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Can FERMI-LAT Measure Gamma Ray Polarization? Bill Atwood, SCIPP/UCSC 8-Oct-2008

Can FERMI-LAT Measure Gamma Ray Polarization? Bill Atwood, SCIPP/UCSC 8-Oct-2008. e -. e +. f. Recoil Nucleus. g. Polarization. Polarization Angle.

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Can FERMI-LAT Measure Gamma Ray Polarization? Bill Atwood, SCIPP/UCSC 8-Oct-2008

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  1. Can FERMI-LAT Measure Gamma Ray Polarization? Bill Atwood, SCIPP/UCSC 8-Oct-2008

  2. e- e+ f Recoil Nucleus g Polarization Polarization Angle If the incident Gamma Ray is linearly polarized, the plane of the e+,e- pair shows a modulation in azimuth, f, about the direction of the Gamma Ray.. e- High Electric Field Pair e+ High Energy Gamma Ray Z=74 Tungsten Details of the LAT Conversion Telescope qOP Silicon Conversions Tungsten Conversion TOP Conversions TUNGSTEN RADIATOR SILICON STRIP DETECTORS First 12 LAT Decks Radiator = 2.8% (68% Convert Here) Silicon = 2 x .4% (20% Convert Here) Trays = .5% (12% Convert Here) BOTTOM Conversions SUPPORT TRAY

  3. Other Angles in the Problem (Eg = 100 MeV) Eg = 100 MeV qop = .017 qMS in mrad, Eg in GeV, c in rad. Len. qMS = 21.1 mrad for Silicon Conversions! (34.5 mrad for Tungsten Conversions) All Conversions Top Silicon Conversions

  4. = T T = Polarization Jargon Asymetries are often used indicating the difference in eg. a rate for two orthogonal incoming polarizations. The “analyzing power” of a polarimeter is given by the theoretical analyzing power times dilution factors. The dilution factors include purity of sample and multiple scattering due to radiator material: Polarization Report Spring-8, 2001 LAT: Xo = .4% AMAX = 9% <A> = 5% Polarization Report Spring-8, 2001

  5. Separation of Silicon Conversions Tray Level Overall THIN THICK TOP BOTTOM Monte Carlo location of conversions Reconstruction The reconstruction places the start of the track in the middle of the lower SSD measuring plane or in the middle of the tungsten radiator above the upper SSD measuring plane.

  6. Separation Analysis Bottom/Top Split: Tkr1TrayZ0 < 0 Top CT (HARD) Bottom CT (easy) These include the Tungsten Conversions

  7. The Selection of Silicon Conversions The multiplicative factor in combination with the statistics (higher purity – fewer events) lead to an optimization Silicon Probability = .7

  8. Putting It All Together … This is probably a bit high as the average Xo is > .4% … As a Polarimeter, LAT has an “analyzing power” of ~ 3% Fraction of events surviving: 755 out of 11661 = 6.5% (Thin radiator & E < 2000, AGs) Efficiency: 664 events correctly tagged / 1545 Silicon Conversions Analysis Efficiency = 42% And so it will take to measure AGammaRays(assumed = 1.0) to

  9. LAT all-sky “First Light” map PRELIMINARY

  10. Prognosis for Vela Polarization Measurement In All Sky Scanning Mode the LAT detects ~ 1000 events/day from Vela Of these >18 are tagged to be Silicon Conversion. In 5 years: NEvents = 5 x 365 x 20. = 33 k events Next Steps Make a real MC simulation INCLUDING polarization effects. It seems that this is not out of the question particularly if one is mainly concerned with the energies above 10 MeV (B. Giebels, private comm.)

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