Incident X-ray Spectrum Research Articles

Vertical matrix perovskite X-ray detector for effective multi-energy discrimination

Multi-energy X-ray detection is sought after for a wide range of applications including medical imaging, security checking and industrial flaw inspection. Perovskite X-ray detectors are superior in terms of high sensitivity and low detection limit, which lays a foundation for multi-energy discrimination. However, the extended capability of the perovskite detector for multi-energy X-ray detection is challenging and has never been reported. Herein we report the design of vertical matrix perovskite X-ray detectors for multi-energy detection, based on the attenuation behavior of X-ray within the detector and machine learning algorithm. This platform is independent of the complex X-ray source components that constrain the energy discrimination capability. We show that the incident X-ray spectra could be accurately reconstructed from the conversion matrix and measured photocurrent response. Moreover, the detector could produce a set of images containing the density-graded information under single exposure, and locate the concealed position for all low-, medium- and high-density substances. Our findings suggest a new generation of X-ray detectors with features of multi-energy discrimination, density differentiation, and contrast-enhanced imaging.

Wide bandgap semiconductor conversion devices for radioisotope microbatteries

Five mesa p+-i-n+ photodiodes (each of 0.126 mm2 area) were investigated as conversion devices for X-ray-voltaics, in order to understand the comparative effects between different semiconductor materials, device structures, and X-ray incident power, and to explore them for use in future radioisotope microbatteries. Three semiconductor materials (AlInP, InGaP, and GaAs) and three i layer thicknesses of one material, AlInP (2 μm, 6 μm, and 10 μm), were investigated under the illumination of various controlled X-ray incident powers. The highest short circuit current was achieved with the GaAs device due to its thickest active layer and highest linear absorption coefficients at the most numerous incident X-ray photon energies (Mo Kα at 17.48 keV; Mo Kβ at 19.6 keV). The highest open circuit voltage was achieved with the 10 μm AlInP device due to its widest bandgap (cf. InGaP and GaAs) and its highest short circuit current (cf. the 2 μm and the 6 μm AlInP devices). The greatest output X-ray power was recorded with the InGaP device due to its highest fill factor (i.e. relatively low series and high shunt resistance) compared to the rest of the devices, although the GaAs device had the highest theoretical output X-ray power. A method for selecting the most suitable semiconductor material and device structure of conversion devices for radioisotope microbatteries (for exhibiting the highest power output) is presented considering the incident X-ray spectrum, while highlighting the importance of non-ideal device effects (reduced charge collection efficiency, increased series resistance, and reduced shunt resistance).

Self-powered multilayer radioisotope identification device.

This is a computational study to develop a rugged self-powered Radioisotope Identification Device (RIID). The principle of operation relies on the High Energy Current (HEC) concept (Zygmanski and Sajo, Med Phys. 43 4-15, 2016) with measurement of fast electron currents between low-Z and high-Z thin-film electrodes separated by nanoporous aerogel films in a multilayer detector structure whose prototypes were previously investigated (Brivio, Albert, Freund, Gagne, Sajo and Zygmanski, Med Phys, 46 4233-4240, 2019), (Brivio, Albert, Gagne, Freund, Sajo and Zygmanski, J Phys D Appl Phys, 53 265303, 2020). Here, we present an optimal detector design that accounts for a wide energy range (keV-MeV) of x-ray-emitting radioisotopes that are of interest to national security and radiation therapy. We studied numerous multilayer detector geometries with N=1..24 basic detector elements composed of 3 electrodes: N x (Al-aerogel-Ta-aerogel-Al). The thicknesses of electrodes and their total number were varied depending on the incident x-ray spectra and its ability to penetrate and interact with the different layers, producing fast electrons. We used radiation transport simulations to find a balanced geometry that accounts for all energies from 10keV to 6MeV in a single design with relatively few detector elements (N=24). In the balanced design, the electrodes have increasing thickness as a function of depth in the detector, ranging from 0.5μm-Ta and 10μm-Al at the entrance to 10mm-Ta and 2.5mm-Al at the exit. Aerogel thickness was fixed at 50μm. Electron currents forming RIID signals were acquired from all Ta electrodes. A model function M(x, Ei ) representing the detector yield as a function of the cumulative Ta thickness (x) for 70 monoenergetic incident beams (E) was derived. We also investigated the detector response to selected radioactive isotopes (Pd-103, I-125, Pu-239, U-235, Ir-192, Cs-137, Co-60). Additional studies were performed with Bremsstrahlung spectra produced by electron beams in kVp tubes and in MV Linacs used in radiology and radiation therapy departments. We investigated different algorithms for radioisotope identification that would work for unknown unshielded as well as shielded sources. Characteristic features of response functions for monoenergetic beams and radioisotopes were determined and used to develop two inverse algorithms of radioisotope identification. Using these algorithms, we were able to identify the unshielded and shielded sources, quantify the minimum, mean and maximum effective energies of the shielded spectra, and estimate the amount of Compton background in the spectrum. A multilayer sensor based on fast electron current was optimized and studied in its abilities as RIID. A balanced design permits the identification of radioisotopes with of a wide range of keV-MeV energies. The device is low cost, rugged, self-powered and can withstand very high dose rates, allowing deployment in difficult conditions, including radiation incidents. The algorithm we developed for radioisotope identification and spectral unfolding is robust and it is an important component in practical applications.

High-rate X-ray spectroscopy in mammography with photon counting detectors using a limited number of energy bins

Direct measurement of mammographic X-ray spectra under clinical conditions can be useful for quality control (QC) in mammography for estimating image quality and patient dose. The measurement of mammographic X-ray spectra using the traditional detectors with multi-channel pulse height analyzer (MCA) systems is challenging because of the limitations imposed on measurements with high energy resolution at high photon counting rates, as well as geometrical restrictions, especially in a hospital environment. Semiconductor photon-counting detectors equipped with faster electronics enable utilization in somewhat high count rate situations at the expense of decreased energy resolution. This work presents a method for estimating mammography X-ray spectra from measurements acquired with a multi-energy bin photon counting detector. Simulation tests were conducted out for different incident X-ray spectra, energy thresholds and for multiple noise realizations assuming an ideal photon-counting X-ray detector with three energy thresholds. In addition, experimental measurements of mammography X-ray spectra were estimated by using a high-count rate Gallium Arsenide (GaAs) photon counting detector (with two or four energy thresholds) and a high resolution X-ray spectroscopy detector with Cadmium Telluride (CdTe) sensor for comparison. Differences between half-value layer values (HVL) obtained from the estimated spectra using the proposed method were found to be within 0.04 mm of the true values. This proposed method could be an attractive tool for laboratory research and calibration of photon-counting detectors.

Dose-Area Product-to-Effective Dose Conversion Coefficients for Pelvic Radiography Using a Monte Carlo Program.

OBJECTIVE. The purpose of this study was to determine dose-area product-to-effective dose (DAP/E) conversion coefficients for a five-view pelvic radiograph series. DAP/E conversion coefficients may be used for radiation dose optimization when designing institutional protocols for pelvic trauma evaluation. MATERIALS AND METHODS. We conducted a retrospective record review of 25 patients at a level 1 trauma center who had sustained pelvic fractures and required a five-view pelvic radiograph series during workup. E values given in International Commission on Radiological Protection Publication 103 were simulated with a PC-based Monte Carlo program in conjunction with anthropomorphic phantoms adjusted on the basis of patient height and weight. Inputs included tube voltage (in kV), tube filtration (in millimeters of aluminum), anode angle, x-ray beam collimation, geometric distances, and angle of projection for each radiograph in the series. An incident polychromatic x-ray spectrum was generated and matched to the corresponding DAP values of each radiograph, and regression analysis was performed for the DAP/E conversion coefficients. RESULTS.E was strongly correlated with DAP independent from body mass index, with a mean global DAP/E conversion coefficient of 0.0125 mSv/dGy · cm2 for all radiographs (R2 = 0.95). Mean DAP/E conversion coefficients were 0.0133, 0.0110, 0.0143, 0.0113, and 0.0101 mSv/dGy · cm2 for anteroposterior, inlet, outlet, Judet left, and Judet right views, respectively (all R2 ≥ 0.94). CONCLUSION. DAP/E conversion coefficients are provided for a five-view pelvic radiograph series to allow reliable estimation of E. Measurement of cumulative E may affirm protocol design changes for the management of pelvic trauma.

A Monte Carlo simulation study on effects of energy weighted dual-energy subtraction images with indirect photon-counting detector in mammography

The purpose of this study is to design an indirect photon-counting detector using a GAGG scintillator with Monte Carlo simulation and to evaluate the efficacy of the result using an energy weighted subtraction method. Factors to validate the efficacy of energy weighted subtraction were the type of scintillator and the profile of the x-ray spectrum. Images were obtained using GAGG and CsI scintillators with incident x-ray spectra split from continuous x-rays and were processed using energy weighted subtraction and energy subtraction. The composition of the phantom for image acquisition consisted of calcium embedded in the breast tissue, and the phantom images were analyzed by comparing the contrast-to-noise ratio (CNR). Based on the simulation results, GAGG demonstrates improved CNR compared to CsI, and the energy weighted subtraction method yields similar or higher CNR values compared to the energy subtraction method except in the case of using the CsI detector in a continuous x-ray spectrum. Moreover, the CNR values were higher for split x-rays than for continuous x-rays. In conclusion, GAGG detectors, the energy weighted subtraction method, and a split spectrum can be used to enhance the contrast of images while augmenting the fine calcification of simulated breast images.

Abstract ID: 189 Uncertainty estimation of MC simulated mammography spectra

Novel breast dosimetry applications require the knowledge of incident X-ray spectra, in mammography energy range [1] . Furthermore, the knowledge of the spectra transmitted through breast tissue is important in the phantom development context. A vast number of MC codes are used as models to generate diagnostic X-ray spectra; however, comparisons of those to experimentally measured X-ray spectra (validations) must be done to assure modelled spectra are representative of real X-ray beams. For these comparisons to be reliable, accurate uncertainty estimation of the counts in the spectra channels is required. This work’s objective was to accurately assess the uncertainties associated to the counts of incident and transmitted mammography MC modelled spectra. In this work, the PENELOPE MC code was used to simulate a CdTe spectrometer’s detection of mammography X-ray spectra, and the attenuation of these spectra after transmission through commercial breast phantoms. Four anode-filter combinations, three commercial breast phantoms and several attenuation thicknesses were used. Altogether, 108 spectra were simulated. The measured spectra emitted by each X-ray generators were used as the MC simulations input spectra, after escape and efficiency distortion effects were corrected by the stripping method. The uncertainty of the modelled spectra’s channels was estimated through MC uncertainty propagation method. For each condition, 25 simulations were performed, varying the input parameters (input spectra and phantom composition) according to their probability density functions. The simulated spectra validation was done through the assessment of the mean weighted squared residuals (MWSR). Except for model’s inaccuracies, good estimation of the uncertainties yields a MWSR equal to 1, which is this quantity’s expected value. The mean of the 108 calculated MWSRs yielded 3.76(03) when only Poisson’s uncertainty was used. Therefore, we conclude Poisson’s uncertainty underestimates the uncertainty of the spectra channels’ counts. The estimation we performed demonstrated better accuracy, it yielded a mean MWSR of 1.17(01).

Technical Note: A model-based sinogram correction for beam hardening artifact reduction in CT.

This study aims to propose a physics-based method of reducing beam-hardening artifacts induced by high-attenuation materials such as metal stents or other metallic implants. The proposed approach consists of deriving a sinogram inconsistency formula representing the energy dependence of the attenuation coefficient of high-attenuation materials. This inconsistency formula more accurately represents the inconsistencies of the sinogram than that of a previously reported formula (called the MAC-BC method). This is achieved by considering the properties of the high-attenuation materials, which include the materials' shapes and locations and their effects on the incident X-ray spectrum, including their attenuation coefficients. Numerical simulation and phantom experiment demonstrate that the modeling error of MAC-BC method are nearly completely removed by means of the proposed method. The proposed method reduces beam-hardening artifacts arising from high-attenuation materials by relaxing the assumptions of the MAC-BC method. In doing so, it outperforms the original MAC-BC method. Further research is required to address other potential sources of metal artifacts, such as photon starvation, scattering, and noise.

Precise material identification method based on a photon counting technique with correction of the beam hardening effect in X-ray spectra

The aim of our study is to develop a novel material identification method based on a photon counting technique, in which the incident and penetrating X-ray spectra are analyzed. Dividing a 40 kV X-ray spectra into two energy regions, the corresponding linear attenuation coefficients are derived. We can identify the materials precisely using the relationship between atomic number and linear attenuation coefficient through the correction of the beam hardening effect of the X-ray spectra.

Quasi-monochromatic imaging in x-ray CT via spectral deconvolution using photon-counting detectors

Photon-counting detectors can obtain the spectral information from an incident x-ray spectrum, although the detected counts may differ from the incident counts due to the detector response. If uncorrected or uncompensated, the response will lead to distortion in CT reconstruction. With the intention of reducing the distortion and exploring the potential of photon-counting detectors, a novel reconstruction strategy with spectral deconvolution, which attempts to set itself apart from traditional material decomposition frameworks, is proposed in this paper. It applies deconvolution to the energy window counts using a calibrated detector response and then uses the post-deconvolution photon counts to reconstruct images in multi-energy windows. The output has a quantitative meaning as a quasi-monochromatic attenuation coefficient, because a relatively narrow energy window width is selected. The deconvolution settings and results are carefully discussed in the numerical simulation. An experimental study is then carried out to verify the effectiveness and robustness. The results show that the reconstructed attenuation coefficients after deconvolution fit the standard reference data very well in most of the energy windows, which implies the feasibility of this quasi-monochromatic imaging method.

Effective X-ray beam size measurements of an X-ray tube and polycapillary X-ray lens system using a scanning X-ray fluorescence method

Size measurements of an X-ray beam produced by an integrated polycapillary X-ray lens (PXL) and X-ray tube system were performed by means of a scanning X-ray fluorescence (SXRF) method using three different metallic wires. The beam size was obtained by fitting the SXRF data with the analytical convolution between a Gaussian and a constant functions. For each chemical element in the wire an effective energy was calculated based on the incident X-ray spectrum and its photoelectric cross section. The proposed method can be used to measure the effective X-ray beam size in XRF microscopy studies.

Non-invasive investigation of the cross-sectional solids distribution in CFB risers by X-ray computed tomography

We apply X-ray computed tomography for non-invasive investigation of the distribution of solids over the cross-section of a pilot plant scale circulating fluidized bed riser and present a novel method to derive volumetric solids concentrations from tomographic raw data. In this, the elimination of low-energetic radiation from the incident X-ray spectrum plays an essential role to avoid detrimental beam hardening effects, which are caused by the pipe material and the fluidized solids. The presented technique is applied for quantitative analysis of the radial solids concentration distribution in a gas-solid circulating fluidized bed riser. The examined operating range covers mean solids concentrations between 2.7vol% and 37.2vol% whilst the superficial gas velocity is varied between 1.5ms−1 and 6.0ms−1. The high spatial resolution of the measured data permits evaluation of the influence of the overall solids holdup and the superficial gas velocity on the radial solids concentration profile. Comparison of the cross-section averaged solids concentrations obtained by X-ray computed tomography with those inferred from differential pressure measurements confirms the high accuracy of the applied method. Based on a comprehensive set of measurement data a model is developed that allows for the prediction of the radial solids concentration distribution in vertical gas-solid upflow.

A preliminary study of an image synthesis method to simulate the change in the incident X-ray spectrum by using thickness information

The objective of this research is to suggest an image synthesis method to simulate the change in the incident X-ray spectrum, based on knowledge of the tissue thickness. The image of a reference phantom was acquired before acquiring the image of an object under the same condition. With knowledge of the thickness of an input pixel, the mapping region could be limited to a specific thickness region in the reference phantom. Taking advantage of the relation between the reference phantom and the imaging object mapped to a limited region, we could synthesize an image acquired at a different tube current and tube voltage. The noise difference was compensated for by the addition of zero-mean Gaussian noise, and RMSE(root mean-square error) between the synthesized image and the actual image was calculated. The average noise difference between the synthesized image and the actual image was about 4.5% (ranging from 0% to 7.7%). RMSE between those images was calculated to be about 1.4% of the average pixel intensity. The calculated results indicate that the proposed method can provide a synthesized image close to the actual image.

Use of a priori spectral information in the measurement of x-ray flux with filtered diode arrays.

Filtered x-ray diode (XRD) arrays are often used to measure x-ray spectra vs. time from spectrally continuous x-ray sources such as hohlraums. A priori models of the incident x-ray spectrum enable a more accurate unfolding of the x-ray flux as compared to the standard technique of modifying a thermal Planckian with spectral peaks or dips at the response energy of each filtered XRD channel. A model x-ray spectrum consisting of a thermal Planckian, a Gaussian at higher energy, and (in some cases) a high energy background provides an excellent fit to XRD-array measurements of x-ray emission from laser heated hohlraums. If high-resolution measurements of part of the x-ray emission spectrum are available, that information can be included in the a priori model. In cases where the x-ray emission spectrum is not Planckian, candidate x-ray spectra can be allowed or excluded by fitting them to measured XRD voltages. Examples are presented from the filtered XRD arrays, named Dante, at the National Ignition Facility and the Laboratory for Laser Energetics.

Spectral response model for a multibin photon-counting spectral computed tomography detector and its applications.

Variations among detector channels in computed tomography can lead to ring artifacts in the reconstructed images and biased estimates in projection-based material decomposition. Typically, the ring artifacts are corrected by compensation methods based on flat fielding, where transmission measurements are required for a number of material-thickness combinations. Phantoms used in these methods can be rather complex and require an extensive number of transmission measurements. Moreover, material decomposition needs knowledge of the individual response of each detector channel to account for the detector inhomogeneities. For this purpose, we have developed a spectral response model that binwise predicts the response of a multibin photon-counting detector individually for each detector channel. The spectral response model is performed in two steps. The first step employs a forward model to predict the expected numbers of photon counts, taking into account parameters such as the incident x-ray spectrum, absorption efficiency, and energy response of the detector. The second step utilizes a limited number of transmission measurements with a set of flat slabs of two absorber materials to fine-tune the model predictions, resulting in a good correspondence with the physical measurements. To verify the response model, we apply the model in two cases. First, the model is used in combination with a compensation method which requires an extensive number of transmission measurements to determine the necessary parameters. Our spectral response model successfully replaces these measurements by simulations, saving a significant amount of measurement time. Second, the spectral response model is used as the basis of the maximum likelihood approach for projection-based material decomposition. The reconstructed basis images show a good separation between the calcium-like material and the contrast agents, iodine and gadolinium. The contrast agent concentrations are reconstructed with more than 94% accuracy.

TH-AB-204-10: Drastic Performance Improvement of a Polychromatic Cone- Beam X-Ray Fluorescence Computed Tomography (XFCT) System Using a Kilowatt-Range X-Ray Source

Purpose: To demonstrate drastic performance improvement of a polychromatic cone-beam x-ray fluorescence computed tomography (XFCT) system using a kilowatt-range x-ray source. Methods: An experimental polychromatic cone-beam XFCT system capable of imaging gold nanoparticles (GNPs) within small-animal-sized objects by stimulating K-shell XRF from gold and detection using a cadmium-telluride detector has been under development. Originally configured with a low-power (∼50 W) x-ray source (105 kVp, 0.45 mA, 0.9-mm tin filter), the system has now been retrofitted with a high-power (∼3 kW) source, operated at 125 kVp, 25 mA. The higher flux allowed the use of a 2-mm tin filter to harden the incident x-ray spectrum, thereby optimizing XRF production. The material detection limit was determined at these settings using GNP/water-containing calibration phantoms (0, 0.1, 0.5, 1, 2 wt. % of GNPs). Subsequently, a 3-cm-diameter PMMA phantom with 0.2, 0.5, and 1 wt. % GNP/water inserts was imaged. The image reconstruction algorithm was modified with Compton-scatter-based background fitting for accurate XRF signal extraction and empirical attenuation correction. Results: Under the optimized conditions, the detection limit was remarkably lower (factor of 2.5), compared to the original configuration, allowing imaging of GNP concentrations as low as 0.2 wt. %. The reconstructed image demonstrated accurate localization of all inserts. Scan time was drastically decreased (factor of 4), enabling imaging of the phantom in 1.5 hours with just one detector translated 11 times. By reverting to the 0.9-mm filter, as used previously, scan time could be lowered to 30 minutes, at the expense of sensitivity. Implementation of additional/array detectors for parallel data acquisition will lower scan time further. Conclusion: By adopting a high-power source for the XFCT system, dramatic improvements were made, while meeting realistic constraints. This work shows the practicality of performing XFCT for routine preclinical ex-/in-vivo imaging using GNPs.

CONSTRAINING THE X-RAY AND COSMIC-RAY IONIZATION CHEMISTRY OF THE TW Hya PROTOPLANETARY DISK: EVIDENCE FOR A SUB-INTERSTELLAR COSMIC-RAY RATE

We present an observational and theoretical study of the primary ionizing agents (cosmic rays and X-rays) in the TW Hya protoplanetary disk. We use a set of resolved and unresolved observations of molecular ions and other molecular species, encompassing eleven lines total, in concert with a grid of disk chemistry models. The molecular ion constraints comprise new data from the Submillimeter Array on HCO$^+$, acquired at unprecedented spatial resolution, and data from the literature, including ALMA observations of N$_2$H$^+$. We vary the model incident CR flux and stellar X-ray spectra and find that TW Hya's HCO$^+$ and N$_2$H$^+$ emission are best fit by a moderately hard X-ray spectra, as would be expected during the "flaring" state of the star, and a low CR ionization rate, $\zeta_{\rm CR}\lesssim10^{-19}$ s$^{-1}$. This low CR rate is the first indication of the presence of CR exclusion by winds and/or magnetic fields in an actively accreting T Tauri disk system. With this new constraint, our best fit ionization structure predicts a low turbulence "dead-zone" extending from the inner edge of the disk out to $50-65$ AU. This region coincides with an observed concentration of millimeter grains, and we propose that the inner region of TW Hya is a dust (and possibly planet) growth factory as predicted by previous theoretical work.

Improving x‐ray fluorescence signal for benchtop polychromatic cone‐beam x‐ray fluorescence computed tomography by incident x‐ray spectrum optimization: A Monte Carlo study

To develop an accurate and comprehensive Monte Carlo (MC) model of an experimental benchtop polychromatic cone-beam x-ray fluorescence computed tomography (XFCT) setup and apply this MC model to optimize incident x-ray spectrum for improving production/detection of x-ray fluorescence photons from gold nanoparticles (GNPs). A detailed MC model, based on an experimental XFCT system, was created using the Monte Carlo N-Particle (MCNP) transport code. The model was validated by comparing MC results including x-ray fluorescence (XRF) and scatter photon spectra with measured data obtained under identical conditions using 105 kVp cone-beam x-rays filtered by either 1 mm of lead (Pb) or 0.9 mm of tin (Sn). After validation, the model was used to investigate the effects of additional filtration of the incident beam with Pb and Sn. Supplementary incident x-ray spectra, representing heavier filtration (Pb: 2 and 3 mm; Sn: 1, 2, and 3 mm) were computationally generated and used with the model to obtain XRF/scatter spectra. Quasimonochromatic incident x-ray spectra (81, 85, 90, 95, and 100 keV with 10 keV full width at half maximum) were also investigated to determine the ideal energy for distinguishing gold XRF signal from the scatter background. Fluorescence signal-to-dose ratio (FSDR) and fluorescence-normalized scan time (FNST) were used as metrics to assess results. Calculated XRF/scatter spectra for 1-mm Pb and 0.9-mm Sn filters matched (r ≥ 0.996) experimental measurements. Calculated spectra representing additional filtration for both filter materials showed that the spectral hardening improved the FSDR at the expense of requiring a much longer FNST. In general, using Sn instead of Pb, at a given filter thickness, allowed an increase of up to 20% in FSDR, more prominent gold XRF peaks, and up to an order of magnitude decrease in FNST. Simulations using quasimonochromatic spectra suggested that increasing source x-ray energy, in the investigated range of 81-100 keV, increased the FSDR up to a factor of 20, compared to 1 mm Pb, and further facilitated separation of gold XRF peaks from the scatter background. A detailed MC model of an experimental benchtop XFCT system has been developed and validated. In exemplary calculations to illustrate the usefulness of this model, it was shown that potential use of quasimonochromatic spectra or judicious choice of filter material/thickness to tailor the spectrum of a polychromatic x-ray source can significantly improve the performance of benchtop XFCT, while considering trade-offs between FSDR and FNST. As demonstrated, the current MC model is a reliable and powerful computational tool that can greatly expedite the further development of a benchtop XFCT system for routine preclinical molecular imaging with GNPs and other metal probes.

Studying the energy dependence of intrinsic conversion efficiency of single crystal scintillators under X-ray excitation

Single crystal scintilators are used in various radiation detectors applications. The efficiency of the crystal can be determined by the Detector Optical Gain (DOG) defined as the ratio of the emitted optical photon flux over the incident radiation photons flux. A parameter affecting DOG is the intrinsic conversion efficiency (nC) giving the percentage of the X-ray photon power converted to optical photon power. nC is considered a constant value for X-ray energies in the order of keV although a non-proportional behavior has been reported. In this work an analytical model, has been utilized to single crystals scintillators GSO:Ce, LSO:Ce and LYSO:Ce to examine whether the intrinsic conversion efficiency shows non proportional behavior under X-ray excitation. DOG was theoretically calculated as a function of the incident X-ray spectrum, the X-ray absorption efficiency, the energy of the produced optical photons and the light transmission efficiency. The theoretical DOG values were compared with experimental data obtained by irradiating the crystals with X-rays at tube voltages from 50 to 140 kV and by measuring the light energy flux emitted from the irradiated screen. An initial value for nC (calculated from literature data) was assumed for the X-ray tube voltage of 50 kV. For higher X-ray tube voltages the optical photon propagation phenomena was assumed constant and any deviations between experimental and theoretical data were associated with changes in the intrinsic conversion efficiency. The experimental errors were below 7% for each experimental setup. The behavior of nC values for LSO:Ce and LYSO:Ce were found very similar, i.e., ranging with values from 0.089 at 50 kV to 0.015 at 140 kV, while for GSO:Ce, nC demonstrated a peak at 80 kV.

Optimizing contrast agents with respect to reducing beam hardening in nonmedical X-ray computed tomography experiments

Iodine is commonly used as a contrast agent in nonmedical science and engineering, for example, to visualize Darcy flow in porous geological media using X-ray computed tomography (CT). Undesirable beam hardening artifacts occur when a polychromatic X-ray source is used, which makes the quantitative analysis of CT images difficult. To optimize the chemistry of a contrast agent in terms of the beam hardening reduction, we performed computer simulations and generated synthetic CT images of a homogeneous cylindrical sand-pack (diameter, 28 or 56 mm; porosity, 39 vol.% saturated with aqueous suspensions of heavy elements assuming the use of a polychromatic medical CT scanner. The degree of cupping derived from the beam hardening was assessed using the reconstructed CT images to find the chemistry of the suspension that induced the least cupping. The results showed that (i) the degree of cupping depended on the position of the K absorption edge of the heavy element relative to peak of the polychromatic incident X-ray spectrum, (ii) (53)I was not an ideal contrast agent because it causes marked cupping, and (iii) a single element much heavier than (53)I ((64)Gd to (79)Au) reduced the cupping artifact significantly, and a four-heavy-element mixture of elements from (64)Gd to (79)Au reduced the artifact most significantly.