hutch had to be moved in order to accommodate our stage! It consists of two 600 mm linear translation stages that sit on a third, wide-base 100 mm translation.
Arthur Woll Cornell High Energy Synchrotron Source, Cornell University
In January 2003, Don Bilderback attended the HASYLAB user meeting and heard about an exciting new development in X-ray instrumentation. Two European groups, one at BESSY II [1] and the other at HASYLAB [2], had demonstrated how to combine two focusing X-ray optics to isolate fluorescence signals from a 3D volume in space. The idea, called confocal X-ray fluorescence microscopy (CXRF), immediately piqued Don’s interest as a possible new application for single-bounce monocapillary X-ray optics, technology Don and his co-workers have developed at CHESS over the last several years. As a new staff scientist on the lookout for exciting projects, I became quickly interested as well. That Spring, Sol Gruner described the technique to Jennifer Mass, a chemist and conservation scientist working for the Winterthur Museum in Delaware, and a collaboration was born.
Research Highlight
Science meets art: Confocal X-ray Fluorescence Microscopy at CHESS
focuses the incident X-rays to a narrow beam. If this beam At the CHESS Users’ Meeting later that year, I invited Jennifer is incident onto a multilayered sample such as that shown to introduce CHESS to the field of conservation science, in Fig 1c, fluorescence will be excited from all of the layers and to discuss what resolution would be required to resolve simultaneously. A second optic is used to limit the area from individual layers in a painting. Her talk [3] was both exciting which fluorescence is collected to a narrow portion of the and humbling. An immense fraction of painted works hide incident beam path, resulting in an active volume several tantalizing, secret treasures – everything from abandoned tens of microns large on each side. sketches to complete works of art that were later painted over – often by the same artist that produced the buried work. Unfortunately, the layer thickness and composition varies tremendously among different artists. We learned that while some artists used paint layers several tens of microns thick, paint-layer thicknesses of 5-10 microns were not uncommon, and some paint layers can be extraordinarily thin – one micron and below. At that time we knew that a depth resolution of ten microns was probably within reach, making certain artists off-limits from the beginning. There are other complications as well. It is estimated [4] that one third of Van Gogh’s entire work lies unseen, beneath his other works. Yet, while he often used very thick layers, he did not always wait for paint to dry before applying the next layer, resulting in dizzyingly complex structures as seen by cross-sectional scanning electron microscopy Fig 1: Schematic illustrations of the confocal X-ray fluorescence geometry. (a) shows the experimental setup, indicating the multilayer optics (ML), slit, ion (SEM). Such structures would be very difficult chamber (IC), beamstop (BS), focusing and collecting optics, and the detector. to reconstruct with CXRF. (b) shows a 3D view of how the confocal active volume is formed from the focal regions of the two optics. (c) indicates, in cross-section, how the sample is scanned through the confocal volume, so that fluorescence from individual layers is resolved.
Ning Gao, a lead scientist at X-ray Optical Systems (XOS, Albany, NY) and collaborator on the CXRF project at HASYLAB, also attended the meeting [3], and described the current state of the art in polycapillary optics, the single most important technological development responsible for making CXRF a practical technique. At the end of the meeting, we all sat down together and planned a first set of experiments, to take place in October 2003. The geometry of CXRF is illustrated in Fig 1b-c. One optic
The first run was conducted at CHESS station D1, using an incident energy of 16 keV, selected by a multilayer monochromator with a d-spacing of 25.5 Ångstroms and bandpass of 1%. The experimental setup is shown in Fig 1a. In our setup, and unlike prior demonstrations, we used one of Don’s single-bounce monocapillary lenses to focus the incident beam. Apart from being readily available at CHESS News Magazine 2005
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Research Highlight
CHESS, these optics have a particular advantage in focal length, which can easily reach several cm. They are also highly efficient, and have a focal-spot size that is nearly independent of energy. Our estimated flux was 1011 photons/second in the focal spot. At this intensity, the detector was easily saturated by fluorescence from the sample, forcing the use of a 1.6 mm Al attenuator to reduce the incident intensity by approximately 20 times. For the collecting optic, we used a polycapillary lens, on loan from XOS. Among the many types of X-ray optics, these optics have large collecting angles – upwards of 25 degrees – and are therefore uniquely suited to collecting fluorescence from a point source. Because of this, all demonstrations of CXRF to date have used polycapillaries as the collecting optic. Our borrowed optic had a collection angle of 15.4 degrees, a working distance of 5 mm, and an efficiency of 13% at 8 keV. Fluorescence was collected by a Rontec Xflash silicon drift detector (Carlisle, MA) placed behind the polycapillary.
The resolution is characterized by a series of thin metal films deposited onto glass slides or silicon substrates. The depth resolution of the microscope, defined by the full-width-half-maximum of a fluorescence peak obtained from such films, was approximately 34 mm above about 8 keV, but increased dramatically below this energy, reaching 55 microns at 2.5 keV. This energy dependence is due to that of the polycapillary. More recently, we have obtained much better resolution, down to 22 microns at 10 keV, by replacing the monocapillary optic with one having improved focal properties. The maximum sensitivity of the system was found by measuring the signal from a lithographically patterned titanium dot on a Si substrate. The Ti Ka count rate from a 16×16×0.02 mm3 dot, corresponding to a mass of 5×10-12 g, was approximately 2200 counts per second. The first results were obtained on several multilayered paint samples on glass slides, which were prepared and characterized before the first run by Christina Bisulca (cover), a master’s student with the University of Delaware’s conservation science program, then beginning her second year. The layer order was chosen to simulate a structure typical of historic paintings, e.g. with more modern pigments, such as those containing titanium, on top of older ones, such as lead-white or mercurycontaining vermillion. Fig 2a shows data collected during a single depth scan on one such sample, having the layer order: titanium white (TiO2)/ orpiment(As2S3)/iron oxide(Fe2O3)/chalk (CaCO3). This sample also incorporated spacer layers of polyvinyl acetate (PVAc) between each distinct pigment. From their location on the horizontal, energy axis, the peaks in Fig 2a are easily identified as the K lines of titanium (4.51, 4.93 keV), iron (6.40, 7.06 keV), calcium (3.69, 4.01 keV), and arsenic (10.54, 11.73 keV); additional peaks in Fig 2a appearing at twice the energy of the two titanium peaks are identified as double-photon-counting events, while the peak at 2.8 keV is a Si escape peak, occurring at 1.7 keV below the Ti Ka line. All of these are detector artifacts. The vertical position of the fluorescence peaks in Fig 2a indicates the depth of each element. Fig 2b shows a cross-section optical micrograph of the same sample, which was cleaved after the data in Fig 2a were taken. The image was scaled to match the depth scale of Fig 2a, and the dashed lines indicated the estimated positions of each interface. The correspondence between the fluorescence peaks in Fig 2a and the layers in Fig 2b is evident. Fig 2c shows the Ka lines for each of the four elements identified in Fig 2a. The solid lines indicate best-fit curves, and density profiles obtained by fitting to a model described in Ref. [5] are also shown in the plot. In brief, the model accounts for the smearing of intensity vs. depth profiles by the finite detector resolution, in order to extract sharply-defined layers for each element. The shaded regions in Fig 2c represent the best-fit thicknesses from this model, which are listed in the third column of Table 1. A conspicuous feature of these profiles is the apparent absence of any pigment between the As and Fe layers (yellow and Fig 2: (a) Fluorescence spectra plotted as depth vs. energy for a multilayered paint sample on a glass slide. Brightness indicates intensity on a logarithmic scale. The main arsenic peak (10.54 keV) has a long tail of intensity penetrating into the sample, corresponding to arsenic in the glass slide. (b) Optical crosssection micrograph of the sample shown in (a), close to the position at which the data in (a) were obtained. The dark red middle layer is attributed to PVAc (see text). The depth scale matches that of (a). Dashed lines indicate the estimated locations of the interfaces. (c) Intensity vs. depth of the four Ka peaks of Ti, As, Fe, and Ca from (a). The Ti Ka and Ca Ka data have been re-scaled for the plot, as indicated by the inset. The calcium peak has a shoulder due to a low-energy “tail” from the bright Ti peak, which is evident in (a). The solid lines and shaded regions represent the best-fit curves and individual layer profiles as extracted from fits to the model described in Ref. [5].
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Table 1 Measured Paint Layer Thickness - Chronology 2 Optical Thickness Measurement: Layer Range /Average (Pm) Titanium White 13-33 / 22 Orpiment 10-36 / 17 PVAc 7-23 / 16 Iron oxide 20-43 / 29 Chalk 7-30 / 17 - Glass slide -
Confocal XRF (Pm) 29 ±5 13±5 21±5 24±5 12±10
Research Highlight
orange regions in Fig 2c). According to the fits of adjacent layers, this unpigmented layer is approximately 20 microns thick. The optical cross section shown in Fig 2b was used to identify this unpigmented region as a particularly thick layer of PVAc, the spacer layer material nominally applied between each layer. Because PVAc does not contain elements that fluoresce in the hard X-ray regime, it shows up as a missing layer in the confocal XRF data. Table 1 compares the other layer thicknesses of this sample as measured by these two techniques. In general, the measurements shown in Table 1 agree very well.
Based on our demonstration work on samples such as that in Fig 2, we applied for and were granted an NSF Instrumentation for Materials Research award (DMR-0415838) to develop CXRF, specifically for the study of antique paintings. The grant solidifies the fruitful collaboration between CHESS, the Winterthur Museum, and the University of Delaware. It also allowed us to purchase the polycapillary optics we had thus far only borrowed, and to design and build a sample manipulator sufficient for studying large paintings. In April 2005, our custom 3D sample manipulator (American Linear Manufacturers, Westbury, NY), shown in Fig 3, arrived at CHESS. CHESS staff members Alan Pauling and Jim Savino contributed key ideas to the manipulator design, and Alan modeled the complete stage in Autocad, which proved critical to the design and optimization process. As it was, a ceiling-mount air conditioner in the D1 hutch had to be moved in order to accommodate our stage! It consists of two 600 mm linear translation stages that sit on a third, wide-base 100 mm translation stage. The easel which is mounted to the stage can accomodate paintings up to 1.1 × 1.4 m2. Vibrations at the scan point are limited by employing a ball transfer just behind the position X-rays strike the sample, which acts as a pivot point. The ball transfer is mounted to the scanning axis, the y-axis in the figure, and presses against a square aluminum sheet, which is mounted to the easel frame. The stage was commissioned at the end of April, and performed very well. On its maiden voyage, initial studies were performed on a 17th century oil on wood panel, The Armorer’s Shop, painted by the Flemish painter David Teniers, in collaboration with conservators Noelle Ocon and Bill Brown from the North Carolina Museum of Art. Other collaborations with the Museum of Modern Art, the Getty, and the Art Institute of Chicago are underway. This work is supported by the NSF under DMR 0415838, and also by CHESS, which is supported by the NSF and the NIH.
Fig 3: Rear view of the new large area 3D manipulator stage with easel. Paintings are mounted to the opposite side of the aluminum plate. X-rays strike the painting at the position of the ball transfer.
References [1] B. Kanngiesser, W. Malzer, and I. Reiche, Nucl. Inst. Meth. B, 211, 259 (2003). [2] K. Proost, K. Janssens, L. Vincze, G. Falkenberg, N. Gao, and P. Bly, HASYLAB Annual report (2002); K.Janssens, K.Proost, and G. Falkenberg, Spectrochimica Acta, 559,1637-164, (2004). [3] http://meetings.chess.cornell.edu//UserMeeting2003/Agenda03.htm#workshops. [4] Joris Dik, Delft University of Technology, Synchrotron Radiation in Art and Archaeology, Workshop of the ESRF 2005 Users Meeting (February 2005). [5] A.R. Woll, J. Mass, C. Bisulca, R. Huang, D. Bilderback, S. Gruner, and N. Gao, Synchrotron Radiation in Art and Archaeology, Workshop of the ESRF 2005 Users Meeting, February 2005. (submitted), A.R. Woll, D.H. Bilderback, S. Gruner, N. Gao, R. Huang, C. Bisulca, and J. Mass, Proceedings of the Materials Research Society, Fall MRS Meeting, 2004, Session OO, Boston, MA (2004). CHESS News Magazine 2005
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