Using Monte Carlo methods to estimate efficiencies of gamma-ray emitters with complex geometries

In the event of a radioactive disaster, one of the biggest tasks is to estimate the radiation dosage received by people to determine the actions of emergency response teams. The first and the most rapid screening method of internally contaminated people in case of an emergency response is to perform in-vivo measurements for gamma-emitters. Development of virtual gamma-ray calibration techniques will be critical for emergency invivo measurements because there are inadequate numbers of phantom types to approximate all body shapes and sizes. The purpose of this project was to find a reliable way to estimate the efficiency of gamma-systems using Monte Carlo computations, and to validate that efficiency by making measurements of a standard geometry. Two geometries, a 5-ml ampoule and a Bottle Manikin Absorption (BOMAB) phantom head, spiked with ⁶⁷Ga were used as standard geometries. The radioactive objects are measured at a number of distances from a high purity germanium (HPGe) detector, and the experimental efficiency for our gamma-spectrometry system is determined. The same set of experiments was then modeled using the Monte Carlo N-Particle Transport Code (MCNP). The conclusion of this project is that computationally derived detector efficiency calibrations can be comparable to those derived experimentally from physical standards.     

The definition of basic TEDS of IEEE 1451.4 for sensors for an electronic tongue and the proposal of new template TEDS for electrochemical devices

It is important to define a standard method to store basic sensor information, such as the type and the structure of sensors for an electronic tongue system and there is no such method defined in the IEEE 1451.4 transducer electronic data sheet (TEDS) so far. The major challenge is to choose a suitable standard template that can be used with sensors for electronic tongues. However, the standard templates provide an imprecise specification when used with sensing devices for electronic tongues. In this paper, we present definitions of the basic TEDS of IEEE 1451.4 for sensors for an electronic tongue system and propose a new template TEDS for IEEE 1451.4 for potentiometric devices.     

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.     

Design and construction of an ultra-low-background 14-crystal germanium array for high efficiency and coincidence measurements

Physics experiments, environmental surveillance, and treaty verification techniques continue to require increased sensitivity for detecting and quantifying radionuclides of interest. This can be done by detecting a greater fraction of gamma emissions from a sample (higher detection efficiency) and reducing instrument backgrounds. A current effort for increased sensitivity in high resolution gamma spectroscopy will produce an intrinsic germanium (HPGe) array designed for high detection efficiency, ultra-low-background performance, and useful coincidence efficiencies. The system design is optimized to accommodate filter paper samples, e.g. samples collected by the Radionuclide Aerosol Sampler/Analyzer (RASA). The system will provide high sensitivity for weak collections on atmospheric filter samples, as well as offering the potential to gather additional information from more active filters using gamma cascade coincidence detection. The current effort is constructing an ultra-low-background HPGe crystal array consisting of two vacuum cryostats, each housing a hexagonal array of 7 crystals on the order of 70% relative efficiency per crystal. Traditional methods for constructing ultra-low-background detectors are used, including use of materials known to be low in radioactive contaminants, use of ultra pure reagents, clean room assembly, etc. The cryostat will be constructed mainly from copper electroformed into near-final geometry at PNNL. Details of the detector design, simulation of efficiency and coincidence performance, HPGe crystal testing, and progress on cryostat construction are presented.     

Efficiency for close geometries and extended sources of a p-type germanium detector with low-energy sensitivity

Typically, germanium detectors designed to have good sensitivity to low-energy photons and good efficiency at high energies are constructed from n-type crystals with a boron-implanted outer contact. These detectors usually exhibit inferior resolution and peak shape compared to ones made from p-type crystals. To overcome the resolution and peak-shape deficiencies, a new method of construction of a germanium detector element was developed. This has resulted in a gamma-ray detector with high sensitivity to photon energies from 14 keV to 2 MeV, while maintaining good resolution and peak shape over this energy range. Efficiency measurements, done according to the draft IEEE 325-2004 standard, show efficiencies typical of a GMX or n-type detector at low energies. The detectors are of large diameter suitable for counting extended samples such as filter papers. The Gaussian peak shape and good resolution typical of a GEM or p-type are maintained for the high count rates and peak separation needed for activation analysis.     

Determination of Pentaerythritol Tetranitrate in Pharmaceuticals by High Performance Liquid Chromatography

Abstract

A high performance liquid chromatographic procedure for the analysis of pharmaceutical formulations containing pentaerythritol tetranitrate including the diluted bulk drug and finished products consisting of uncoated tablets and timed-release capsules and tablets was developed. The method employs a reversed-phase system with UV detection at 230 nm. Replicate analyses of 11 commercial formulations (5 diluted bulk drugs and 6 dosage forms gave precision values (CV) having a range of 0.17 to 1.80%. Recovery values obtained from these commercial preparations via fortification ranged from 98.8 to 102.0% while recoveries from 3 synthetic mixtures varied from 99.2 to 100.8%. The detector response for the analyte was observed to be linear over a 50-fold concentration range using nitroglycerin as the internal standard. The proposed HPLC method is specific, easy to perform and exhibits excellent accuracy and precision. Seven different brands of HPLC columns were evaluated for possible use with the method.     

Optimization of HANARO cold neutron induced prompt gamma activation analysis system by using Monte Carlo code

Prompt gamma activation analysis (PGAA) is a nuclear analytical technique for non-destructive determination of elemental and isotopic compositions. The principle of PGAA technique is based on detection of captured gamma-ray emitted from an analytical sample while being irradiated with neutrons. Use of a cold neutron beam guide greatly reduces the gamma-ray background at the analytical sample while maintaining a neutron capture rate is comparable to that of standard thermal neutron PGAA. A new cold neutron induced prompt gamma activation analysis (CN-PGAA) system has been under construction since April of 2009 at the HANARO Cold Neutron Building (KAERI, Republic of KOREA). In this study, the Compton suppression factor of the CN-PGAA system was estimated to be 5.5 using a ⁶⁰Co radioactive source in conjunction with the MCNPX simulations. Several parameters of the CN-PGAA system were studied to estimate and optimize the performance of the system: scintillation material in the guarded detector of a Compton suppression spectrometer (CSS); the relative positions of the HPGe detector and annular detector; and the distance between the HPGe detector and back catcher BGO detectors of the CSS. In addition, the neutron ray-trace simulation package, McStas, was adopted to predict the neutron flux and wavelength distribution at the end of the cold neutron beam guide. These results served as input for the MCNPX simulation of the CN-PGAA system.     

Optimization of pulse processing parameters for HPGe gamma-ray spectroscopy systems used in extreme count rate conditions and wide count rate ranges

The need to perform gamma-ray spectroscopy measurements at high count rates with HPGe detectors is more common than many believe. Examples exist in safeguards, radiochemistry, nuclear medicine, and neutron activation analysis. In other applications wide dynamic ranges in count rate may be encountered, for example samples taken after a nuclear accident are counted on a system normally used for environmental monitoring. In a real situation, it may not be possible to reduce count-rates by increasing the distance or using collimators. The challenge is to obtain the “best” data possible in the given measurement situation. “Best” is a combination of statistical (number of counts) and spectral quality (peak width and position) considerations over a wide range of count rates. The development of multichannel analyzers (MCA) using digital signal processing (DSP) has made possible a much wider range of values for shaping times as well as the processing of the detector signal in various ways to improve performance with pulse-by-pulse adjustments. The pulse processing time is directly related to the shaping time. The throughput is related to the pulse processing time and the duration of the detector signal. Longer shaping times generally produce better peak resolution. However, the longer shaping times mean larger dead times and lower throughput. The ability to select the best compromise between throughput and resolution is possible with DSP MCAs. In addition, the dead-time-per-pulse can be reduced by changing the digital filter without significant impact on the full-width at half-maximum. To evaluate the improvements and to suggest an approach to optimization of system performance, a small and a large GEM (p-type) coaxial HPGe detector were selected for measurements to determine the performance at various input count rates and wide range of rise times and flattops in the DSPEC 50 MCA. Results will be presented for the throughput measured at dead times from 30 to 99.9 % with and without the use of the ORTEC Enhanced Throughput Mode.     

Possibility of scanning electron microscope observation and energy dispersive X-ray analysis in microscale region of insulating samples using diluted ionic liquid

The possibility of scanning electron microscope (SEM) observation and energy dispersive X-ray (EDX) spectrometry analysis in microscale regions of insulating samples using diluted ionic liquid was investigated. It is possible to obtain clear secondary electron images of insulating samples such as a rock and mineral at 5,000 times magnification by dropping 10 μL of 1 wt% of 1-ethyl-3-methylimidazolium acetate (EMI-CH?COO) diluted with ethanol onto the samples. We also obtained EDX spectra of the samples in microscale regions (~5 μm2) without overlapping EDX spectra of other minerals with different composition. It might be possible to perform quantitative analysis of the samples if a method that does not need standard samples is applied or an X-ray detector sensitive for light elements was attached. The method of dropping 1 wt% EMI-CH?COO diluted with ethanol onto insulating samples is useful for SEM observation, EDX analysis in microscale regions, and the preservation of scarce rock and mineral samples because ionic liquid can be easily removed with acetone.     

Performance evaluation of evaporative light scattering detection and charged aerosol detection in reversed phase liquid chromatography

In pharmaceutical industry ultraviolet (UV) detection is often used as the preferred detection technique in HPLC analysis since most pharmaceutical compounds possess a UV-absorbing chromophore. However, in case the active pharmaceutical ingredient (API) does not have a UV-absorbing chromophore, or if some of the impurities present lack a chromophore, they will not be detected in routine HPLC analysis employing only a UV detector and alternative detection schemes have to be used. Refractive index detection or mass spectroscopy (MS) can be used but these detectors have their intrinsic weaknesses, such as lack of sensitivity or high cost. With the appearance of semi-universal techniques such as evaporative light scattering detection (ELSD), and more recent, charged aerosol detection (CAD), detection of non-UV-absorbing compounds became feasible without having to resort to such complex or costly detection methods. This paper evaluates the different performance characteristics such as sensitivity, linearity, accuracy and precision of both the ELSD and CAD detector coupled to HPLC. One disadvantage of this type of detector is the non-linear response behaviour which makes direct linear regression for making calibration curves inaccurate.     

Quantitative phase analysis on a single crystal X-ray diffractometer equipped with a two-dimensional flat detector

The work describes the procedure for performing a quantitative powder X-ray diffraction analysis in the Debye-Scherrer scheme on a single crystal diffractometer equipped with a flat two-dimensional detector. Specially prepared mixtures of polycrystalline phases (6h-A1₂O₃, Si, 6h-SiO₂, and W) with substantially different linear absorption coefficients are analyzed. It is shown that even when crystallites are most prone to preferred orientation, it is possible to perform measurements with an accuracy no worse than traditional 5 wt.%.     

Conformations of the transition metal complexes

A historical overview is given on the structural and conformational studies of tris(diamine)metal system. Studies on various coordination compounds revealed that energy minimization calculations can predict the detailed geometries of the complexes. The computed geometry of a coordination compound agrees with that observed in the crystal structure to within several standard deviations. Differences in thermodynamic properties between different conformers are well reproduced. Equilibrium distribution of conformers can be reasonably accounted for on the basis of the minimized strain energies.     

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.     

Impulse-response functions of several detectors used in flow-injection analysis

A procedure for the determination of the impulse-response function of a detector is given. Its application to photometers, ion-sensitive field effect transistors, a potentiometric detector at constant current and a voltammetric detector shows that the impulse-response function can be used to obtain specific information about the performance of the detector in the manifold. This function clearly shows the contribution of the detector to the peak broadening and how the detector generates the final signal from the presented concentration profile. From this information one could derive improvements to the detector, such as changing the construction of the detector cell, minimizing the influence of other parts of the manifold or adapting the attached electronics.     

Characterization of HPGe gamma spectrometers by geant4 Monte Carlo simulations

Two coaxial and a low-energy HPGe detector were characterized with Monte Carlo simulations, using the geant4 toolkit. The geometry of the detectors, including the dimensions of the crystal and the internal structural parts, were initially taken from the factory specifications and from X-ray radiographies, and were later fine-tuned. The detector response functions, with special emphasis on the absolute full-energy peak efficiencies and peak-to-total ratios, were calculated and compared to experimental data taken at different measurement geometries. Between 150 keV and 11 MeV an agreement within 1–2 standard deviation has been achieved, whereas systematic deviations were experienced at lower energies.     

Detector resolution required for accurate identification in common gamma-ray masking situations

Accurate nuclide identification depends on the ability to determine if specific peaks are present in the spectrum. Several current handheld nuclide identifiers and portal monitors use a variant of a peak quality value for this. The peak quality is usually calculated as the peak area divided by the uncertainty of the peak area and when this quotient is above a threshold value, the peak is said to be present. Other works [Terracol et al. In: 2004 IEEE Nuclear Science Symposium Conference Record, Rome, Italy, 2004, Ryder In: Scanning Electron Microscopy/1977 V. 1, Proceedings of the Workshop on Analytical Electron Microscopy, Chicago, 1977] have developed a formalism to calculate the peak uncertainty for interfering peaks based on the detector resolution, background, individual peak areas, and peak separation. The threshold on peak uncertainty determines the minimum activity that will be identified or detected. Care must be used in the selection of the threshold in order to comply with the false positive and false negative requirements of the detection system regime, or “concept of operations”. The performance standards for the handheld identifiers and portal monitors specify the nuclides required to be identified. From this list and other commonly expected nuclides, the energies of the expected gamma rays can be tallied, yielding a table of the separations of adjacent peaks possible in the collected spectrum. Using the formalism, the peak quality value can be determined as a function of the detector resolution, peak area and background for the energy separations in the table determined above. Results are shown for the cases of HEU and plutonium with the masking nuclides of NORM, ¹³³Ba, or ⁵⁷Co for both germanium and sodium iodide detectors. Typical resolutions, efficiencies and counting times were used.     

色谱/原子吸收联用系统中卧管式微火焰原子化离子化同步检测器的研究与应用

 介绍了一种新型的卧管式微火焰原子化离子化同步检测器 ,对其结构、工作原理及性能进行了研究。将其应用到色谱 /原子吸收联用系统中 ,实现了有机金属化合物以及与其共存的有机化合物的同步检测。有机金属化合物 (二乙基汞 )的原子吸收信号检出限为 2 5× 10 -11g/s;有机化合物 (苯 )的离子化信号检出限为 1 0× 10 -11g/s。     

Orientation of molecular groups of fibers in nonoriented samples determined by polarized ATR-FTIR spectroscopy

A method based on polarized attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy is proposed for determining the infrared dichroic absorption ratio of a single fiber from a sample deposited flat on a germanium crystal without the requirement of fiber orientation. The method shows its efficiency on cellulose fibers of paper and has been applied to protein fibers (type I collagen and β-amyloid) and polysaccharide fibers (cellulose and starch). The method gives access to the dichroic ratio of strong absorptions bands, which is not easily accessible with conventional absorption techniques. Then, the orientation of the molecular groups of organic fibers can be easily determined by polarized ATR-FTIR spectroscopy. By extension, this method will be useful to determine the molecular orientation of fibers in structured complex samples, such as biological tissues and plants. Spatially resolved information on the organization of the fiber network will be easily extracted by utilizing a focal plane array detector for imaging measurements.     

Mitigation of fluorescence and scattering in reflection convex-crystal X-ray spectrometers

X-ray diagnostics in today's high-energy density environments must contend with intense and energetic X-ray background levels. In this work, we address the issues of X-ray fluorescence and scattering in reflection-geometry X-ray crystal spectrometers. In this geometry, the detector can capture not only a dispersed X-ray spectrum but also fluorescence and/or scattered X-rays from the diffracting crystal and crystal mounts. Studies to optimally reduce these sources of spectral contamination have been performed using the HENEX spectrometer. Variables that mitigate such unwanted background include filtration, collimation and judicious selection of crystal and detector materials.