness-of-fit’ and is expressed as function of the %RSD.     

Accurate and precise spectrochemical analysis of Bi-Pb-Sr-Ca-Cu-O high-temperature superconductor materials

For quality tests of ceramic Bi-Pb-Sr-Ca-Cu-O superconductor materials (precursors and final products) accurate stoichiometric determinations of the metallic major components are necessary. Three methods were developed during the last decade in our group. Their properties and the results obtained are compared. The first classical analytical procedure requires to much manpower. The second combined chemical-spectrometric procedure is a routine method today, but also unsuitable for rapid tests. A new fully spectrometric procedure was recently developed using a simultaneously working Echelle spectrometer with CID detector, autosampler and a special self-made data evaluation software. The basis of this method are multi-line measurements of each analyte element, a new method of spectral line selection for main component precision determinations, multi-component calibrations, and frequent external standardizations. For this method the sum of the confidence intervals of all element determinations was less than 1%, and no systematic error was detected.     

Effects of pulse strength, width, and sample spinning speed on the spectral spin diffusion of multiquantum coherences of spin-3/2 quadrupolar nuclei

The effects of radio-frequency pulse strength, width, and sample spinning speed on the spin-diffusion spectrum of half-integer quadrupolar spins in solids have been studied by theoretical, numerical, and experimental investigations. It is revealed that the line shape of the cross peaks changes nonmonotonically with respect to the change of pulse strength, pulse width, or sample spinning speed. It is also found that the sample spinning speed has much more pronounced influence on the spin diffusion spectral line shape. In many cases of practical importance, the effect of sample spinning must be included in spectral analysis, in contrast to the practice of previous studies. Moreover, this effect can be exploited to further improve the precision in the determination of relative orientation of the electric-field gradient tensors of the exchange partners.     

Direct determination of closely overlapping drug mixtures of diflunisal and salicylic acid in serum by means of derivative matrix isopotential synchronous fluorescence spectrometry

A direct method for the simultaneous fluorimetric determination of two anti-inflammatory drugs in serum is proposed. The combination of matrix isopotential synchronous fluorescence (MISF) and first derivative technique provides good analytical results and permits the simultaneous determination of diflunisal and salicylic acid in human serum. MISF spectra are obtained by calculating the isopotential trajectory in the three-dimensional fluorescence spectrum for a serum solution. In the spectral contour, the trajectory is taken to be the portion of the line that passes by the fluorescence maxima of both compounds ensuring a sensitivity level similar to that of a direct determination in absence of background fluorescence. Analysis was carried out in water using a pH of 7.2 provides by 0.1 M sodium dihydrogen phosphate buffer. Serum samples are diluted 100 times and provide linear calibration plots at diflunisal and salicylic acid concentrations up to 800 ng mL⁻¹. The goodness of the analytical signal was checked by using variance analysis. Signals recorded throughout the calibration range were subjected to three calibrations per each analyte, both in the absence and in the presence of variable amounts of the other analyte. Differences between individual calibrations and slopes were compared with those within individual calibrations. Based on the results, diflunisal and salicylic acid can be accurately quantified in the presence of each other. The limit of detection calculated according to Clayton who uses error propagation throughout the calibration curve and a non-centralized security factor was 36.8 and 37.3 ng mL⁻¹ for diflunisal and salicylic acid, respectively.     

Estimation of individual ultraviolet spectra in incomplete two-component separations by high-performance liquid chromatography

A method is described for the estimation of spectral features in a two-component chromatographic peak by means of a u.v. diode-array detector. The calculation relies on the assumption that the front of a fused chromatographic peak contains a single pure component. The spectrum of this component is used in calculating the concentration profile of the other component, thus allowing the determination of a solution band for the spectrum of the second component. The boundaries of the solution band are based on non-negativity restrictions of chromatographic and spectral features. The method does not require the use of principal components analysis.     

Characterisation of optical detectors using high-accuracy instruments

The facilities of the Metrology Research Institute at the Helsinki University of Technology, and methods for characterisation of optical detectors for spectral radiant intensity and irradiance responsivity, are described. The instrumentation for such characterisations includes a reference spectrometer with a number of auxiliary set-ups, and equipment for the spectral irradiance measurements with a filter radiometer based on a trap detector. The methods of realising the spectral responsivity scales based on an absolute cryogenic radiometer in house are addressed. The procedures and results of characterisation of a multipoint measuring system of photosynthetically active radiation, by employing the available facilities, are briefly described. The absolute irradiance responsivity of the device is determined by using a photometric lamp, whose spectral irradiance has been measured with the filter radiometer. The combined standard uncertainty of this set of calibrations is 3.6% at the 1σ level. The uncertainty is caused almost completely by the multipoint measuring system.