Transmissive XBPM developments at PSF/BESSY Martin R. Fuchs
Acknowledgments PSF Martin Fieber-Erdmann Ronald Förster Uwe Müller BESSY Karsten Blümer Karsten Holldack Gerd Reichardt Franz Schäfers BIOXHIT, ESRF Thierry Martin John Morse
Contents Transmissive XBPMs based on fluorescent foils and Position Sensitive Devices Introduction Fluorescent foil XBPM Sedimentation deposition of phosphors Electrophoretic deposition Position Sensitive Detector XBPM
Requirements on a transmissive beam monitor Opaque monitors: Commissioning, debugging Automatic correction of slow drifts << 1 Hz Transmissive monitors: Automatic alignment / optimization of beamline Stabilization of fast drifts / vibrations Requirements: Measurement of beam position, intensity and profile High time resolution (from slow drifts up to 100 Hz) For > 1 monitor in beamline: Absorption < 10 % over a: Broad photon energy range Radiation hard to fluxes of typically 10 11 photons/s (up to 10 14 photons/s)
Transmissive X-ray beam position monitors Imaging monitors Pixel based photocurrent detector monitors Fluorescent foil monitors: Phosphor powder coated screens Scintillators fluorescing CVD Diamond beam intensity, position, profile limited time resolution Blade monitors beam position (limited) high time resolution high flux capacity
Center of mass position monitors Back-fluorescence quadrant diode array beam intensity, position Quadrupole ionization chamber high time resolution Quadrant electrode photocurrent monitors Thinned Position Sensitive Detector
Fluorescent screen XBPM fluorescing foil observed via lens coupled CCD time-resolved imaging monitor: - beam intensity - center of mass position - 2-dimensional beam shape XBPM design in use at BESSY and PSF beamlines [K. Holldack et al., 2000, BESSY Annual Report] Current monitor control GUI (VC++) alternative LabVIEW GUI in use at BESSY
Transmissive fluorescent screen XBPM Extension to existing beam-occluding beam monitor A C Phosphor screen (A) and deflection mirror (B) holder with 3-point arrest (C), modified for transmission operation B A original occluding SPOT setup new transmittant setup normal incidence imaging of beam spot: thinnest possible screen thickness compact pneumatically retractable design microscope with zoom objective large numerical aperture possible due to 90 degree deflection
Phosphor materials Requirements on phosphor materials: absorption < 10 % over photon energy range 6-20 kev phosphor grain size < 3 µm radiation hard for prolonged exposure times fast excitation decay, negligible afterglow must work in high vacuum conditions Calculated X-ray transmission curves for various phosphor bulk materials for a foil thickness 1µm
Gravity settled screens Screens coated by gravity settling Phosphors: Y 2 O 3 :Eu YVO 4 :Eu Y 3 Al 5 O 12 :Ce (YAG, P46) Substrates: Kapton Parylene-C Sedimentation screens Y2O3:Eu / Parylene-C un-aluminized (left) aluminized (right) [Applied Scintillation Technologies, UK]
ElectroPhoretic Deposition (EPD) Phosphor-deposition on Al-coated substrate: Phosphor powder supension in salt solution. Accelerate charged phosphor particles towards cathode (Al-coated Mylar) to deposit. Control of layer thickness via: applied voltage (amplitude, duration) thickness of Al-coating Possibility to optimize for small particle size by letting the suspension settle. Phosphor tested: Y 2 O 2 S:Eu / Mylar [J. D. McGee et al., Adv. Electr. Electr. Physics, 1966] [in Collaboration with G. Reichardt, BESSY]
1140 x 860 µm 140 x 110 µm Y 2 O 3 :Eu Kapton Y 2 O 2 S:Eu Mylar (EPD)
Phosphor efficiency -6dB Absorption at 10 kev 100% 9-12% Y 2 O 3 :Eu Al-coating efficiency enhancement 4% EPD screen (Y 2 O 2 S:Eu) +6dB ~ 5% 0 +12dB > 60% CVD diamond screen good for high flux sections [Element Six, UK]
Phosphor screen absorption Wavelength dependence of screen absorption
Image comparison of screens P43 Quartz Y 2 O 2 S:Eu Mylar (EPD??) Y 2 O 2 S:Eu Mylar (EPD) Y 2 O 3 :Eu Parylene Y 2 O 3 :Eu Parylene Al 100x200 µm focus 2 mm hole normal incidence
Center of mass position monitors Back-fluorescence quadrant diode array beam intensity, position Quadrupole ionization chamber high time resolution Quadrant electrode photocurrent monitors Thinned Position Sensitive Detector
Position sensitive detector (PSD) Transmissive XBPM: thinned-down PSD: Center-of-mass positional information from differential measurements of photo diode surface resistance Potentiometer principle Photocurrents (1D PSD) Position conversion via relative photocurrents (difference over sum): difference / sum (1D PSD) [PSD Illustrations (c) Hamamatsu Photonics K. K. 2003]
low dark current high speed response small position detection error high position resolution more difficult to manufacture
Transmissive PSD Development project Joint development project with Micron Semiconductor, UK Project Partners: BESSY, DIAMOND, ESRF, PETRA, SOLEIL Specifications: duolateral design active area 1x1, 3x3, 5x5 or 10x10 mm 2 active area thinned down to 5 µm thickness variation +-0.3 µm dark current pa - few na operation at full depletion [REFLEC]
Outlook: active stabilization [in Collaboration with F. Höft, BESSY]