POLAR: A novel Xray polarimeter

1
POLAR A novel X-ray polarimeter

• Daniel Haas
• ISDC Versoix
• ECRS 2008
• Kosice, Slovakia

• Outline
• GRBs and their Models
• Why POLAR The Detection Concept
• POLAR Design
• Testbench First Results
• POLAR EQM
• Outlook/Challenges Conclusions

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Gamma Ray Bursts...

• Gamma ray bursts (GRB) flashes of gamma rays at
  random places in the sky and at random times
• Brightest events in the universe
• About 1 GRB/day observed

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...Origin of GRBs...
See papers discussing various modelsT.Piran,
A.Dar A. De Rujula, M.Lyutikov, D.Eichler,
G.Ghisellini, D.Lazzatti, M.Medvedev, E.Rossi etc.

• Not clear, different models

From M.Lyutikov, 2003
Plin 10-20
Plin 0 - 100
Plin 50
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...GRB Polarization...

• Polarized ?s produced by
• Synchrotron radiation
• Cyclotron emission
• Bremsstrahlung
• Compton Scattering
• Magnetic photon splitting

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...GRB Polarization - Measurements

• Polarization of the prompt ?-ray emission from
  the ?-ray burst of 6 December 2002 (RHESSI),
  Coburn, W Boggs, S. E., Nature, 2003, 423, 415
  (122 citations) Plin 80 20 (highly
  significant detection!)
• Re-analysis of polarization in the ?-ray flux of
  GRB 021206, Rutledge, R. E. Fox, D. B., MNRAS,
  2004, 350, 1288
• Statistical Uncertainty in the Re-Analysis of
  Polarization in GRB021206, Coburn, W
  Boggs, S. E., 2003astro.ph.10515B
• Gamma-Ray Burst Polarization Limits from RHESSI
  Measurements, Wiggler, C. et al, ApJ, 2004, 613,
  1088
• Evidence of polarisation in the prompt ?-ray
  emission from GRB 930131 and GRB 960924
  (BATSE/GRO), Willis, D. R. et al, 2005, AA, 439,
  245
• Polarisation studies of the prompt ?-ray emission
  from GRB 041219a using the spectrometer aboard
  INTEGRAL, McGlynn, S., 2007, AA, 466, 895

Large Uncertainty Controversial, poor sample of
events, Polarization should be added to priority
programs for GRBs
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Detection principle...

• Compton scattering
• photons tend to scatter at a right anglew.r.t.
  the initial polarization vector
• ? azimuthal scatter angle
• ? Compton scatter angle
• Klein-Nishina
• Level of Polarization

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...Detection principle

• Use two largest energy deposits with E gt 5 keV
• GRB position is known or reasonably estimated
• Analysing power?100A100/B100 (mod. factor)
  modulation amplitude for 100 polarization
• MC predicts clear modulation signal with period ?
• Unpolarized photons create pattern with period
  ?/2

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POLAR design...

• Requirements
• Compton polarimeter
• Simple, compact instrument
• relies on given burst position and spectrum
• or provides a crude estimate in case of unique
  observation
• Dedicated for GRB observations only
• Large area
• Large modulation factor
• Large field of view
• Energy range for incoming photons
• 50 keV - 500 keV

• Design
• 40x40 uniform scintillator array
• Light, fast and low Z plastic
• Scintillator size 6x6x200 mm
• Matching Hamamatsu MAPM8500
• Aeff 400 cm2
• ?100 35
• FoV 1/3 of the sky
• Passive shielding
• Main electronics below detector

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...POLAR design
Electronics housing
Passive Shielding
25 DetectorModules
Module in Carbon Fiber 8x8 barsMAPM H8500
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Testbench...
CAMAC/NIM readout of a prototype module
Prototype Module and scintillator bar testing
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...Testbench
Demonstrator Electronicswith one prototype
target (custom made FPGA boardwith ASICS and
place for 2 PMs)
Photon polarizer from Cs137 with NaI detector as
scatterer (290 keV ?-rays)
Source
Scatterer
90o
Trigger ReadoutSignal
Detector(here only a 2nd NaI)
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First Results...

• Tests with L-shape of 88 bars
• Polarized photons (290 keV/60) from Cs137 after
  90o scatterer
• Coupling to MAPM H8500
• (mod.fac x degree of polarization)
• Asymmetry up to 12 confirms
  proof-of-principle

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...First Results...

• Light collection studies
• Simulation predicts
• 45 ?s reach PM
• 10-20 difference top/bottom
• Surface quality essential

• Measurements confirm simulation
• different wrapping tested
• use Vikuiti (Alu-like, but even higher
  reflectivity

very good surface quality
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...First Results (MC)

• Minimal detectable polarizationE10-5 erg/cm2
• MDP3? 10 for strong bursts
• Several measurements per year

• Modulation factor as function of energy and theta
• events from the side are harder to detect than
  from the top
• low energies are favored

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EQM
Assembly studies
Frontend Design
MAPM in TVT
Module in Carbon Fiber 8x8 barsMAPM H8500
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Outlook Challenges

• Demonstrator will be tested on Polarizer soon
• Preliminary qualification tests of EQM mechanics
  underway (vibration, thermal stress etc.)
• Beamtests at SLS by end 2008 (PSI Villigen/CH)
  and ESRF (Grenoble/F) by end 2009
• Optical crosstalk on PM pads is high (10),
  Trigger/Online rejection has to take care of this
• Gain calibration of 1600 scintillator bars is
  difficult
• In-flight calibration still an issue
• Localisation of GRBs stand-alone is challenging
• rough estimation seems feasible, more studies
  ongoing

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Conclusions

• Accurate and statistical meaningful polarization
  measurements of GRBs can
• constrain theoretical models
• give crucial information about GRBs central
  engine
• POLAR - a compton hard X-ray GRB polarimeter may
  answer these questions
• compact 40x40 array of low Z scintillators
  (6x6x200 mm)coupled to Hamamatsu MAPM H8500
• MDP3? 10 for GRBs of 105 erg/cm2
• Tens of detections/year expected
• Proof-of-principle on polarizer testbench
• Demonstrator will be used in testbeams for
  further studies
• EQM under design, to be ready by 2010

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The POLAR Collaboration

• Centre des Physique des Particules de Marseilles,
  (CPPM, France)
• Département de Physique Nucléaire et
  Corpusculaire (DPNC Genève, Switzerland)
• Institute of High Energy Physics (IHEP Beijing,
  China)
• INTEGRAL Science Data Centre (ISDC Versoix,
  Switzerland)
• Laboratoire dAnnecy de Physique des Particules
  (LAPP, France)
• Paul Scherrer Institut, (PSI Villigen,
  Switzerland)
• Andrzej Soltan Institute for Nuclear Studies (IPJ
  Warsaw, Poland)