M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 3 6 – 4 8

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Development of a lab-scale, high-resolution,

tube-generated X-ray computed-tomographysystem for three-dimensional (3D)materials characterization

J.C.E. Mertens, J.J. Williams, Nikhilesh Chawla⁎

Materials Science and Engineering, Arizona State University, Tempe, AZ 85287–5604, USA

A R T I C L E D A T A

⁎ Corresponding author. Tel.: 480 965 2402; faE-mail addresses: james.mertens@asu.ed

http://dx.doi.org/10.1016/j.matchar.2014.03.001044-5803/© 2014 Elsevier Inc. All rights rese

A B S T R A C T

Article history:Received 6 December 2013Received in revised form5 February 2014Accepted 3 March 2014Available online 12 March 2014

The design and construction of a modular high resolution X-ray computed tomography (XCT)system is highlighted in this paper. The design approach is detailed formeeting a specified set ofinstrument performance goals tailored towards experimental versatility and high resolutionimaging. The XCT tool is unique in the detector and X-ray source design configuration, enablingcontrol in the balance betweendetection efficiency and spatial resolution. The systempackage isalso unique: The sample manipulation approach implemented enables a wide gamut of in situexperimentation to analyze structure evolution under applied stimulus, by optimizing scanconditions through a high degree of controllability. The component selection and design processis detailed: Incorporated components are specified, customdesigns are shared, and the approachfor their integration into a fully functional XCT scanner is provided. Custom designs discussedinclude the dual-target X-ray source cradle whichmaintains position and trajectory of the beambetween the two X-ray target configurations with respect to a scintillator mounting andpositioning assembly and the imaging sensor, aswell as anovel large-format X-ray detectorwithenhanced adaptability. The instrument is discussed fromanoperational point of view, includingthe details of data acquisition and processing implemented for 3D imaging via micro-CT. Theperformance of the instrument is demonstrated on a silica-glass particle/hydroxyl-terminated-polybutadiene (HTPB) matrix binder PBX simulant. Post-scan data processing, specificallysegmentation of the sample's relevantmicrostructure from the 3D reconstruction, is provided todemonstrate the utility of the instrument.

© 2014 Elsevier Inc. All rights reserved.

Keywords:X-ray microtomography3D materials scienceCompositeMultiscale

1. Introduction

3D imaging of material microstructures has typically beenlimited to destructive techniques such as serial-sectioning [1];in more recent times, the nondestructive approach of X-raymicro-computed tomography (XCT) has demonstrated unique3D imaging capabilities in the study of microstructures and

x: 480 727 9321.u (J.C.E. Mertens), jason.w

2rved.

material behavior phenomena [2,3] X-ray-tomography tech-niques have long been used in the medical field, but havebecome increasingly applicable to materials science researchas the imaging resolution has approached a scale which issuitable to studymaterialmicrostructures [4]. Advanced facilitieswhich employ synchrotron light sources in XCT experiments(ex. Beamline 2-BM, Advanced Photon Source, Argonne National

illiams@asu.edu (J.J. Williams), nchawla@asu.edu (N. Chawla).

37M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 3 6 – 4 8

Laboratories) are high in demand due to a limited number offacilities resulting from their high cost, requiring users to runexperiments over short time frames and at low frequencies.Accordingly, lab-scale devices (Xradia, Inc., CA, USA, BrukerCorporation, WI, USA) have been commercially developed forhigh resolution XCT studies without synchrotron light.

Bench-top systemshave fallen short of synchrotron facilitiesprimarily due to thenature of theX-ray source [5]. In-houseXCTsystems utilize braking-radiation derived X-rays, rather thansynchrotron derived X-ray beams, leading to a less brilliant,polychromatic cone-beam rather than a highly brilliant, mono-chromatic parallel beam. The brilliance affects the necessaryscan time, whereas the conic polychromatic beam leads toimage artifacts that must be corrected [6]. The resolution ofthese in-house systems has been limited by, among otherissues, the X-ray source and detector limitations [7]. It isunderstood that advances in components for X-ray generationand imaging are continually advancing in performance, furtherbridging the gap between synchrotron imaging. However, theseare slow to arrive in commercial systems which are extremelyexpensive. Furthermore, systems are sought which have ahigher degree of modularity for experimental versatility and insitu capability.

Hence, the goal is to design and construct a customsystembyincorporating state-of-the-art components for high-resolution3D imaging. Previous researchers [5,7–11] have shown thepotential and efficacy of custom built detectors as well as(almost) full lab-scale CT instruments, and have providedvaluable insight to many crucial aspects of the design process.Ofwhich studies, for those that have focused on customdetectordesign theory, the approach has either been focused onapplication to synchrotron beamline imaging [9,10], or havefocused on concepts for the fiber-optic scintillator-lens couplingdesign [7]. Of those studies that have been focused on detectoroptimization for lab-scale system application, a mere survey ofcommercially available detectors has been performed [8,11].Although Flynn et al. [5] have detailed the importance ofmaximizing X-ray flux through the sample in lab-scale systemsfor optimizing signal, a design approach for optimizing detectionefficiency from the detector parameters has not been provided.This system and the following description of the instrument'sdesign provide a consolidation of these design principles to yieldanew, truly optimized lab-scale X-raymicrotomography system.

This system, and the followingdescriptionof the instrument'sdesign, provides a consolidation of these design principles toyield: a) the optimized lab-scale system in terms of imagingresolution, b) the brightness of the X-ray source, and c) thedetection efficiency. Specifically, detector design considerationsfor improvement in regards to efficiency and resolution areoffered by considering a lens-coupled design, where the charac-teristics of the optically phosphorescent (scintillating) medium,optical lens itself, anddigital sensor are employed, in conjunctionwith the intensity and resolution limits of a dual-target X-raytube and the scintillatingmedium's response to the polychromaticX-ray beam. Additionally, the design and component selectionfor more accurate alignment of the X-ray source, detector androtation axis is offered for accurate reconstruction and increasedcapacity for sample and experiment adaptability.

In this manuscript, we fully detail the design and construc-tion of a unique, lab-scale system for in situ, high-resolution

and multi-scale, material studies. The novelty of this designapproach is realized by the complete optimization of compo-nents for lab-scale imaging resolution and detection efficiency.The novelty of this instrument's performance is summarized bya capability to be tailored for experimental and imaging goals athand, where system characteristics are adjusted all the wayfrom the X-ray source generation mechanism energy andpower, the scintillator composition and thickness, the opticalcoupling lens' magnification, resolution, numeric aperture andfield of view, the X-ray flux through the sample, the X-raymagnification, control over data acquisition via a customprogrammed interface, and in the future, the painless incorpo-ration of new exotic equipment which has yet to be realized.Other lab-scale systems, whether commercial or researchderived, do not realize this magnitude of controllability andexperimental versatility, and have not fully optimized theperformance characteristics of the deployed equipment withrespect to one another. In this discussion, we focus on thedesign process for high resolution and efficient detection inlab-scale tomography, aswell as the basic systemprogrammingnecessary for data acquisition and processing in X-raymicro-tomographic imaging. We use a glass sphere reinforcedpolymer composite as a model material to demonstrate theusefulness of the current system.

2. System Performance Goals

To justify the design and construction of this system, theperformance goals established for the tool are outlinedprior to adiscussion of the approach to achieve the goals. The process ofresource allocation and optimization will not be discussed butshould be noted.

The most fundamental components for lab-scale XCTsystem are the X-ray source, sample rotation stage, detectionsystem, and computing systems. Many options for each existedcommercially, and all affect the performance of the system.

The design of this system is motivated by the authors' needto study materials wherein many features of interests are evensmaller than one micrometer. In cone-beam radiography, thespatial resolution can be limited by blurring from the X-raysource, the resolution in the scintillating medium, the physicalsize of the detector elements, and the resolution limits of anyoptics. In tomography, the resolution of the 3D reconstructionmay commonly be limited by imperfections in object rotation,the number of projections acquired with unique orientation,algorithmic accuracy, and artifact presence. This tool's designto minimize these limitations is discussed.

Another goal of this work is to accommodate of sample sizeswhich may be of interest. This system's design targets imagingsamples of tens of microns to several centimeters in diameter:An example of eachmay be an industry-scale solder-bump or a1 mm dog-bone tensile specimen contained within an in situmechanical loading jig 5 cm in diameter. The design approachfor achieving this is discussed.

The design also targeted a flexibility to adapt the system toa wide gamut of experimental chambers and new equipment:A prime example is mechanical, thermal, or environmentalchambers which may be sample dependent and unique.This concern coupled with maintenance and component

38 M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 3 6 – 4 8

upgradability concerns corroborated the logicality of imple-menting a modular design, wherein commercially availablecomponents are integrated with custom components to yielda fully functional instrument.

After an investigation into the limitations of lab-scale XCT,the performance targets of the XCT system were realisticallyrefined. The XCT system in this study (Fig. 1) was designedwith the following performance goals:

• Spatial resolution of less than 1 μm• Imaging of specimens of up to 10 mm in diameter• Imaging of high atomic number sample compositions• Capacity for in situ chamber up to 5 kg and 10 cm dia.• Programmable and modular for incorporation of nearly anycomponent for future advancements, replacements, andextended lifetime

• Minimal total consumption of resources

3. Design Theory

In any digital imaging system, there is a trade-off between theFOV and the feature resolution in any given single frame,where the compromise is made by optimizing the magnifica-tion [12]. This is due to limitations on the digital detectionsystem's chip pixel count and spatial sampling frequency. Theability to resolve features is also affected by the contrast in theimage between the feature of interest and the surroundings aswell as the feature spacing, commonly shown using thepoint-spread function, line-spread function, edge-spread func-tion, modulation-transfer function, contrast-transfer function,or the optical-transfer function, and usually with the full-widthat one-half the peak's maximum signal criterion for resolution[4,13] In radiography, the contrast between features is largelycontrolled by the difference in the average linear X-rayattenuation coefficient of the feature of interest and that ofthe surrounding at the energy of the incident X-rays, and theexposure time. Thedifference indetected intensity, I, between apixel sampling a feature and that of a pixel sampling thesurrounding area determines the contrast of that feature. Thus,

Fig. 1 – Interior view of the constructed X-ray microCT scanner. Sleft), the sample rotation stage on the X–Y–Z translating motioncamera and scintillator assembly mounted on X–Y–Z translatingthe detector assembly is 2 in. tall.

higher counts in longer exposures can yield better contrastfor features in thicker/denser samples or for features with alow difference in X-ray attenuation compared to that of thesurrounding medium. The transmission, I/I0, where I0 is thetransmitted intensity, of monochromatic X-ray through thesample, decays exponentially through the thickness, x, and is afunction of the average density, ρ, and mass attenuationcoefficient, (μ/ρ), related through the Beer–Lambert law as:

I=I0 ¼ exp −μ=ρ

� �ρ x

h i: ð1Þ

A detected intensity, I, is determined for every detectorelement over many known orientations of the sample. Theusefulness of longer exposures is limited by thermal, elec-tronic, or electromagnetic noise at the digital detector as wellas bit-depth of the detector, beam stability, and throughputnecessity. Thicker samples, with a larger x, require largervalues of I0, or longer exposures for the same detectedintensity. Therefore, in addition to requiring a larger FOVand thus experiencing a lower spatial sampling frequency, theresolution possible to obtain in large samples is furtherchallenged. To detect a given number of counts throughlarger samples, higher X-ray energies/intensities are required,degrading the resolution by penumbral blurring as describedby Chaney et al. [14] and G. Schena et al. [15]. Imagingresolution is highly dependent on the contrast of the imagedfeature, as discussed by S. Paciornik et al. [12], which in thecase of X-ray imaging is dictated by dissimilarity in thefeature's and surroundings' effective X-ray attenuation coef-ficient. This difference in attenuation is typicallymeasuredusingfluorescing mediums which emit optical light with an intensityproportional to the incidentX-ray intensity. Therefore, the spatialresolutionof a systemrelying onphosphorescence canbe limitedby the spatial sensitivity of the phosphor. In the case of lens-coupled single-crystal-scintillator X-ray detection systems, theimaging resolution is also limited by the optical characteristics ofthe light conversionmediumand numeric aperture of the opticalrelay components, due to refraction in the scintillator, the opticaldepth-of-field, and classical diffraction-limited lens resolution asdescribed by A. Koch et al. [9].

hown is the X-ray source mounted on custom cradle (upperstack (lower left), and the detector assembly including CCDmotion stack (right). For scale, the light shaded oval mirror in

39M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 3 6 – 4 8

The ideal X-ray source for accurate CT is one which emitsparallel, monochromatic X-rays such as at synchrotronbeam-lines. The use of monochromatic X-rays allows fordetermination of the actual linear X-ray attenuation coefficientin a voxel, whereas with an untreated X-ray tube such accuracyis precluded by a polychromatic nature. X-ray tubes, used inbench-top systems, produce Bremsstrahlung radiation in acone of emission. To reconstruct the 3Dattenuation structure ofa sample from projections acquired with cone-beam X-rays,reconstruction algorithms such as the Fourier-transform-based Feldkamp–Davis–Kress (FDK) [6] or iterative Algebraic-Reconstruction-Techniques (ART) [16] are implemented, whichapproximate the attenuation coefficient at each voxel byaccounting for the variation in the angle of the beam pathfrom pixel to pixel and back projecting into the sample spaceusing known scan geometries. These algorithms commonly useweighting functions, such as the Shepp–Logan or Ram-Lak filterfunction, to improve signal-to-noise and sharpen edges in areconstructed sample plane image, as discussed by T. Taylor etal. [17] The accuracy of these algorithms depends very stronglyon physical parameters such as component perpendicularity,axis alignment, beam-to-central-axis angle at the extremitiesof the rotating sample, and the amount of orientation informa-tion obtained. Accurate reconstructions are more challengingshould aspects of the sample move out of the detector's FOVduring rotation, discussed by R.M. Lewitt et al. [18] Hence, itmust be ensured that samples intended for imaging can berotated as perfectly as possible, and positioned very accuratelyover a full rotationwhile residingwithin the FOVof thedetector.

Fig. 2 – The minimum system resolution (R) limited by focalspot size (F) and the detector pixel size (P) as a function ofsystem magnification (M) for this system, with the15 μm pixel detector under different focal spot sizeconditions. An optimal system magnification using a 1 μmfocal spot size is seen to be 31×, alternatively determinedwith Eq. (2).[7,14].

4. Primary Component Selection andOptimization Methods

4.1. X-ray Source

In the selection of an X-ray source for high resolution imaging,the selection is limited to a microfocus X-ray source. That is,the X-rays are emitted from a micrometer-sized (typically oneto several micrometers) area in the target. In the evaluation ofmicrofocus X-ray sources, all parameters were considered,including power range, voltage range, focal spot size as afunction of power, the minimum focus-to-object distance(FOD) which can limit X-ray flux through the sample, the typeof the target, the target composition whether the target iscooled, rotated or interchangeable, the lifetime of the tube inthe case of a vacuum sealed source, or the lifetime of thefilament in the case of a vacuum-pumped source, and themaneuverability and compatibility with the system design.

The resolution limit of the system based on the X-ray focalspot size conditions and the pixel size of the selected imagingchip (15 μm) is shown in Fig. 2: This concept is critical fordetermining the optimal system magnifications for a givencombination of X-ray source/generating conditions and digitalimaging chip deployed into a system. This magnification,M, canbedetermined for a given focal spot size, F, andpixel size, Pby thesuccinctly summarized equation shown by G. Schena et al. [7]:

M ¼ 2P=Fð Þ þ 1 ð2Þ

The total system magnification can be determined for agiven X-ray focal spot size and imaging sensor's physical pixelsize at which neither parameter is limiting the resolution ofthe system. The total system magnification is a function ofX-ray geometric magnification and optical magnification:

MSystem ¼ MOptical � MX‐Ray ð3Þ

One application of Fig. 2 and Eq. (3) is the determination ofthe system magnification, X-ray generating target configura-tion (transmission vs. reflection), the X-ray tube acceleratingvoltage and X-ray target current to achieve a particulartargeted spatial resolution: The system magnification re-quired to reach a certain resolution can be determined fromthe sensor pixel size (see G. Schena et al. [7]). The focal spotsize can then be determined which does not limit resolution,and the X-ray accelerating voltage, target current, and targettype can be adjusted to achieve a focal spot of this size (seeChaney and Hendee [14]). A direct application of this relation isconsidering the focal spot size of any candidate X-ray source asa function of target power or X-ray intensity in conjunctionwiththe imaging goals at hand, and designing the detector toachieve the necessary magnification. The achievable X-raymagnification can be limited by the component separationdistances necessary, the size of the X-ray detecting screen andthe camera's FOV. The relevant component separation dis-tances are the Source-to-Detector-Distance (SDD) and Source-to-rotation-Axis Distance (SAD), and are related to the X-raymagnification by

MX‐Ray ¼ SDD=SAD: ð4Þ

The emission area of a source's X-ray producing target, i.e.the focal spot, limits the resolution of the system at largemagnifications. Transmission targets limit lateral spreadingof the electron beam within the target by being thinner thanthe normal X-ray producing interaction depth in a bulk

40 M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 3 6 – 4 8

material, effectively yielding the smallest focal-spot for thehighest resolution imaging. Conversely the reflection-typetarget configuration can be loaded at much higher powerdensities yielding a more energetic or brighter beam fordenser samples, as discussed by B.C. Masschaele et al. [19].Ultimately, this amounts to detail detectabilities of hundredsof nanometers via a transmission target configuration, and inthe range of 1 μm with the reflection target configuration forthicker/denser samples.

The X-ray source implemented in this system (X-RAY WorXGmbH XWT-160-SE/TC, Garbsen, Germany) was selected basedon an ability to meet the system performance goals: This sourceis capable of 160 kV of accelerating potential yielding high energyX-rays to maximize transmission through large samples, iscapable of a very small focal spot size for high resolution imaging,andhas adual-target (transmission/reflection) interchangeabilityto performmulti-scale imaging. The manufacturer of this sourcespecifies a minimum detail detectability of <0.3 μm and amaximum power of 3 W with the high resolution transmissiontarget and a minimum detail detectability of <2.0 μm and amaximum power of 280 Wwith the reflection target.

4.2. X-ray Detector

There exist awidevarietyof configurations for digitizing sampledX-ray intensities, most relying on scintillating materials toconvert X-rays of a given intensity to a proportional amount ofoptical light, coupled to a light sensitive imaging chip. Phosphorscan vary in microstructure from single crystal to columnar-grained topowder-compacted [4,13,20–22], imaging chipsnamelyinclude complimentary metal-oxide semiconductor (CMOS) andcharge-coupled devices (CCD) types [23], and the mode oftransferring the scintillated light to the light-sensitive detectorelements can vary from lens to fiber-optic to direct contact of thephosphor and chip.[10,24] The imaging system in this design isthat of lens-coupled configuration, providing what was deter-mined to be the highest resolution, light transfer efficiency, andversatility. Specific to the decision of a fiber optic versus lenscoupled sensor/phosphor design, three factors were mostinfluential: The ease with which the scintillating medium canbe interchanged in the lens-coupled design, the ability toconstruct the lens-coupling in-house, and the added magnifi-cation variability of the lens-coupled configuration. Anotherfunction of the sensor–phosphor coupling is the capability toremove the light sensor out of the X-ray path, which can giverise to ‘zingers’ in X-ray images caused by a non-signal X-rayinteraction with the sensor itself. [31] With fiber coupledconfigurations, the fiber-bundle length should allow for cameraremoval from the beam path. In the case of lens-coupledconfiguration, mirrors have been used for optical signaldiversion from the X-ray path to the sensor. Two obvioussolutions exist in placing a right-angle mirror in front of thecamera lens [32] or behind the camera lens [10], where the priorhas the added advantage of also removing the lens from thebeam path potentially preventing burnishing (a darkening orbrowning of the optical glass [9] which leads to a loss in opticaltransmission). For the first solution, a longer working distancelens is required, and for the second a longer optical path behindthe lens is required. The longer optical path behind the lens,although easily accomplished with infinity corrected objective

lens, may affect the range of magnification possible withfinite-focus lens.

The selected camera (Alta U230, Apogee Imaging Systems,Inc., CA, USA) in this system contains a high cosmetic-gradelarge format 2048 × 2048 back-illuminated CCD array(CCD230-42, e2v technologies inc., UK) composed of 15 μmsquare pixels, 16-bit dynamic range, and a peak quantumefficiency of about 95% at 550 nm corresponding to the peakemission spectra of LuAG:Ce (cerium-doped lutetium alumi-num garnet), YAG:Ce (cerium-doped yttrium aluminum gar-net), and CsI:Tl (thallium doped cesium iodide). Selecting ascintillator with a peak emission wavelength near that of thecandidate imaging chip's peak quantum efficiency (QE) helpsto maximize count rates and minimize scan time for a givenprojection of a given sample. High QE is achieved by theback-lit architecture, a green-sensitive ‘mid-band’ coating tothe CCD chip, and thermoelectric cooling to −30 °C. A largedynamic range sensor is important for X-ray detectors inorder to maximize the number of counts possible before pixelsaturation. This is critical when attempting to achieve contrastbetween phases with similar x-ray attenuation coefficients atthe X-ray energy used for imaging. The camera's large-formatCCD chip did come with the challenge of identifying a suitablelens for high-resolution imaging which was also able to utilizethe entire area of such a large chip. The lens selected for thispurpose (Micro-Symmar 2.8/50 mm, Schneider Optics, Inc., NY,USA) is a finite-focus macro-lens selected on imaging resolu-tion, a relatively large numeric aperture, low image distortion,and magnification variability. As a finite-focus lens, themagnification of the lens system can be tailored by alteringthe length of the optical track length to the chip, which alsochanges to working distance of the lens. This lens' imagequality is optimized for a magnification factor of 3.5×, althougha magnification range of 2× to 5× is practically achievable. Theoptical track length can be changed using the adjustable lengthoptical tube, or by adding and removing optical extension tubes,both of which are visible behind the lens in Fig. 1.

Selecting the optimum scintillator requires careful consider-ation of many parameters including composition, thickness,emission spectra, density and light yield for maximizingdetection efficiency over a range of X-ray energy balancedagainst factors such as the thickness, index of refraction andnumeric aperture of the lens system (if applicable) for highresolution imaging capability.[13,22,25] Particularly, the thick-ness of the selected scintillator must be carefully optimized inthe trade-off between X-ray conversion efficiency and theresolution limit of the system based on the spatial samplingfrequency of the camera system and the resolution of the X-raysource. Due to that fact that scintillator thickness can limit theresolution, but also determines the amount of signal achievablein a given time frame of data acquisition, extreme care must betaken to optimize the thickness of the scintillator with theperformance goals in mind.

Many studies provide great insight into the resolutionachievable using lens-coupled single crystal scintillators, show-ing that thin scintillator screens arenecessary in order to achievethe highest imaging resolution.[9,20] These studies also showthat lenses implemented with a very low numeric-aperture (NA)limit the resolution from light diffraction with a small depen-dence on scintillator thickness, and that with high NA lenses

41M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 3 6 – 4 8

lower resolution is possible but require thinner scintillators torealize. Numerical simulations for imaging with a YAG:Cescintillator, where refractive index n = 1.95, emission wave-length λ = 550 nm, and assuming a lens of some NA is focusedon the scintillator, show that a suitablemodel for calculating thespatial resolution in terms of the full width at 50% of theintegrated line-spread-function (LSF) is

R ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffip=NAð Þ2 þ qzNAð Þ2

qð5Þ

where the constants p = 0.18 μmandq = 0.075, z is the thicknessis micrometers, and NA is the numeric aperture of thelens system used [9]. The lens implemented in this system hasNA = 0.1758, so numerical approximations of achievable resolu-tion using YAG:Ce of varying thickness can be demonstrated, asshown in Table 1. This should be considered for all candidatescintillatingmaterials using the appropriate values formodeling,and the reader is referred to the approach used in the derivationof Eq. (5) and an involveddiscussion on this topic byA. Koch et al.[9].

A competing factor for determining the optimum thicknessof a scintillator implemented for a given task is the fraction ofabsorbed X-rays within the scintillator itself, which are able toparticipate in photoelectric absorption and ultimately thescintillation of optical light. [9] The ultimate consequence ofthis compromise is a degraded detector quantum efficiency(DQE) in high-resolution capable detectors. This trade-off forcesthe instrument designer to determinewhat spatial resolution isdesired, and whether the resulting DQE with a scintillator of anappropriate thickness to achieve that resolution provides areasonable required exposure time per projection consideringthe fluence of the X-ray source used (which is limited by thetarget resolution in the case of vacuum-tube x-ray sources). Thegoal of this design was to achieve one micrometer imagingresolution capability, while simultaneously optimizing thedetector's DQE for high energy X-ray scanning conditions.Using the approach of Koch et al. [9] the DQE of this systemwas analyzed as a function of energy under different hypothet-ical scintillator compositions of varying thickness in conjunc-tion with the candidate image sensors and lens couplingoptions, as shown in Fig. 3 for the selected CCD camera andlens, using Eq. (6).

DQE ¼ SNR2out

SNR2in

≅ηabs 1þ1þ 1

�ην=e

ηcoll Ex=Eν

� �ηx=ν

0@

1A

−1

ð6Þ

The DQE was calculated as a function of energy byconsidering the photoelectric X-ray attenuation coefficient ofeach material as a function of X-ray energy considering thedensity and thickness of the scintillator to determine theabsorption efficiency (ηabs), the X-ray to light conversionefficiency of the scintillator (ηx/ν) in conjunction with thephoton energy-in to energy-out ratio, the light capturing

Table 1 – Resolution achievable at 50% contrast basedon thelens currently in this system (NA = 0.1758) system usingYAG:Ce of varying thickness based on the approach of A.Koch et al. [9] using Eq. (5).

YAG:Ce thickness (μm) 5 25 50 100 250 500Resolution (μm) 1.03 1.08 1.22 1.67 3.45 6.67

efficiency of the lens implemented (ηcoll), and the integratedquantum efficiency of the CCD implemented over theemission spectrum of the scintillator being modeled (ην/e).The density (6.73 g/c.c. for LuAG:Ce and 4.55 g/c.c. for YAG:Ce)[4], the light yield of the scintillating material per keV ofdeposited energy (20 photons/keV for LuAG:Ce, 45 photons/keV for YAG:Ce) [3], the refractive index (1.82 for LuAG:Ce, 1.85for YAG), the relative emission spectra available from productvendors, and the photoelectric mass attenuation coefficientas a function of energy (calculated from NIST's XCOM X-rayabsorption calculator) were polled and used to calculate theDQE of the detector being designed in order to aid in theselection of an optimal scintillator. The polychromatic natureof the X-ray source is not considered, i.e., calculations weremade at every X-ray energy and not for each operating tubevoltage (Fig. 3). As the X-ray energy-number distributiondepends on varying factors such as the filtering used, it wasdetermined most useful to analyze the DQE behavior in termsof X-ray energy, with the realization that a distribution ofX-ray energies are being used for imaging. Another simplifi-cation was made by considering the lens and CCD window tobe fully transparent at all optical wavelengths.

While many scintillator compositions and thicknesses wereconsidered, ultimately, the selection analysiswas limited to thecompositions and dimensions which were commercially avail-able. Another realistic restriction is the size of imaging area of acertain scintillator of a certain thickness. The curves in Fig. 3 areshown only for LuAG:Ce and for YAG:Ce of select thickness, butwere constructed for a wide number of candidate scintillatingmaterials and thicknesses, including Bismuth Germanate (BGO)and Cesium Iodide (CsI).

Given the multi-scale yet high resolution goals of thisinstrument, the approach of interchangeable scintillators ofdifferent composition and thickness was adopted, which is apractical necessity for systems to be used for samples of differentsize/compositionunderdifferent target resolutions.Nonetheless,a scintillator composition and thickness was identified in orderto bridge all the performance goals of this system, aided bycalculations of the resulting DQE when integrated with theselected CCD chip and lens system. The higher attenuation of

Fig. 3 – Estimation of the DQE resulting from the selected lens(NA = 0.1758) and CCD chip (QE > 90% at 550 nm) inconjunction with YAG:Ce and LuAG:Ce single crystalscintillator screens of varying thickness using Eq. (6).

42 M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 3 6 – 4 8

LuAG:Ce at high X-ray energy for a given thickness is preferredfor high energy radiography. Note that this is not the case forsystems, such as synchrotron facilities, which use low energyX-rays near 20–30 kV. The thickness was selected as a compro-mise between the theoretically achievable imaging resolutionand the theoretically achievable DQE.

Another close candidate material is that of the columnar-grain structured CsI:Tl, which also has a peak emissionwavelength which is well matched with the simultaneouslyselected detector and a high light yield [4,13,20,26]: Thismaterialwas not pursued due to a lack of confidence in resolutioncapabilities on theorder of 1 μm,and fromchallenges identifyingsuppliers for this material less than 0.3 mm thick.

Finally, the scintillation medium used in the current tool isa single crystal LuAG:Ce circular disk, selected based onachievable resolution, the resulting DQE with the cameraand lens, and commercial availability in a wide range ofthicknesses. The material was selected over YAG:Ce based onthe resulting calculated QE of the detector at high energy(Fig. 3). The scintillator currently in use is a 250 μm thickLuAG:Ce crystal mounted in an aluminum ring (Crytur, spol. sr.o., Turnov, Czech Republic), chosen as the best singleoptimized thickness for our initial performance goals, as theresolution was determined to safely coincide with thenominal effective pixel size of the CCD/lens combination(4.29 μm) adopted using the aforementioned modeling ap-proach. A thinner scintillator can be used where a higherresolution is needed at the scintillating medium, and con-versely a thicker scintillator can be used where detectionefficiency is more of a concern than a spatial resolution.

4.3. Rotation Stage

XCT reconstructions rely on acquiring projections at manysample orientations. A stage which provides perfect rotationwithout any physical distortion and which provides preciseand accurate positioning is desired. It is also ideal that thesample rotation stage have a small footprint and a high loadcapacity. In actual rotation stages, some undesirable proces-sion, tilt, wobble or run-out may occur during the rotation,and the precision of positioning is finite. The rotation stage inthis system (ORT-101-L Air Rotary Air Bearing with a Delta TauMAC-MC-1A-SD Controller, Nelson Air Corp., NH, USA) wasselected to maintain the system's imaging resolution, provid-ing 27 kg load capacity, high positioning precision within0.00001°, and low distortions of less than +/− .025 μm at11 rpm 6″ from the spindle face.

4.4. Workstation Computing

Two workstations have been implemented in this system inorder to increase the flow of data: One for the control of the dataacquisition software and equipment, and a second for rawimage calibration, running reconstruction algorithms, andperforming post-reconstruction image processing for micro-structural segmentation, visualization, and quantification. Theperformance specifications of the reconstruction-algorithmrunning computer are of most technical significance: The CPUon this workstation utilizes an Intel Core i5-2500K 3.3 GHzquad-core processor (Intel Corporation, CA, USA) for the

reconstruction of the 3D volume. The system employs a 448core 6 GB CUDA enabled GPU (Tesla c2075, NVIDIA Corporation,CA, USA for the graphic hardware accelerated reconstruction.Of critical importance for performing reconstruction on largedatasets without the use of a computer cluster is the size of theRAM on the system. Roughly twice the projection data set, insingle precision, plus 5–10 GB is required to run cone-beamreconstructions, that is, without breaking the data up intosmaller subsets. This system has 32 GB of RAM. To give aperspective, a 1441 image dataset (i.e., 1/8° steps over 180°)acquired using 2048 × 2048 elements per frame is roughly22.5 GB,making it necessary to reconstruct such a large datasetin subsets.

The workstation used for data acquisition primarily acts asa scan control station, using routines written in LabVIEW,fromNational Instruments Corporation, to communicate withthe X-ray source, CCD camera, four linear axes, and rotationsaxis. One aspect of a workstation used for this purpose whichshould not be overlooked is the number of communicationports available for the desired motion controllers, X-raysource, detector, and in situ instrumentation. Without insitu equipment requirements, this system currently con-sumes three USB ports, one Ethernet port, and one 9-pinRS232 serial port.

5. Auxiliary Component Selection and SystemIntegration Methods

In addition to theaforementioned critical components of theXCTsystem, several other components are essential to make XCTscanning practical. Sufficiently thick X-ray shielding is required,depending on the X-ray source, and the design of the chambercan be a challenge. Equipment space, chamber ventilation,access, and assembly sitemust be considered. Another necessityis the ability to shield the sensitive components of the XCTsystem from environmental vibrations. A vibration-dampingbreadboard table top and air-floating vibration isolator supportcolumns (RS2000-36-8 & S-2000A-428, Newport Corporation, CA,USA) were selected as the foundation for the system compo-nents, based on an ability to minimize any vibrations andfacilitate component mounting/assembly. The top also allowsfor very large X-ray magnifications due to its 6 ft. (~2 m) length.The breadboard table facilitates positioning of the three maincomponent ‘stacks’ – the X-ray source, sample manipulation,and X-ray detector assemblies – virtually anywhere on the 3′ × 6′(~1 m × ~2 m) surface with one-inch increments. Although theX-ray source stack is essentially static once placed, the samplemanipulation stack andX-ray detector stack are both designed tobe able tomove in three translational directions to coincide withthe X-ray beam axis.

Obtaining reference images, where the beam is engagedunder the same conditions for imaging but with no sample inplace, in order to calibrate the data for any defects in the opticalsystem and variability in the CCD elements' sensitivity, isanother practical necessity. This is accomplished in this systemby utilizing a high-load, long-range motorized linear stage withhigh repeatability (XA10A-R2, Kohzu Precision, Alio Industries,CO, USA) established orthogonal to the central axis of the X-raybeam. This is critical in order to remove the sample from the

Fig. 4 – Computer-aided design model showing the systemconfiguration with both the (a) transmission and (b) reflectionX-ray target heads being implemented. The tube cradle, towhich the X-ray source is mounted in either case, wascustom-designed to maintain focal spot position and beamtrajectory while supporting the 36 kg source with negligibledeflection. The two dark positioners mounted on the rotationstage are the centering stages, and the large linear stagepositioned below the rotation stage is used for acquiringreference images. The rectangular block in line with the beamaxis was replaced with the U200-A3H mirror mount inconstruction, and houses the LuAG:Ce scintillator.

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beampath at predefined increments, acquire a ‘reference frame’,and return the sample accurately to its original position forscanning. This stage has a 20 kg load capacity and a ±0.2 μmrepeatability for this purpose.

Additionally, other long-range linear stages (UniSlideA6012, Velmex, Inc., NY, USA) were implemented in order toalign and position the sample and detector with each otherand with the central axis of the X-ray cone. Thus, the longrange linear manipulators acting parallel to the central axis ofthe cone-beam emission were selected based on the range ofmagnifications desired and the necessary load capacity tosupport the components assembled upon each.

Centering the specimen to be imaged on the center of therotation stage is a practical necessity to optimize the spatialresolution for a given field of view. One requirement for thesestages is a high load capacity to enable large in situ experimen-tation chambers orheavy samples. Another requirement for thecentering stages is that their footprint be contained within theplatformof the rotation stage, limiting the selection to relativelysmall manipulators. Although the rotation stage has a capacityof approximately 27 kg, it is also quite heavy (2.1 kg); with thefully assembled sample translation stack, excluding the center-ing stages, the actual weight capacity of the entire stack islimited by the Kohzu XA10A positioner, which has an excesscapacity of 15 kg. implying that any centering stage need belight weight but also need not have a load capacity larger thanapproximately 14 kg. Two linear-axis piezo centering stages of5 kg load capacity and 2 nmresolution (PPS-28,MICOS,USA andLLC, CA, USA) were identified as a solution, load capacity andfootprint area considered. The load capacity falls just shy of theload capacity goal considering the weight of the uppermanipulator itself acting on the lower manipulator, whereasthe positioning precision is much higher than required center-ing. It would of course be possible to remove the centeringstages in the event a heavier than 5 to 15 kg sample or chamberbe required, albeit with no mechanism for sample centering.

The selection and verification of component compatibility,and integration into the system design, was made possible byuse of computer aided design (CAD) and simulation, byincorporating part models made available from equipmentmanufacturers. Using CAD tools, it was possible to design thedual-target X-ray cradle in this system to maintain beamposition and emission axis when switching between X-raysource target heads, critical for multi-scale 3D imaging (Fig. 4).This was accomplished using tight tolerance machining ofprecision ground aluminum plate and steel rod to construct thecradle, wherein mounting positions were determined via X-raytarget position per computer model within the X-ray headsrespectively. Other implementations of CAD design includedpositioning and alignment trials of all the components consid-ered for selection, and the design of component-to-componentadapter plates in both the sample and the detector component‘stack’.

In-situ experimental capabilities are made possible byspecially designed, experiment-specific, stimulus-applying(mechanical load, electrical current, thermal cycling) cells;the accommodation of such experimental cells into the designof the overall system is simplified by using CAD analysis. Theprimary consideration in the design of any experimental cellis versatility (in terms of sample size/geometry and the type of

sample stimulus applied) and programmability. In previousstudies [27,28], in situ XCT tensile cells have been designed foruse in a synchrotron beamline, where a main designcompromise is the cell's X-ray transparency balanced againstload frame rigidity. In lab scale cells, an added design concernis the footprint of the cell, which limits the X-ray source torotation axis distance, leading to lower X-ray flux through thesample and longer necessary exposure times for a givenorientation. Hence, the primary considerations for enablingaccommodation of these cells into the imaging system is thevertical alignment with the X-ray source/detector, the X-raytransparency of the cell, the footprint, the method formounting on the centering/rotation stages, and the loadcapacity of the motion stages in the specimen motion stack.

The CAD approach was also instrumental to the cameraand scintillator-housing design and their physical linkage(Figs. 5 & 6). A camera support bracket was designed to mountonto a larger plate, which also supports the scintillator. Thescintillator housing is required to provide a stable support forthe scintillator, and provide alignment capability for rotationand pitch in order to realize a scintillator which is perpendic-ular to both the X-ray source cone axis and the camera/lensassembly. Another requirement is the ability to adjust thespacing between the scintillator and the camera lens, i.e., theworking distance. For this mode of control, it is desirable tohave highly linear motion in order to maintain a flat, focusedimage field on the scintillator. Another important consider-ation is keeping dust from settling on the scintillator surface,which results in dead-zones in the detection system. Thisnuisance can be alleviated by proper scintillator containmentin the housing design, the approach here being to contain thescintillator within a sealed optical tube.

The scintillator and its supporting substrate weremountedwithin an optical tube (SM1L10, THORLABS Inc., NJ, USA)

Fig. 5 – Scintillator tube housing assembly in CAD design andphotograph: (a) Optical tube (b) Scintillator (c) Retaining Rings(d) Kapton film sealing front with graphite spray for opticalseal, shown as transparent.

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between two low stress retaining rings (SM1LTRR, THORLABSInc., NJ, USA) as shown in Fig. 5. The ‘front’ of the scintillatorassembly was sealed off by a 0.001″ thick layer of Kapton film,pinned to the front of the tube with a third retaining ring. Theassembly was necessarily performed in a clean room envi-ronment. The back of the scintillator tube was sealed by a 6 mmleaded glass window: Any dust settled on this surface is well outof focus, 2 mm away from the scintillator. Additionally, theleaded glass offers minor shielding of the lens from X-rays, buttransmitting scintillating light. The tube was optically sealed atthe front by a graphite spray, and in the back by a bellows linkingthe tube to the lens. The material at the front of the tubeattenuates X-rays to some extent, and must be minimized.This was accomplished by using very thin, low attenuating

Fig. 6 – Detector assembly in CAD software: (a) Camera (b)Camera support and shield (c) Base plate (d) Long-rangeextension plate, for changing extension tubes (e) Focusingstage (f) Mirror-mount-to-focusing-stage adapter (g)Mirror-mount post (h) Mirror mount (i) Shaft collar supportfor (j) Optical tube assembly (k) Bellows material (not shown)(l) Lens (m) Extension tubes and adjustable length collar (n)Right-angle mirror housing (o) Right-angle mirror.

film. The scintillator containing tube is mounted within amirrormanipulator (U200-A3H, Newport Corporation, CA, USA) toadjust pitch. Other than the scintillator tube assembly, and theshaft collar and manipulator which contain it, no additionalmechanism was established to prevent lens burnishing. Designapproaches for accomplishing this are mentioned in Section 4.2.After approximately one year of operation, a darkening of thelens is apparent but has yet to be fully quantified, although anychanges in exposure requirements therefrom are not apparent.

The scintillator assembly is incorporated into the fulldetector assembly as shown in Fig. 6. A fourth motorized linearstage {XA04A-R2, KohzuPrecision, Alio Industries, CO, USA}wasimplemented to aid in focusing the optical lens systemonto thescintillator. This stage has a 10 mm travel range with 2 μm oftravel permotor step and is sufficient for focusing: Longer travelranges were enabled by a custom slider plate onto which thefocusing stage was mounted. This stage's 4 kg load capacityeasily accommodated the scintillator assembly herein de-signed. Aligning the scintillator tube axis with the lens axis,where several components and adapter plates are required,would have been a much bigger challenge without access tovirtual part design and assembly.

6. Operational Methods

The X-ray radiographic projections were acquired using thissystem's acquisition program, written in LabVIEW. The scanprogramenables control of the exposure time, CCDbinning, scanrotation start angle, scan rotation end angle, scan rotationincrement, number of frames to acquire for each orientation ifaveraging is to be used, the frequency of reference and dark fieldimaging for calibration, the position of the reference stage forreference imaging, the position of the reference stage for sampleimaging, the X-ray source accelerating voltage, the X-ray sourcetarget current, and the X-ray source electron beam centeringinterval. Reconstruction was performed using two sources forfast-Fourier-transformback-projection algorithms using the FDKmethod via MATLAB computational software by MathWorks.The firstmethod implementedwas theOpen-Source Cone-beamApparatusReconstructor (OSCaR) developedoutof theUniversityof Toronto [29]. Another approach implemented enabling morecontrol of memory allocation and GPU functionality is by usingopen-source FDK reconstruction code made available by KyungSang Kim on the MathWorks' Matlab Central File Exchangewebsite [30].

7. Results

A composite material of interest was used to demonstrate the3D imaging capabilities of this system. The material ofinterest in this case is a plastic-bonded-explosive (PBX)material simulant (PBXS), wherein spherical silica glassparticles are representative of energetic particles which areembedded in a hydroxyl-terminated poly-butadiene (HTPB)matrix. The microstructure of this material is shown in themicrograph of a representative area taken of a sectioned PBXSspecimen in Fig. 7. It is known from optical microscopy thatthere is a bimodal distribution of glass spheres, with nominal

Fig. 8 – Two orthogonal projections (a) 0° and (b) 90° of thePBX sample acquired in the 2048 format using 40 kVP and700 μA during the 180° scan with 1/8° sample rotationincrements of projection.

45M A T E R I A L S C H A R A C T E R I Z A T I O N 9 2 ( 2 0 1 4 ) 3 6 – 4 8

diameters for each of the two distributions at 30 μm and350 μm. The samples were prepared with approximately56 vol.% large particles, 19 vol.% small particles, and 25 vol.%of polymeric binder.

In order to characterize the 3 day microstructure of thissystem, prior to studying damage initiation and evolution infour dimensions, it was necessary to reconstruct the initialsample microstructure and optimize parameters for dataacquisition. This sample, about 2 mm × 3 mm in cross section,was imaged 1441 times corresponding to each unique angularposition having a 1/8° multiple between 0° and 180°, where 0°position corresponds to the starting position of the sample inthe first image. Two orthogonal, calibrated, radiographicperspectives acquired in the 1441-projection-scan are shownin Fig. 8. These radiographs were acquired by X-ray emission at40 kVP with 700 μA of target current, with an X-ray magnifica-tion of 1.8× and a lens magnification of 3.6×. The X-raymagnification was selected such that the sample filled but didnot overfill the FOVof the detector, given the 3.6×magnificationof the lens used, which is the standard magnification factor forthe lens. The SAD in this initial scan was not minimized, wasmeasured to be 37.4 mm, and resulted in long, 90 s exposures,with no binning, due to a low flux through the sample. Eachpixel corresponds to a (2.3 μm)2 area in the sample. Eachprojection was calibrated by the use of a master dark and amaster flat frame resulting in the images shown in Fig. 8.

Reconstruction of the sample's 3D structure was per-formed using cone-beam FDK FFT back-projection algorithmsimplemented using MATLAB script. The full reconstructedvolume without cropping is a 2048 × 2048 × 2048 array (deter-mined by the detector format) of single precision numbers(determined by the computational precision), representing

Fig. 7 – Optical micrograph of glass-mock plastic bondedexplosive sample acquired at 50× magnification. The lightlyshaded circular objects are the glass particles simulatingenergetic material, and the dark matrix phase is the HTPBparticle binder. One challenge with imaging this specimenusing traditional techniques requiring traditional samplepreparation is the tendency for the glass spheres to debondfrom the polymer binder, evident in this image by the largedark pores. This is not an issue in XCT.

the average calculated X-ray attenuation coefficient for thematerial sampled within each voxel. Each voxel, or number,within the 3D array corresponds to a (2.3 μm)3 volumeelement within the sample, determined by the spatialsampling frequency from the detector pixel size and X-raymagnification.

It was determined that the reconstructed volume accu-rately represents the sample's 3D microstructure by usingknowledge gleaned from 2D optical analysis. The grayscalecontrast between voxels representing portions of the largeglass spheres (350 μm nominally), and those representingportions of the matrix, was large enough to yield satisfactorysegmentation of the glass particles by a global threshold. Thesame approach was also sufficient for the larger of the smallglass spheres (30 μm, nominally), but was not effective inaccurately segmenting the smallest glass particles. For thistask, a more sophisticated segmentation technique is re-quired, such as a live-wire or local-thresholding approach.The effect of thresholding the grayscale values in thereconstruction volume is shown in Fig. 9 for the Y–Z slice,where columns are parallel to the rotation axis, and rowsperpendicular.

8. Discussion

Thresholding allows for separate analysis of the entire solidcomposite, the silica glass phases, and polymeric matrix phase.Measurements which are of interest, and may represent theextent of damage, for the entire solid include macroscopicvolume, contained voids, and shape. 3D measurements on theglass particle size distribution, shape, and spacing are of interest,while interconnectivity and cross-sectional-area-fraction are ofinterest in the matrix. On a segmented 3D dataset, these mea-surements are easily performed using either 3D viewer softwareor array analysis functions in MATLAB. Discretely determiningthe phase represented by each voxel, performed for this datasetand shown in Fig. 10, allows for the segmentedmicrostructure tobe analyzed quantitatively, deployed in modeling studies andultimately to be used in 4D Digital-Volume-Correlation studies,where particle tracking in 3D over time and under stimulus canbe used to quantify local strain.

Fig. 9 – Different binary image results from thresholding (a) the grayscale reconstruction for analyzing either (b) the entire solidor (c) only the silica glass particles from (a) the gray scale reconstruction, shown on the central Y–Z slice (top) and for theresulting 3D volume (below). Scale bar applies to the top 2D images only.

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9. Conclusion

The motivation for designing and constructing a modularX-ray computed tomography system has been established. Byunderstanding the limitations and trade-offs for micro-XCT,and implementing a modular design approach, realisticperformance goals were outlined for the system: The ap-proach for meeting these goals has been highlighted. Theprocess for component selection and the design of the systembased on the selected components has been detailed. Theselected components themselves have been discussed, aswell as the approach for integrating both critical and auxiliarycomponents into a fully functional microCT system. A noveldetector design has been described which has met designgoals for field of view, resolution, and efficiency, by optimiz-ing X-ray magnification, lens magnification, lens resolvingpower, scintillator resolution, X-ray detection efficiency, andfocal spot size through adjustable parameters and dependingon the sample and imaging goals: The level of controlachieved by this instrument is unmatched by any identifiablelab-scale X-ray computed tomography instrument. Thedetector's lens system provides adjustable optical magnifica-tion in conjunction with interchangeable scintillator assem-blies of different, optimal scintillator thickness, and a methodfor precise focusing of the lens on the scintillator has beenachieved: Together with the X-ray source characteristics,these tunable parameters provide an unrivaled control overtrue imaging resolution balanced against detection efficiency.

Unlike other lab-scale systems, this system is componentupgradeable to extend useful lifetime and to maintainstate-of-the-art functionality. Along the same lines, thisinstrument is unmatched in terms of in situ experimentalversatility. High precision alignment was easily accomplishedfor accurate reconstruction in this instrument through thetube cradle design and independent 3-axis sample anddetector control. Background calibration of data was accom-plished in a streamlined fashion by implementing automatedroutines during the scan which take advantage of motorizedmotion stages. High energy X-ray scans have been enabled bythe capable X-ray source and custom detector design remov-ing the CCD chip from X-ray beam which can cause noisedegradation of images. The approach for data acquisition anddata processing has been outlined (Fig. 11).

The total construction cost of the system as detailed isapproximately one-third of a comparable commercial systemtoauthors' understanding, primarily considering X-ray tubeaccelerating voltage range, detector field of view, and claimedspatial resolution. It is worth noting that nearly half of the totalsystem's resource allocation was directed towards the dual-target microfocus X-ray source because of its performancespecifications and their compatibility with the goals of thediscussed tomography, as previously detailed in Sections 2, 3,and 4. Approximately ¾ of the total cost was directed towardsthe primary components, and the remaining ¼ towards theauxiliary components. It is also worth noting that the nearestcommercial system, on comparison of imaging performanceand sample accommodation, was retailed for approximately

Fig. 10 – 3D rendering of the segmentedmicrostructure in thePBX sample using the scanning and reconstructionparameters herein described. The blue (dark) sphericalobjects represent the silica glass particles whereas the gold(light) interconnected phase represents the HTPB polymerbinder. The dimensions of the cropped bounding box X–Y–Zis 3755 μm–3223 μm–4741 μm, respectively.

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three times the cost of this custom system at the time ofconstruction.

The steps of data acquisition and processing for 3D samplereconstruction were demonstrated on a silica-glass particle/hydroxyl-terminated-polybutadiene matrix binder plastic bond-ed explosive simulant. The utility of the constructed system asper the outlined performance goals has been demonstratedthrough a discussion on themicrostructure phase segmentationresults for 3D material survey. 3D renderings of the discussedsample's reconstructedmicrostructure as determined by thres-hold segmentation have been provided for completeness.The approach described herein has resulted in the con-struction of a nondestructive imaging apparatus whichprovides a high degree of experimental utility for in-situhigh-resolution materials-oriented research.

Fig. 11 – The relative cost of the components within thecustomX-ray computed tomography system to the total cost,displayed as a pie chart.

Acknowledgments

We would like to gratefully acknowledge the funding andsupport from the Security and Defense Systems InitiativeArizona State University (ASU), the Semiconductor ResearchCorporation, and the Air Force Office of Scientific Researchthrough a collaboration with NextGen Aeronautics. We wouldlike to acknowledge the support of Dr. Werner Dahm and Mr.Mark Giddings at ASU, Dr. Mario Pacheco (Intel Corporation,Chandler, AZ, USA) and Dr. Stephanie O'Keefe (Next GenAeronautics, Torrance, CA, USA). We would also like toacknowledge the support of X-RAY WorX and their staff. Ourgratitude is also expressed for the peer review process forextracting informative details which would have been omit-ted otherwise.

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