X-ray Conversion Research Articles

Analogous Lubberts effect in photon counting detectors.

For a light burst generated by an x ray-scintillator interaction, the quasi-isotropic spreading of optical quanta determines that both the light collection efficiency and the spread function of the light burst depend on the depth (z) of interaction: the closer the initial position of light burst to the light sensor, the less light loss and spatial resolution degradation a detector system encounters. For scintillator-based indirect-conversion detectors, the variation of the x-ray conversion gain and the MTF along the z direction contributes to a significant fraction of DQE loss and is known as the Lubberts effect. Understanding the physics behinds the effect and developing solutions to reduce the effect have led to tremendous improvements in the performance of indirect-conversion detectors. In contrast, for direct-conversion photon counting detectors (PCDs), since no optical quanta generation or transport is involved, there is no classical Lubberts effect. However, the transport properties of the generated charge carriers in semiconductors do depend on their travel distance. As a result, the performance of direct conversion detectors may depend on z. This dependence can be considered as a counterpart of the classical Lubberts effect. The purpose of this work is to analyze the contributions of a variety of physical mechanisms that may contribute to a Lubberts-like effect in direct-conversion PCDs. The results show that the Lubberts-like effect is primarily caused by the depth-dependent charge trapping and recombination likelihood. Based upon the physics analysis, PCD electrode design can be optimized to minimize the Lubberts-like effect and the associate DQE loss.

The experimental investigation of the hohlraum energetics of two-entrance holes spherical hohlraum at the 100 kJ level laser facility

The two-laser entrance hole (LEHs) spherical hohlraum energetic experiments with all 48 laser beams and two laser pulse shapes at the 100 kJ level laser facility were investigated. In this work, the time-resolved radiation temperature measured by multi-angle x-ray diodes agreed well with LARED simulations, and the peak radiation temperature was up to 260 eV with the laser power of 45 TW. Meanwhile, the backscattered laser energy fraction was less than 5% in the majority of shots, which proposed a low level of laser–plasma interaction (LPI) effect in the spherical hohlraum. However, in the shaped pulse shots with capsules, the stimulated Raman scattering (SRS) of smaller incident angle lasers was significantly increased to 15%. The measured SRS spectrum and LARED simulations showed that the increase in the LPI effects caused by the ablated CH plasma was around 0.1Nc (Nc is the critical density). In summary, according to the experimental results, the x-ray conversion efficiency of the vacuum spherical hohlraum was 85%–88% in 3 ns square pulses and 89%–93% in 3.6 ns shaped pulses. It was closer to that of the two-LEH cylindrical hohlraum at the Shen-Guang and NIF facilities.

Simulations of X-ray conversion into primary electrons in GEM-based detector

The paper presents the results of simulations of the interaction of X-rays of different discrete energies (1–25 keV) with the structure of the GEM chamber. The simulations were performed using Geant4 software. This program allowed taking into account phenomena such as the emission of photoelectrons, Auger electrons and fluorescence X-rays. As a result, the energy distributions of primary electrons formed in the absorption region of the detector (drift gap) were obtained. The distributions of primary electrons, which are the source of electron avalanches in the detector under study, are directly responsible for the distribution of pulses on the detector reading anode. Some measurements were also performed, confirming the compatibility of simulated spectra and experimental ones. The main conclusion from the simulation is the need to eliminate copper from the detector structure, especially from GEM foils and from the reading electrode, if there is an intension to record X-rays with energies above 9 keV . An observable parasitic contribution to the spectrum recorded by the detector is also given by several other materials.

An investigation of the L-shell x-ray conversion efficiency for laser-irradiated tin foils

We have used the Shenguang II laser in third harmonic (351 nm) to investigate the emission of L-shell radiation in the 3.3–4.4 keV range generated using thin foils of Sn coated onto a parylene substrate with irradiation of order 1015 W cm−2 and nanosecond pulse duration. In our experiment, we have concentrated on assessing the emission on the non-laser irradiated side as this allows an experimental geometry relevant to experiments on photo-ionised plasmas where a secondary target must be placed close to the source, to achieve x-ray fluxes appropriate to astrophysical objects. Overall L-shell conversion efficiencies are estimated to be of order 1%, with little dependence on Sn thickness between 400 and 800 nm.

Luximos: A 768×64 900-fps Tileable Pipelined X-Ray CMOS Image Sensor for Dental Imaging With 2.6 LSB/nGy Sensitivity

In this letter, a stitched array of $768\times 64$ pixels for X-ray dental imaging is presented. The sensor architecture allows to increase the frame rate up to 900 fps thanks to a pipeline approach that minimizes the deadtime between consecutive frames. A column-based ADC converts pixel values in digital form while an external FPGA performs the operation of time-delay integration accumulation, thus increasing the signal-to-noise ratio. Internal programmable gain allows to achieve a maximum sensitivity of 2.6 LSB/nGy and, at the same time, to extend the dynamic range of the sensor in case of high intensity. The final system consists of two sensors each one having a size of $74.5\times 9$ mm, assembled in a PCB and coupled with a high sensitive columnar thallium-doped cesium iodide (CsI(Tl)) scintillator for X-ray conversion. The sensor, fabricated in a standard CMOS 0.15- $\mu \text{m}$ technology with a stitching approach, shows a pixel size of 97 $\mu \text{m}$ .

Bremsstrahlung conversion efficiency and gamma - neutron generation from polypropylene, aluminum, iron, and lead bombarded by 10 MeV electrons

An electron beam from the UELR-10-15S2 accelerator (average energy of 9.92 ? 0.48 MeV) was applied to irradiate food and medical items at the Research and Development Center for Radiation Technology, Vietnam Atomic Energy Institute, Vietnam. The materials are under an electron beam window, such as irradiation products, conveyor, magnet and shielding material for the magnet coil, bombarded by electrons and generated X-ray (bremsstrahlung effect). In this article, X-ray conversion efficiency from polypropylene, aluminum, iron, and lead bombarded by an electron beam from the UELR-10-15S2 accelerator is measured by the film dosimeter and simulated by the MCNP4c2 code, and there is good agreement between the calculation and measurement results. The results show that X-ray conversion efficiency is the highest from lead (4.3 %), so the gamma - neutron reaction (Q-value of -6.74 MeV for 207Pb) has to be studied in food and medical items irradiated by a 10 MeV eelectron beam.

ION-INDUCED STRUCTURAL-PHASE TRANSFORMATIONS IN AUSTENITE STAINLESS STEEL 12Cr18ni10Ti, AISI 316 AND AISI 304

In this paper the ion-induced structural-phase transformations in austenitic stainless steels 12Cr18Ni10Ti, AISI 316 and AISI 304 have been studied by the Mössbauer spectroscopy of conversion electrons, X-ray diffraction and scanning microscopy. It has been established, that the electronic structure of the Mössbauer atoms in stainless steels AISI 304 and AISI 316 are different. Consequently, molybdenum does not affect the short-range order of 57Fe in AISI 316 steel. Implantation of its own atoms (57Fe) in AISI 316 steel leads, as in case with AISI 304 and 12Cr18Ni10Ti steel, to martensitic transformations. Among three steels AISI 316 was found to be the most stable to the ion-induced structural phase transformations.

Rubidium Doping to Enhance Carrier Transport in CsPbBr3 Single Crystals for High-Performance X-Ray Detection.

The direct band gap CsPbBr3 perovskite is regarded as a promising alternative for low-cost and high-performance X-ray radiation detectors. Despite the fact that CsPbBr3 nanocrystals have been shown to be good scintillators in the indirect conversion mode, the direct X-ray conversion with CsPbBr3 single crystals is expected to yield higher spatial resolution. Here, rubidium (Rb) doping is demonstrated to be an efficient approach to improve carrier transport and X-ray detection performance in the direct-conversion X-ray detectors based on Cs(1-x)RbxPbBr3 single crystals. Electrical properties' characterizations as combined with X-ray photoelectron spectroscopy (XPS) measurements have revealed that Rb doping in Cs(1-x)RbxPbBr3 single crystals can enhance the atomic interaction and orbital coupling between Pb and Br atoms, leading to an enhancement of carrier transport and X-ray detection performance. X-ray detectors based on a small amount (0.037%) of Rb-doped Cs(1-x)RbxPbBr3 single crystals exhibited a high X-ray sensitivity of 8097 μC Gyair-1 cm-2. This work offers a feasible strategy to improve the X-ray detection performance by chemical doping in all-inorganic perovskite X-ray detectors.

Synthesis and characterization of Gd2O2 S: Tb3+ phosphor powder for X-ray imaging detectors

Gadolinium oxysulfide phosphor doped with trivalent terbium have been synthesized using urea homogenous precipitation and followed by sulfurization at 800 °C under argon atmosphere. Structural and morphological of synthesized phosphor powder were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectrometry (FT-IR). Hexagonal structure of Gd2O2S:Tb3+ phosphor was confirmed by XRD. Compositional analysis were carried out by energy dispersive x-ray (EDX) and particle induced x-ray emission (PIXE). Photoluminescence emission spectra was measured by fluorescent spectrometer. A sedimentation technique is used to deposit the phosphor powder directly on the glass substrate using poly vinyl alcohol as a paste. A number of phosphor layers have been synthesized with the layer thickness ranging from 150 to 268 µm. Measurement results of x-ray conversion efficiency for layers were investigated using 300 kVp X-ray tube in which the maximum light output and contrast were observed for layer with a thickness of 193 µm. Oxysulfide phosphor layer was analyzed by ion beam induced luminescence (IBIL). Emitting of green light from phosphor layer confirm its luminescence property.

Sensitivity of X-ray image detectors: Space charge effects

A model for calculating the sensitivity of direct conversion X-ray image detectors is developed incorporating the space charge effect. The space charge perturbs the applied electric field across the thickness of the photoconductor, and thus the electric field becomes nonuniform. First, Poisson's equation is solved to obtain the electric field distribution. Then, the continuity equations for both holes and electrons are solved considering the perturbed electric field. Finally, the collected charge is determined from the charge carrier distributions and electric field profile using the Shockley–Ramo theorem. The developed X-ray sensitivity model is applied to polycrystalline mercuric iodide based detectors. The electric field collapses for a critical amount of trapped charge if the ratio of trapped electrons to holes is high (∼0.5). The pinch in electric field slows down the drift of free electrons and hence reduces the sensitivity of the detector. The developed model is fitted with the published experimental data, and the fitted values of effective mobility-lifetime products of charge carriers agree well with the published results. It is found that the space charge reduces the sensitivity in the screen-printed sample more significantly than in the vacuum deposited sample.

Investigation of high X-ray conversion efficiency Kr filled gas sources at the National Ignition Facility

We report on the performance of high x-ray fluence Kr K-shell sources that are being developed for high energy density experiments. These targets are 4.1 mm in diameter 4.4 mm tall hollow epoxy tubes having a 40 μm thick wall holding 1.5 atm of Kr gas. For these shots, the National Ignition Facility laser delivered a nominally constant total energy of ≈750 kJ of 351 nm (3ω) light at the three power levels [e.g., ≈120 (low), ≈145 (medium), and ≈210 TW (high)]. The Kr K-shell (Ephoton = 8–20 keV) x-ray radiant intensity and radiant energy (kJ/sr) of these sources were found to increase as a function of laser power but began to plateau at the highest laser power. The Kr K-shell radiant energy increased from ≈1 kJ/sr at ≈120 TW to ≈2 kJ/sr at ≈210 TW. Radiation hydrodynamics simulations predict radiant energies to be always higher than these measurements. The increase in K-shell emission is attributed to its strong dependence on the electron temperature. Electron temperature distributions were inferred from measured Heα and Lyα line emission through the use of a genetic algorithm and Scram modeling. The inferred temperatures from the experiment are 20% to 30% higher than those predicted from modeling.

The depth-dose curve depends on the inhomogeneity of the material irradiated by electron beam

Electron linear accelerator with average energy 9.92 ± 0.48 MeV, UELR-10-15S2, was set up and operated at the Research and Development Center for Radiation Technology, Vietnam Atomic Energy Institute, for irradiating foods and medical devices directly without X-ray converter. Inside the homogenous products, the depth-dose curve depends on electron beam energy and product density, and moreover, it also depends on the inhomogeneity of the irradiated material. In this article, the depth-dose distribution is calculated by MCNP simulation code and measured by a film dosimeter inside the inhomogeneous products. The results show that the maximum deviation of the depth-dose curve between inhomogeneous and homogeneous products with the same density is about 20%. So the area density limit for irradiating double sided on the electron beam (9.92 ± 0.48 MeV) is in the range from 6.1 to 9.7 g/cm2 instead of 8.5 g/cm2 in general.

Mechanisms of luminescence decay in YAG-Ce,Mg fibers excited by γ- and X-rays

Time-resolved luminescence of YAG-Ce (150 ppm) fiber crystal with Mg co-doping was studied under pulsed X-ray excitation and γ-rays (Cs 662 keV). The initial part of decay kinetics under X-ray excitation is faster than for direct cerium excitation (63 ns). Decay kinetics is also characterized by the presence of slow components with at least two characteristic times longer than Ce3+ radiation time. The slowest one which dominates for t > 500 ns in YAG-Ce without Mg co-doping practically vanishes for samples with 50 ppm co-doping. Decay kinetics under γ-rays are characterized by slower rise time than that under X-rays. These properties can be explained by competition of energy transfer and energy losses in track regions. The distribution of excitations in tracks produced by X-rays differs from that in tracks produced by γ-rays, since the energy of primary electron after γ-quantum conversion is much higher than after X-ray absorption. The stopping power for energetic electrons decreases with increase of electron energy, and therefore the density distribution after X-ray conversion is shifted to higher densities, Therefore, the acceleration of recombination and quenching of excitations is more prominent under X-ray excitation. Specific role of Ce4+ induced by Mg co-doping is also discussed in the paper.

Simulation and design of folded perovskite x-ray detectors

A variety of medical, industrial, and scientific applications requires highly sensitive and cost-effective x-ray detectors for photon energies ranging from keV to MeV. Adapting the thickness of polycrystalline or single crystal conversion layers especially to high-energy applications increases the complexity of fabrication and potentially decreases the performance of conventional direct conversion x-ray detectors. To tackle the challenges with respect to the active layer thickness and to combine the superior performance of single crystal materials with the low-cost nature of polycrystalline conversion layers, we investigate thin film x-ray detector technologies based on a folded device architecture. Analytical models simulating the sensitivity and the detective quantum efficiency (DQE) are used to evaluate the performance of folded detectors based on polycrystalline organic-inorganic perovskite semiconductors in various layout configurations and for different photon energies. Simulations of folded perovskite devices show high sensitivities. The DQE analysis introduces additional noise related boundary conditions for the folding length. A comparison with conventional detectors based on state of the art conversion materials at different photon energies demonstrates the potential of the folded detector layout as simulated sensitivities are comparable to single crystal detectors.

The REX irradiation facility and its applications

Abstract The REX system is based on a linear electron accelerator and a removable head containing an electron to X-ray converter with its collimator. The machine and the irradiation chamber were manufactured and commissioned at ENEA Frascati Research Center. The activity of the Laboratory for Development of Particle Accelerators and Medical Applications is dedicated to the development of particle accelerators (electrons and protons) for medical, industrial and research applications, realized as sections of complex equipment or as stand-alone machine and offers related services to the scientific community. The REX facility is a versatile irradiation machine thought to be used for experimental campaigns in different research fields thanks to its customizable beam. Some of the cases we performed are related to irradiation of biological systems, development of in-beam monitoring devices, test of beam monitors, material radiation damage studies and cultural heritage. Experiments are carried out for specimen treatment with the two-radiation quality: electrons and X-rays. An overview of the laboratory and of the activities performed with the REX facility in its present status are given in this work and specific attention is devoted to the application on the cultural heritage for the preservation and conservation of artefacts.

Study of the X-Ray Radiation in Metal-Containing Composites Irradiated with Neutrons and Photons

Secondary X-ray radiation is studied upon the neutron and photon irradiation of composites. The characteristic X-ray radiation and conversion electrons arising from the capture of slowing neutrons of the DT reaction by liquid-crystal DNA–Gd structures are studied for their effects on biological cells. The liquid-crystal structure in the form of microparticles with a volume of 0.1 µm3 contains 10 000 Gd atoms per DNA molecule. During the neutron irradiation of biological samples with a concentration of 1000 DNA-Gd particles per cell, cells are damaged by secondary radiation at a thermal neutron fluence of ~1011 n/cm2. A high-purity germanium (HPGe) spectrometer is used to study the radiation-protective properties and characteristic X-ray radiation of a Pb-containing composite upon irradiation with photons from cobalt-57 with an activity of 100 kBq.

Non-stationary model of oblique x-ray incidence in amorphous selenium detectors: I. Point spread function.

In previous work, a theoretical model of the point spread function (PSF) for oblique x-ray incidence in amorphous selenium (a-Se) detectors was proposed. The purpose of this paper is to develop a complementary model that includes two additional features. First, the incidence angle and the directionality of ray incidence are calculated at each position, assuming a divergent x-ray beam geometry. This approach allows the non-stationarity of the PSF to be modeled. Second, this paper develops a framework that is applicable to a digital system, unlike previous work which did not model the presence of a thin-film transistor (TFT) array. At each point on the detector, the incidence angle and the ray incidence direction are determined using ray tracing. Based on these calculations, an existing model for the PSF of the x-ray converter (Med Phys. 1995;22:365-374) is generalized to a non-stationary model. The PSF is convolved with the product of two rectangle functions, which model the sampling of the TFT array. The rectangle functions match the detector element (del) size in two dimensions. It is shown that the PSF can be calculated in closed form. This solution is used to simulate a digital mammography (DM) system at two x-ray energies (20 and 40keV). Based on the divergence of the x-ray beam, the direction of ray incidence varies with position. Along this direction, the PSF is broader than the reference rect function matching the del size. The broadening is more pronounced with increasing obliquity. At high energy, the PSF deviates more strongly from the reference rect function, indicating that there is more blurring. In addition, the PSF is calculated along the polar angle perpendicular to the ray incidence direction. For this polar angle, the shape of the PSF is dependent upon whether the ray incidence direction is parallel with the sides of the detector. If the ray incidence direction is parallel with either dimension, the PSF is a perfect rectangle function, matching the del size. However, if the ray incidence direction is at an oblique angle relative to the sides of the detector, the PSF is not rectangular. These results illustrate the non-stationarity of the PSF. This paper demonstrates that an existing model of the PSF of a-Se detectors can be generalized to include the effects of non-stationarity and digitization. The PSF is determined in closed form. This solution offers the advantage of shorter computation time relative to approaches that use numerical methods. This model is a tool for simulating a-Se detectors in future work, such as in virtual clinical trials with computational phantoms.

Technical challenges in generalizing calibration techniques for breast density measurements.

PurposeWe are developing a calibration methodology for full‐field digital mammography (FFDM). Calibration compensates for image acquisition technique influences on the pixel representation, ideally producing improved inter‐image breast density estimates. This approach relies on establishing references with rigid breast tissue‐equivalent phantoms (BTEs) and requires an accurate estimate of the compressed breast thickness because the system readout is nominal. There is also an attenuation mismatch between adipose breast tissue and the adipose BTE that was noted in our previous work. It is referred to as the “attenuation anomaly” and addressed in this report. The objectives are to evaluate methods to correct for the compressed breast thickness and compensate for the attenuation anomaly.MethodsThickness correction surfaces were established with a deformable phantom (DP) using both image and physical measurements for three direct x‐ray conversion FFDM units. The Cumulative Sum serial quality control procedure was established to ensure the thickness correction measurements were stable over time by imaging and calibrating DPs biweekly in lieu of physical measurements. The attenuation anomaly was addressed by evaluating adipose image regions coupled with an optimization technique to adjust the adipose calibration data. We compared calibration consistency across matched left and right cranial caudal (CC) mammographic views (n = 199) with and without corrections using Bland–Altman plots. These plots were complemented by comparing the right and left breast calibrated average (μa) and population distribution mean (ma) with 95% confidence intervals and difference distribution variances with the F‐test for uncorrected and corrected data.ResultsThickness correction surfaces were well approximated as tilted planes and were dependent upon compression force. A correction was developed for the attenuation anomaly. All paddles (large and small paddles for all units) exhibited similar tilt as a function of force. Without correction, ma = 0.92 (−1.77, 3.62) was not significantly different from zero with many negative μa samples. The thickness correction produced a significant shift in the μa distribution in the positive direction with ma = 13.99 (11.17, 16.80) and reduced the difference distribution variance significantly (P < 0.0001). Applying both corrections in tandem gave ma = 22.83 (20.32, 25.34), representing another significant positive shift in comparison with the thickness correction in isolation. Thickness corrections were stable over approximately a 2‐year timeframe for all units.ConclusionThese correction techniques are valid approaches for addressing technical problems with calibration that relies on reference phantoms. The efficacy of the calibration methodology will require validation with clinical endpoints in future studies.

Cascaded systems analysis of a-Se/a-Si and a-InGaZnO TFT passive and active pixel sensors for tomosynthesis

Medical imaging systems like full field digital mammography (FFDM) and digital breast tomosynthesis (DBT) commonly use amorphous selenium (a-Se) based passive pixel sensor (PPS) direct conversion x-ray detectors. On one hand, direct conversion detectors inherently offer better resolution characteristics in terms of a higher modulation transfer function (MTF), in comparison to the indirect CsI:Tl PPS x-ray imager. On the other hand, especially at lower doses, this superior performance of the direct imager is seldom retained in its detective quantum efficiency (DQE) curves.It is well known that a-Se PPS x-ray imagers suffer from high additive electronic noise originating from the from the amorphous silicon (a-Si) thin film transistor (TFT) array that is being used in the current back-plane technology. This degrades the noise power spectrum (NPS) and subsequently the overall DQE. To address this deficiency, we propose to replace the PPS back-plane by active pixel sensor (APS) back-plane technology, which has the potential to reduce the back-plane electronic noise by amplifying the input signal, especially at low doses. The proposed APS is based on amorphous In–Ga–Zn–O (a-IGZO) TFT technology, which can offer high mobility (5–20 cm2 V−1 s−1), low leakage current (<10−13 A) and low flicker noise (Hooge’s parameter αH ~ 1.5 10−3), leading to better imager noise performance.To test our hypothesis, we used linear cascaded systems analysis to model the imaging performance (MTF, NPS and DQE) of the PPS and APS a-Se direct imagers. This model was first validated using experimentally measured data obtained for a 85 µm pixel pitch a-Se/a-Si TFT PPS imager. Using this model, we analyzed the noise performance of the direct a-Se and indirect CsI:Tl x-ray a-IGZO APS imagers at different dose and electronic noise levels. Obtained results clearly showed that lowering back-plane electronic noise can significantly improve the performance of the a-Se/a-IGZO TFT APS imager. Our simulated results showed that a higher DQE at lower radiation doses (maximum DQE of 0.6 can be achieved at an exposure level of 1 µGy) can be achieved with the a-Se detector, thereby making this combination a promising candidate for low dose applications like DBT.

Generalized breast density metrics

Mammograms represent data that can inform future risk of breast cancer. Data from two case-control study populations were analyzed. Population 1 included women (N = 180 age matched case-control pairs) with mammograms acquired with one indirect x-ray conversion mammography unit. Population 2 included women (N = 319 age matched case-control pairs) with mammograms acquired from 6 direct x-ray conversion units. The Fourier domain was decomposed into n concentric rings (radial spatial frequency bands). The power in each ring was summarized giving a set of measures. We investigated images in raw, for presentation (processed) and calibrated representations and made comparison with the percentage of breast density (BD) determined with the operator assisted Cumulus method. Breast cancer associations were evaluated with conditional logistic regression, adjusted for body mass index and ethnicity. Odds ratios (ORs), per standard deviation increase derived from the respective breast density distributions and 95% confidence intervals (CIs) were estimated.A measure from a lower radial frequency ring, corresponding 0.083–0.166 cycles mm−1 and BD had significant associations with risk in both populations. In Population 1, the Fourier measure produced significant associations in each representation: OR = 1.76 (1.33, 2.32) for raw; OR = 1.43 (1.09, 1.87) for processed; and OR = 1.68 (1.26, 2.25) for calibrated. BD also provided significant associations in Population 1: OR = 1.72 (1.27, 2.33). In Population 2, the Fourier measure produced significant associations for each representation as well: OR = 1.47 (1.19, 1.80) for raw; OR = 1.38 (1.15, 1.67) for processed; and OR = 1.42 (1.15, 1.75) for calibrated. BD provided significant associations in Population 2: OR = 1.43 (1.17, 1.76). Other coincident spectral regions were also predictive of case-control status.In sum, generalized breast density measures were significantly associated with breast cancer in both FFDM technologies.