WIDE FIELD COMPACT DETECTOR FOR HARD X-RAY POLARIZATION MEASUREMENTS

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J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing

WIDE FIELD COMPACT DETECTOR FOR HARD X-RAY POLARIZATION MEASUREMENTS

J.-P. Vialle 1, W. Hajdas 2*, 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, France2PSI, 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

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INTRODUCTION

• Polarization observations in -rays were 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 !

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GRB POLARIZATION

• Fireball Modelhigh values excluded

Plin ~ 10-20 %

• Cannonball Modelfull range possible

Plin = 0 - 100%(depends only on .)

• Electromagnetic Modelwell defined, moderatePlin ~ 50 %

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.

From M.Lyutikov, 2003

Polarization measurement should be added to priority programs for GRB’s

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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.

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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

)(cos)(sin1 2220

rd

d

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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

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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

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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

1 2 3 4 5 6 7 8 9 100.0

0.1

0.2

0.3

0.4

0.5

0.6

Fra

ctio

n (

%)

Number of reacting bars

Photon Energy 20 keV 40 keV 80 keV 140 keV 260 keV

0 5 10 15 20 250.0

0.2

0.4

0.6

0.8

1.0

Distance (bar width unit)

Co

un

ts (

a.u

.)

Distance between two largestenergy depositions

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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

0 30 60 90 120 150 180100

150

200

250

300

350

400

Co

un

ts (

a.u

.)

Azimuth Angle

Modulation Curve

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MODULATION FACTOR

• Peak 100 reaches 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

0 100 200 300 400 500 6000.0

0.1

0.2

0.3

0.4

0.5

Mo

du

lati

on

Fac

tor

(%)

Energy (keV)

Modulation Factor at 00

0 10 20 30 40 50 60 700.18

0.20

0.22

0.24

0.26

0.28

0.30

0.32

0.34

0.36

0.38

Mo

du

lati

on

Fac

tor

(%)

Polar Angle (deg)

Mean Modulation Factor

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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

0 50 100 150 200 250 3000

50

100

150

200

250

300

350

Eff

ecti

ve A

rea

(cm

2 )

Energy (keV)

Effective Area

at 00

at 450

0 100 200 300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

Fig

ure

of

Mer

it (

Fo

M)

Energy (keV)

Figure of Merit at 00

BA /100

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BACKGROUND SOURCES

0 100 200 300 400

10-4

10-3

10-2

10-1

100

Ph

oto

n F

lux

(a.u

.)

Energy (keV)

Diffuse Crab GRB (10-6 erg/cm2/s)

• Cosmic rays removed by upper energy threshold

• No measurements inside belts though discrimination possible

• Diffuse background E,bg>10 keV Fdif=2.46 /cm2/sr/s – 430 coinc./s

• Non-GRB sources – e.g. Crab FCrab=0.7 /cm2/s

• S/C induced ’s – ISGRI estimated Find=0.02 /cm2/sr/s

• Weaker GRBs specially at lower energies background dominated

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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 !

10-7 10-6 1x10-5 1x10-40

20

40

60

80

100

MD

P (

%)

Energy fluence (erg/cm2)

MDP with 3 level short GRB long GRB

TBSSn /)(/ 100

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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

20 40 60 80 100 120 1401

10

100

Co

un

ts (

a.u

.)

Channel (1)

Source position (57Co) close far

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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)

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THRESHOLD and NON-UNIFORMITY

0 50 100 150 200 250 300 350

25

50

75

100

125

150

Co

un

ts (

1)

Channel (1)

X-ray source Rb Ag

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

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-RAY POLARIZER and ASYMMETRY DATA

10 20 30 40 500.00

0.04

0.08

0.12

0.16

Asy

mm

etry

(1)

Distance (mm)

Polarized 290 keV rays

bars 6x6 mm2

• 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

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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

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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

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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%

25 50 75 100 125 150 175

1

10

100

Co

un

ts (

a.u

.)

Channel (1)

Source position (241Am) far close

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Band parametrization of GRB’s Energy spectrum

GRB 990123

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Polarization and Compton scattering

WIDE FIELD COMPACT DETECTOR FOR HARD X-RAY POLARIZATION MEASUREMENTS

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