Page 1
1
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
Page 2
2
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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 !
Page 3
3
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 4
4
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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.
Page 5
5
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 6
6
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 7
7
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 8
8
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 9
9
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 10
10
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 11
11
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 12
12
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 13
13
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 14
14
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 15
15
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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)
Page 16
16
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 17
17
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
-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
Page 18
18
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 19
19
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 20
20
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
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
Page 21
21
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
Band parametrization of GRB’s Energy spectrum
GRB 990123
Page 22
22
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
Page 23
23
J.P. Vialle – SF2 workshop – Aug. ‘06 – Beijing
Polarization and Compton scattering