Discrete Photon Energies Research Articles

Development and validation of a Medaka fish voxel-based model for internal ionizing radiation dosimetry

In this paper, we present a voxel-based phantom of Medaka fish that can be used to assess the internal radiation doses that would be absorbed by different organs of this fish species if exposed to radioactive wastewater released into the ocean. The geometric model for fish was generated based on available Wavefront Object files for smooth-bodied Medaka fish organs, whereas due to the lack of Medaka fish material specification, the material model was constructed using material data appropriate to ICRP 110 adult male voxel-based phantom. Absorbed Fractions (AFs) and Specific Absorbed Fractions (SAFs) were calculated for eight organs of major interest as sources and for each organ as target at a set of discrete photon, electron, alpha and neutron energies. To validate the present model the calculated AFs in the studied organs were compared to ones obtained in similar organs in a voxel-based phantom of another teleost fish species called Limanda limanda. The results presented are consistent with the reference dosimetric data. We concluded that the Medaka model can be used in radioecology research to improve marine radiation protection.

Dissociative photoionization of NO across a shape resonance in the XUV range using circularly polarized synchrotron radiation.

We report benchmark results for dissociative photoionization (DPI) spectroscopy and dynamics of the NO molecule in the region of the σ* shape resonance in the ionization leading to the NO+(c3Π) ionic state. The experimental study combines well characterized extreme ultraviolet (XUV) circularly polarized synchrotron radiation, delivered at the DESIRS beamline (SOLEIL), with ion-electron coincidence 3D momentum spectroscopy. The measured (N+, e) kinetic energy correlation diagrams reported at four discrete photon energies in the extended 23-33 eV energy range allow for resolving the different active DPI reactions and underline the importance of spectrally resolved studies using synchrotron radiation in the context of time-resolved studies where photoionization is induced by broadband XUV attosecond pulses. In the dominant DPI reaction which leads to the NO+(c3Π) ionic state, photoionization dynamics across the σ* shape resonance are probed by molecular frame photoelectron angular distributions where the parallel and perpendicular transitions are highlighted, as well as the circular dichroism CDAD(θe) in the molecular frame. The latter also constitute benchmark references for molecular polarimetry. The measured dynamical parameters are well described by multichannel Schwinger configuration interaction calculations. Similar results are obtained for the DPI spectroscopy of highly excited NO+ electronic states populated in the explored XUV photon energy range.

Optimal Pulse Processing, Pile-Up Decomposition, and Applications of Silicon Drift Detectors at LCLS

Silicon drift detectors (SDDs) revolutionized spectroscopy in fields as diverse as geology and dentistry. For a subset of experiments at ultra-fast, x-ray free-electron lasers (FELs), SDDs can make substantial contributions. Often the unknown spectrum is interesting, carrying science data, or the background measurement is useful to identify unexpected signals. Many measurements involve only several discrete photon energies known a priori, allowing single event decomposition of pile-up and spectroscopic photon counting. We designed a pulse function and demonstrated that the signal amplitude and rise time are obtained for each pulse by fitting, thus removing the need for pulse shaping. By avoiding pulse shaping, rise times of tens of nanoseconds resulted in reduced pulse pile-up and allowed decomposition of remaining pulse pile-up at photon separation times down to hundreds of nanoseconds while yielding time-of-arrival information with precision of 10 nanoseconds. Waveform fitting yields simultaneously high energy resolution and high counting rates (2 orders of magnitude higher than current digital pulse processors). We showed that pile-up spectrum fitting is relatively simple and preferable to pile-up spectrum deconvolution. We developed a photon pile-up statistical model for constant intensity sources, extended it to variable intensity sources (typical for FELs) and used it to fit a complex pile-up spectrum. We subsequently developed a Bayesian pile-up decomposition method that allows decomposing pile-up of single events with up to 6 photons from 6 monochromatic lines with 99% accuracy. The usefulness of SDDs will continue into the x-ray FEL era of science. Their successors, the ePixS hybrid pixel detectors, already offer hundreds of pixels, each with similar performance to an SDD, in a compact, robust and affordable package

Effective Dose Rate Coefficients for Immersions in Radioactive Air and Water.

The Oak Ridge National Laboratory Center for Radiation Protection Knowledge (CRPK) has undertaken a number of calculations in support of a revision to the United States Environmental Protection Agency (US EPA) Federal Guidance Report on external exposure to radionuclides in air, water and soil (FGR 12). Age-specific mathematical phantom calculations were performed for the conditions of submersion in radioactive air and immersion in water. Dose rate coefficients were calculated for discrete photon and electron energies and folded with emissions from 1252 radionuclides using ICRP Publication 107 decay data to determine equivalent and effective dose rate coefficients. The coefficients calculated in this work compare favorably to those reported in FGR12 as well as by other authors that employed voxel phantoms for similar exposure scenarios.

Analysis and Monte Carlo modelling of radio-opaque personal protective fabrics

The composition of a second generation radio-opaque fabric and its performance relative to established characterised materials are described. The protection offered by this fabric has been examined using a wide range of discrete gamma photon energies. Data indicate improved attenuation in the range 50–300 keV relative to earlier lightweight personal protective materials and conventional personal protection fabrics. Statistically significant improvements were not observed for gamma photons possessing energies > 300 keV. Experimentally benchmarked models represent powerful tools for the investigation and optimisation of radio-opaque fabrics. In the present work, Monte Carlo simulations using the Monte Carlo N-Particle Transport Code, Version 5 software produced single layer fabric attenuation results for first and second generation fabrics within the uncertainties associated with the experimental data. This model was then used to assess secondary X-ray production and consider the attenuation performance of alternative fabric compositions.

Monte Carlo-based revised values of dose rate constants at discrete photon energies.

Absorbed dose rate to water at 0.2 cm and 1 cm due to a point isotropic photon source as a function of photon energy is calculated using the EDKnrc user-code of the EGSnrc Monte Carlo system. This code system utilized widely used XCOM photon cross-section dataset for the calculation of absorbed dose to water. Using the above dose rates, dose rate constants are calculated. Air-kerma strength Sk needed for deriving dose rate constant is based on the mass-energy absorption coefficient compilations of Hubbell and Seltzer published in the year 1995. A comparison of absorbed dose rates in water at the above distances to the published values reflects the differences in photon cross-section dataset in the low-energy region (difference is up to 2% in dose rate values at 1 cm in the energy range 30–50 keV and up to 4% at 0.2 cm at 30 keV). A maximum difference of about 8% is observed in the dose rate value at 0.2 cm at 1.75 MeV when compared to the published value. Sk calculations based on the compilation of Hubbell and Seltzer show a difference of up to 2.5% in the low-energy region (20–50 keV) when compared to the published values. The deviations observed in the values of dose rate and Sk affect the values of dose rate constants up to 3%.

Accurate characterization of thin films on rough surfaces by spectroscopic ellipsometry

Spectroscopic ellipsometry (SE) is the technique of choice to determine material properties of thin films in a fast and non-destructive way. However, its ability to accurately extract the material properties of thin films deposited on rough substrate surfaces still remains a challenge, due to depolarization of specularly reflected light. In this paper we present a method based on SE to determine the properties of thin films on rough surfaces. It is shown that, by analyzing SE data only at discrete photon energies, information such as the film thickness and the refractive index at these photon energies can be extracted. The discrete photon energies selected in this method can be related to destructive interference of light at the thin-film surfaces, which significantly reduces the depolarization problem. The method is demonstrated using thin amorphous hydrogenated silicon nitride (a-SiNx:H) films deposited on both polished and rough silicon wafer substrates. By varying the a-SiNx:H film thickness, it is shown that the depolarization effect of light is strongly dependent on the incident photon energies due to light interference at the surface of the a-SiNx:H film. Based on this observation, energy selective ellipsometry (ESE) is proposed as a technique for analyzing thin films on random rough surfaces. It is shown that a-SiNx:H films with a wide range of thickness and optical properties on rough silicon surfaces can be studied accurately by means of ESE. This technique is expected to have useful practical applications, for example in photovoltaics where films are typically deposited onto textured substrates as a means of enhancing light trapping.

Skeletal dosimetry in a voxel‐based rat phantom for internal exposures to photons and electrons

The skeleton makes a significant contribution to the whole body absorbed dose evaluation of rats, since the bone marrow and bone surface in the skeleton express high radiosensitivity and are considered to be important dose-limiting tissues. The bone marrow can be categorized as red bone marrow (RBM) and yellow bone marrow (YBM). It is important to investigate the bone marrow in skeletal dosimetry. Cryosectional color images of the skeleton of a 156 g rat were segmented into mineral bone (including cortical bone and trabecular bone), RBM, and YBM. These three tissue types were identified at 40 different bone sites and integrated into a previously developed voxel-based rat computational phantom. Photon and electron skeletal absorbed fractions were then calculated using the MCNPX Monte Carlo code. Absorbed fraction (AF) and specific absorbed fraction (SAF) for mineral bone, RBM, and YBM at the 40 different bone sites were established for monoenergetic photon and electron sources placed in 18 organs and seven bone sites. Discrete photon energy was varied from 0.01 to 5.0 MeV in 21 discrete steps, while 21 discrete electron energies were studied, from 0.1 to 10.0 MeV. The trends and values found were consistent with the results of other researchers [M. G. Stabin, T. E. Peterson, G. E. Holburn, and M. A. Emmons, "Voxel-based mouse and rat models for internal dose calculations," J. Nucl. Med. 47, 655-659 (2006)]. S-factors for the radionuclides 169Er, 143Pr, 89Sr, 32P, and 90Y, located in 18 organs and seven bone sites for the skeleton, were calculated and are provided in detail. For internal dose calculations, the AF data reveal that the mineral bone in the rat skeletal system is responsible for significant attenuation of gamma rays, especially at low energies. The photon SAF curves of RBM show that, for photon energies greater than 0.6 MeV, there is an increase in secondary photons emitted from the mineral bone as photon energy increases. The SAF values calculated in this study can also be used to determine the absorbed dose to the skeletal system of rats. The S-factors generated here will be useful in preclinical targeted radiotherapy experiments.

Angular sensitivity of gated microchannel plate framing cameras

Gated, microchannel-plate-based (MCP) framing cameras have been deployed worldwide for 0.2–9 keV x-ray imaging and spectroscopy of transient plasma phenomena. For a variety of spectroscopic and imaging applications, the angular sensitivity of MCPs must be known for correctly interpreting the data. We present systematic measurements of angular sensitivity at discrete relevant photon energies and arbitrary MCP gain. The results can been accurately predicted by using a simple two-dimensional approximation to the three-dimensional MCP geometry and by averaging over all possible photon ray paths.

X-ray attenuation coefficients of high-atomic-number, hexanuclear transition metal cluster compounds: A new paradigm for radiographic contrast agents

The purpose of this study was to examine the radiologic attenuation properties of the parent cluster compounds, particularly attenuation as a function of discrete photon energy, before investigating ligand substitutions, which are necessary to improve cluster biocompatibility and to impart desirable physicochemical properties. The linear attenuation coefficients for solutions of the cluster compounds Ta6Br14, K8Ta6O19, and (H3O)2W6Cl14 were determined at 60, 80, 103, 122, and 140 keV from gamma-ray transmission measurements with americium-241, xenon-133, gadolinium-153, cobalt-57, and technetium-99m radioactive sources. Transmission measurements were obtained for a fixed time interval that ensured a statistically accurate count distribution exceeding 20,000 counts through the sample for each trial. On a strictly mole per liter basis, a 0.075 mol/L aqueous solution of K8Ta6O19 showed 1.08 times the attenuation of 0.063 mol/L aqueous iohexol at 60 keV and 3.30 times the attenuation at 80 keV. Similarly, a 0.05 mol/L methanolic solution of (H3O)2W6Cl4 showed 0.96 times (96%) the attenuation of 0.063 mol/L aqueous iohexol at 60 keV but 3.09 times the attenuation of the iohexol solution at 80 keV. Attenuations of 0.063 mol/L aqueous iohexol and 0.0125 mol/L Ta6Br14 (ie, at approximately one-fifth the iohexol concentration) were comparable at greater than 60 keV. These results confirm the theoretic potential for use of early transition metal cluster compounds as radiographic contrast agents. At higher x-ray energies, cluster compounds demonstrate multiplied x-ray attenuation relative to iodinated contrast agents.

Characterization of thin films by X-ray transmission measurements

We have used synchrotron radiation in order to determine the mass densities and the elemental composition of single and multilayered solid films. Soft X-rays were used to measure the spectral transmittance in a certain photon energy range, i.e. between 50 eV and 10 keV, and the local transmission profile at discrete photon energies within this range. To give an example, we present some results obtained for an optical filter which is to be used in the focal plane PN CCD-camera of ESAs XMM (X-ray Multi Mirror) astronomy satellite. Moreover, we established a laboratory set-up for transmittance measurements using the discrete line radiation emitted from a solid target X-ray source. Thus, we are able to control layer thicknesses of thin films produced by vacuum evaporation condensation. We describe the compact X-ray test facility, and give the obtained performance data of the filters.

Calculation of absorbed energy in the gastrointestinal tract.

One goal of this research was to reproduce the photon specific absorbed fraction calculations of Cristy and Eckerman using their gastrointestinal (GI) tract model. A second goal was to calculate photon specific absorbed fraction values for their GI tract model using electron tracking techniques. A final goal was to calculate electron absorbed fraction values for their GI tract model. This paper summarizes the work performed using the currently accepted model of the GI tract provided by Cristy and Eckerman. Their model was coded into the Electron Gamma Shower 4 (EGS4) computational package for calculation of photon specific absorbed fraction values. To benchmark the initial code, the EGS4 program was run so that all secondary particles deposited their energy at the site of the primary photon interaction (i.e., without electron tracking). The results obtained from these preliminary calculations were compared to those provided by Cristy and Eckerman to verify and benchmark the program. Next, specific absorbed fraction values were calculated for twelve discrete photon energies using the electron tracking capabilities of EGS4. These photon specific absorbed fraction values were compared to those calculated without electron tracking. Finally, absorbed fraction values were calculated for twelve discrete electron energies. The electron absorbed fraction values were compared to those calculated without electron tracking. Finally, absorbed fraction values were calculated for twelve discrete electron energies. The electron absorbed fraction values were compared to the ICRP "one-half assumption" for electron energy deposition in the wall of the GI tract.

Determination of the dielectric function of strongly anisotropic crystals in reflection

We suggest that the optical dielectric function of absorbing and strongly anisotropic crystals can be determined by combining ellipsometry and polarimetry in reflection experiments. The advantage of ellipsometry is its sensitivity to average optical properties, whereas polarimetry detects the optical anisotropy most sensitively. The concept is systematically tested with different dielectric materials, and it is applied to a detwinned YBa2Cu3O7−x single crystal at discrete photon energies between 1.2 and 2.7 eV. The results show evidence for structural variations of the Cu–O chains. A polarimetric measurement on a (110) face of CuO demonstrates the ease of detecting the absence of two-dimensional ℘ symmetry in the reflecting surface.

Site selectivity of DAFS analysis on hexaferrite at Fe K edge by using both monochromatic optics and energy dispersive optics

DAFS (Diffraction Anomalous Fine Structure) analysis combines the chemical and short range structural sensitivity of absorption spectroscopy and the long range structural selectivity of diffraction, so it can provide site selective spectroscopy information or chemically sensitive structural one. We have undertaken our experiment in the vicinity of the Fe K absorption edge on a single crystal of Y-type hexaferrite BaZnFe6O11 to distinguish Fe cation contributions on tetrahedral and octahedral sites. For these experiments, diffraction intensities were measured as a function of the X-ray energy by scanning over the Fe K absorption edge using both energy dispersive and classical monochromatic optics. In the latter case, intensities of 10 Bragg reflections were collected at about one hundred discrete photon energies with a step width of a few eV over a spectral range of 800 eV. We also report the first DAFS measurements obtained by energy/angular “dispersive diffraction”. During these “energy/angular dispersive diffraction” experiments, a simultaneous collection of the same Bragg reflection intensities was performed over a continuous 250 eV energy range using dispersive focusing optics and a two dimensional detector. Results obtained with both experimental setups agree well and were analyzed using a multi-wavelength refinement procedure. Since in the case of hexaferrite sample, Fe atoms in different crystallographic sites contribute to the reflections, intensity analyses were performed using a simultaneous refinement procedure of several reflections at all energies. We report the determination of anomalous scattering factors f′(E), f″(E) for tetrahedral and octahedral sites.

Specific absorbed fractions of energy from internal photon sources in brain tumor and cerebrospinal fluid

Transferrin, when injected intracranially into glioblastoma multiforme lesions, acts as a cytotoxic substance. Transferrin, radiolabeled with In-111, can be coinjected and subsequent scintigraphic imaging can demonstrate the biokinetics of the cytotoxic transferrin. The administration of 111In transferrin into a brain tumor results in distribution of radioactivity in the brain, brain tumor, and the cerebrospinal fluid (CSF). Information about absorbed radiation doses to these regions, as well as other nearby tissues and organs, is important for evaluating radiation-related risks from this procedure. The radiation dose is usually estimated for a mathematical representation of the human body. We have included source/target regions for the eye, lens of the eye, spinal column, spinal CSF, cranial CSF, and a 100-g tumor within the brain of an adult male phantom developed by Cristy and Eckerman. The mathematical models of the spinal column, spinal CSF, and the eyes were developed previously, however, these source/targets have not been routinely included in photon transport simulations. Specific absorbed fractions (SAFs) as a function of photon energy were calculated using the ALGAMP computer code, which utilizes Monte Carlo techniques for simulating photon transport. The ALGAMP code was run three times, with the source activity distributed uniformly within the tumor, cranial CSF, and the spinal CSF volumes. These SAFs, which were generated for 12 discrete photon energies ranging from 0.01 to 4.0 MeV, were used with decay scheme data to calculate S-values needed for estimating absorbed doses. S-values for 111In are given for three source regions (brain tumor, cranial CSF, and spinal CSF) and all standard target regions/organs, the eye and lens, as well as to tissues within these source regions. S-values for the skeletal regions containing active marrow are estimated. These results are useful in evaluating the radiation doses from intracranial administration of 111In transferrin. The SAFs are also generally useful for calculation of absorbed dose from any radionuclide in these source regions.

Photon-assisted tunneling through a quantum dot.

We study single-electron tunneling in a two-junction device in the presence of microwave radiation. We introduce a model for numerical simulations that extends the Tien-Gordon theory for photon-assisted tunneling to encompass correlated single-electron tunneling. We predict sharp current jumps which reflect the discrete photon energy hf, and a zero-bias current whose sign changes when an electron is added to the central island of the device. Measurements on split-gate quantum dots show microwave-induced features that are in good agreement with the model.

On Temperature Dependence of Charge Photogeneration in Holecular Crystals

Abstract A simple analytical expression for the charge photogeneration yield (φ) is derived which is based on the Onsager theory of geminate recombination. The expression shows non-Arrhenius temperature (T) dependence φ∼exp(-A/½) with A=2(ezα/4πϵ ϵk)½, where α is a characteristic parameter in the spatial (r) distribution function of thermalized electron-hole pairs g(r) = = (α/4πr2)exp(-αr). The values of α obtained on its basis from the literature experimental data for anthracene crystals are interpreted in terms of collision cross section for primary carriers. The discrete photon energy dependence of α can be explained by crystal anisotropy for the carrier motion.

Far-infrared magnetoabsorption in HgTe–CdTe and Hg1−xMnxTe–HgTe superlattices

We report far-infrared (FIR) magnetoabsorption measurements on two Hg-based superlattices: HgTe/CdTe and Hg0.90Mn0.10 Te/HgTe grown by molecular-beam epitaxy. The latter one is novel and unique in that it is a superlattice of a zero-gap semiconductor and a narrow-gap diluted magnetic semiconductor. The FIR experiments were carried out at discrete photon energies between 2.5 and 12.8 meV in the temperature range 1.6 K<T<200 K. Rich spectra of inter- and intraband transitions were observed, the intensities and positions of the transitions showing a dramatic temperature dependence associated with the thermal redistribution of electrons within the energy level scheme of the superlattices.

Dose-rate conversion factors for external exposure to photon emitters in soil.

Dose-rate conversion factors have been calculated for external exposure above ground to monoenergetic photon emitters in soil. These factors give external dose rates per unit source concentration in soil. The calculations are based on the point-kernel integration method and assume that the source concentration at any depth in soil is uniform over an infinite surface parallel to the ground plane. Dose-rate factors in air at a height of 1 m above ground are tabulated for discrete photon energies between 0.01 and 10 MeV and for source depths in soil between 0 and 300 cm. Application of the results for plane sources in soil to the calculation of photon dose rates from distributions of sources with depth in soil is described, and dose-rate factors are tabulated for the particular cases of uniform slab sources of finite thickness and sources which are exponentially distributed with depth. We also demonstrate how dose-rate factors in air for monoenergetic photon sources are used to estimate dose-rate factors for body organs of exposed individuals and for the spectrum of photons from radioactive decay. The calculations in this paper show that allowing for downward migration of radionuclides in soil can result in significant reductions in external dose compared with the usual assumption that radionuclides which are deposited on the ground surface remain there until removal by radioactive decay.

Damping Effects in Angular-Resolved Ultraviolet Photoemission Spectroscopy on Cu(111)

Previously published photoemission spectra taken at several discrete photon energies from the (111)-surface of copper are analyzed with the purpose of extracting information on the broadening effects involved. In the low energy case (6.40 eV) the effect of surface disorder on the spectral shape of a surface state is explained as the result of a reduced coherence length parallel to the surface, while sensitivity to disorder normal to the surface is the prevailing effect for the bulk transition of the same spectrum. The use of angular instead of frequency tuning is proposed as a feasible alternative to extract electronic lifetimes in favourable cases. Numerical values of inverse lifetimes are derived and found to agree well with the trend in recent measurements of energy dependent inverse lifetimes. Finally, accounting for our instrumental broadening, the residual K ||-dependent width of a dispersive peak is compared with values predicted from the band structure of copper and published lifetimes measured in normal emission. Inhomogeneous damping is proposed as a possible explanation of a deviation seen from the calculated widths as the final state crosses a Brillouin zone boundary.