WIDE FIELD COMPACT DETECTOR FOR HARD X-RAY POLARIZATION MEASUREMENTS

1. WIDE FIELD COMPACT DETECTOR FOR HARD X-RAY POLARIZATION MEASUREMENTS J.-P. Vialle 1, W. Hajdas2*, N. Produit 3, F. Barao 4, C.Casella 5, K.Deiters 2, S. Deluit 3, C. Leluc 5, A. Mschedlishvili 2, M. Pohl 5, D. Rapin 5, E. Suarez-Garcia 2, R. Walter 3, C. Wigger 2 and A. Zehnder 2 1LAPP/IN2P3/CNRS, Annecy, France 2PSI, Villigen, Switzerland; 3ISDC, Universite de Geneve, Switzerland; 4LIP, Lisboa, Portugal; 5DPNC, Universite de Geneve, Switzerland; * Most of this talk comes from W. Hadjas. Many thanks to him

2. INTRODUCTION • Polarization observations in -rayswere mostly neglected • Most of the missions had difficulties with unambiguous measurements • Recent reports of high polarization in GRB (RHESSI) alerted the community • Present GRB data cannot pin down its mechanism • Large set of observations, including energy and time dependence, is needed • Several new polarimeters are either under construction or being studied !

3. GRB POLARIZATION • Fireball Model high values excluded Plin ~ 10-20 % • Cannonball Modelfull range possible Plin = 0 - 100% (depends only on .) • Electromagnetic Modelwell defined, moderate Plin ~ 50 % 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. Polarization measurement should be added to priority programs for GRB’s

4. OTHER POLARIMETER PROJECTS • GRAPE Gamma Ray Polarization Experiment: Low Z - high Z hybrid, 50-300 keV; M.McConnell et al. • PoGO Polarized Gamma-ray Observer: phoswich of slow-fast units with AC, 30-100 keV; T.Mizuno et al. • SGD Soft Gamma-ray Detector: Compton telescope of Si-strips and CdTe pixels and AC, E<300 keV; H.Tajima • CIPHER Coded Imager and Polarimeter for High Energy Radiation: CdTe array, E<1 MeV; R.Curado da Silva • RHESSI High Energy Solar Spectrometric Imager: 9 large Ge, active/passive modes, E>10 keV; M.McConnell, C.Wigger • Coronas-F, INTEGRAL, etc.

5. DETECTOR REQUIREMENTS • Designed for polarization measurement only • Simple measurement of polarization with compact instrument • optimized for GRB observations • Relies on given burst position and spectrum (GCN, extra det.) • Large area, large modulation factor and wide field of view • Utilizes large angle Compton scattering, (tolerates small one) • Signal to background ratio to be carefully studied and optimized

6. POLAR DETECTOR • 48x48 uniform scintillator array • Light, fast and low Z plastic • Tests and prototype with BC400: rad-hard and chemically stable • Element size for study: 4x4x200 mm3; matches space-used MAPM H6568 • Final version size 6x6x200 mm3;fits new MAPM H8500 • Optical insulation and thin (≈ 1 mm) carbon fiber outside shielding • No active shielding; but outer layers can be used if needed

7. MC SIMULATIONS and RESPONSE MATRIX • GEANT4 package (CERN) • Target size 192x192x200 mm3 • Carbon shield outside/top and dummy MAPM/electronics below • Total mass less than 12 kg • Polarized photon flat field • Angles:  =(0,70), =0, 45 • Energies: single or Band-like (=-1, =-2, Epeak=200 keV) • Dominance of Compton events (and photon escape) observed

8. MULTIPLICITY and TRIGGER CONCEPT • Most photons deposit energy in several bars • Threshold set at Emin= 5 keV • Upper threshold Esum< 300+ keV (TBC) • Trigger activation: at least 2 channels • Selection of two highest E deposits • Reacting pixels define geometry • Further (on/off-line) cuts possible

9. POLARIZATION RESPONSE • Analysis uses two largest energy deposits with Ethr= 5 keV • GRB position is known (e.g. GCN) • Fit function: N=A·cos(2(-0)+½)+B • Parameters:100=A100/B100 - modulation factor for 100% polarization0 – polarization direction • MC predicts clear modulation signal • Unpolarized photons create pattern with period /2

10. MODULATION FACTOR • Peak 100reaches about 40% • High plateau between 100 and 250 keV; • Lower energy threshold around 35 keV; no limits for higher one • Constant values kept up to = 30 for off-axis GRB • 30% sky coverage possible • 100 values comparable with other polarimeters

11. EFFECTIVE AREA and FIGURE OF MERIT • Maximum effective area ·A = Aeff≈ 200 cm2 for 4x4 mm2 bars • Aeff ≈ 80 cm2 – 22% of total area for Band-like spectrum • Polar angular dependence varies within 15% only • Increase by a factor of 2 for bar size 6x6 mm2 • Figure of merit FoM =B – background rate replaced by detector volume (scaling) • Effective X-ray energies < 50 keV

12. BACKGROUND SOURCES • Cosmic rays removed by upper energy threshold • No measurements inside belts though discrimination possible • Diffuse background E,bg>10 keVFdif=2.46 /cm2/sr/s – 430 coinc./s • Non-GRB  sources – e.g. CrabFCrab=0.7 /cm2/s • S/C induced ’s – ISGRI estimated Find=0.02 /cm2/sr/s • Weaker GRBs specially at lower energies background dominated

13. MINIMUM DETECTABLE POLARIZATION • MDP = B - background rates from S/C induced and diffuse ≈ 500 s-1 n= 3, Aeff = 100 cm2, 100 = 30%,T = 0.3, 20 s (short/long GRB) S – signal rate from Band spectrum EPeak= 320 keV, = -1.6, = -2.5 • E=10-5 erg/cm2→ MDP3≈ 10%example LTC GRB060418 by RHESSI • Several measurements per year !

14. TESTS WITH 16 SMALL BARS (4x4 mm2) • 4x4 flat MAMP H6568 (Hamamatsu) • BC400 plastic bars 4x4x200 mm3 • Bicron 620 reflective paint • 241Am (59keV) and 57Co (122 keV) radioactive sources • Charge sensitive ADC CAMAC • Large attenuation of the pulse high amplitude along the bar length • Pulse height variations among anodes within a factor of 2

15. TESTS WITH 8x8 BARS (6x6 mm2) • Single module assembly with 6x6x200 mm3 BC400 plastics • Teflon wrapping and Al foil isolation of each bar • Coupling to H8500 8x8 MAPM (pitch 6 mm, anode size 5.8 mm) • XY table and sources tester • The same DAQ as for 4x4 mm2 bars • Mono-energetic fluorescence sources exposures (6-44 keV)

16. THRESHOLD and NON-UNIFORMITY Teflon wrapping • Signal top/bottom ratio 1.31 • Amplitude variations among anodes by ½ (factory ⅓) • Threshold 5 keV feasible Signal with sources: Rb at 13.4keV & Ag at 22.2keV • New test with Al wrapping • Signal top/bottom ratio 1.19 • Signal higher by 20% wrt Teflon

17. -RAY POLARIZER and ASYMMETRY DATA • Simple -ray polarizer constructed • 90 scattering of photons on large scintillator • Strong 137Cs source (37 MBq) • Collimated, partially polarized -rays (E90deg≈ 290 keV, Pmax≈ 60%) • Triple coincidences for trigger • Coincidence pattern and CS ADC readout (CAMAC) • Asymmetry up to 12% depending on distance between plastics; draft data

18. IN PREPARATION: FAST and SLS • FAST (Muon lifetime experiment at PSI) equipment available from this summer on • Detector equivalent to smaller POLAR:- 12.8 x 19.2 x 20 cm3 scintillator array- 1536 BC400 bars (4 x 4 x 200 mm3) - 96 H6568 MAPM • DAQ: FD, TAC, event-by-event • Test with polarized -rays from source • PSI SLS synchrotron polarized -rays • Various energies up to many tens keV • 8x8 detector array (6x6x200 cm3) tests

19. SUMMARY • POLAR – Compton hard X-ray GRB polarimeter using low Z scintillators • 48x48 homogeneous array of 6x6x200 mm3 plastic bars • FoV ≈ ⅓ of the sky and low energy threshold Emin < 50 keV • Aeff≈ 200 cm2 and 100≈ 40% at 200 keV • MDP3≈ 10% for GRB total energy of 10-5 erg/cm2; tens of detections/year • First asymmetry results obtained demonstrating polarimetric capability • Discussions underway to include POLAR in forthcoming satellite experiments

20. LIGHT COLLECTION WITH WS-FIBERS • Wavelength shifters implementation across the bar length • Bicron BC620 with diameter 1 mm, glued using BC600 optical cement • Two shifter fibers per plastic bar • Overall double coat reflective paint • Good signal uniformity obtained • Active volume reduction by 12%

21. Band parametrization of GRB’s Energy spectrum GRB 990123

23. Polarization and Compton scattering