X-ray based methods for non-destructive

Nuclear Instruments and Methods in Physics Research A 591(2008)14–18X-ray based methods for non-destructive testing and materialcharacterizationRandolf Hanke Ã,Theobald Fuchs,Norman UhlmannFraunhofer Entwicklungszentrum fu ¨r Ro ¨ntgentechnik,Dr.-Mack-Strasse 81,90762Fu ¨rth,GermanyAvailable online 20March 2008AbstractThe increasing complexity and miniaturization in the field of new materials as well as in micro-production requires in the same wayimprovements and technical advances in the field of micro-NDT to provide better quality data and more detailed knowledge about the internal structures of micro-components.Therefore,non-destructive methods like radioscopy,ultrasound,optical or thermal imaging increasingly gain in importance with respect to ongoing product and material development in the different phases like material characterization,production control or module reliability testing.Because of the manifold different application fields,i.e.,certain physical NDT methods applied to material inspection,characterization or reliability testing,this contribution will focus on the radioscopic-based methods related to their most important applications.Today,in modern industrial quality control,X-ray transmission is used in two different ways:Two-dimensional radioscopic transmission imaging (projection technique),usually applied to inline inspection tasks in application fields like lightweight material production,electronic component soldering or food production.Computed tomography (CT)for generation of three-dimensional data,representing spatial information and density distribution of objects.CT application fields are on the one hand the understanding of production process failure or component and module inspection (completeness)and on the other hand the dimensional measuring of hidden geometrical outlines (metrology).This paper demonstrates the methods including technical set-ups (X-ray source and detector),imaging and reconstruction results and the methods for high speed and high-resolution volume data generation and evaluation.r 2008Elsevier B.V.All rights reserved.PACS:07.79.7c;68.37.yz;81.70.Tx;87.57.Q À;87.59.Àe;87.59.B À;87.59.bd;87.59.bf Keywords:X-ray;Radioscopy;Micro-computed tomography;Image processing1.IntroductionRadioscopic inspection has become one of the most powerful tools in the field of non-destructive testing for industrial material inspection since the discovery of X-rays by Wilhelm Conrad Ro ntgen in the year 1895(Nobel Prize 1901).Already at this time,W.C.Ro ntgen had to face the same problems,which are still today regarded as majorchallenges with respect to advanced research and develop-ment for non-destructive testing:The high dynamic range of measured intensities caused by the exponential attenuation law for radiation in matter. The superposition of object structures along the radia-tion beam direction,caused by projective geometrical imaging (projection technique).Long exposure times,essential for a sufficient signal-to-noise ratio (SNR).Loss in contrast by diffuse background,generated by scattered radiation,which occurs during Compton/locate/nima0168-9002/$-see front matter r 2008Elsevier B.V.All rights reserved.doi:10.1016/j.nima.2008.03.016ÃCorresponding author.Tel.:+49911580617510;fax:+49911580617599.E-mail address:randolf.hanke@iis.fraunhofer.de (R.Hanke).interaction of photons with electrons of the outer atomic shell.The strong attenuation by metals compared to weak absorption of human skin,organs and tissue.2.X-ray beams—a short introduction2.1.X-ray generationX-rays are generated during acceleration of electrons, e.g.,in synchrotrons(synchrotron radiation)or by deceleration of electrons in solid-state materials(Brems-strahlung).Synchrotron radiation provides a highly brilliant beam (up to some1018photons/mrad s)especially capable for diffraction or for experiments which need coherent light (phase contrast method)but is unfortunately only available at some few research facilities in Europe.1On the other hand,X-ray tubes,which essentially create Bremsstrahlung,provide a considerably lower photonflux (some1012photons/mrad s),however are transportable and thus available in every laboratory.As this paper primarily will discuss industrial applications,hence the beam generation is focused on X-ray tubes.Depending on the focal spot size,macro-focus tubes (usually sealed tubes with spot sizes above100m m)or micro-focus tubes(open tubes with spot sizes down to 1m m)are commercially available.The principle of beam generation is shown in Fig.1(by courtesy of Viscom AG). Fig.1demonstrates the generation of Bremsstrahlung with a reflection target,i.e.,X-ray photons are emitted from a massive target anode usually used in a901angle to the direction of the impacting electron beam.As can be seen from thefigure,due to the target angle the size of the optical X-ray focus is different from the electron focus and also the characteristics of the Bremsstrahl spectrumchanges with the length of the absorption path due to varying target absorption of the generated X-rays(Heel effect).An alternative geometry is the transmission target solution,which means that electrons are impacting on a thin metal sheet target and the generated X-rays are emitted in the same direction as the electron beam.This solution offers of course a more homogeneous photon intensity characteristics and enables the realization of smaller focal spot sizes,however creates lower photon intensities compared to the reflection target due to a worse heat dissipation.2.2.X-ray detectionBesides the conventionalfilm radiography,digital imaging became more and more relevant in industrial radioscopy within the last20years.Today,most industrial digital detectors are still based on indirect conversion technology via scintillation effect(cf.Fig.2),which means integrating the detected photon intensity over all energies and converting this intensity into a local signal of visible light,which in turn is detected by silicon semiconductor detectors.Standard cameras are image intensifiers,a-Siflat panels or CMOS detectors,coated with scintillators(GdOx,CsI and others).Depending on the sensor pixel size,the coating thickness has to be adapted to achieve the best detective quantum efficiency(DQE)on the one hand[1]and to gain the optimal spatial resolution of the sensor on the other hand.Another way for X-ray photon detection is the direct conversion of the photons by semiconductors,which means,the impacting photons directly are converted into electron/hole pairs.Because of the progress of these alternative direct converting sensors within the last10 years,this technology may be expected to substitute or at least to partially replace the indirect converting technologyFig. 1.Generation of Bremsstrahlung by impact of electrons on a reflection target(courtesy of Viscom AG).Fig.2.Principle of direct(top)and indirect(bottom)detection of X-ray photons.1/als/synchrotron_sources.html.R.Hanke et al./Nuclear Instruments and Methods in Physics Research A591(2008)14–1815step by step within the next years.The major advantage is the much higher photon sensitivity(single photon counter) and the capability of resolving photon energy.Currently applied sensor semiconductor materials are CdTe, CdZnTe,Si,and GaAs.Compared to indirect converting detectors,today the bad economic conditions(price vs.active sensor area)may still be an advantage for the conventional integrating detectors.However,the significant physical advantages, e.g.,the capability of single photon counting for applica-tions like Nano CT with X-ray sources,having a very low photonflux and thus requiring long exposure times,will foster this method and hopefully provide competitive detector solutions even under economic aspects within the near future.3.Industrial radioscopic systemsAs already mentioned above,the main applications of industrial radioscopy can be differentiated by two-dimen-sional radiography and three-dimensional computed tomo-graphy(CT)(Fig.3).3.1.RadiographyThe main task in thisfield is the transfer of,e.g.,new detection technologies into the industrial production process for100%non-destructive testing, e.g.,of light alloy products,electronic components or even food[2,3]. Therefore,a radiographic system(Fig.4)has to provide X-ray images with highest achievable signal-to-noise ratio at simultaneously short exposure times.In addition, modern inspection systems have to allow for fully automated image processing and evaluation in order to become powerful and reliable tools for inline inspection (Fig.5).X-ray inspection is a method for visualizing a three-dimensional body in a two-dimensional plane,and hence, three-dimensional object details are superposed in a radio-graphic projection.These super-impositions may cause structures resembling defects in the X-ray images,so-called artifacts.Consequently,those artifacts as well as real structures have to be distinguished from real defects by image processing[4].puted tomographyThefield of computed tomography(CT)has attracted tremendous attention in the recent past[5,6].This is mainly due to three reasons:New detector technologiesHigh-performance computersHigh-performance X-ray tubesToday,in general,industrial CT can be differentiated into axial and planar CT depending on the geometry of the objects under inspection(cf.Fig.3).In particular,research activities are currently focused on Helical CT,High-Energy CT,Micro/Nano CT,Inline CT,and Robot CT.In this context,the success of system realization will strongly depend on the features of new detectors as a key component for CT!Regarding especially the Robot CT(Fig.6),which shall be applied for non-destructive X-ray laminographic inspection of large and bulky objects like aircraft fuselages orfins,large detectors with high sensitivity at low energiesFig.3.Different methods in industrialradioscopy.Fig.4.Schematic set-up for an industrial X-ray inspectionchain. Fig.5.Realization of an intelligent system for automated radioscopy (ISAR)for fully automated inspection of light alloy wheels.R.Hanke et al./Nuclear Instruments and Methods in Physics Research A591(2008)14–18 16are of utmost importance in order to provide quantitative means for testing carbonfiber reinforced components. Inline CT will need high-speed area array detectors, capable to provide images with a sufficiently high SNR and enabling readout frequencies of at least20frames per second,i.e.,exposure times below50ms.Nano CT systems are planned to be realized by enhancing an electron probe micro analyzer(EPMA)to a real nano focus X-ray source with focal spot sizes down to 100nm.The comparable low source intensity in combina-tion with a high demand on spatial resolution requires the use of highly sensitive and directly converting detectors with pixel sizes below10m m.In case of large parts,which show difficult geometries and aspect ratios,for instance rotor blades,Helical CT technique will be applied as a practical method to handle such objects.4.Applications in industrial material inspectionIn this chapter,some results for applications in different businessfields are presented.4.1.AerospaceFig.7shows two typical examples for the aerospacefield, representing the challenges in radiographic testing.In case of rotor blades,X-ray technology is applied to carbonfiber composites with low absorbing materials and low atomic numbers,whereas the turbine blades consist of strongly absorbing material(nickel-based alloys).Chal-lenges with these objects and materials are varying physical effects like radiation scattering or beam hardening.The development of CT systems,which are capable to detect, e.g.,pores or smallest delaminations within the composites or to measure small and hidden coolant boreholes of the turbine blades,currently is in progress.4.2.AutomotiveIn thisfield of business,the application of radioscopy is mainly focused on inline inspection accompanying the mass production of safety relevant components by advanced casting techniques.Thus,one major task will be the development and industrial realization of high-speed CT in order to generate and evaluate CT volume data,as demonstrated in Fig.8,within some10s for each part. 4.3.ElectronicsThe challenge in electronic component inspection is on the one hand the inapplicability of axial CT due to the geometry of,e.g.,printed circuit boards(PCB).On theFig.6.Schematic representation of a CT system with two communicating robots for X-ray source anddetector.Fig.7.Examples of radioscopy and CT inspection tasks in the aerospace businessfield.Fig.8.3D visualization of a cone beam CT reconstruction of an aluminum steering gear case;dark spots represent small blowholes.R.Hanke et al./Nuclear Instruments and Methods in Physics Research A591(2008)14–1817other hand,there is the need of highest resolution in therange of1m m and below to characterize the quality and reliability of wire bondings directly on chips and semi-conductor components.In order to check a PCB,laminography or digital tomosynthesis is applied to this inspection task.This means that only data from a limited range of projection angles are used for reconstruction.Even though there is a loss of information due to the incomplete set of data,Fig.9 demonstrates clearly the capability of this method to separate the different layers with a spacing of the single layers of approximately50m m.Nevertheless,due to the lack of information,a(negligible)contribution of blurring still is visible.Fig.10shows the sub-micro CT reconstruction of a CMOS chip.The size of one reconstructed volume element (voxel)in each of the three spatial dimensions is approximately900nm.The reconstruction was based on X-ray cone beam data,acquired during a single rotation of the object.AcknowledgmentsThis project was supported by the Freistaat Bayern and the European Community.References[1]K.Stierstorfer,M.Spahn,Self-normalizing method to measure thedetective quantum efficiency of a wide range of X-ray detectors,Med.Phys.(1999)26;Am.Assoc.Phys.Med.(1999)1312.[2]R.Hanke,T.Wenzel,Image processing for inline digital industrialradiology,J.Nondestruct.Eval.,Indian Soc.NDT(India)20/3(2001).[3]R.Hanke,T.Wenzel,U.Hassler,A new radioscopic system for postcast perfection,Foundry Trade J.176(3594)(2002).[4]T.Wenzel,R.Hanke,Texture analysis for classification of cast defectsin X-ray images,Second Workshop on NDT in Progress,Prag,2003.[5]R.Hanke,Microanalysis by volume computed tomography,in:G.I.T.Imaging&Microscopy,April2003,pp.40–43.[6]T.Fuchs,W.A.Kalender,Detectors for CT scanners,in:Alberto DelGuerra(Ed.),Ionizing Radiation Detectors for Medical Imaging, ISBN:981-238-674-2,2004,pp.125–147.Fig.9.Digital tomosynthesis applied to a multilayerPCB.Fig.10.Visualization of a cone beam CT reconstruction of a smallfraction(left)of a CMOS memory chip.R.Hanke et al./Nuclear Instruments and Methods in Physics Research A591(2008)14–18 18。