High-energy Measurements Research Articles

Composition- and temperature-dependent liquid structures in Al–Cu alloys: an ab initio molecular dynamics and x-ray diffraction study

The composition- and temperature-dependent liquid structures in eight Alrich–Cu binary alloys (from hypoeutectic Al93Cu7 to hypereutectic Al70Cu30) have been experimentally and computationally studied by x-ray diffraction (XRD) experiments and ab initio molecular dynamics (AIMD) simulations. The remarkable agreements of structure factors for all liquid Alrich–Cu alloys obtained from high-temperature high-energy XRD measurements and AIMD simulations have been achieved, which consolidates the analyses of structural evolutions in Alrich–Cu liquids during the cooling processing by AIMD simulations. The heat capacity of liquid Alrich–Cu alloys continuously increases and presents no abnormal peak when reducing the temperature, which differs from the reported prediction for 55-atom Alrich–Cu nanoliquids. The diffusivities of Al and Cu undergo an increasing deviation from Arrhenius behavior by tuning Cu concentration from 7 to 30 atomic percentages, correlated to the local ordering in these liquids by means of coordination number, bond-angle distribution, Honeycutt–Andersen index, bond-orientational order and Voronoi tessellation analyses. Upon cooling, the microstructure of the liquid Alrich–Cu alloys inclines to form Al2Cu crystal-like local atomic ordering, especially in the hypereutectic liquids. The favorable short-range ordering between Cu and Al atoms could cause the non-Arrhenius diffusion behavior.

A QTC-based signal readout for position-sensitive multi-output detectors

A new signal readout method for position-sensitive multi-output detectors, such as those in high-energy spectroscopy measurement and nuclear imaging, was developed by combining the charge division circuit, summing circuit and charge-to-time conversion (QTC) circuit. The 64 outputs of a Hamamatsu H8500 position-sensitive photomultiplier tube were processed, and three digital pulses were generated. The widths of digital pulses were determined using the time-to-digital converter in an field programmable gate array. The energy and position information of incident γ-rays is estimated based on the proportionality between the width of digital pulses and input charge created by γ-photons. A prototype was built using discrete components and tested, and the energy and position resolutions were improved compared with that obtained with standard ADCs. This method greatly simplifies the front-end electronics and the digital interface. It enables a compact electronics system and an easy integration into an ASIC.

Sci‐Sat AM: Radiation Dosimetry and Practical Therapy Solutions ‐ 11: Commissioning of a system for the measurement of electron stopping powers

Purpose:To develop the techniques required to experimentally determine electron stopping powers for application in primary standards and dosimetry protocols.Method and Materials:A large‐volume HPGe detector system (>80% efficiency) was commissioned for the measurement of high energy (5–35 MeV) electron beams. As a proof of principle the system was used with a Y‐90/Sr‐90 radioactive source. Thin plates of absorbing material (< 0.1 gcm‐2) were then placed between the source and detector and the emerging electron spectrum was acquired. The full experimental geometry was modelled using the EGSnrc package to validate the detector design, optimize the experimental setup and compare measured and calculated spectra.Results:The biggest challenge using a beta source was to identify a robust spectral parameter to determine for each measurement. An end‐point‐fitting routine was used to determine the maximum energy, Emax, of the beta spectrum for each absorber thickness t. The parameter dEmax/dt is related to the electron stopping power and the same routine was applied to both measured and simulated spectra. Although the standard uncertainty in dEmax/dt was of the order of 5 %, by taking the ratio of measured and Monte Carlo values for dEmax/dt the uncertainty of the fitting routine was eliminated and the uncertainty was reduced to less than 2 %. The agreement between measurement and simulation was within this uncertainty estimate.Conclusion:The investigation confirmed the experimental approach and demonstrated that EGSnrc could accurately determine correction factors that will be required for the final measurement setup in a linac beam.

Energy Spectrum Measurement of High Power and High Energy (6 and 9 MeV) Pulsed X-ray Source for Industrial Use

Background: Industrial X-ray CT system is normally applied to non-destructive testing (NDT) for industrial product made from metal. Furthermore there are some special CT systems, which have an ability to inspect nuclear fuel assemblies or rocket motors, using high power and high energy (more than 6 MeV) pulsed X-ray source. In these case, pulsed X-ray are produced by the electron linear accelerator, and a huge number of photons with a wide energy spectrum are produced within a very short period. Consequently, it is difficult to measure the X-ray energy spectrum for such accelerator-based X-ray sources using simple spectrometry. Due to this difficulty, unexpected images and artifacts which lead to incorrect density information and dimensions of specimens cannot be avoided in CT images. For getting highly precise CT images, it is important to know the precise energy spectrum of emitted X-rays. Materials and Methods: In order to realize it we investigated a new approach utilizing the Bayesian estimation method combined with an attenuation curve measurement using step shaped attenuation material. This method was validated by precise measurement of energy spectrum from a 1 MeV electron accelerator. In this study, to extend the applicable X-ray energy range we tried to measure energy spectra of X-ray sources from 6 and 9 MeV linear accelerators by using the recently developed method. Results and Discussion: In this study, an attenuation curves are measured by using a step-shaped attenuation materials of aluminum and steel individually, and the each X-ray spectrum is reconstructed from the measured attenuation curve by the spectrum type Bayesian estimation method. Conclusion: The obtained result shows good agreement with simulated spectra, and the presently developed technique is adaptable for high energy X-ray source more than 6 MeV.

Structure of the Photo-catalytically Active Surface of SrTiO3.

A major goal of energy research is to use visible light to cleave water directly, without an applied voltage, into hydrogen and oxygen. Although SrTiO3 requires ultraviolet light, after four decades, it is still the "gold standard" for the photo-catalytic splitting of water. It is chemically robust and can carry out both hydrogen and oxygen evolution reactions without an applied bias. While ultrahigh vacuum surface science techniques have provided useful insights, we still know relatively little about the structure of these electrodes in contact with electrolytes under operating conditions. Here, we report the surface structure evolution of a n-SrTiO3 electrode during water splitting, before and after "training" with an applied positive bias. Operando high-energy X-ray reflectivity measurements demonstrate that training the electrode irreversibly reorders the surface. Scanning electrochemical microscopy at open circuit correlates this training with a 3-fold increase of the activity toward the photo-induced water splitting. A novel first-principles joint density functional theory simulation, constrained to the X-ray data via a generalized penalty function, identifies an anatase-like structure as the more active, trained surface.

High-energy in-beam neutron measurements of metal-based shielding for accelerator-driven spallation neutron sources

Metal-based shielding plays an important role in the attenuation of harmful and unwanted radiation at an accelerator-driven spallation neutron source. At the European Spallation Source, currently under construction in Lund, Sweden, metal-based materials are planned to be used extensively as neutron guide substrates in addition to other shielding structures around neutron guides. The usage of metal-based materials in the vicinity of neutron guides however requires careful consideration in order to minimize potential background effects in a neutron instrument at the facility. Therefore, we have carried out a combined study involving high-energy neutron measurements and Monte Carlo simulations of metal-based shielding, both to validate the simulation methodology and also to investigate the benefits and drawbacks of different metal-based solutions. The measurements were carried out at The Svedberg Laboratory in Uppsala, Sweden, using a 174.1 MeV neutron beam and various thicknesses of aluminum-, iron-, and copper-based shielding blocks. The results were compared to geant4 simulations and revealed excellent agreement. Our combined study highlights the particular situations where one type of metal-based solution may be preferred over another.

Impact of neighborhood design on energy performance and GHG emissions

This paper presents an innovative and holistic approach to the analysis of the impact of selected design parameters of a new solar community on its environmental performance, in terms of energy efficiency and carbon footprint (green-house gas (GHG) emissions). The design parameters include energy performance level of buildings, density, type of the neighborhood (mixed-use vs residential), location of the commercial center relative to residential areas and the design of the streets. Energy performance is measured as the balance between overall energy consumption for building operations (assuming an all-electric neighborhood) and electricity generation potential through integration of PV panels on available roof surfaces. Greenhouse gas emissions are those associated with building operations and transport.Results of simulations carried out on prototype neighborhoods located in the vicinity of Calgary, Alberta, Canada indicate that, while adopting high-energy efficiency measures can reduce the buildings’ impact by up to 75% in terms of energy consumption and GHG emissions, transport still has a large environmental impact. The parameters of highest impact on transport and its associated GHG emissions are the design of the neighborhood and the distance to the business center. Density, as isolated parameter, has a modest effect on the selected mode of transportation, in terms of using private or public transportation.While this study relates to a specific location and a range of design assumptions, the methodology employed can serve as a template for evaluating design alternatives of new sustainable developments and their environmental impact.

Perspectives of the GAMMA-400 space observatory for high-energy gamma rays and cosmic rays measurements

The GAMMA-400 gamma-ray telescope is intended to measure the fluxes of gamma-rays and cosmic-ray electrons and positrons in the energy range from 100 MeV to several TeV. Such measurements concern the following scientific tasks: investigation of point sources of gamma-rays, studies of the energy spectra of Galactic and extragalactic diffuse emission, studies of gamma-ray bursts and gamma-ray emission from the Sun, as well as high precision measurements of spectra of high-energy electrons and positrons. Also the GAMMA- 400 instrument provides the possibility for protons and nuclei measurements up to knee. But the main goal for the GAMMA-400 mission is to perform a sensitive search for signatures of dark matter particles in high-energy gamma-ray emission. To fulfill these measurements the GAMMA-400 gamma-ray telescope possesses unique physical characteristics in comparison with previous and present experiments. The major advantage of the GAMMA-400 instrument is excellent angular and energy resolution for gamma-rays above 10 GeV. The GAMMA-400 experiment will be installed onboard of the Navigator space platform, manufactured by the NPO Lavochkin Association. The expected orbit will be a highly elliptical orbit (with apogee 300.000 km and perigee 500 km) with 7 days orbital period. An important profit of such an orbit is the fact that the full sky coverage will always be available for gamma ray astronomy.

THENuSTAREXTRAGALACTIC SURVEY: FIRST DIRECT MEASUREMENTS OF THE ≳10 keV X-RAY LUMINOSITY FUNCTION FOR ACTIVE GALACTIC NUCLEI ATz> 0.1

We present the first direct measurements of the rest-frame 10-40 keV X-ray luminosity function (XLF) of Active Galactic Nuclei (AGNs) based on a sample of 94 sources at 0.1 < z <3, selected at 8-24 keV energies from sources in the NuSTAR extragalactic survey program. Our results are consistent with the strong evolution of the AGN population seen in prior, lower-energy studies of the XLF. However, different models of the intrinsic distribution of absorption, which are used to correct for selection biases, give significantly different predictions for the total number of sources in our sample, leading to small, systematic differences in our binned estimates of the XLF. Adopting a model with a lower intrinsic fraction of Compton-thick sources and a larger population of sources with column densities N_H ~ 10^{23-24} /cm2 or a model with stronger Compton reflection component (with a relative normalization of R ~ 2 at all luminosities) can bring extrapolations of the XLF from 2-10 keV into agreement with our NuSTAR sample. Ultimately, X-ray spectral analysis of the NuSTAR sources is required to break this degeneracy between the distribution of absorbing column densities and the strength of the Compton reflection component and thus refine our measurements of the XLF. Furthermore, the models that successfully describe the high-redshift population seen by NuSTAR tend to over-predict previous, high-energy measurements of the local XLF, indicating that there is evolution of the AGN population that is not fully captured by the current models.

Synchrotron X-ray Scattering Measurements of Disordered Materials

Abstract With the advent of third-generation synchrotron sources and the development of light source techniques, X-ray scattering techniques have become feasible, leading to new approaches for studying the structures of disordered materials in a quantitative manner. We introduce a dedicated diffractometer for high-energy total X-ray scattering measurement and a newly developed anomalous X-ray spectrometer at SPring-8. As advanced methodologies for the measurement of liquids, we now offer three state-of-art levitation instruments for aerodynamic levitation, electrostatic levitation, and acoustic levitation at the SPring-8 beamlines, covering a wide temperature range of −40–3000 ℃. Furthermore, scientific investigations of glasses, liquids, and amorphous materials reported in the last five years at SPring-8 are reviewed.

Extending resonant inelastic X-ray scattering to the extreme ultraviolet

In resonant inelastic X-ray scattering (RIXS), core hole resonance modes are used to enhance coupling between photons and low energy electronic degrees of freedom. Resonating with shallow core holes accessed in the extreme ultraviolet (EUV) can provide greatly improved energy resolution at standard resolving power, but has been found to often yield qualitatively different spectra than similar measurements performed with higher energy X-rays. This paper uses experimental data and multiplet-based numerical simulations for the M-edges of Co-, Ni- and Cu-based Mott insulators to review the properties that distinguish EUV RIXS from more commonly performed higher energy measurements. Key factors such as the origin of the strong EUV elastic line and advantages of EUV spectral functions over soft X-ray RIXS for identifying intrinsic excitation line shapes are discussed.

Linearity estimation of high accurate energy detector for excimer laser

High accurate pulse energy detection for excimer laser is one of the most important technologies in deep ultra violet (DUV) lithography machine for precise dose control. In order to estimate the linearity of the SIOM proprietary energy detector (ED), a modified correlation method and the corresponding system have been developed. In the proposed method, an identical home-made ED was used as reference in the experimental system showing benefit as pulse energy fluctuation elimination. Secondly, this method provides a solution of high accurate measurement with a reference detector which is the same as the tested detector. The linearity estimation system contains an excimer laser, a beam expander, an adjustable diaphragm, a beam splitter, a reference ED (R-ED), a set of attenuators and a tested ED (T-ED). Experimental results show that, with linear least-squares fitting, the measured nonlinearity factor of the ED is 0.617% in a dynamic range of about 1–10nJ per pulse, and the combined uncertainty is 1.140%. It is proved that the SIOM proprietary ED can be used in high accurate pulse energy measurement of excimer lasers.

Integrating β-Pb0.33V2O5 Nanowires with CdSe Quantum Dots: Toward Nanoscale Heterostructures with Tunable Interfacial Energetic Offsets for Charge Transfer

Achieving directional charge transfer across semiconductor interfaces requires careful consideration of relative band alignments. Here, we demonstrate a promising tunable platform for light harvesting and excited-state charge transfer based on interfacing β-PbxV2O5 nanowires with CdSe quantum dots. Two distinct routes are developed for assembling the heterostructures: linker-assisted assembly mediated by a bifunctional ligand and successive ionic layer adsorption and reaction (SILAR). In the former case, the thiol end of a molecular linker is found to bind to the quantum dot surfaces, whereas a protonated amine moiety interacts electrostatically with the negatively charged nanowire surfaces. In the alternative SILAR route, the surface coverage of CdSe nanostructures on the β-PbxV2O5 nanowires is tuned by varying the number of successive precipitation cycles. High-energy valence band X-ray photoelectron spectroscopy measurements indicate that “mid-gap” states of the β-PbxV2O5 nanowires derived from the stereoactive lone pairs on the intercalated lead cations are closely overlapped in energy with the valence band edges of CdSe quantum dots that are primarily Se 4p in origin. Both the midgap states and the valence-band levels are in principle tunable by variation of cation stoichiometry and particle size, respectively, providing a means to modulate the thermodynamic driving force for charge transfer. Steady-state and time-resolved photoluminescence measurements reveal dynamic quenching of the trap-state emission of CdSe quantum dots upon exposure to β-PbxV2O5 nanowires. This result is consistent with a mechanism involving the transfer of photogenerated holes from CdSe quantum dots to the midgap states of β-PbxV2O5 nanowires.

The effects of extending the spectral information acquired by a photon-counting detector for spectral CT

Photon-counting x-ray detectors with pulse-height analysis provide spectral information that may improve material decomposition and contrast-to-noise ratio (CNR) in CT images. The number of energy measurements that can be acquired simultaneously on a detector pixel is equal to the number of comparator channels. Some spectral CT designs have a limited number of comparator channels, due to the complexity of readout electronics. The spectral information could be extended by changing the comparator threshold levels over time, sub pixels, or view angle. However, acquiring more energy measurements than comparator channels increases the noise and/or dose, due to differences in noise correlations across energy measurements and decreased dose utilisation. This study experimentally quantified the effects of acquiring more energy measurements than comparator channels using a bench-top spectral CT system. An analytical and simulation study modeling an ideal detector investigated whether there was a net benefit for material decomposition or optimal energy weighting when acquiring more energy measurements than comparator channels. Experimental results demonstrated that in a two-threshold acquisition, acquiring the high-energy measurement independently from the low-energy measurement increased noise standard deviation in material-decomposition basis images by factors of 1.5–1.7 due to changes in covariance between energy measurements. CNR in energy-weighted images decreased by factors of 0.92–0.71. Noise standard deviation increased by an additional factor of due to reduced dose utilisation. The results demonstrated no benefit for two-material decomposition noise or energy-weighted CNR when acquiring more energy measurements than comparator channels. Understanding the noise penalty of acquiring more energy measurements than comparator channels is important for designing spectral detectors and for designing experiments and interpreting data from prototype systems with a limited number of comparator channels.

Multiparticle production in nuclear collisions using effective-energy approach

The dependencies of charged particle pseudorapidity density and transverse energy pseudorapidity density at midrapidity on the collision energy and on the number of nucleon participants, or centrality, measured in nucleus-nucleus collisions are studied in the energy range spanning a few GeV to a few TeV per nucleon. The study is based on the earlier proposed model, combining the constituent quark picture together with Landau relativistic hydrodynamics and shown to interrelate the measurements from different types of collisions. Within this picture, the dependence on the number of participants in heavy-ion collisions are found to be well described in terms of the effective energy defined as a centrality-dependent fraction of the collision energy. The effective energy approach is shown to reveal a similarity in the energy dependence for the most central and centrality data in the entire available energy range. Predictions are made for the forthcoming higher-energy measurements in heavy-ion collisions at the LHC.

Measurement of high exciton binding energy in the monolayer transition-metal dichalcogenides WS2 and WSe2

Monolayer transition-metal dichalcogenides are direct gap semiconductors with great promise for optoelectronic devices. Although spatial correlation of electrons and holes plays a key role, there is little experimental information on such fundamental properties as exciton binding energies and band gaps. We report here an experimental determination of exciton excited states and binding energies for monolayer WS2 and WSe2. We observe peaks in the optical reflectivity/absorption spectra corresponding to the ground- and excited-state excitons (1s and 2s states). From these features, we determine lower bounds free of any model assumptions for the exciton binding energies as E2sA−E1sA of 0.83eV and 0.79eV for WS2 and WSe2, respectively, and for the corresponding band gaps Eg≥E2sA of 2.90 and 2.53eV at 4K. Because the binding energies are large, the true band gap is substantially higher than the dominant spectral feature commonly observed with photoluminescence. This information is critical for emerging applications, and provides new insight into these novel monolayer semiconductors.

Effective-energy budget in multiparticle production in nuclear collisions

The dependencies of charged particle pseudorapidity density and transverse energy pseudorapidity density at midrapidity on the collision energy and on the number of nucleon participants, or centrality, measured in nucleus–nucleus collisions are studied in the energy range spanning a few GeV to a few TeV per nucleon. The approach in which the multiparticle production is driven by the dissipating effective energy of participants is introduced. This approach is based on the earlier proposed consideration, combining the constituent quark picture together with Landau relativistic hydrodynamics shown to interrelate the measurements from different types of collisions. Within this picture, the dependence on the number of participants in heavy-ion collisions are found to be well described in terms of the effective energy defined as a centrality-dependent fraction of the collision energy. For both variables under study, the effective-energy approach reveals a similarity in the energy dependence obtained for the most central collisions and centrality data in the entire available energy range. Predictions are made for the investigated dependencies for the forthcoming higher-energy measurements in heavy-ion collisions at the LHC.

Energetic Electron Enhancements below the Radiation Belt and X-Ray Contamination at Low-Orbiting Satellites

The work concerns a problem of electron-induced contaminant at relatively low latitudes to high-energy astrophysical measurements on board the low-orbiting satellites. We show the results of a statistical analysis of the energetic electron enhancements in energy range 30–300 keV observed by a fleet of NOAA/POES low-orbiting satellites over the time period from 1999 to 2012. We demonstrate geographical distributions of great and moderate long-lasting enhancements caused by different type of the solar wind drivers.

On the local electronic and atomic structure of Ce1−xPrxO2−δ epitaxial films on Si

The local electronic and atomic structure of (111)-oriented, single crystalline mixed Ce1−xPrxO2−δ (x = 0, 0.1 and 0.6) epitaxial thin films on silicon substrates have been investigated in view of engineering redox properties of complex oxide films. Non-destructive X-ray absorption near edge structure reveals that Pr shows only +3 valence and Ce shows only nominal +4 valence in mixed oxides. Extended x-ray absorption fine structure (EXAFS) studies were performed at K edges of Ce and Pr using a specially designed monochromator system for high energy measurements. They demonstrate that the fluorite lattice of ceria (CeO2) is almost not perturbed for x = 0.1 sample, while higher Pr concentration (x = 0.6) not only generates a higher disorder level (thus more disordered oxygen) but also causes a significant reduction of Ce–O interatomic distances. The valence states of the cations were also examined by techniques operating in highly reducing environments: scanning transmission electron microscopy-electron energy loss spectroscopy and X-ray photoemission spectroscopy; in these reducing environments, evidence for the presence of Ce3+ was clearly found for the higher Pr concentration. Thus, the introduction of Pr3+ into CeO2 strongly enhances the oxygen exchange properties of CeO2. This improved oxygen mobility properties of CeO2 are attributed to the lattice disorder induced by Pr mixing in the CeO2 fluorite lattice, as demonstrated by EXAFS measurements. Thus, a comprehensive picture of the modifications of the atomic and electronic structure of Ce1−xPrxO2−δ epitaxial films and their relation is obtained.

Off-centering of a Bi ion in cubic phase of (Bi1/2Na1/2)TiO3

The paraelectric cubic structure of (Bi1/2Na1/2)TiO3 was analyzed precisely by high-energy synchrotron radiation X-ray powder diffraction measurement and the maximum entropy method (MEM)/Rietveld method. The 〈100〉-favored rotator-like thermal behavior of the Bi ion was observed. The 〈100〉 off-centering of Bi led to the Bi–O distance of ∼2.5 Å that was shorter than the average Bi–O distance in the structure of the Bi on-centered model. The off-centering of Bi can be attributed to the orbital hybridization between the Bi and O ions.