Conference on Applied Digital Imaging Techniques for Understanding the Palimpsest X-ray Fluorescence Imaging Following Synchrotron Beam Excitation Uwe Bergmann Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center April 1-4, 2004 Walters Art Museum Baltimore, Maryland
Outline introduction to synchrotron radiation (SR) x-ray transmission imaging x-ray fluorescence (XRF) imaging examples of SR based XRF imaging details and numbers proposed plan
Synchrotron Radiation Spectrum
SR Facilities Around the World 54 operational light source rings in 19 countries 8 rings in construction 11 in advanced stages of design/planning For a list of SR facilities around the world see: http://ssrl.slac.stanford.edu/sr_sources.html
Four Largest US Synchrotron Sources Stanford Synchrotron Radiation Laboratory (SSRL) Stanford Linear Accelerator Center (1974) National Synchrotron Light Source (NSLS) Brookhaven National Laboratory (1982) Advanced Light Source (ALS), Lawrence Berkeley National Laboratory (1993) Advanced Photon Source (APS), Argonne National Laboratory (1996)
Different Types of Synchrotron Sources schematics of a synchrotron lab
Properties of Synchrotron Sources? - very intense - tunable - polarization - short pulses
Concept of X-ray Transmission Imaging 1 CaFeZn, 2µm - measure transmission at two energies: below and above absorption edge of element of interest - take difference to enhance contrast - chemical dependence Transmission 0.98 0.96 0.94 0.92 plus: - stereographic imaging is possible - line sources: faster imaging 0.9 1.5 (a) 4 6 8 10 12 Energy [kev] Fe 2 O 3 Fe (acac) 3 minus: - large ink concentration necessary - sample has to be sufficiently transparent to x-rays Normalized Absorption 1.0 0.5 Fe 2+ Fe 3+ 1.5 1.0 0.5 Fe 3+ FeCp 2 FePO 4 0.0 0.0 7120 7140 7160 7180 7120 7140 7160 7180 Energy (ev)
Concept of X-ray Fluorescence Imaging transmission x-ray energy sample e - level diagram Fe valence levels Kß 2,5, Kß 3p fluorescence detector x-ray fluorescence incident x-rays 2p 1s Kß 1,3, Kß Ka 1.2 incident x-ray photon knocks out inner-shell electron hole is immediately filled from electron in outer shell fluorescence photon is emitted, predominantly Ka radiation fluorescence is detected for imaging plus: greatly enhanced sensitivity compared to transmission imaging
Choice of X-ray Energy 1 0.98 CaFeZn, 2µm Ca Ka Fe Ka Zn Ka Transmission 0.96 0.94 0.92 0.9 4 6 8 10 12 Energy [kev]
Examples of XRF Imaging with SR Investigation of calcifications in breast tissue using EXAFS and XRF Sayers et al
Examples of XRF Imaging with SR X-Ray Fluorescence Studies of Elements in Human Lung Tissue M.Z. Kiss, D.E. Sayers, A.C. Thompson, S.A. McHugo, P. Pleshanov, E. Pleshanova1, Yu. A. Bykovsky
Detection of Trace Impurities 10 6 hv = 11200 ev α = 0.085 o Intensity / (5000 sec) 10 5 10 4 1000 Si Ag Cl Fe Cu Baur et al 100 2000 4000 6000 8000 1 10 4 1.2 10 4 Energy (ev) sensitivity to extremely low concentrations much less than millionth of a mono layer!
Schematic Experimental Setup fluorescence from element in ink x scanning directions z detector X-ray beam from synchrotron
Schematic Experimental Setup top view transmission detector front view - continuous x-scans - steps in z-direction z x x detector x-ray beam polarization palimpsest example: 10 * 10 cm 2 scanning area, 100 µm resolution 1/100 sec illumination per data point 10 seconds/line, 1000 lines ~ 3 hours total scanning time intense x-ray beam is important
Some Details - scans are performed with high precision computer-controlled translation stages (Huber) -fast scan ability * - large area Ge detector for fluorescence detection available - incident intensity is recorded for normalization - automated system that closes beam shutter if scanning gets interrupted - tests on sample texts for best x-ray parameters and potential radiation damage - palimpsest can be placed in airtight frame (glove box for loading available) - data analysis programs are available (e.g. Matlab) blue = available now green = still to be done
Proposed Plan build and test all the scanning/imaging hardware identify one or several elements for imaging (e.g. Fe, Ca) study a test object of composition similar to the original text test for appropriate sample protection, X-ray energy, beam size and exposure times (radiation damage) study the original text