A compton-suppressed phoswich detector for gamma spectroscopy

A phoswich detector with two scintillation layers has been designed and assembled at Oregon State University. This detector is able to identify and reject Compton events and ultimately reduce the Compton continuum in gamma energy spectra. In this detector, CsI(Tl) crystal is used to primarily detect photoelectric events. The CsI(Tl) crystal is partially surrounded by a BGO crystal layer to capture and identify Compton-scattered photons. Both crystals are optically coupled to a single photomultiplier tube. A real-time, FPGA-based digital pulse shape analysis was developed to discriminate and reject Compton-induced pulses from the CsI(Tl) crystal. All the digital pulse processing functions including pulse shape discrimination analysis, pile-up rejection and energy measurement were implemented in an on-board FPGA device. In this paper, the results of recent measurements using radioactive lab sources will be presented and discussed.     

A new algorithm of piecewise automated beam search for peak alignment of chromatographic fingerprints

The alignment of chromatographic signals is an important preprocessing step before further multivariate analysis. This paper presents a method, automated peak alignment by beam search (Auto-PABS), to solve the problem of peak shift in chemical chromatographic fingerprints by piecewise shifting and linearly interpolating. It is characterized by searching an adaptive range for the values of shifting and linearly interpolating of each segment. This search range is estimated by the calculation of fast Fourier transform cross correlation between the sample segment and its corresponding reference segment. Thus, arbitrary peak alignment is avoided when the real peak shifts are unknown in a large data set. Since the maximum of search range is close to the real shift, more accurate beam search is adopted to accomplish the optimization process. Simulated data and herbal medicine fingerprints of HPLC and GC are selected for evaluation. The output matrix of aligned chromatographic profiles is used directly for principal components analysis, yielding satisfactory results on real samples.     

An automatic sample changer for alpha spectroscopy with a surface barrier detector

Semiconductor particle detectors are ideal for alpha spectroscopy due to their compact size, low noise, and high resolution. This paper describes the construction and testing of an automatic sample changer for use with such a detector. The changer was constructed from locally available services and materials. It holds up to 24 samples of alpha emitting material deposited on 22 mm stainless steel counting planchets. The vacuum chamber can be evacuated to less than 10 μm mercury in 10 to 15 minutes. Once the chamber has been evacuated and detector bias has been applied, any sample in the chamber may be selected for analysis, either automatically or manually. Continuous automatic analysis of up to 24 samples is possible. Variation in efficiency from position to position was found to be 3.25% at the detector-sample spacing of 4.8 mm, and 2.31% at 27 mm. Shielding between the adjacent samples not under analysis and the detector was acceptable. Published with the approval of the Director of the Arkansas Agric. Exp. Stn., Fayetteville. This project was supported in part by grants from ERDA and Arkansas Power and Light Co. and is gratefully acknowledged. Received June 18, 1979. Use of trade name does not imply endorsement or guarantee of that product to the exclusion of other products of similar nature.     

Natural radioactivity levels of limestone rocks in northern Iraq using gamma spectroscopy and nuclear track detector

The activity concentration of radionuclides, such as ²³⁸U, ²²⁶Ra and ⁴⁰K of limestone rocks in northern Iraq was measured using gamma spectroscopy. The radionuclide activities were obtained and discussed. CR-39 nuclear track detector was used to measure the radon exhalation rates as well as the effective radium contents of these samples and are found to correspond with uranium concentration values measured by NaI(Tl) detector in the corresponding limestone rocks samples. The absorbed gamma dose rates in air due to the presence of ²³⁸U, ²²⁶Ra, ⁴⁰K and cosmic ray contribution varied between 105.3 and 223.11 nGy/h. The annual effective dose of each sample has been calculated. The correlation between activities of ²²⁶Ra, ²²²Rn exhalation rates and ²³⁸U is explained. Results show a symmetrical distribution of activity concentrations of primordial of radionuclides in selected samples. The values of all studied radionuclides are considered to be a typical level of natural background and compared with results of similar investigations carried out else where.     

Fibre optic coupler as a detector for microfluidic applications

A new construction of a fibre optic coupler is presented in the paper. Two polymer optical fibres were used to build a coupler in which coupling efficiency of optical power depends on the refractive index of liquid delivered to a microchannel formed by the fibres. The coupler was tested as a detector in saccharose concentration measurements, and was used in absorbance measurements. A red light emitting diode and a spectrometer were used as a light source and a photodetector, respectively. Experiments confirmed that the coupler can be used for the real time monitoring of the changes in the refractive index of a saccharose solution exhibiting repeatable changes in the signal, with no hysteresis. Absorbance tests were performed with a solution of bromothymol blue at different pH.     

Surface ionization detector with a supersonic free jet for gas chromatography some applications

A new design for a gas chromatographic surface ionization detector based upon hyperthermal positive surface ionization has been developed: There were two requirements: supersonic free jet nozzle and the high work function surface of Re-oxide. This detector, which is highly sensitive in response to all organic compounds, can be operated as an universal detector with an additional selectivity towards some species that have low ionization energy, but with selectivity to a much lesser degree than a conventional surface ionization detector. The minimum detectable amount of toluene is ca. 10⁻¹² g/s with a linearity greater than 10⁴. Some applications are demonstrated using three examples for the analysis of different formulations: (1), terpene mixture, (2), polycyclic aromatic hydrocarbon mixture and (3), alkyl alcohol mixture.     

Methods and software for predicting germanium detector absolute full-energy peak efficiencies

High-purity germanium (HPGe) and lithium drifted germanium (Ge(Li)) detectors have been the detector of choice for high resolution gamma-ray spectroscopy for many years. This is primarily due to the superior energy resolution that germanium detectors present over other gamma-ray detectors. In order to perform quantitative analyses with germanium detectors, such as activity determination or nuclide identification, one must know the absolute full-energy peak efficiency at the desired gamma-ray energy. Many different methods and computer codes have been developed throughout history in an effort to predict these efficiencies using minimal or no experimental observations. A review of these methods and the computer codes that utilize them is presented.     

Modeling a gamma spectroscopy system and predicting spectra with Geant-4

An activity predictor software was previously developed to foresee activities, exposure rates and gamma spectra of activated samples for Radiation Science and Engineering Center (RSEC), Penn State Breazeale Reactor (PSBR), Neutron Activation Analysis (NAA) measurements. With Activity Predictor it has been demonstrated that the predicted spectra were less than satisfactory. In order to obtain better predicted spectra, a new detailed model for the RSEC NAA spectroscopy system with High Purity Germanium (HPGe) detector is developed using Geant-4. The model was validated with a National Bureau of Standards certified ⁶⁰Co source and tree activated high purity samples at PSBR. The predicted spectra agreed well with measured spectra. Error in net photo peak area values were 8.6–33.6%. Along with the previously developed activity predictor software, this new model in Geant-4 provided realistic spectra prediction for NAA experiments at RSEC PSBR.     

Application of a simple peak shape function for the evaluation of gamma and X-ray spectra

The computational method for the rapid evaluation of gamma and X-ray spectra observed by semiconductor detectors in radiometric analysis in presented using a simple peak shape function. The computer programme includes a minimizing routine based on the modified Newton-Raphson method giving a minimized vector of the parameters more rapidly than the conventional approach. The spectra of some gamma-ray emitting fission products and X-ray spectra excited by radioisotopes of materials from copper ore processing were studied. It was found that the simple Gaussian peak function gives the results equivalent to the total peak area method.     

Pulse rise-time characterization of a high pressure xenon gamma detector for use in resolution enhancement

High pressure xenon ionization chamber detectors are possible alternatives to traditional thallium doped sodium iodide [NaI(Tl)] and hyperpure germanium as gamma-spectrometers in certain applications. Xenon detectors incorporating a Frisch grid exhibit energy resolutions comparable to cadmium/zinc/telluride (CZT) (e.g., 2% @ 662 keV) but with far greater sensitive volumes. The Frisch grid reduces the position dependence of the anode pulse rise-times, but it also increases the detector vibration sensitivity, anode capacitance, voltage requirements and mechanical complexity. We have been investigating the possibility of eliminating the grid electrode in high-pressure xenon detectors and preserving the high energy resolution using electronic rise-time compensation methods. A two-electrode cylindrical high pressure xenon gamma-detector coupled to time-to-amplitude conversion electronics was used to characterize the pulse rise-time of deposited gamma-photons. Time discrimination was used to characterize the pulse rise-time versus photo peak position and resolution. These data were collected to investigate the effect of pulse rise-time compensation on resolution and efficiency.     

Fourier transform infrared spectroscopy with a sample deposition interface as a quantitative detector in size-exclusion chromatography

Micellar electrokinetic capillary chromatography was developed to analyze plant hormones including gibberellic acid, abscisic acid, indole-3-acetic acid, alpha-naphthaleneacetic acid, 2,4-dichlorophenoxyacetic acid, kinetin-6-furfurylaminopurine and N6-benzyladenine. The influences of some crucial parameters including buffer concentration, pH value, micelle concentration and applied voltage on electrophoretic separation were investigated. Under optimum conditions (50 mM borate as the running buffer containing 50 mM sodium dodecylsulfate, pH 8.0; separation voltage: -15 kV; injection: hydrodynamic injection, 5 s at 50 mbar; temperature: 25 degrees C), a complete separation of seven plant hormones was accomplished within 30 min. Emphasis was placed on improving detection sensitivity in order to detect small amounts of hormones in plant tissue. Multiple wavelength detection and expanded bubble cell capillary were used with enrichment factors of 2 and 3, respectively. In addition, an on-line concentration method of large volume sample stacking was designed. Enrichment factors of up to approximately 10-600 were achieved for these hormones with detection limits down to 0.306 ng/ml. The method was successfully applied to analyzing abscisic acid in flowers of transgenic tobacco.     

Detection of trace materials with Fourier transform infrared spectroscopy using a multi-channel detector

FTIR spectroscopy is one of the most powerful methods for material characterization. However, the sensitivity of this analytical tool is often very limited especially for materials with weak infrared absorption or when spectral bands of the targeted trace material overlap with the spectral bands of major components. Fortunately, for heterogeneous samples, there is an opportunity to improve the sensitivity of detection by using an imaging approach. This paper explores the opportunity of enhancing the sensitivity of FTIR spectroscopy to detect trace amounts of materials using the FTIR imaging approach based on a focal plane array (FPA) detector. Model sample tablets of ibuprofen in hydroxypropyl methylcellulose (HPMC) have been used to exemplify the detection limits of FTIR spectroscopy using: (a) a conventional mercury cadmium telluride (MCT) detector and (b) a FPA detector. The sensitivity level was compared and it has been found that for this particular set of samples, the lowest concentration of ibuprofen in HPMC that can be detected using attenuated total reflection (ATR) measuring mode with the single element MCT detector was 0.35 wt% while using the FPA detector, the presence of drug has been detected in a sample that contains as little as 0.075 wt% of drug. The application of using this enhanced sensitivity offered by the multi-channel detector to probe trace amounts of drug particles left on the surface of a finger after handling a small amount of the drug has also been demonstrated. These results have broad implications for forensic, biomedical and pharmaceutical research.     

A comparison of a direct search method and a genetic algorithm for conformational searching

We present results from the application of two conformational searching methods: genetic algorithms (GA) and direct search methods for finding low energy conformations of organic molecules. GAs are in a class of biologically motivated optimization methods that evolve a population of individuals in which individuals who are more “fit” have a higher probability of surviving into subsequent generations. The parallel direct search method (PDS) is a type of pattern search method that uses an adaptive grid to search for minima. Both methods found energies equal to or lower than the energy of the relaxed crystal structure in all cases, at a relatively small cost in CPU time. We suggest that either method would be a good candidate to find 3-D conformations in a large scale screening application. © 1996 by John Wiley & Sons, Inc.     

Curve fitting using a genetic algorithm for the X-ray fluorescence measurement of lead in bone

Summary We investigated the potential application of the genetic algorithm in the analysis of X-ray fluorescence spectra from measurement of lead in bone. Candidate solutions are first designed based on the field knowledge and the whole operation, evaluation, selection, crossover and mutation, is then repeated until a given convergence criterion is met. An average-parameters based genetic algorithm is suggested to improve the fitting precision and accuracy. Relative standard deviation (RSD%) of fitting amplitude, peak position and width is 1.3-7.1, 0.009-0.14 and 1.4-3.3, separately. The genetic algorithm was shown to make a good resolution and fitting of K lines of Pb and g elastic peaks.     

A micro electron capture detector for temperature programmed analysis with capillary columns for a wide range of applications

Summary The electron capture detector (ECD) has become a indispensable and widely used tool for the detection of halogenated compounds in gas chromatography. The ECD is successfully used for routine analysis particularly in the field of environmental survey, residue determination in the food and agriculture chemistry and even more so in clinical chemistry. However, its application with high resolution glass capillary columns revealed a substantial drawback of existing ECD's, primarily the relatively large internal detector volume. The micro ECD described, with its very small cell volume (140 mm³), particularly designed for use with high resolution capillary columns, minimizes drastically the loss of separation efficiency since diffusion effects are negligible. The micro ECD permits the use of splitless or split injection techniques, as well as temperature programmed analysis to optimize sample separation. Very stable baseline and low noise conditions lower the detection limits considerably (2 pg Lindane full scale). The micro ECD can be operated either with direct current, pulsed frequency or constant current modes. Examples demonstrating the capabilities of this detector include the detection of halogenated organic volatiles in water at subnanogram levels, the determination of PCB's and pesticide residues in food as well as selected analysis in clinical chemistry, such as bile acids and amino acids in body fluids.     

Tabu search algorithm for DNA sequencing by hybridization with isothermic libraries

In this paper, a problem of isothermic DNA sequencing by hybridization (SBH) is considered. In isothermic SBH a new type of oligonucleotide libraries is used. The library consists of oligonucleotides of different lengths depending on an oligonucleotide content. It is assumed that every oligonucleotide in such a library has an equal melting temperature. Each nucleotide adds its increment to the oligonucleotide temperature and it is assumed that A and T add 2 degrees C and C and G add 4 degrees C. The hybridization experiment using isothermic libraries should provide data with a lower number of errors due to an expected similarity of melting temperatures. From the computational point of view the problem of isothermic DNA sequencing with errors is hard, similarly like its classical counterpart. Hence, there is a need for developing heuristic algorithms that construct good suboptimal solutions. The aim of the paper is to propose a heuristic algorithm based on tabu search approach. The algorithm solves the problem with both positive and negative errors. Results of an extensive computational experiment are presented, which prove the high quality of the proposed method.     

Development of an algorithm for peak detection in comprehensive two-dimensional chromatography

A method for peak detection in two-dimensional chromatography is presented. The algorithm applies first the methods developed for peak detection in one-dimensional chromatography to detect peaks in one dimension. In a second step, a decision tree is applied to decide which one-dimensional peaks are originated from the same compound and have to be 'merged' into one two-dimensional peak. To this end, different features of the peaks (second-dimension peak regions and second-dimension retention times) are compared and different criteria (common peak regions, retention time differences, unimodality in the first dimension) are applied. Different options can be used, depending on the nature of the data. The user controls this decision tree by establishing several options and "switches". The algorithm was tested with GCxGC chromatograms obtained for a commercial air-freshener sample, detecting and merging the modulated peaks belonging to the same compound. Recommendations for the set of options and switches are given. A utility that calculates and sums peak areas from merged peaks is added to facilitate automated quantification. Although the algorithm was developed for GCxGC, its application to comprehensive two-dimensional liquid chromatography (LCxLC) data should at most require minor modifications.     

High resolution applications of laser-induced breakdown spectroscopy for environmental and forensic applications

Laser-induced breakdown spectroscopy (LIBS) has been used in the elemental analysis for a variety of environmental samples and as a proof of concept for a host of forensic applications. In the first application, LIBS was used for the rapid detection of carbon from a number of different soil types. In this application, a major breakthrough was achieved by using a multivariate analytical approach that has brought us closer towards a “universal calibration curve”. In a second application, it has been demonstrated that LIBS in combination with multivariate analysis can be employed to analyze the chemical composition of annual tree growth rings and correlate them to external parameters such as changes in climate, forest fires, and disturbances involving human activity. The objectives of using this technology in fire scar determinations are: 1) To determine the characteristic spectra of wood exposed to forest fires and 2) To examine the viability of this technique for detecting fire occurrences in stems that did not develop fire scars. These examples demonstrate that LIBS-based techniques are inherently well suited for diverse environmental applications. LIBS was also applied to a variety of proof of concept forensic applications such as the analysis of cremains (human cremation remains) and elemental composition analysis of prosthetic implants.