Photoelectric Absorption Cross Sections Research Articles

Evaluation and synthesis of perovskite crystals as high-Z sensors for hybrid pixel detectors

High-energy photon imaging experiments are crucial techniques in synchrotron facilities, often employing hybrid pixel detectors for these operations. These detectors combine a photo-sensitive semiconductor component with a pixelated microelectronic Application Specific Integrated Circuit (ASIC) for signal processing and image formation. However, detecting photons above 90 keV poses significant challenges, even for heavy semiconductors, due to lower photoelectric absorption cross-section at this energy range. Nevertheless, lead-based perovskites, such as CsPbBr3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CsPbBr}}_3$$\\end{document}, are remarkable alternatives as they present excellent cross-section values and noteworthy transport properties, contributing to increased high-energy detection efficiencies. Here, we employ a chemical synthesis route for CsPbBr3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CsPbBr}}_3$$\\end{document} single-crystals, enabling experimental measurements of carrier mobility of 100.7 cm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{cm}}^{2}$$\\end{document}/Vs. We also developed a simulation algorithm to calculate the current pulses generated on pixelated electrodes. Our simulations evaluate CsPbBr3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CsPbBr}}_3$$\\end{document}’s performance coupled with the latest photon-counter ASIC developed by CERN, the Timepix4. Our findings indicate that CsPbBr3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CsPbBr}}_3$$\\end{document} crystals require intense applied electric fields, around 1 kV/mm, for accurate signal integration. Furthermore, we observed no correlation between incident energy and induced pulse width. Through microelectronics simulations, we demonstrate that the signal formation behavior of CsPbBr3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CsPbBr}}_3$$\\end{document} is compatible with Timepix4 ASICs, consequently establishing operational guidelines for employing this promising material as sensors in hybrid pixel detectors.

Identification of carbonate sedimentary facies from well logs with machine learning

Sedimentary facies identification is critical for carbonate oil and gas reservoir development. The traditional method of sedimentary facies identification not only be affected by the engineer's experience but also takes a long time. Identifying carbonate sedimentary facies based on machine learning is the trend of future development and has the advantages of short time consuming and reliable results without engineers' subjective influence. Although many references reported the application of machine learning to identify lithofacies, but identifying sedimentary facies of carbonate reservoirs is much more challenging due to the complex sedimentary environment and tectonic movement. This paper compares the performance of the carbonate sedimentary facies identification using four different machine learning models, and the optimal machine learning with the highest prediction accuracy is recommended. First, the carbonate sedimentary facies are classified into the lagoon, shallow sea, shoal, fore-shoal, and inter-shoal five tags based on the well loggings. Then, five well log curves including spectral gamma ray (SGR), uranium-free gamma ray (CGR), photoelectric absorption cross-section index (PE), true formation resistivity (RT), shallow lateral resistivity (RS) are used as the input, and the manual identified carbonate sedimentary facies are used as the output of the machine learning model. The performance of four different machine learning algorithms, including support vector machine (SVM), deep neural network (DNN), long short-term memory (LSTM) network, and random forest (RF) are compared. The other two wells are used for model validation. The research results show that the RF method has the highest accuracy of sedimentary facies prediction, and the average prediction accuracy is 78.81%; the average accuracy of sedimentary facies prediction using SVM is 77.93%. The sedimentary facies predictions using DNN and LSTM are less satisfying compared with RF and SVM, and the average accuracy is 69.94% and 73.05%, respectively. The predicted carbonate sedimentary facies by LSTM are more continuous compared with other machine learning models. This study is helpful for identifying compelx sedimentary facies of carbonate reservoirs from well logs.

Effect on dark matter exclusion limits from new silicon photoelectric absorption measurements

Recent breakthroughs in cryogenic silicon detector technology allow for the observation of single electron-hole pairs released via particle interactions within the target material. This implies sensitivity to energy depositions as low as the smallest band gap, which is $\sim1.2$ eV for silicon, and therefore sensitivity to eV/$c^2$-scale bosonic dark matter and to thermal dark matter at masses below 100 MeV/$c^2$. Various interaction channels that can probe the lowest currently accessible masses in direct searches are related to standard photoelectric absorption. In any of these respective dark matter signal models any uncertainty on the photoelectric absorption cross section is propagated into the resulting exclusion limit or into the significance of a potential observation. Using first-time precision measurements of the photoelectric absorption cross section in silicon recently performed at Stanford University, this article examines the importance having accurate knowledge of this parameter at low energies and cryogenic temperatures for these dark matter searches.

Photoelectric absorption cross section of silicon near the bandgap from room temperature to sub-Kelvin temperature

The use of cryogenic silicon as a detector medium for dark matter searches is gaining popularity. Many of these searches are highly dependent on the value of the photoelectric absorption cross section of silicon at low temperatures, particularly near the silicon bandgap energy, where the searches are most sensitive to low mass dark matter candidates. While such cross section data have been lacking from the literature, previous dark matter search experiments have attempted to estimate this parameter by extrapolating it from higher temperature data. However, discrepancies in the high temperature data have led to order-of-magnitude differences in the extrapolations. In this paper, we resolve these discrepancies by using a novel technique to make a direct, low temperature measurement of the photoelectric absorption cross section of silicon at energies near the bandgap (1.2 eV–2.8 eV).

Improvement of petrophysical workflow for shear wave velocity prediction based on machine learning methods for complex carbonate reservoirs

Shear wave velocity (S-wave velocity or Vs) is one of the most critical issues for carbonate reservoirs characterization, because of its complexity of rock compositions and pore structures. The Xu-Payne petrophysical model is a commonly used method to predict S-wave velocity. However, the model excessively depends on the interpretation accuracy of rock compositions, pore structures, fluid properties and fluid saturation, which makes the prediction result potentially uncertain. With the development of intelligent algorithms, a machine learning method (Long-short Term Memory Neural Network, LSTM) is proposed to improve the traditional petrophysical workflow. With the capabilities of deep learning and data mining, the LSTM model can deeply mine the rich information in the wireline logs and then establish the relationships between S-wave velocity and wireline logs. In order to illustrate the prediction effect, the carbonate reservoir in the Majiagou Formation Member 5(Ma 5 Member) which belongs to the Ordovician system in the D area of Sulige gas field is taken as an example. Six kinds of sensitive wireline logs, including compensated acoustic log (AC), natural gamma ray log(GR), photoelectric absorption cross-section log (PE), density log (DEN), deep lateral resistivity log(RLLD), neutron log(CNL), are selected as the input data of the LSTM model. The results reveal that the accuracy of LSTM method is up to 98.9%, whereas the Xu-Payne model is only 73%. With the predicted result of all wells in the areas using LSTM, the sensitive elastic properties, including P-and S-wave velocity, P- and S-wave impendences, ratio of P- and S-wave velocity, Poisson's ratio, lame coefficient and fluid sensitivity factor, are also concluded. Compared with the Xu-Payne model, the newly proposed workflow is proved to be convenient and suitable for the prediction of S-wave velocity in carbonate reservoirs.

The photon interaction cross sections of blood

The photon interaction cross sections which contain the photoelectric absorption, coherent (Rayleigh) and incoherent (Compton) scattering are calculated for blood in the energy range of 1–150 keV. The molecular photoelectric absorption cross section is achieved from atomic cross section data. The molecular coherent (Rayleigh) and incoherent (Compton) scattering cross sections are estimated using relativistic modified form factor and incoherent scattering function approximation, respectively. The molecular form factors updated including molecular interference effects and incoherent scattering functions were also calculated. With the inclusion of the molecular interference effects in the coherent (Rayleigh) scattering, the more accurate data of radiation scattered from blood will be provided. The results are compared with references values in literature. It is found that the theoretical attenuation coefficients are in good agreement with experimental values.

Abstract 670: Specific ROS generation and monitoring in cancer using radioactive gold nanoparticle

Abstract Radiotherapy is a key treatment and is beneficial in the treatment of about 50% of all cancer patients. Such treatment relies on the deposition of energy (the dose) in tumour cells, typically by irradiation with either high-energy gamma rays or X-rays (photons), or energetic beams of ions, sufficient to damage the cancer cells or their vasculature and thus induce tumour death or nutrient starvation. However, like chemotherapy, photon radiotherapy is non-specific, since a significant dose can be delivered to healthy tissue along the track of the photons, in front and behind the tumour. Therefore, recently several research groups have proposed the use of specifically targeted noble-metal nanoparticles irradiated by radiation as an effective therapeutic tool for treating various malignancies with minimum collateral damage to healthy tissue. In general, the photoelectric absorption cross sections of radiosensitive materials directly depend on atom radius, and thus their radiotherapy effects also depend on atom radius. This physical principle determines that gold (atomic-number Z=79) has a stronger radiation enhancement effect by photoelectric effect. Loading with gold nanoparticles (AuNPs) is one of the promising candidates in this area. Gold NPs have a larger atomic radius than gold atom (~0.17 nm), and thus should have a stronger radiation enhancement effect than gold atom. During irradiation, this results in an enhancement of the energy deposition in the vicinity of the gold particles generate the ROS on the surface due to the generation of photoelectrons, Auger electrons, and characteristic X-rays. ROS play important roles in various cellular process. In particularly, at sufficiently high concentrations, ROS could become an effect that is utilized for various therapeutic application such as apoptosis. Herein, we synthesized the radiosensitizer PEGylated gold nanoparticles (RPAuNPs) for generation and measuring of ROS on surface by radiation exposure. First, gold nanoparticles (AuNPs) was prepared by one-pot synthesis and the sharpness and size were efficiently controlled by adjusting the amount of reducing agent. Furthermore, they were replaced with heterobifunctional PEG (COOH-PEG-SH) not only to serve as a biocompatible stabilizer (PAuNPs) and but also to conjugate Dihydrorhodamine 123 (DHR 123) (RPAuNPs) as ROS sensor. Well-engineered formation of RPAuNPs was verified by transmission electron microscopy (TEM) showing their overall size of 50 nm. In addition, UV-Vis data strongly demonstrated successful synthesis. We evaluated their functional use in inducing ROS and the results showed that our radiosensitizing agents exhibited the feasibility of effective ROS generation under x-ray radiation. And we investigated there in vitro/in vivo imaging and therapeutic efficacies, optical properties, biocompatibility for tumor cell. Citation Format: Jihye Choi, Yoon Woo Koh, Hyung Kwon Byeon, Guillem Pratx, Jaemoon Yang. Specific ROS generation and monitoring in cancer using radioactive gold nanoparticle [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 670.

Duvernay shale lithofacies distribution analysis in the West Canadian Sedimentary Basin

In the West Canadian Sedimentary Basin (WCSB), Duvernay shale is considered to contribute most of the Canadian shale gas reserve and production. According to global shale gas exploration and development practice, reservoir property and well completion quality are the two key factors determining the shale gas economics. The two key factors are strongly depending on shale lithofacies. On the basis of inorganic mineralogy theory, all available thin section, X-ray diffraction, scanning electron microscope (SEM), energy dispersive spectrometer (EDS) data were used to assist lithofacies analysis. Gamma ray (GR), acoustic (AC), bulk density (RHOB), neutron porosity (NPHI) and photoelectric absorption cross-section index (PE) were selected for log response analysis of various minerals. Reservoir representative equation was created constrained by quantitative core analysis results, and matrix mineral percentage of quartz, carbonate, feldspar and pyrite were calculated to classify shale lithofacies. Considering the horizontal continuity of seismic data, rock physics model was built, and acoustic impedance integrated with core data and log data was used to predict the horizontal distribution of different lithofacies. The results indicate that: (1) nine lithofacies can be categorized in Duvernay shale, (2) the horizontal distribution of different lithofacies is quite diversified, siliceous shale mainly occurs in Simonette area, calcareous shale is prone to develop in the vicinity of reef, while calcareous-siliceous shale dominates in Willesdon Green area.

Sedimentary control on the formation of a multi-superimposed gas system in the development of key layers in the sequence framework

Based on core observations, well logs and test results of siderite-bearing mudstone from the Benxi Formation to the Member 2 of the Shanxi Formations in the Linxing block, northeastern Ordos Basin, a logging identification model for siderite-bearing mudstone (key layer) was established. The porosity characteristics and sealing property were quantitatively evaluated by logging data. Sedimentary control on the formation of multi-superimposed gas-bearing system in the development of key layers in the sequence framework was also discussed. The results showed that the siderite-bearing mudstone has obvious logging response characteristics, e.g., high photoelectric absorption cross-section index (PE), high density (DEN), high amplitude natural gamma ray (GR), low acoustic (AC), low resistivity (M2RX) and low neutron porosity (CNCF). The quantitatively evaluated results of the porosity characteristics and sealing property for the key layer showed that the key layer has the characteristics of low porosity (with an average of 1.20 percent), low permeability (with an average of 2.29 × 10−8μm2), and high breakthrough pressure (with an average of 12.32 MPa) in the study area. This layer acts as an impermeable gas barrier in a multi-superimposed gas system. The results also indicated that the material composition of the multi-superimposed gas-bearing system can be established by the sequence stratigraphic framework. The sedimentary evolution results in a cyclic rhythm of material composition vertically. The spatial distribution of the corresponding transgressive event layer near the maximum flooding surface (MFS) in the sequence framework restricts the spatial distribution of the key layer with high breakthrough pressure and low porosity, which constitutes the gas-bearing system boundary. The siderite-bearing mudstone formed near the MFS in the second-order sequence and constitutes a stable comparison of the first-order gas-bearing system boundary, which has a wide range of regional distribution and stable thickness. The siderite-bearing mudstone formed near the MFS in the third-order sequence is often incompletely preserved due to the late (underwater) diversion channel erosion and cutting. This layer forms the coal-bearing reservoirs, which we termed as a second-order gas-bearing system in adjacent third-order sequences to form a uniform gas-bearing system.

On a new mechanism on [formula omitted]-ray attenuation strengthened by new data

Introduction We extended our previous detailed γ -ray attenuation measurements of 60Co γ rays in Pb and Fe to include also Cu, Hg, Al and H2O. Purpose To introduce a new mechanism on γ -ray attenuation in matter. Materials and methods 60Co radiation source, slices of the absorbers and a Geiger Muller detector counter in broad and thin beam geometries. Experiments We performed detailed measurements of the detected radiation versus the thickness of the absorber and in parallel Monte Carlo simulations for each experiment. Results Monte-Carlo calculations were in qualitative agreement with experiment but quantitatively predicted systematically more radiation at the target. We related the differences with the parameters Eγ and Zeff of the photoelectric absorption cross section which guided us to propose a new mechanism on the absorption of γ rays in matter. The γ radiation creates a huge number of electrons and ions passing through the absorber which form a steady state plasma condition increasing slightly the parameters Eb and Zeff of the lattice which in turn can increase measurably the photoelectric absorption thus restoring the agreement between Monte Carlo calculations and experiment. Conclusion Based on our large set of data which included absorbers in a wide range of Zeff we strengthened the need for a new mechanism on γ-ray absorption in matter.

Accurate X-ray Absorption Spectra of Dilute Systems: Absolute Measurements and Structural Analysis of Ferrocene and Decamethylferrocene

X-ray absorption fine structure (XAFS) of ferrocene (Fc) and Decamethylferrocene (DmFc) have been determined on an absolute scale using transmission measurements of multiple solutions of differing concentrations (15 mM, 3 mM, pure solvent) at operating temperatures of 10–20 K. Mass attenuation coefficients and photoelectric absorption cross sections are measured and tabulated for both molecules for an extended energy range in excess of 1.5 keV from the Fe K-shell absorption edge. At these temperatures, the minimization of of dynamic disorder has enabled a critical determination of the oscillatory absorption structures created by multiple-scattering paths of the excited photoelectron. These oscillatory structures are highly sensitive to the local conformation environment of the iron absorber in organometallic structures. Crystallographic and scattering studies have reported both structures characterized by staggered cyclopentadienyl rings, in contrast with low temperature crystallography and recent density functional theoretical predictions. Phase changes in the crystallographic space groups are reported for Fc at different temperatures, raising the possibility of alternative conformation states. Robust experimental techniques are described which have allowed the measurement of XAFS spectra of dilute systems by transmission at accuracies ranging from 0.2% to 2%, and observe statistically significant fine structure at photoelectron wavenumbers extending to >12 Å–1. The subtle signatures of the conformations are then investigated via extensive analysis of the XAFS spectra using the full multiple scattering theory as implemented by the FEFF package. Results indicate a near-eclipsed D5h geometry for low-temperature Fc, in contrast with a staggered D5d geometry observed for DmFc. The ability of this experimental approach and data analysis methodology combined with advanced theory to investigate and observe such subtle conformational differences using XAFS is a powerful tool for future challenges and widens the capacity of advanced XAFS to solve a broad range of challenging systems.

Brittleness index estimation in a tight shaly sandstone reservoir using well logs

Premise evaluation of the ability of fracture network forming using hydraulic fracturing is crucial for the development of tight shaly sandstone reservoirs. X-ray diffraction, petrographic analysis integrated with petrophysical well logs was used to provide insights to the brittleness of Chang 7 shaly sandstone in the Ordos basin. The mineralogical composition of this unit is quartz, clay, carbonates and feldspar with minor quantities of pyrite. Quartz and carbonate are the critical brittle minerals. The brittleness index defined in terms of mineralogy and geomechanical properties was investigated respectively. The geomechanical brittleness derived from dipole acoustic logs has a wide range from 0.98% to 93.21%, averaging 58.07%. In contrast, the mineralogical brittleness index from X-ray diffraction results is in the range of 28.26–78.85%, averaging 59.31%. The geomechanical brittleness index could be easily calculated using the dipole acoustic and bulk density logs. For wells without dipole acoustic logs, predicting the brittleness index using conventional well log suits is of great importance. The mineralogical brittleness index was then correlated with their well-log signatures. Statistical regression analysis was carried out to find the relations between brittleness index and conventional well logs, and it is found that the ratio of gamma ray to photoelectric absorption cross section index (GR/Pe) shows a good correlation with brittleness index. Therefore, the brittleness index could be estimated using conventional logs (GR/Pe). The geomechanical brittleness index and the test oil results confirm the reliability of the brittleness index estimation model. Favorable layers for hydraulic fracturing are mainly associated with the intervals with high values of brittleness index. This work is valuable for the evaluation of hydraulic fracturing effects in unconventional oil and gas reservoirs in the future.

Logging identification of the Longmaxi mud shale reservoir in the Jiaoshiba area, Sichuan Basin

Compared with conventional gas reservoirs, shale gas reservoirs are not sensitive to petrophysical properties, making it much difficult to identify this kind of reservoirs with well logging technologies. Therefore, through a comparison of the logging curves of the Lower Silurian Longmaxi marine shale in the Jiaoshiba area, Sichuan Basin, it is found that the mud shale on conventional log curves generally features high gamma ray, high uranium, low thorium, low kalium, relative high resistivity, high interval transit time, low neutron, low density and low photoelectric absorption cross section index, while on elements logging curves, it features an increase of silicon content and a decrease of aluminum and iron content. Based on the logging response characteristics of mud shale, the logging curves most sensitive to shale, gamma ray, neutron and density logging were selected and overlaid to identify mud shale effectively. On the basis of qualitative identification, the density logging value can identify the non-organic-rich mud shale from organic-rich mud shale, because the former has a density of 2.61–2.70 g/cm3, while the latter has a density of less than 2.61 g/cm3. The identification results agree well with the results of field gas content test, TOC experiment, and gas logging, so this study can provide reference for the logging interpretation.

Linear attenuation coefficients of tissues from 1 keV to 150 keV

The linear attenuation coefficients and three interaction processes have been computed for liver, kidney, muscle, fat and for a range of x-ray energies from 1keV to 150keV. Molecular photoelectric absorption cross sections were calculated from atomic cross section data. Total coherent (Rayleigh) and incoherent (Compton) scattering cross sections were obtained by numerical integration over combinations of F2m(x) with the Thomson formula and Sm(x) with the Klein–Nishina formula, respectively. For the coherent (Rayleigh) scattering cross section calculations, molecular form factors were obtained from recent experimental data in the literature for values of x<1Å−1 and from the relativistic modified atomic form factors for values of x≥1Å−1. With the inclusion of molecular interference effects in the coherent (Rayleigh) scattering, more accurate knowledge of the scatter from these tissues will be provided. The number of elements involved in tissue composition is 5 for liver, 47 for kidney, 44 for muscle and 3 for fat. The results are compared with previously published experimental and theoretical linear attenuation coefficients. In general, good agreement is obtained. The molecular form factors and scattering functions and cross sections are incorporated into a Monte Carlo program. The energy distributions of x-ray photons scattered from tissues have been simulated and the results are presented.

Determination of Atomic Number Exponent for Elemental Absorber at Am– 241 Energy in X–Ray Attenuation Experiments

In X-ray absorption experiment, the dependence of the photoelectric absorption cross-section varied with atomic number Z. In this paper, the dependence on Z of the cross – section or coefficient of photoelectric absorption is first evaluated from published data for (20) elements ranging from Al–Sn, and then the cross–section for Fe is obtained experimentally by measuring the X-ray intensity with and without the Fe absorber and compared with the theoretical value, the variation of the photoelectric absorption coefficient (µ/ρm) showed less dependence on Z for the two tested of Z regions. This is because of the inclusion of mass density (ρm) of the absorber in the coefficient. The values of m obtained in the present work increases linearly with energy for both ranges of elements.

Radiological characterization and water equivalency of genipin gel for x-ray and electron beam dosimetry

The genipin radiochromic gel offers enormous potential as a three-dimensional dosimeter in advanced radiotherapy techniques. We have used several methods (including Monte Carlo simulation), to investigate the water equivalency of genipin gel by characterizing its radiological properties, including mass and electron densities, photon interaction cross sections, mass energy absorption coefficient, effective atomic number, collisional, radiative and total mass stopping powers and electron mass scattering power. Depth doses were also calculated for clinical kilovoltage and megavoltage x-ray beams as well as megavoltage electron beams. The mass density, electron density and effective atomic number of genipin were found to differ from water by less than 2%. For energies below 150 keV, photoelectric absorption cross sections are more than 3% higher than water due to the strong dependence on atomic number. Compton scattering and pair production interaction cross sections for genipin gel differ from water by less than 1%. The mass energy absorption coefficient is approximately 3% higher than water for energies <60 keV due to the dominance of photoelectric absorption in this energy range. The electron mass stopping power and mass scattering power differ from water by approximately 0.3%. X-ray depth dose curves for genipin gel agree to within 1% with those for water. Our results demonstrate that genipin gel can be considered water equivalent for kilovoltage and megavoltage x-ray beam dosimetry. For megavoltage electron beam dosimetry, however, our results suggest that a correction factor may be needed to convert measured dose in genipin gel to that of water, since differences in some radiological properties of up to 3% compared to water are observed. Our results indicate that genipin gel exhibits greater water equivalency than polymer gels and PRESAGE formulations.

Delayed Iron Lines in Centaurus X‐3

We present the study of the aperiodic time variation of the binary X-ray pulsar Centaurus X-3 observed with the Rossi X-Ray Timing Explorer (RXTE). We extracted light curves from six energy bands and calculated cross-spectral functions between the energy band containing the iron K lines (iron band) and the remaining bands. We discovered, for the first time, that the temporal variation of iron band emission is delayed as compared with that of the other energy X-rays by 0.39 ± 0.10 ms (1 σ error). We also discovered that the time difference shows a general trend such that the higher energy X-rays are advanced in comparison with the lower energy X-rays with the exception of the iron band. Assuming that the iron emission lines are fluorescent lines in origin and are emitted by circumstellar matter, we derived 5.6 ± 1.5 ms for the time delay of the temporal variation of the iron line from the average temporal variation in the energy band above the iron absorption edge, taking into account the energy spectral shape and the photoelectric absorption cross section. This result leads to a determination of (1.7 ± 0.5) × 108 cm for the distance between the original X-ray source and the reprocessor, which emits the iron lines. The general trend of the hard X-ray advance is considered to be related to the X-ray emission mechanism on the neutron star.

Direct observation of scattering contributions in X-ray attenuation measurements, and evidence for Rayleigh scattering from copper samples rather than thermal-diffuse or Bragg–Laue scattering

High-precision attenuation measurements can now lead to a direct assessment of scattering models in the X-ray regime. This is true not just at high energies but even when scattering cross-sections typically lie below 0.1% of the photoelectric absorption cross-section. This allows model-independent results to be obtained in these attenuation experiments for the first time.

Detailed Tabulation of Atomic Form Factors, Photoelectric Absorption and Scattering Cross Section, and Mass Attenuation Coefficients in the Vicinity of Absorption Edges in the Soft X-Ray (Z=30–36, Z=60–89, E=0.1 keV–10 keV), Addressing Convergence Issues of Earlier Work

Reliable knowledge of the complex x-ray form factor [Re(f ) and f″] and the photoelectric attenuation coefficient (σPE) is required for crystallography, medical diagnosis, radiation safety, and XAFS studies. Discrepancies between currently used theoretical approaches of 200% exist for numerous elements from 1 to 3 keV x-ray energies. The key discrepancies are due to the smoothing of edge structure, the use of nonrelativistic wave functions, and the lack of appropriate convergence of wave functions. This paper addresses these key discrepancies and derives new theoretical results of substantially higher accuracy in near-edge soft x-ray regions. The high-energy limitations of the current approach are also illustrated. The energy range covered is 0.1 to 10 keV. The associated figures and tabulation demonstrate the current comparison with alternate theory and with available experimental data. In general, experimental data are not sufficiently accurate to establish the errors and inadequacies of theory at this level. However, the best experimental data and the observed experimental structure as a function of energy are strong indicators of the validity of the current approach. New developments in experimental measurement hold great promise in making critical comparisons with theory in the near future.

X‐Ray Transmission in Cold Matter: Nonrelativistic Corrections for Compton Scattering

Obscuration by cold matter is frequently observed in a variety of X-ray sources. With the improved sensitivity and energy resolution of the new generation of X-ray detectors, such as those aboard ASCA, it may not always be possible to neglect electron scattering for column densities much greater than ~1023 cm-2. We derive simple, approximate, analytic expressions for a spherical geometry, which enable the transmitted flux to be corrected for nonrelativistic Compton scattering. The method allows the use of arbitrary element abundances, photoelectric absorption cross sections, and input spectra and is computationally fast enough for use in a spectral fitting code. The approximations can be used for column densities up to ~5 × 1024 cm-2. The maximum observed energy for which the approximations are valid depends on the column density and is ~18.5 keV for NH = 1023 cm-2 and ~10 keV for NH = 5 × 1024 cm-2. The model provides a significantly improved description of the transmitted spectrum compared to the usual model, in which only attenuation due to the absorption opacity is considered, and scattering is neglected altogether. The latter leads to errors in the transmitted spectrum that can exceed 1 order of magnitude, especially around the Fe K edge.