High Spatial Resolution X-ray Imaging Research Articles

High-resolution X-ray Imaging Based on Pixel-structured CsI:Tl Scintillating Screens for Indirect X-ray Image Sensors

We introduce the development of pixel-structured screens with a thallium-doped CsI (CsI:Tl) scintillator for indirect digital X-ray imaging sensors. The indirect-conversion detection method based on the pixel-structured CsI:Tl scintillator provides high spatial resolution X-ray imaging without sacrificing the light spread in thick scintillation layers. The scintillation screens were fabricated by using a vacuum deposition process and filling the CsI:Tl scintillating powders into a two-dimensional pixel-structured silicon array. Pixel structures with 100 μm and 50 μm pixel sizes with wall widths of 20 μm and 200 μm thickness were prepared and the fabricated CsI:Tl scintillating powders were filled into the trench of the pixel structure through a vacuum process. The final scintillation screens with 2.5 cm × 2.5 cm size were prepared and directly coupled to a CCD image sensor with an optical lens for performance evaluation of X-ray imaging. The imaging performance of the samples was investigated in terms of the relative light intensity, the X-ray linearity and the spatial resolution under practical X-ray exposure conditions. These preliminary results imply that pixel-structured CsI:Tl scintillating screens show high spatial resolution by less lateral spread of the emitted visible photons within pixel-structured silicon arrays. However, these X-ray detectors still require improved X-ray sensitivity by coating the reflective layer onto an inner silicon wall surface and filling the scintillating power into pixel structures completely.

Novel nanocrystalline Gd2O3(Eu) scintillator screens with a micro-pixel structure for high spatial resolution X-ray imaging

We developed a novel pixel-structured scintillation screen with nanocrystalline Gd2O3:Eu particle sizes for high spatial resolution X-ray imaging detectors. Nanocrystalline Gd2O3:Eu scintillators were successfully synthesized with a hydrothermal method and a subsequent calcination treatment, which were used as a material for converting incident X-rays into visible light. In this work, silicon-based pixel structures with different 100, 50 and 30 μm pixel sizes, a 10 μm wall width and a 120 μm thickness were prepared with the standard photolithography and the deep reactive ion etching (DRIE) process. Subsequently, a micro-pixel-structured scintillation screen was fabricated by adding the synthesized nanocrystalline Gd2O3:Eu scintillating phosphor to pixel-structured silicon arrays. Additionally, X-ray imaging performance such as relative light intensity, X-ray to light response and the spatial resolution in terms of modulation transfer function (MTF) were measured by using an X-ray source and a lens-coupled charge coupled device (CCD) camera system. The light intensity of the pixel-structured nanocrystalline Gd2O3:Eu screen was much higher than that of a pixel-structured sample made with a commercial microcrystalline Gd2O3:Eu product due to the density of the nanocrystalline Gd2O3:Eu scintillating powder-filled silicon structure. As the pixel size of the pixel-structured silicon decreased, the light intensity decreased. However, as the pixel size decreased, the spatial resolution significantly improved with no evident crosstalk from the emitted optical photons between adjacent scintillating pixels. The MTF of pixel-structured nanocrystalline Gd2O3:Eu screens with a 100 and a 50 μm pixel size was 20% and 30% at 6 lp/mm, respectively. As a result, this new technology showed that a microchannel structure based on a nanocrystalline Gd2O3:Eu scintillator could provide higher light intensity and high spatial resolution imaging compared to conventional microcrystalline scintillating phosphor.

F and F-aggregates colour centres in lithium fluoride for high spatial resolution x-ray imaging

Stable formation of primary F and aggregate F2 and F3+ colour centres in LiF crystals irradiated with a X-ray table-top source was investigated by optical absorption and photoluminescence measurements as a function of the irradiation dose. The F absorption band intensity follows a power law behaviour as a function of dose with a single exponent equal to 0.77, a value in excellent agreement with the predictions of a unified model for colour centres formation. The aggregate defects photoluminescence response appears suitable for applications in high dynamic range imaging plate X-ray detectors based on photoluminescence of coloured LiF. The choice of polycrystalline LiF thin films grown on different substrates allows one to obtain high resolved images even when more penetrating X-rays are used, as the colour centres formation is limited in depth by the film total thickness.

ACHANDRASTUDY OF THE ROSETTE STAR-FORMING COMPLEX. II. CLUSTERS IN THE ROSETTE MOLECULAR CLOUD

We explore here the young stellar populations in the Rosette Molecular Cloud (RMC) region with high spatial resolution X-ray images from the Chandra X-ray Observatory, which are effective in locating weak-lined T Tauri stars as well as disk-bearing young stars. A total of 395 X-ray point sources are detected, 299 of which (76%) have an optical or near-infrared (NIR) counterpart identified from deep FLAMINGOS images. From X-ray and mass sensitivity limits, we infer a total population of about 1700 young stars in the survey region. Based on smoothed stellar surface density maps, we investigate the spatial distribution of the X-ray sources and define three distinctive structures and substructures within them. Structures B and C are associated with previously known embedded IR clusters, while structure A is a new X-ray-identified unobscured cluster. A high mass protostar RMCX #89 = IRAS 06306+0437 and its associated sparse cluster is studied. The different subregions are not coeval but do not show a simple spatial-age pattern. Disk fractions vary between subregions and are generally 20% of the total stellar population inferred from the X-ray survey. The data are consistent with speculations that triggered star formation around the HII region is present in the RMC, but do not support a simple sequential triggering process through the cloud interior. While a significant fraction of young stars are located in a distributed population throughout the RMC region, it is not clear they originated in clustered environments.

X-ray Imaging Technology using Field Emission Array

We proposed a novel CdTe X-ray image sensor, which was driven by the FEA, to obtain high spatial resolution X-ray images and have demonstrated the principle operation by using the CdTe image sensor with one pixel. We have also fabricated a FEA matrix with 12×12 pixels to obtain X-ray images. For the further improvement of spatial resolution in the CdTe image sensor, we have proposed a novel double-gated FEA with a focusing lens, which was fabricated by using the etch-back method. The double-gated FEA showed a good focusing characteristic without significant decrease of the emission current, when the height of the focus electrode was optimized. The CdTe image sensor driven by the double-gated FEA is promising for an ultra-high-resolution X-ray image sensor.

Nano particle fluidisation in model 2-D and 3-D beds using high speed X-ray imaging and microtomography

Nanoparticles and nanocomposites have become a major focus of interest in science and technology due to exceptional properties they provide. However, handling and processing of ultra-fine powders is very challenging because they are extremely cohesive. Fluidization is one of techniques available to process powders. It has become increasingly important to understand how these nanoparticles can be handled and processed to benefit from their favourable properties. A high spatial (down to 400 nm) and temporal resolution (down to 1 ms) X-ray imaging apparatus has been designed to study nanoparticles in fluidized beds under different gas flow velocities. The mean volume distribution of the nanoparticle agglomerates was determined with X-ray microtomography. The X-ray microtomography technique provides valuable in situ, non-destructive structural information on the morphological changes that take place during fluidisation of powder samples.

AChandraACIS Study of 30 Doradus. I. Superbubbles and Supernova Remnants

We present an X-ray tour of diffuse emission in the 30 Doradus star-forming complex in the Large Magellanic Cloud using high spatial resolution X-ray images and spatially resolved spectra obtained with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. The dominant X-ray feature of the 30 Doradus nebula is the intricate network of diffuse emission generated by interacting stellar winds and supernovae working together to create vast superbubbles filled with hot plasma. We construct maps of the region showing variations in plasma temperature (T = 3-9 million degrees), absorption [NH = (1-6) × 1021 cm-2], and absorption-corrected X-ray surface brightness [SX = (3-126) × 1031 ergs s-1 pc-2]. Enhanced images reveal the pulsar wind nebula in the composite supernova remnant N157B, and the Chandra data show spectral evolution from nonthermal synchrotron emission in the N157B core to a thermal plasma in its outer regions. In a companion paper we show that R136, the central massive star cluster, is resolved at the arcsecond level into almost 100 X-ray sources. Through X-ray studies of 30 Doradus the complete life cycle of such a massive stellar cluster can be revealed.

In situ x-ray imaging of nanoparticle agglomeration in fluidized beds

A high spatial (down to 400 nm) and temporal resolution (down to 1 ms) x-ray imaging apparatus has been designed to study the agglomeration of arc plasma synthesized zinc oxide nanoparticles (average diameter of 50 nm) in fluidized beds under different gas flow velocities. The mean volume distribution of the nanoparticle agglomerates was determined with x-ray microtomography and found to correspond to a lognormal distribution with a mean value of 0.70×109μm3 and a variance of 3.6×1021 (μm3)2. The average density of the agglomerates was found to be 2.9gcm−3 compared to 5.6gcm−3 for the individual nanoparticles. The powder assembly was then dynamically imaged using an x-ray image intensifier coupled to a digital camera using a field of view of 24.20 mm by 32.25 mm and a temporal resolution of 40 ms. Sequential frames were captured into computer memory for a range of gas flow velocities from 0.026ms−1 to 0.313ms−1. The breakup energy of the agglomerates was calculated to be approximately 2×10−8J using a combination of dynamic observations and physical properties of the agglomerate system extracted from the x-ray microtomographic data.

ChandraX-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/7715: Tidal Ultraluminous X-Ray Sources, Emergent Wind, and Resolved HiiRegions

We present high spatial resolution X-ray imaging data for the interacting galaxy pair NGC 7714/7715 (Arp 284) from the Chandra X-ray telescope. In addition to the unresolved starburst nucleus, a variable point source with LX ≈ 1040 ergs s-1 was detected 15 (270 pc) to the northwest of the nucleus, coincident with a blue, extremely optically luminous (MV ≈ -14.1) point source on Hubble Space Telescope images. Eleven other candidate pointlike ultraluminous X-ray sources (ULXs) were also detected in the vicinity of NGC 7714/7715, two of which exceed 1040 ergs s-1. Ten of these appear to be associated with interaction-induced features, but only two are associated with star formation regions. We also found diffuse emission with LX ≈ 3 × 1040 ergs s-1 extending 11'' (1.9 kpc) to the north of the nucleus. Its spectrum can be fitted with either a two-temperature MEKAL function (kT = 0.59 and 8 keV) or a 0.6 keV MEKAL function plus a power law (Γ = 1.8 ± 0.2). The hard component may be due to high-mass X-ray binaries (HMXBs) with possible contributions from inverse Compton radiation, while the soft component is likely from a superwind. Superbubble models imply an expansion age of ≈15 Myr, supporting previous assertions of an intermediate-age nuclear stellar population in addition to a 5 Myr starburst. We also detected extended X-ray emission associated with four extranuclear H II region complexes. The emission from these H II regions and the nuclear starburst could be due to either an enhanced population of HMXBs relative to Local Group galactic averages or to diffuse gas heated by winds from supernovae, if the X-ray production efficiency LX/Lmech is high (≈5%). To estimate LX/Lmech, we collected published data for well-studied H II regions and superbubbles in nearby galaxies. For H II regions with ages less than 3.5 Myr, the median LX/Lmech ≈ 0.02%, while for older star formation regions, LX/Lmech ≈ 0.2%–7%. Thus, it is possible that gas heating by supernovae may be sufficient to account for the observed X-rays from these H II regions. In galaxies much more distant than NGC 7714, for example, the Cartwheel galaxy, H II region complexes similar to those in NGC 7714 will be unresolved by Chandra and will mimic ULXs. No X-ray emission was detected from the Type Ib supernova SN 1999dn, with an upper limit of ≈2 × 1038 ergs s-1.

Development of background reduced Fresnel phase zone plate

In study of hot and dense plasma, a high spatial resolution (a few microns) x-ray imaging is very important to observe these plasmas. The Fresnel phase zone plate (FPZP) consists of alternately material and transparent circular annuli placed concentrically, which image x rays using diffraction x rays from all annuli. FPZP have imaged 4.7–4.77 keV x rays with 2.2 μm spatial resolution. However FPZP has a problem that background level is comparable to signal level. In subtraction of background, the error of 10% is caused. For the accurate background subtraction, we designed new FPZP, which consist of three β layers of a transparent zone and two material zones. The new design FPZP parameters (thickness of material zones, each zone width) have been optimized, and in that optimum design signal-to-background ratio is 4 times better than conventional two layers FPZP.

10 MK Gas in M17 and the Rosette Nebula: X‐Ray Flows in Galactic HiiRegions

We present the first high spatial resolution X-ray images of two high-mass star forming regions, the Omega Nebula (M17) and the Rosette Nebula (NGC 2237-2246), obtained with the Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer instrument. The massive clusters powering these H II regions are resolved at the arcsecond level into more than 900 (M17) and 300 (Rosette) stellar sources similar to those seen in closer young stellar clusters. However, we also detect soft diffuse X-ray emission on parsec scales that is spatially and spectrally distinct from the point-source population. The diffuse emission has luminosity LX 3.4 × 1033 ergs s-1 in M17 with plasma energy components at kT 0.13 and 0.6 keV (1.5 and 7 MK), while in Rosette it has LX 6 × 1032 ergs s-1 with plasma energy components at kT 0.06 and 0.8 keV (0.7 and 9 MK). This extended emission most likely arises from the fast O star winds thermalized either by wind-wind collisions or by a termination shock against the surrounding media. We establish that only a small portion of the wind energy and mass appears in the observed diffuse X-ray plasma; in these blister H II regions, we suspect that most of it flows without cooling into the low-density interstellar medium. These data provide compelling observational evidence that strong wind shocks are present in H II regions.

Hard x-ray imaging using free-standing spherically bent crystals

We report the attempt to prepare free-standing spherically bent Si crystals with radii of curvature as small as 30 cm and large uniform working areas up to several cm2. We also report on the use of these crystals in the Laue geometry to record two-dimensional high spatial resolution x-ray images in the hard x-ray region (λ∼0.5 Å). We discuss how these bent crystals were made, and show how these spherically bent crystals were used in the Laue geometry to obtain hard x-ray images of test objects at the silver K-alpha wavelength at two different magnifications. The mean spatial resolution of this x-ray imaging scheme, determined from the recorded image traces, was found to be better than 10 μm over a field of view of 1.5×1.5 mm. Contributions of possible focusing mechanisms (dynamical and polychromatic) are discussed. Additional theoretical understanding of how such a scheme is working is needed.

Development of nondestructive evaluation methods for hot gas filters

Rigid ceramic hot gas candle filters are currently under development for high-temperature hot gas particulate cleanup in advanced coal-based power systems. The ceramic materials for these filters include monolithics (usually non-oxides), oxide and non-oxide fiber-reinforced composites, and recrystallized silicon carbide. A concern of end users in using these types of filters, where over 3000 may be used in a single installation, is the lack of a data base on which to base decisions for reusing, replacing or predicting remaining life during plant shutdowns. One method to improve confidence of usage is to develop nondestructive evaluation (NDE) technology to provide surveillance methods for determination of the extent of damage or of life-limiting characteristics such as thermal fatigue, oxidation, damage from ash bridging such as localized cracking, damage from local burning, and elongation at elevated temperatures. Although in situ NDE methods would be desirable in order to avoid disassembly of the candle filter vessels, the possible presence of filter cakes and/or ash bridging, and the state of current NDE technology prevent this. Thus, off-line NDE methods, if demonstrated to be reliable, fast and cost effective, could be a significant step forward in developing confidence in utilization of rigid ceramic hot gas filters. Recently, NDE methods have been developed which show promise of providing information to build this confidence. Acousto-ultrasound, a totally nondestructive method, together with advanced digital signal processing, has been demonstrated to provide excellent correlation with remaining strength on new, as-produced filters, and for detecting damage in some monolithic filters when removed from service. Thermal imaging, with digital signal processing for determining through-wall thermal diffusivity, has also been demonstrated to correlate with remaining strength in both new (as-received) and in-service filters. Impact acoustic resonance using a scanning laser vibrometer has been demonstrated to allow detection of changes in frequency which may be correlated to remaining strength. These methods have been shown to be applicable to clay-bonded SiC filters, recrystallized SiC filters, CVI–SiC composite filters and oxide composite filters. Other NDE methods under development include: (a) fast, high spatial-resolution X-ray imaging for detecting density variations and dimensional changes; (b) air-coupled ultrasonic methods for determining through-thickness compositional variations; and (c) acoustic emission technology with mechanical loading for detecting localized bulk damage.