Showing papers on "Photon energy published in 2006"
TL;DR: It is experimentally demonstrated that using semiconductor nanocrystals the authors can reduce this energy loss to a nearly absolute minimum allowed by energy conservation by producing multiple excitons per single photon.
Abstract: The performance of photovoltaic and photochemical devices is directly linked to the efficiency with which absorbed photons are converted into electron−hole pairs (excitons). A usual assumption is that one photon produces a single exciton, while the photon energy in the excess of the material's energy gap (the gap that separates the conduction from the valence band) is wasted as heat. Here we experimentally demonstrate that using semiconductor nanocrystals we can reduce this energy loss to a nearly absolute minimum allowed by energy conservation by producing multiple excitons per single photon. Specifically, we generate seven excitons from a photon with an energy of 7.8 energy gaps, which corresponds to only ∼10% energy loss, while in the normal scenario (one photon produces one exciton) ∼90% of the photon energy would be dissipated as heat. Such large yields of charge carriers (photon-to-exciton conversion efficiency up to 700%) has the potential to dramatically increase the performance of photovoltaic ce...
502 citations
TL;DR: In this article, the complete two-loop expression for the jet function of soft-collinear effective theory is presented, including non-logarithmic terms, which provides the basis for a calculation of the effect of a photon-energy cut in the measurement of the B ¯ → X s γ decay rate at next-to-next-to leading order in renormalization-group improved perturbation theory.
179 citations
TL;DR: At the 1.7-GeV electron storage ring BESSY II, a first source of synchrotron radiation with 100 fs pulse duration, variable (linear and circular) polarization, tunable photon energy, and excellent signal-to-background ratio was constructed and is now in routine operation.
Abstract: At the 1.7-GeV electron storage ring BESSY II, a first source of synchrotron radiation with 100 fs pulse duration, variable (linear and circular) polarization, tunable photon energy (300 to 1400 eV), and excellent signal-to-background ratio was constructed and is now in routine operation.
175 citations
TL;DR: In this paper, the photosphere of an ultrarelativistic flow with internal dissipation of energy was explored by calculating the spectra for a large range of the characteristics of the flow, and an accurate fitting formula was given that provided the photospheric spectral energy distribution in the ~10 keV to ~10 MeV energy range (in the central engine frame) as a function of the basic physical parameters of flow.
Abstract: We explore the observational appearance of the photosphere of an ultrarelativistic flow with internal dissipation of energy as predicted by the magnetic reconnection model. Previous study of the radiative transfer in the photospheric region has shown that gradual dissipation of energy results in a hot photosphere. There, inverse Compton scattering of the thermal radiation advected with the flow leads to powerful photospheric emission with spectral properties close to those of the observed prompt GRB emission. Here, we build on that study by calculating the spectra for a large range of the characteristics of the flow. An accurate fitting formula is given that provides the photospheric spectral energy distribution in the ~10 keV to ~10 MeV energy range (in the central engine frame) as a function of the basic physical parameters of the flow. It facilitates the direct comparison of the model predictions with observations, including the variability properties of the lightcurves. We verify that the model naturally accounts for the observed clustering in peak energies of the E*f(E) spectrum. In this model, the Amati relation indicates a tendency for the most luminous bursts to have more energy per baryon. If this tendency also holds for individual GRB pulses, the model predicts the observed narrowing of the width of pulses with increasing photon energy.
165 citations
TL;DR: The 19ID undulator beamline of the Structure Biology Center has been designed and built to take full advantage of the high flux, brilliance and quality of X-ray beams delivered by the Advanced Photon Source and allows optimal strategies to be adopted in protein crystallographic experiments, thus maximizing the chances of their success.
Abstract: The 19ID undulator beamline of the Structure Biology Center has been designed and built to take full advantage of the high flux, brilliance and quality of X-ray beams delivered by the Advanced Photon Source. The beamline optics are capable of delivering monochromatic X-rays with photon energies from 3.5 to 20 keV (3.5–0.6 A wavelength) with fluxes up to 8–18 × 1012 photons s−1 (depending on photon energy) onto cryogenically cooled crystal samples. The size of the beam (full width at half-maximum) at the sample position can be varied from 2.2 mm × 1.0 mm (horizontal × vertical, unfocused) to 0.083 mm × 0.020 mm in its fully focused configuration. Specimen-to-detector distances of between 100 mm and 1500 mm can be used. The high flexibility, inherent in the design of the optics, coupled with a κ-geometry goniometer and beamline control software allows optimal strategies to be adopted in protein crystallographic experiments, thus maximizing the chances of their success. A large-area mosaic 3 × 3 CCD detector allows high-quality diffraction data to be measured rapidly to the crystal diffraction limits. The beamline layout and the X-ray optical and endstation components are described in detail, and the results of representative crystallographic experiments are presented.
148 citations
TL;DR: In this article, the authors investigated the Raman spectra of (Mn, Co)-codoped ZnO films as functions of laser line and temperature, and found that the intensity of different phonon modes exhibits redshift with temperature increasing, which can be attributed to anharmonic effect in the material.
Abstract: Raman spectra of (Mn, Co)-codoped ZnO films were investigated as functions of laser line and temperature. It is shown that the Raman shifts for different phonon modes exhibit redshift with temperature increasing, which can be attributed to the anharmonic effect in the material. Strong resonant Raman spectra of multi-LO phonons were observed as well for the films with the exciting photon energy higher than the direct band gap. Compared with the Raman spectra for ZnO films, the Mn, Co codoping effect on the spectra is revealed by the presence of additional phonon modes at 275 and 642cm−1 and another intensive phonon mode at around 524cm−1. With postannealing, Raman intensity of these phonon modes decrease, owing to the incorporation of oxygen into the films. Origins of the different phonon modes in the spectra are discussed as well.
138 citations
TL;DR: In this article, the authors showed that the rate of impact ionization in PbSe nanocrystals has a strong energy dependence and that the relaxation energy increases linearly with the carrier excess energy.
Abstract: In the impact ionization process, a hot carrier relaxes by generating an exciton. We present tight binding calculations showing that the rate of this process in PbSe nanocrystals has a strong energy dependence and that the relaxation energy increases linearly with the carrier excess energy. The impact ionization can be extremely fast $(\ensuremath{\sim}\mathrm{fs})$ but it is not enhanced by the confinement in contrast to the usual belief. It explains the multiple exciton generation observed experimentally in these dots except at high photon energy where more complex many-particle phenomena are involved.
124 citations
TL;DR: In this paper, an electron-density scaling method was shown to predict primary photon attenuation in media other than water to within 1-2% for all the materials studied and for energies up to 5 MeV.
Abstract: Photon dose calculation algorithms (such as the pencil beam and collapsed cone, CC) model the attenuation of a primary photon beam in media other than water, by using pathlength scaling based on the relative mass density of the media to water. In this study, we assess if differences in the electron density between the water and media, with different atomic composition, can influence the accuracy of conventional photon dose calculations algorithms. A comparison is performed between an electron-density scaling method and the standard mass-density scaling method for (i) tissues present in the human body (such as bone, muscle, etc.), and for (ii) water-equivalent plastics, used in radiotherapy dosimetry and quality assurance. We demonstrate that the important material property that should be taken into account by photon dose algorithms is the electron density, and not the mass density. The mass-density scaling method is shown to overestimate, relative to electrondensity predictions, the primary photon fluence for tissues in the human body and water-equivalent plastics, where 6%-7% and 10% differences were observed respectively for bone and air. However, in the case of patients, differences are expected to be smaller due to the large complexity of a treatment plan and of the patient anatomy and atomic composition and of the smaller thickness of bone/air that incident photon beams of a treatment plan may have to traverse. Differences have also been observed for conventional dose algorithms, such as CC, where an overestimate of the lung dose occurs, when irradiating lung tumors. The incorrect lung dose can be attributed to the.incorrect modeling of the photon beam attenuation through the rib cage (thickness of 2-3 cm in bone upstream of the lung tumor) and through the lung and the oversimplified modeling of electron transport in convolution algorithms. In the present study, the overestimation of the primary photon fluence, using the mass-density scaling method, was shown to be a consequence of the differences in the hydrogen content between the various media studied and water. On the other hand, the electron-density scaling method was shown to predict primary photon fluence in media other than water to within 1%-2% for all the materials studied and for energies up to 5 MeV. For energies above 5 MeV, the accuracy of the electron-density scaling method was shown to depend on the photon energy, where for materials with a high content of calcium (such as bone, cortical bone) or for primary photon energies above 10 MeV, the pair-production process could no longer be neglected. The electron-density scaling method was extended to account for pair-production attenuation of the primary photons. Therefore the scaling of the dose distributions in media other than water became dependent on the photon energy. The extended electron-scaling method was shown to estimate the photon range to within 1% for all materials studied and for energies from 100 keV to 20 MeV, allowing it to be used to scale dose distributions to media other than water and generated by clinical radiotherapy photon beams with accelerator energies from 4 to 20 MV.
118 citations
TL;DR: In this article, the photoelectron angular distribution non-dipole parameters associated with the terms of the second order O(kr ) 2 ] (k is the photon energy and r is the radius of the ionized atomic shell) for both unpolarized and linearly polarized radiation are presented.
113 citations
TL;DR: These calculations show that PECD in such randomly oriented samples can be understood in the electric dipole approximation and that, unlike the case pertaining in core-shell ionization, in valence shell ionization there is a significant additional influence contributed by the initial orbital density.
Abstract: An electron imaging technique has been used to study the full angular distribution of valence photoelectrons produced from enantiomerically pure molecular beams of camphor when these are photoionized with circularly polarized light. In addition to the familiar beta parameter, this provides a new chiral term, taking the form of an additional cosine function in the angular distribution which consequently displays a forward-backward electron ejection asymmetry. Several ionization channels have been studied using synchrotron radiation in the 8.85-26 eV photon energy range. With alternating left and right circularly polarized radiations the photoelectron circular dichroism (PECD) in the angular distribution can be measured and shows some strong dynamical variations with the photon energy, depending in sign and intensity on the ionized orbital. For all orbitals the measured PECD has a quite perfect antisymmetry when switching between R and S enantiomers, as expected from theory. In the HOMO(-1) channel the PECD chiral asymmetry curves show a double maxima reaching nearly 10% close to threshold, and peaking again at approximately 20% some 11 eV above threshold. This is attributed to a resonance that is also visible in the beta parameter curve. Newly optimized CMS-Xalpha photoionization dynamics calculations are also presented. They are in reasonably good agreement with the experimental data, including in the very challenging threshold regions. These calculations show that PECD in such randomly oriented samples can be understood in the electric dipole approximation and that, unlike the case pertaining in core-shell ionization-where a highly localized achiral initial orbital means that the dichroism arises purely as a final state scattering effect-in valence shell ionization there is a significant additional influence contributed by the initial orbital density.
104 citations
TL;DR: In this article, the influence of the photon energy on the optoelectronic properties presented by n-type bottom-gate thin-film transistors based on indium zinc oxide was shown.
Abstract: Insensitivity to light irradiation is desirable for conventional applications of thin-film transistors, i.e., the active matrices of displays. However, if one produces a device presenting controlled sensitivity to light, many other applications can benefit or can even be created. In this work it is shown the influence of the photon energy on the optoelectronic properties presented by n-type bottom-gate thin-film transistors based on indium zinc oxide. In the dark, the devices present very good electrical properties, working in the enhancement mode, exhibiting on–off ratios higher than 10 7 and channel mobility above 30 cm 2 /V s. Remarkable results were achieved when the devices were exposed to light radiation, the most striking one is the possibility to switch between enhancement (in the dark) and depletion (illuminated) modes, with different threshold voltages and on/off ratios, function of the light power density and wavelength used. This type of behavior permits to design circuits where one can have the same transistor working either in the enhancement or depletion modes, function of the light beam and intensity impinging on it, highly important for short wavelength detector applications.
TL;DR: A tabletop soft x-ray laser is applied for the first time as a high energy photon source for chemical dynamics experiments in the study of water, methanol, and ammonia clusters through time of flight mass spectroscopy.
Abstract: A tabletop soft x-ray laser is applied for the first time as a high energy photon source for chemical dynamics experiments in the study of water, methanol, and ammonia clusters through time of flight mass spectroscopy. The 26.5 eV/photon laser pulse time duration of 1n s is employed as a single photon ionization source for the detection of these clusters. Only a small fraction of the photon energy is deposited in the cluster for metastable dissociation of cluster ions, and most of it is removed by the ejected electron. Protonated water, methanol, and ammonia clusters dominate the cluster mass spectra. Unprotonated ammonia clusters are observed in the protonated cluster ion size range 2n22. The unimolecular dissociation rate constants for reactions involving loss of one neutral molecule are calculated to be 0.6‐2.710 4 , 3.6‐6.010 3 , and 0.8‐2.010 4 s 1 for the protonated water 9n24, methanol 5n10, and ammonia 5n18 clusters, respectively. The temperatures of the neutral clusters are estimated to be between 40 and 200 K for water clusters 10n21, and 50‐100 K for methanol clusters 6n10. Products with losses of up to five H atoms are observed in the mass spectrum of the neutral ammonia dimer. Large ammonia clusters NH3n n3 do not lose more than three H atoms in the photoionization/ photodissociation process. For all three cluster systems studied, single photon ionization with a 26.5 eV photon yields near threshold ionization. The temperature of these three cluster systems increases with increasing cluster size over the above-indicated ranges. © 2006 American Institute of Physics. DOI: 10.1063/1.2202314
TL;DR: In this paper, the effect of annealing on optical properties and surface structure of ZnO thin films was investigated by spectroscopic ellipsometry (SE) and atomic force microscopy (AFM).
TL;DR: The surface impedance of a superconductor changes when energy is absorbed and Cooper pairs are broken to produce single electron (quasiparticle) excitations, which can be sensitively measured using a thin-film resonant circuit called a microwave kinetic inductance detector (MKID) as discussed by the authors.
Abstract: The surface impedance of a superconductor changes when energy is absorbed and Cooper pairs are broken to produce single electron (quasiparticle) excitations. This change may be sensitively measured using a thin-film resonant circuit called a microwave kinetic inductance detector (MKID). The practical application of MKIDs for photon detection requires a method of efficiently coupling the photon energy to the MKID. The authors present results on position sensitive x-ray detectors made by using two aluminum MKIDs on either side of a tantalum photon absorber strip. Diffusion constants, recombination times, and energy resolution are reported. MKIDs can easily be scaled into large arrays.
TL;DR: By irradiating He and Ne atoms with 3mJ, 12fs, near infrared laser pulses from a tabletop laser system, the authors generated spatially and temporally coherent x rays up to a photon energy of 3.5keV.
Abstract: By irradiating He and Ne atoms with 3mJ, 12fs, near infrared laser pulses from a tabletop laser system, the authors generated spatially and temporally coherent x rays up to a photon energy of 3.5keV. With this source it is possible to use high-harmonic radiation for x-ray absorption spectroscopy in the keV range. They were able to clearly resolve the L absorption edges of titanium and copper and the K edges of aluminum and silicon. From the fine structure of the x-ray absorption they estimated the interatomic distances.
TL;DR: In this article, the optical constants (absorption coefficient (α), refractive index (n), extinction coefficient (k), real and imaginary part of dielectric constant) have been studied for a-Se96−xTe4Agx (where x = 0, 4, 8, 12) thin films as a function of photon energy in the wavelength range.
TL;DR: This version of the Fano test avoids the need of photon regeneration and the calculation of photon energy absorption coefficients, and is intended to help users to adopt an optimal configuration that guarantees both a high-accuracy calculation of the absorbed dose and a reasonably short computing time.
Abstract: The stability of the electron transport algorithm implemented in the Monte Carlo code PENELOPE with respect to variations of its step length is analysed in the context of the simulation of ion chambers used in photon and electron dosimetry. More precisely, the degree of violation of the Fano theorem is quantified (to the 0.1% level) as a function of the simulation parameters that determine the step size. To meet the premises of the theorem, we define an infinite graphite phantom with a cavity delimited by two parallel planes (i.e., a slab) and filled with a 'gas' that has the same composition as graphite but a mass density a thousand-fold smaller. The cavity walls and the gas have identical cross sections, including the density effect associated with inelastic collisions. Electrons with initial kinetic energies equal to 0.01, 0.1, 1, 10 or 20 MeV are generated in the wall and in the gas with a uniform intensity per unit mass. Two configurations, motivated by the design of pancake- and thimble-type chambers, are considered, namely, with the initial direction of emission perpendicular or parallel to the gas-wall interface. This version of the Fano test avoids the need of photon regeneration and the calculation of photon energy absorption coefficients, two ingredients that are common to some alternative definitions of equivalent tests. In order to reduce the number of variables in the analysis, a global new simulation parameter, called the speedup parameter (a), is introduced. It is shown that setting a = 0.2, corresponding to values of the usual PENELOPE parameters of C1 = C2 = 0.02 and values of WCC and WCR that depend on the initial and absorption energies, is appropriate for maximum tolerances of the order of 0.2% with respect to an analogue, i.e., interaction-by-interaction, simulation of the same problem. The precise values of WCC and WCR do not seem to be critical to achieve this level of accuracy. The step-size dependence of the absorbed dose is explained in the light of the properties of PENELOPE's transport mechanics. This work is intended to help users to adopt an optimal configuration that guarantees both a high-accuracy calculation of the absorbed dose and a reasonably short computing time.
TL;DR: In this article, a Monte Carlo (MC) simulated continuum and line spectra of monochromatic and continuum photons in a bulk outflow from a compact object is presented.
Abstract: In Paper by Titarchuk & Shrader the general formulation and results for photon reprocessing (downscattering) that included recoil and Comptonization effects due to divergence of the flow were presented. Here we show the Monte Carlo (MC) simulated continuum and line spectra. We also provide an analytical description of the simulated continuum spectra using the diffusion approximation. We have simulated the propagation of monochromatic and continuum photons in a bulk outflow from a compact object. Electron scattering of the photons within the expanding flow leads to a decrease of their energy which is of first order in V/c (where V is the outflow velocity). The downscattering effect of first order in V/c in the diverging flow is explained by semi-analytical calculations and confirmed by MC simulations. We conclude that redshifted lines and downscattering bumps are intrinsic properties of the powerful outflows for which Thomson optical depth is greater than one. We fitted our model line profiles to the observations using four free parameters, \beta=V/c, optical depth of the wind \tau, the wind temperature kT_e and the original line photon energy E_0. We show how the primary spectrum emitted close to the black hole is modified by reprocessing in the warm wind. In the framework of the our wind model the fluorescent iron line K_alpha is formed in the partly ionized wind as a result of illumination by central source continuum photons. The demonstrated application of our outflow model to the XMM observations of MCG 6-30-15, and to the ASCA observations of GRO J1655-40, points out a potential powerful spectral diagnostic for probes of the outflow-central object connection in Galactic and extragalactic BH sources.
TL;DR: For certain photon energies, this work produces photoexcited NC ensembles with a nearly pure single multiplicity that can be tuned from 1 to 7, which can find applications ranging from lasing and nonlinear optics to photovoltaics and photocatalysis.
Abstract: We analyze distributions of exciton populations in PbSe nanocrystal (NC) ensembles as a function of excitation wavelength. For photon energies that result in carrier multiplication, these distributions are non-Poissonian and are characterized by two dominant exciton multiplicities that are determined by the ratio of photon energy to NC energy gap. For certain photon energies, we produce photoexcited NC ensembles with a nearly pure single multiplicity that can be tuned from 1 to 7. This result can find applications ranging from lasing and nonlinear optics to photovoltaics and photocatalysis.
TL;DR: In this article, the authors presented converged, completely ab initio calculations of the triple differential cross sections for double photoionization of aligned H2 molecules for a photon energy of 75.0 eV.
Abstract: We present converged, completely ab initio calculations ofthe triple differential cross sections for double photoionization ofaligned H2 molecules for a photon energy of 75.0 eV. The method ofexterior complex scaling, implemented with both the discrete variablerepresentation and B-splines, is used to solve the Schroedinger equationfor a correlated continuum wave function corresponding to a single photonhaving been absorbed by a correlated initial state. Results for a fixedinternuclear distance are compared with recent experiments and show thatintegration over experimental angular and energy resolutions is necessaryto produce good qualitative agreement, but does not eliminate somediscrepancies. Limitations of current experimental resolution are shownto sometimes obscure interesting details of the crosssection.
TL;DR: A tilted angle CZT detector is proposed in this work for applications in photon counting/energy weighting x-ray and CT imaging and provides higher spatial and energy resolution, and shorter charge collection time, which potentially enables fast energy resolving x-rays imaging.
Abstract: X-ray imaging with a photon counting/energy weighting detector can provide the highest signal to noise ratio (SNR). Scanning slit/multi-slit x-ray image acquisition can provide a dose-efficient scatter rejection, which increases SNR. Use of a photon counting/energy weighting detector in a scanning slit/multi-slit acquisition geometry could provide highest possible dose efficiency in x-ray and CT imaging. Currently, the most advanced photon counting detector is the cadmium zinc telluride (CZT) detector, which, however, is suboptimal for energy resolved x-ray imaging. A tilted angle CZT detector is proposed in this work for applications in photon counting/energy weighting x-ray and CT imaging. In tilted angle configuration, the x-ray beam hits the surface of the linear array of CZT crystals at a small angle. This allows the use of CZT crystals of a small thickness while maintaining the high photon absorption. Small thickness CZT detectors allow for a significant decrease in the polarization effect in the CZT volume and an increase in count rate. The tilted angle CZT with a small thickness also provides higher spatial and energy resolution, and shorter charge collection time, which potentially enables fast energy resolving x-ray image acquisition. In this work, the major performance parameters of the tilted angle CZT detector, including its count rate, spatial resolution and energy resolution, were evaluated. It was shown that for a CZT detector with a 0.7 mm thickness and 13 degrees tilting angle, the maximum count rate can be increased by 10.7 times, while photon absorption remains >90% at photon energies up to 120 keV. Photon counting/energy weighting x-ray imaging using a tilted angle CZT detector was simulated. SNR improvement due to optimal photon energy weighting was 23% and 14% when adipose contrast element, inserted in soft tissue with 10 cm and 20 cm thickness, respectively, was imaged using 5 energy bins and weighting factors optimized for the adipose. SNR improvement was 42% and 31% when CaCO(3) contrast element, inserted in soft tissue with 10 cm and 20 cm thickness, respectively, was imaged using 5 energy bins and weighting factors optimized for CaCO(3). The SNRs of the photon counting single-kVp dual-energy subtracted images of CaCO(3) and adipose were higher by 2.04 and 2.74 times, respectively, as compared to currently used dual-kVp dual-energy subtracted images. Experiments with a CZT crystal with 2 mm thickness have shown significant decrease in the tailing effect of the CZT pulse spectrum at 59 keV and 122 keV photon energies, when the tilting angle configuration was used. Finally, feasibility of the tilted angle CZT detector for photon counting cone beam breast CT imaging was demonstrated.
TL;DR: Recently, Sud et al. as mentioned in this paper have developed new approaches including using distorted wave Born approximation (DWBA) theory to compute pair production cross sections in the intermediate energy region (5.0-10.0 MeV) on a firmer theoretical basis.
TL;DR: In this paper, the surface impedance of a superconductor changes when energy is absorbed and Cooper pairs are broken to produce single electron excitations, which can be sensitively measured using a thin-film resonant circuit called a microwave kinetic inductance detector (MKID).
Abstract: The surface impedance of a superconductor changes when energy is absorbed and Cooper pairs are broken to produce single electron (quasiparticle) excitations. This change may be sensitively measured using a thin-film resonant circuit called a microwave kinetic inductance detector (MKID). The practical application of MKIDs for photon detection requires a method of efficiently coupling the photon energy to the MKID. We present results on position sensitive X-ray detectors made by using two aluminum MKIDs on either side of a tantalum photon absorber strip. Diffusion constants, recombination times, and energy resolution are reported. MKIDs can easily be scaled into large arrays.
TL;DR: The inactivation efficiency for indirect action was greater than that for direct action over the photon energy range and the ion LET range tested, and a significant contribution of direct action was also found for the increased RBE in the low photon energy region.
Abstract: Ito, A., Nakano, H., Kusano, Y., Hirayama, R., Furusawa, Y., Murayama, C., Mori, T., Katsumura, Y. and Shinohara, K. Contribution of Indirect Action to Radiation-Induced Mammalian Cell Inactivation: Dependence on Photon Energy and Heavy-Ion LET. Radiat. Res. 165, 703–712 (2006). The contribution of indirect action mediated by OH radicals to cell inactivation by ionizing radiations was evaluated for photons over the energy range from 12.4 keV to 1.25 MeV and for heavy ions over the linear energy transfer (LET) range from 20 keV/μm to 440 keV/μm by applying competition kinetics analysis using the OH radical scavenger DMSO. The maximum level of protection provided by DMSO (the protectable fraction) decreased with decreasing photon energy down to 63% at 12.4 keV. For heavy ions, a protectable fraction of 65% was found for an LET of around 200 keV/μm; above that LET, the value stayed the same. The reaction rate of OH radicals with intracellular molecules responsible for cell inactivation was nearly co...
TL;DR: A new mechanism of molecular ionization near the appearance intensity that produces a sequence of peaks in the nuclear kinetic energy spectrum separated by the photon energy is measured and explained.
Abstract: We have measured and explained a new mechanism of molecular ionization near the appearance intensity that produces a sequence of peaks in the nuclear kinetic energy spectrum separated by the photon energy. Our interpretation is based on an internally consistent model for the nuclear motion during an intense laser pulse. Within this model, the same concepts and language can be used for both dissociation and ionization, leading to a more unified understanding of the dynamics.
TL;DR: The far-infrared electromagnetic response of Cu2O is monitored via broadband terahertz pulses after ultrafast resonant excitation of three-dimensional 3p excitons to report the first observation of stimulated emission of terAhertz radiation from internal transitions of exciton.
Abstract: We report the first observation of stimulated emission of terahertz radiation from internal transitions of excitons. The far-infrared electromagnetic response of Cu{sub 2}O is monitored via broadband terahertz pulses after ultrafast resonant excitation of three-dimensional 3p excitons. Stimulated emission from the 3p to the energetically lower 2s bound level occurs at a photon energy of 6.6 meV, with a cross section of {approx} 10{sup 14} cm{sup 2}. Simultaneous excitation of both exciton levels, in turn, drives quantum beats which lead to efficient terahertz emission sharply peaked at the difference frequency.
TL;DR: A cascaded-systems approach is used that employs a complex structure of parallel cascades to describe signal and noise transfer through the photoelectric effect in terms of the modulation transfer function, Wiener noise power spectrum, and detective quantum efficiency (DQE), which overstates the DQE when the electron range cannot be ignored.
Abstract: Image quality in diagnostic x-ray imaging is ultimately limited by the statistical properties governing how, and where, x-ray energy is deposited in a detector. This in turn depends on the physics of the underlying x-ray interactions. In the diagnostic energy range (10-100 keV), most of the energy deposited in a detector is through photoelectric interactions. We present a theoretical model of the photoelectric effect that specifically addresses the statistical nature of energy absorption by photoelectrons, K and L characteristic x rays, and Auger electrons. A cascaded-systems approach is used that employs a complex structure of parallel cascades to describe signal and noise transfer through the photoelectric effect in terms of the modulation transfer function, Wiener noise power spectrum, and detective quantum efficiency (DQE). The model was evaluated by comparing results with Monte Carlo calculations for x-ray converters based on amorphous selenium (a-Se) and lead (Pb), representing both low and high-Z materials. When electron transport considerations can be neglected, excellent agreement (within 3%) is obtained for each metric over the entire diagnostic energy range in both a-Se and Pb detectors up to 30 cycles/mm, the highest frequency tested. The cascaded model overstates the DQE when the electron range cannot be ignored. This occurs at approximately two cycles/mm in a-Se at an incident photon energy of 80 keV, whereas in Pb, excellent agreement is obtained for the DQE over the entire diagnostic energy range. However, within the context of mammography (20 keV) and micro-computed tomography (40 keV), the effects of electron transport on the DQE are negligible compared to fluorescence reabsorption, which can lead to decreases of up to 30% and 20% in a-Se and Pb, respectively, at 20 keV; and 10% and 5%, respectively, at 40 keV. It is shown that when Swank noise is identified in a Fourier model, the Swank factor must be frequency dependent. This factor decreases quickly with frequency, and in the case of a-Se and Pb, decreases by up to a factor of 3 at five cycles/mm immediately above the K edge. The frequency-dependent Swank factor is also equivalent to what we call the "photoelectric DQE," which describes signal and noise transfer through photoelectric interactions.
TL;DR: In this article, a hombohedral structure p-type semiconductor Cr2O3 nanowires were generated by sol-gel template technology with the diameters in the range of 100-300nm and the lengths ca. 10μm.
Abstract: Rhombohedral structure p-type semiconductor Cr2O3 nanowires were generated by sol-gel template technology with the diameters in the range of 100–300nm and the lengths ca. 10μm. A sharp ultraviolet photoluminescence band (full width at half maximum=13.8nm) at the wavelength of 385nm (3.22eV in photon energy) was observed, which was attributed to the transition involving 3d3 electron of the Cr3+ ions.
TL;DR: In this article, the third-order nonlinear susceptibilities associated with intersubband transition are theoretically calculated for ZnS/CdSe cylindrical quantum dot quantum well, and the eigenenergies of electrons and wave functions in QDQW have been calculated under the effective-mass approximation by solving a three-dimensional nonlinear Schrodinger equation.
Abstract: The third-order nonlinear susceptibilities associated with intersubband transition are theoretically calculated for ZnS/CdSe cylindrical quantum dot quantum well. The eigenenergies of electrons and wave functions in QDQW have been calculated under the effective-mass approximation by solving a three-dimensional nonlinear Schrodinger equation, and by means of compact density matrix method, the third-order nonlinear susceptibilities for third harmonic generation and quadratic electro-optic effects have been calculated for different sizes of QDQW. By increasing the width of shell well, the peaks of χ THG ( 3 ) and χ QEOE ( 3 ) as functions of pump photon energy ℏ ω will shift to the lower energy, and the intensities of the peaks will increase.
TL;DR: It is shown that significant dose enhancement is possible by altering the linear accelerator target and filtration, but the magnitude is highly dependent on contrast medium concentration.
Abstract: Contrast-enhanced radiation therapy (CERT) is a treatment approach involving the irradiation of tumours containing high atomic number (Z) contrast media, using low-quality x-ray beams. This work describes the experimental generation of x-ray beams using a linear accelerator with low-Z target materials (beryllium and aluminium), in order to produce photon energy spectra appropriate for CERT. Measurements were made to compare the experimental beams to conventional linear accelerator photon beams in terms of per cent depth dose. Monte Carlo simulation was used to model the generation of each beam, and models were validated against experimental measurement. Validated models were used to demonstrate changes in photon spectra as well as to quantify the variation of tumour dose enhancement with iodinated contrast medium concentration in a simulated tumour volume. Finally, the ratio of the linear attenuation coefficient for iodinated contrast medium relative to water was determined experimentally as a function of iodine concentration. Beams created with low-Z targets show significant changes in energy spectra compared to conventional beams. For the 4 MeV/Be beam, for example, 33% of photons have energies below 60 keV. Measurements and calculation show that both the linear attenuation coefficient ratio and dose enhancement factor (DEF) increase most rapidly at concentrations below 46 mg I ml−1. There is a significant dependence of DEF on electron energy and a lesser dependence on target material. The 4 MeV/Be beam is the most promising in terms of magnitude of DEF—for example, DEF values of 1.16 and 1.29 are obtained for concentrations of 20 mg I ml−1 and 50 mg I ml−1, respectively. DEF will increase or decrease, respectively, for shallower or deeper tumours at a rate of approximately 1.1% cm−1. In summary, we show that significant dose enhancement is possible by altering the linear accelerator target and filtration, but the magnitude is highly dependent on contrast medium concentration.