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TPCs in Astronomical Polarimetry

TPCs in Astronomical Polarimetry. A brief review of efforts to develop TPCs as polarimeters and as components of polarization-sensitive instruments in high-energy astrophysics. Kevin Black Forbin Scientific and NASA’s Goddard Space Flight Center. Polarimetry in High-Energy Astrophysics

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TPCs in Astronomical Polarimetry

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  1. TPCs in Astronomical Polarimetry A brief review of efforts to develop TPCs as polarimeters and as components of polarization-sensitive instruments in high-energy astrophysics Kevin Black Forbin Scientific and NASA’s Goddard Space Flight Center

  2. Polarimetry in High-Energy Astrophysics (1 keV - 10 GeV) • Remains the only largely unexploited tool • Instruments have not been sensitive enough to be seen to be the best return on investment • Two unambiguous measurements of one source (Crab nebula) at 2.6 and 5.2 keV • Best chance for pathfinder (SXRP on SXG mission ~1993) never flew • Interest and development efforts have exploded in the last 10 years • As other observational techniques have matured, need for polarimetry has become apparent • New techniques are lowering the technical barriers • Tantalizing results of polarization in GRB, solar flare Timing Imaging Spectroscopy

  3. The Experimental Landscape An Overview of Development Efforts • Dedicated Polarimeters: • Thompson Scattering • Gas Pixel Detectors (2001-) • Dichroic Materials (2006-) • TPCs (2006-) • Dedicated Polarimeters: • Low-Z converter, High-Z absorber • Numerous development efforts (~8) • No TPCs • Advanced Compton Telescopes: • Numerous technical approaches (~6) • All have good polarization sensitivity • TPCs a component in some X-ray Softg Medium g Hard g • Pair (and triplet) Telescopes: • Next generation soon to launch • No polarization sensitivity • Only notion for polarimetry w/TPCs

  4. L Polarimetry Terminology • Polarization signature:A + B cos2fdistribution of interaction products • Analyzing power:scales from 0 to 1 • of the interaction: • of an instrument (modulation): • Polarimeter Figure of Merit: • but, systematics are important! Photon Energy (keV)

  5. TPCs as X-ray Polarimeters

  6. X-ray E f Auger electron Photoelectron Photoelectric X-ray Polarimetry • Based on: correlation between the X-ray electric field vector and the photoelectron emission direction (L=1) • Challenge: both good modulation and high QE • Scattering mean free path < 0.1% X-ray absorption depth • Ideal polarimeter is an track imagerwith: • resolution elements < mean free path • active depth >= absorption depth • => active depth > 103 resolution elements

  7. Photoelectron y x The TPC as an X-ray Polarimeter • GEM with strip readout • Track images formed by time projection by binning arrival times • Active depth (~10 cm) >> resolution elements (~100 micron) • Drift distance (diffusion limit) determined by degree of X-ray beam collimation Readout Strips Drift Electrode GEM CSAs e- Drift y Differentiated Waveforms Digitized Waveforms Image x X-ray Trigger z

  8. Strip number Time Interaction Point End Point Demo TPC polarimeter at GSFC Prototype polarimeter • Made from off-the-shelf components: • 130 micron pitch • 13mm(w) x 30mm(d) active area • 24-channel ADC • drift velocity: 40 nsec bin = 130 microns • 460 Torr Ne:DME (50:50) Reconstructed 6.4 keV track images

  9. Demo TPC Polarimeter Test Results • Uniform response • Good modulation • No false modulation • An encouraging start unpolarized 5.9 keV polarized 6.4 keV at 0o polarized 6.4 keV at 45o polarized 6.4 keV at 90o Black et al, in preparation

  10. TPC Polarimeters for X-ray Telescopes • High efficiency enables sensitive observations of extragalactic sources, even in a small mission • Adjustable optical depth allows TPC to be used in conjunction with focal plane instrument in a large multi-purpose mission Dedicated polarimeter at telescope focus on a Small Explorer Focal plane detector TPC Focal plane detector behind TPC on a larger mission Jahoda et al, 2006 Sensitivity estimates

  11. Wide Field-of-View Polarimeter for Gamma Ray Bursts • TPCs can be constructed in large volumes, enabling large effective-area wide-field cameras Wide-field TPC polarimeter (40o x 40o) Sensitivity of a two-camera mission J.E. Hill et al, 2006

  12. 3 liter TPC polarimeter Imaging Polarimeter for Solar Flares • Polarimetry of solar flares requires arc second imaging to distinguish arc from “footprint” • Enabled by large volume TPC with rotation modulation collimator B. Dennis et al, 2006 Rotation Modulation Collimator provides few arc second imaging of extended sources with a non-imaging detector

  13. Compton Polarimetry

  14. L Compton Scattering Polarimetry • Azimuthal scattering angle is preferentially normal to electric field of incoming photon • Analyzing power depends on energy, polar angle f

  15. Compton Polarimetry 50-500 keV • Many development efforts for dedicated polarimeters • GRAPE, POGO,GIPSI, CIPHER, POLAR, SGD…… • Acceptance optimized for high scatter angles • Low-Z active scattering elements, high-Z calorimeters • Collimated, coded-aperture, RMC imaging • No obvious role for TPCs The GRAPE Concept ML McConnell et al, SPIE Conf. Proc. 5165 (2004)

  16. “Well” geometry captures large-angle, polarization-sensitive events standard geometry Polarimetry with Compton Telescopes (0.5 - 30 MeV) • Compton scattering used to image sources over wide field-of-view • Past telescopes lacked polarization sensitivity due to poor geometry • Many new concepts being developed, with many detector technologies, most with good polarization sensitivity

  17. q E1,r1 E2,r2 Imaging with Compton Telescopes • Two types of telescope: • Electron Tracking – extensible to pair production • Silicon strips, Xe TPC • Better imaging, lower background • Non-tracking • Thick silicon, segmented Ge, liquid Xe TPC • Better effective area Imaging with a Compton telescope. The initial photon direction is related to the scatter direction and the energies of the products. Measuring the direction of the scattered electron reduces the event circle to an arc.

  18. Non-Tracking TPC Compton Telescope, (LXeGRITcousin of XENON WIMP detector) • Liquid Xe, 2d charge readout, scintillation trigger • Good energy and spatial resolution, with large effective area • Modulation near L above 1 MeV From E. Aprile et al, NIM A, 461 (2001) 256. From E. Aprile et al, ApJS, 92 (1994) 689.

  19. Electron Tracking Compton Telescopes with Gas TPCs • Two development programs at Kyoto and GSFC/UNH • Kyoto effort maturing rapidly: lab results, recent balloon flight • Based on micropattern detectors with 2d readout Imaging with the Kyoto electron tracking Compton telescope From T. Tanimori et al, New Astronomy Reviews 48, (2004) 263

  20. Non-tracking Tracking with silicon strips Tracking with TPC Bloser et al, New Astronomy Reviews 48, (2004) 299 Benefits of Telescopes with Gas TPCs • Gas TPC provides most accurate electron tracking • Best point spread function (= sinf Df Dq) of all Compton telescopes, especially at large scatter angles • Improved signal/background improves polarization sensitivity (for equal effective areas) Comparison of polarization sensitivity for three types of advanced Compton telescopes assuming equal effective areas.

  21. Pair Production Polarimetry with TPCs

  22. Pair Production Polarimetry • Cross section depends on azimuthal angle of e+e- pair plane with respect to photon polarization plane (L = 0.14): • Multiple scattering exponentially suppresses m • Triplet production has same L, • but with much larger angles Wojtsekhowski et al, NIM A 515 (2003) 605

  23. Astronomical Pair Production Polarimetry • “Don’t give up!” – M. Weisskopf (SLAC Polarimetry conference 2004) • Even most advanced, imminent missions (AGILE, GLAST) have negligible polarization sensitivity due to multiple scattering • Only one evolving futuristic concept (UNH/GSFC) holds out hope: • Segmented negative ion TPC with Ar (enhance triplet production) • Energy measured from multiple scattering (no calorimeter) • Simulations show Xe/CO2 unlikely to work Extending TPC Compton to pair production UNH/GSFC TPC pair telescope concept

  24. Raw events With detector response With response + diffusion TPC Pair Polarimeter Simulations • GEANT4 simulation results for Xe TPC pair polarimeter: m = .08 m = .02 (not significant) No modulation Courtesy Peter Bloser, UNH • Simulation efforts continuing (at very low level): • Using Ar/CS2 to increase triplet production and reduce diffusion • Triplet production being implemented in GEANT4 • Needs better reconstruction algorithm near vertex • There is hope!

  25. Summary • Polarimetry is being established in high-energy astrophysics with new polarimeters and instruments with polarimetric capability • Role of TPCs: • 2 – 50 keV: dedicated photoelectric polarimeters • High efficiency detectors for X-ray telescopes • Large volume devices for GRBs and solar flares • 0.5 – 30 MeV: components of advanced polarization-sensitive Compton telescopes • Liquid Xe TPC scatterer/absorber for high efficiency • Electron-tracking TPC for enhanced imaging, sensitivity • Above 30 MeV: advanced negative-ion TPCs are currently the only (albeit faint) hope for polarization-sensitive pair telescopes

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