Challenges and techniques to effectively characterize the efficiency of broad-energy germanium detectors at energies less than 45 keV

With the now common availability of large-volume thin-window germanium detectors, it is possible to routinely measure very low energy (<45 keV) gamma and X-rays while maintaining good sensitivity for high-energy gamma rays. The effective calibration of such detectors down to these low energies is often problematic or not possible because of the lack of calibrated sources or knowledge of the source geometry. New methods have been recently developed that extend Canberra’s ISOCS/LabSOCS mathematical efficiency computation methods down to energies as low as 10 keV. Key to these developments is the capability to characterize the efficiency versus spatial location of a detector at the factory and thus eliminate the requirement to have “in the field” low-energy source standards. In this paper, the challenges for performing reliable efficiency characterizations below 45 keV and techniques developed to overcome these challenges are discussed. Response characterization results are presented for various types of low-energy and broad-energy detectors manufactured by Canberra.     

Evaluation of 500 keV proton irradiation of bilayer graphene on SiC by the use of SRIM code, a Monte Carlo simulation method for stopping and range of ions in matter

Graphene is experienced its golden age in the world of nanotechnology. Despite the fact that it takes key roles in the very complex areas, it is a simple two-dimensional material which is formed by only carbon atoms with a honeycomb form on especially another material. Graphene monolayer is usually supported by a known SiC substrate. SiC is a valuable material for both electronics and nuclear researches because of the excellent shielding and conduction properties. We simulated 500 keV proton irradiation effects on bilayer graphene on SiC by SRIM code in this paper. SRIM is a very detailed code capable of modeling ranging from very thick materials to single layer structures. This code is based on sending ions with specific energy to a target material. We presented damages and displacements caused by 500 keV protons to bilayer graphene/SiC target in the view of promising face of space and solar cell technology.     

Monte Carlo simulation and electrochemical performance corrosion inhibition whid benzimidazole derivative for XC48 steel in 0.5 M H2SO4 and 1.0 M HCl solutions

The inhibition of 1-(4-methoxybenzyl)-2-(4-methoxyphenyl)-1H-benzimidazole (MMB) on corrosion of XC48 steel in solutions 1.0 M HCl and 0.5 M H₂SO₄ were studied by potentiodynamic polarization and electrochemical impedance spectroscopy techniques (EIS). Potentiodynmic polarization curves revealed that MMB acts as a mixed-type inhibitor in both acidic media. The impedance results indicated that the corrosion process occurs under activation control. Furthermore, MMB shows a higher inhibition efficiency in HCl (97%) than in H₂SO₄ (92%) at 10^?4 M MMB. The values of ΔG°_ads, ΔH_a, E_a and ΔS_a in temperature range 293–323 K indicated that MMB strongly retarded the corrosion of XC48 steel in both solutions by a chemisorptions process. The adsorption of Benzimidazole (MMB) on carbon steel surface followed Langmuir adsorption isotherm. Scanning electron microscopy (SEM) analysis confirmed that there is an adsorbed film on the surface of XC48 steel. The results of Monte Carlo simulations studies confirmed the inhibition action of MMB.     

Li(i=1–3) sub-shell x-ray relative intensities for some compounds of 66Dy at 22.6 and 59.54 keV incident photon energies

The intensity ratios, I_Lk/I_Lα (k=l,β_1,4,β_3,6,β_2,7,9,10,15,γ_1,5,γ_2,3,γ₄,β,γ), have been measured for some compounds of ₆₆Dy, namely, Dy₂O₃, Dy₂(CO₃)₃, Dy₂(SO₄)₃, DyI₂ and Dy metal by creating the L_i(i=1–3) sub-shell vacancies in widely different proportions at two incident photon energies of 22.6 keV and 59.54 keV, in order to check the predicted dependence of these ratios on the incident photon energy and also investigate the influence of chemical effects on these ratios for an f-block element. The measurements were performed using the EDXRF spectrometer involving disk type radioactive sources of Cd¹⁰⁹ and Am²⁴¹ and a Peltier cooled Si PIN x-ray detector arranged in the 90° reflection geometry. The measured intensity ratios have been compared with the theoretical I_Lk/I_Lα values and those calculated using the two sets of fluorescence and Coster–Kronig yields available in literature in order to check the reliability of the theoretical/calculated values. Further, the measured ratio, I_Lγ4/I_Lα, was found to depend on the oxidation state of ₆₆Dy as well as nature of ligand attached to it in a given compound.     

Characterization of the water-soluble comb–linear interpolyelectrolyte nanoaggregates by Monte Carlo simulations and fluorescence probe techniques

Insight into the microstructure of the aggregate formed by the Coulombic interaction between the cationic comb copolymer poly(acrylamide-co-[3-(methacryloylamino) propyl]trimethylammonium chloride)-graft-polyacrylamide, P(AM-co-MAPTAC)-g-PAM, and the anionic polyelectrolyte poly(sodium acrylate), NaPA, was provided by Monte Carlo simulations and the fluorescence probe technique. The computational outcome revealed a core–shell organization of the comb copolymer, with an indefinite boundary between the inner and outer region. The copolymer had a spherical shape, and its backbone moiety adopted an extended conformation. The spatial extension of the copolymer and the core region contracted when association with the oppositely charged polyelectrolyte occurred. The fluorescence probes 2-dimethylamino-6-propionylnaphthalene, PRODAN, and 1-anilinonaphthalene-8-sulfonic acid, ANS, exhibited a specific interaction with the complex. A lower polarity in the polyelectrolyte complex as compared with the water polarity was sensed by the fluorescence probes, a feature which was attributed to a certain compaction of the AM-co-MAPTAC part of comb copolymer.     

Comparison of four types of gamma- and X-ray detectors for environmental applications in the 10–450 keV energy range

The performance of four types of - and X-ray radiation detectors for environmental applications was evaluated in the 10–450 keV energy range. Two cadmium zinc telluride (CdZnTe) room temperature semiconductor detectors were evaluated along with a cryogenically cooled semiconductor detector and two different types of scintillation detectors. The energy resolution, absolute peak detection efficiency and peak-to-background ratio of each of the detectors were calculated and intercompared. The advantages and disadvantages of environmental applications of each detector, along with their performance results, are summarized.     

Comparison between a radio-frequency and direct current glow discharge in argon by a hybrid Monte Carlo–fluid model for electrons,argon ions and fast argon atoms

A hybrid Monte Carlo–fluid model has been developed for the electrons, argon ions and fast argon atoms in an argon glow discharge, either operated in the dc mode or the capacitively coupled rf mode. Typical working conditions for rf GD-OES are considered, i.e. approximately 6 torr argon gas pressure and approximately 10 W power. Typical results of the model, like the potential distributions, densities, fluxes and ionization rates, will be presented and compared between the two operation modes. It will be demonstrated that the rf discharge yields more efficient ionization than the dc discharge, and hence the rf discharge requires lower voltages to obtain the same amount of power, which is in good correspondence to experimental observations.     

An interlaboratory study of quantitation procedures for the analysis of water and wastewater for organo-chlorine pesticides by gas chromatography–electron capture detector

An interlaboratory study was conducted to assess two widely used procedures for estimating quantitation levels. Six laboratories participated in the analysis of artificially prepared water samples for organo-chlorine compounds by liquid-liquid extraction followed by gas chromatography–electron capture detector using USEPA Method 608. The study consisted of three phases, including six months of results from analyte free samples, the replicate analysis of fortified samples at a single concentration by the laboratory, and finally the analysis of blind fortified samples prepared by a third party. Estimated detection and quantitation limits (Currie's L_C and L_Q and USEPA's MDL and ML) were determined for each laboratory-method-analyte combination and then compared to the observed detection and quantitation limits. The overwhelming majority of analyte free samples had a reported value of zero. As a result, observed quantitation and detection limits were frequently zero. When they were not zero, the observed quantitation limits were sometimes less than the observed detection limits and when they were not, there was no observed fixed ratio between the quantitation and detection limits. The variability between days of analysis and the use of noise reducing techniques proved to be a significant source of the observed non-normal distribution of results from distilled water samples with a concentration of zero. Conventional procedures and their underlying analytical and statistical assumptions did not provide useful predictions of laboratory quantitation based upon the results of this study. Rather than one time statistical determinations, ongoing verification of quantitation limits may be a better approach.     

The simulation of the adsorption of unsaturated cyclic hydrocarbons on the (001)-2 × 1 reconstructed silicon surface by the Monte Carlo and transfer-matrix methods

A model of the adsorption of cyclic unsaturated molecules on the Si(001)-2 × 1 reconstructed surface was developed for the example of 1,4-cyclohexadiene, which can be differently adsorbed on surface adsorption centers. Calculations were performed for a grand canonical ensemble by the Monte Carlo and transfer matrix methods. The structure of the ordered phases formed and the conditions of their appearance were studied in detail. It was shown that the suggested model reproduced all the qualitative special features of the system studied and similar systems.     

Adsorption of CO2 and N2 in Na–ZSM-5: effects of Na+ and Al content studied by Grand Canonical Monte Carlo simulations and experiments

Zeolite crystals with cations present, such as ZSM-5, are widely used for gas sequestration, separations, and catalysis. One possible application is as an adsorbent to separate CO₂ from N₂ in flue gas mixtures. Typically, the zeolite framework is of a SiO₂ composition, but tetravalent Si atoms can be replaced with trivalent Al atoms. This change in valence creates a charge deficit, requiring cations to maintain the charge balance. Experimental studies have demonstrated that cations enhance adsorption of polar molecules due to strong electrostatic interactions. While numerous adsorption studies have been performed for silicalite-1, the all-silica form of ZSM-5, fewer studies on ZSM-5 have been performed. Grand Canonical Monte Carlo simulations were used to study adsorption of CO₂ and N₂ in Na–ZSM-5 at T = 308 K, which is ZSM-5 with Na⁺ counter-ions present. The simulations suggest that a lower Si/Al ratio (or higher Na⁺ and Al content) substantially increases adsorption at low pressures. At high pressures, however, the effect of the Al substitutions is minor, because the Al^?/Na⁺ sites are saturated with guest molecules. Similarly, a lower Si/Al ratio also increases the isosteric heat of adsorption at low loading, but the isosteric heats approach the silicalite-1 reference values at higher loadings. Comparison of simulations and experimental measurements of the adsorption isotherms and isosteric heats points to the importance of carefully considering the role of charge on the Na⁺ cations, and suggest that the balancing cations in ZSM-5, here Na⁺, only have partial charges.     

The SAMPL5 host–guest challenge: computing binding free energies and enthalpies from explicit solvent simulations by the attach-pull-release (APR) method

The absolute binding free energies and binding enthalpies of twelve host–guest systems in the SAMPL5 blind challenge were computed using our attach-pull-release (APR) approach. This method has previously shown good correlations between experimental and calculated binding data in retrospective studies of cucurbit[7]uril (CB7) and β-cyclodextrin (βCD) systems. In the present work, the computed binding free energies for host octa acid (OA or OAH) and tetra-endo-methyl octa-acid (TEMOA or OAMe) with guests are in good agreement with prospective experimental data, with a coefficient of determination (R²) of 0.8 and root-mean-squared error of 1.7 kcal/mol using the TIP3P water model. The binding enthalpy calculations achieve moderate correlations, with R² of 0.5 and RMSE of 2.5 kcal/mol, for TIP3P water. Calculations using the newly developed OPC water model also show good performance. Furthermore, the present calculations semi-quantitatively capture the experimental trend of enthalpy-entropy compensation observed, and successfully predict guests with the strongest and weakest binding affinity. The most populated binding poses of all twelve systems, based on clustering analysis of 750 ns molecular dynamics (MD) trajectories, were extracted and analyzed. Computational methods using MD simulations and explicit solvent models in a rigorous statistical thermodynamic framework, like APR, can generate reasonable predictions of binding thermodynamics. Especially with continuing improvement in simulation force fields, such methods hold the promise of making substantial contributions to hit identification and lead optimization in the drug discovery process.     

Rate constants for reactions O(3P) + X2 → OX + X (X = Br,I) determined by an RRKM-type statistical algorithm employing Monte Carlo simulation

Thermal and crossed molecular beam experiments have independently given information on the reactions O(³P) + Br₂ → OBr + Br and O(³P) + I₂ → OI + I, which proceed through long-lived reaction complexes. Our previously described statistical RRKM-type simulation algorithm is now modified to calculate detailed and macroscopic rate constants for these reactions. Good agreement is found with the experimental thermal rate constant for O + I₂, while reasonable results are found for O + Br₂ when a potential energy barrier with a height of 4.2 kJ/mol is included in the entrance channel. A somewhat larger energy barrier may be needed to reach good agreement with experiments. Angular momentum restrictions are shown to considerably lower the rate constants for these reactions. It is demonstrated that the detailed reaction model largely influences the energy dependence of the detailed rate constants, and also that extrapolations to other temperatures for thermal rate constants depend strongly on the potential energy model used.     

In situ acetylation dispersive liquid–liquid microextraction followed by gas chromatography–mass spectrometry for the simultaneous determination of musks,triclosan and methyl-triclosan in wastewaters

A rapid and environmental-friendly analytical method for the simultaneous analysis of different personal care products (PCPs) namely synthetic musks (nitro-, polycyclic and macrocyclic musks) and the disinfectant triclosan (TCS) and its transformation product methyl-TCS in wastewater samples has been developed. The method combines dispersive liquid–liquid microextraction with in situ aqueous derivatisation using acetic anhydride (Ac2O) prior to ?gas chromatography–mass spectrometry analysis. Several parameters affecting both extraction and derivatisation efficiency (e.g. type and volume of extractor and dispersive solvents, volume of derivatising reagent, etc.) were optimised to achieve reliable conditions. Validation of the method for all compounds under study showed good linearity with coefficient of correlation > 0.9947. Limits of quantification (LOQs) ranged between 2 and 72 ng/L for musks and 28 and 31 ng/L for TCS and methyl-TCS, respectively. Accuracy, expressed as the average recoveries, ranged between 76% and 87%, and precision, expressed in terms of intraday repeatability (%RSD), was better than 13% for all analytes. The application of the method to the analysis of 24 wastewater samples enabled the detection of all the target PCPs at concentration levels up to 2.7 μg/L, being galaxolide (HHCB) and tonalide (AHTN) the more prevalent, present in 88% and 46% of the samples, respectively.