DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIRE POLAR POLAR: A novel X-ray polarimeter Daniel...

DEPARTEMENT DE PHYSIQUE NUCLEAIRE ET CORPUSCULAIREPOLAR

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

Daniel Haas, ECRS ‘08, Kosice


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



...Origin of GRBs...• Not clear, different models

From M.Lyutikov, 2003

See papers discussing various models:T.Piran, A.Dar & A. De Rujula, M.Lyutikov, D.Eichler, G.Ghisellini, D.Lazzatti, M.Medvedev, E.Rossi etc.

Plin ~ 10-20% Plin = 0 - 100%Plin ~ 50%



...GRB Polarization...

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are needed to see this picture.

Polarized ’s produced by:– Synchrotron radiation– Cyclotron emission– Bremsstrahlung– Compton Scattering– Magnetic photon splitting



...GRB Polarization - Measurements• Polarization of the prompt γ-ray emission from the γ-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 γ-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 γ-ray emission from GRB 930131 and GRB 960924 (BATSE/GRO), Willis, D. R. et al, 2005, A&A, 439, 245

• Polarisation studies of the prompt γ-ray emission from GRB 041219a using the spectrometer aboard INTEGRAL, McGlynn, S., 2007, A&A, 466, 895

Large Uncertainty & Controversial, poor sample of events, Polarization should be added to priority programs for GRB’s



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:



0 30 60 90 120 150 180100

150

200

250

300

350

400

Counts (a.u.)

Azimuth Angle ζ

Modulation Curve...Detection principle

• Use two largest energy deposits with E > 5 keV

• GRB position is known or reasonably estimated

• Analysing power:100=A100/B100 (mod. factor)

modulation amplitude for 100% polarization

• MC predicts clear modulation signal with period

• Unpolarized photons create pattern with period /2



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



...POLAR designElectronics

housing

Passive Shielding

25 DetectorModules

Module in Carbon Fiber

8x8 barsMAPM H8500



Testbench...

CAMAC/NIM readout of aprototype module

Prototype Moduleand scintillator bar testing



...TestbenchDemonstrator 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)

Trigger ReadoutSignal

Source

Scatterer

Detector(here only a 2nd NaI)

90o



First Results... Tests with L-shape of 8+8 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











...First Results...Light collection studies• Simulation predicts:

– ~45% ’s reach PM– 10-20% difference top/bottom– Surface quality essential

Light Collection for 50 keV photons

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Incoming photon position

Opt Phot: Detected/ProducedAir Ref PaintTeflon Alu+AirTefl+Air

• Measurements confirm simulation – different wrapping tested– use Vikuiti (Alu-like, but

even higher reflectivity

very good surface quality



D e p e n d e n c e o f

1 0 0 o n E n e r g y a n d a n g le θ

0 .0 0

0 .0 5

0 .1 0

0 .1 5

0 .2 0

0 .2 5

0 .3 0

0 .3 5

0 .4 0

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0

E n er g y ( k e V )

μ100

T h 0 P h i 0

T h 4 5 P h i 0

T h 6 0 P h i 0

T h 7 5 P h i 0

...First Results (MC)• Minimal detectable polarization

E=10-5 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





MAPM in TVT





Frontend Design

Module in Carbon Fiber

8x8 barsMAPM H8500

Assembly studies

EQM



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



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



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 d’Annecy de Physique des Particules

(LAPP, France)• Paul Scherrer Institut, (PSI Villigen, Switzerland)• Andrzej Soltan Institute for Nuclear Studies (IPJ

Warsaw, Poland)

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