Transcript Slide 1

POLAR
A WIDE FIELD COMPACT DETECTOR
FOR POLARIZATION MEASUREMENTS
of HARD X-RAYS from GRB
Giovanni Lamanna1,
S.Basa6, S.M. Gierlik2, D. Haas5, W. Hajdas3, R. Hermel1, H. Hofer4,
C. Lechanoine-Leluc4, R. Marcinkowski2, A. Mazure6,A. Mtchedlishvili3,
S. Orsi4, M. Pohl4, N. Produit5, D. Rapin4, E. Suarez4, J.P. Vialle1
1LAPP
(Laboratoire d’Annecy-le-vieux de Physique des Particules) ,
Annecy, FRANCE
2IPJ,
Swierk/Otwock, Poland;
Villigen, Switzerland;
4DPNC, Universite de Geneve, Switzerland;
5ISDC, Universite de Geneve, Switzerland;
6LAM (INSU/CNRS), Marseille, FRANCE;
3PSI,
G. LAMANNA
Polarimetry days in Rome
16-17 October 2008
Polarimetry
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Electromagnetic radiation (EM) has 4 measurable quantities:
– Direction
– Intensity
– Energy
– Polarization
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Measurements of polarization provide information about the radiation mechanism but they
have not been enough explored
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Difficult to do with actual detectors
Wrong measurements published (claim 80%+- 20%) (GRB 021206 accidental coincidences)
A lot of theoretical interests ( 122 citations)
POLAR
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POLAR is a Compton hard X-ray GRB polarimeter using proven technologies:
40x40 homogeneous array of 6x6x200 mm3 low Z scintillator plastic bars and PM detectors
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Measuring the PROMPT signal of the GRBs within a wide FoV (≈ ⅓ of the sky)
Perform first ever successful polarization measurement of hard X photons in space with
high statistical significance and controlled systematic effects.
G. LAMANNA
Polarimetry days in Rome
16-17 October 2008
Gamma Ray Burst (GRB)
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Currently about 2 to 3 GRBs detected per
week. They are flashes of gamma rays, at
random places in the sky and at random times.
GRB 910412, 12 April 1991
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Energetic bursts that mark the most violent,
cataclysmic explosions in the universe
G. LAMANNA
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Transient events ranging in duration from
a fraction of a second up to a few
hundred seconds. Two types of GRBs:
1) Soft-spectrum Long burst
(death of massive stars)
2) Hard spectrum Short burst
(merger of compact star binaries:
ns-ns; ns-bh..)
Polarimetry days in Rome
16-17 October 2008
Gamma Ray Burst (GRB)
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Both result in a Black Hole formation and regardless of the progenitor a “fireball” central
engine would be responsible of a relativistic jet from the center of the explosion (bulk
Lorentz factor G > 100.)
Zhang & Meszaros
Swift
POLAR will use polarimetry to analyse the “PROMPT SIGNAL” : the only possible
means of probing the structure of the central engine of the expanding fireball closest
to the nascent black hole.
G. LAMANNA
Polarimetry days in Rome
16-17 October 2008
Gamma Ray Burst (GRB)
Theories on the GRB production mechanism can be constrained by
different degrees of linear polarization (P):
P ~ > 80% Inverse Compton jet model
20%< P <60% synchrotron emission is the dominant source of radiation. (EM model)
Low degrees of polarization: flux with a high degree of polarization experiencing partial
depolarization e.g. electrons in a randomly oriented magnetic field. (IS model)
In general models fall into:
-Physical: globally ordered B field and
synchrotron emission-> net polarization
-Geometrical: random B field and electrons->
No-net polarization except…when
viewing ~ 1/G outside the jet cone (loss of emission symmetry)
(but random viewing angles make high P significantly smaller)
Other POLAR scientific Goals: Solar Flares, Soft Gamma Repeaters….
G. LAMANNA
Polarimetry days in Rome
16-17 October 2008
The POLAR detector
1600 PS bars: 25 x 8x8*6mm*6mm*20cm
Light, fast and low Z plastic
(in favor of Compton effect)
BC400: rad-hard and
chemically stable
G. LAMANNA
MAPM H8500
(8*8 anode pixel)
PS
Bars
PMT
(mm)
Weight
(kg)
Dimension
(cm)
40*40
5*5
20
30*30
Wavelength
(Peak)
420 nm
Uniformity
2-3
Polarimetry days in Rome
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The POLAR detector
Requirements:
- relies on given burst position and spectrum
- or provides a crude estimate in case of unique observation
Dedicated for GRB observations only thanks to:
- Large area
- Large modulation factor
- Large field of view
The scientific objectives lead to the following instrument specifications:
Effective detection area
Incoming photon energy
A large modulation factor
A large FOV (deep space view)
Total Mass
Mean power Consumption
Telemetry (continuous readout)
400 cm2 (at 200 keV)
50 keV to 500 keV
~40% (at 200 keV)
1/3 of the sky
< 30 kg
< 30 W
< 100 kBit/s
While the exact data format has not yet been fixed, a preliminary evaluation of
the event size is 128 bits per event at a mean background rate of around 500 Hz
(to be compared with ~100 kHz during GRB events).
G. LAMANNA
Polarimetry days in Rome
16-17 October 2008
COMPTON POLARIMETRY: Basic Principles
Geometry of the large angle Compton scattering.
The strategy is to look for the two bars where interactions occur:
first Compton and second Compton or Photoelectric.
G. LAMANNA
Polarimetry days in Rome
16-17 October 2008
Monte Carlo Results
(GEANT4 full simulation)
400
Modulation Curve
350
Analysis uses two largest energy deposits with Ethr= 5 keV
(corresponding to electron recoil energy from 50 keV photon
scattered at 90o) (trigger activation: at least 2 channels)
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GRB position is known
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Optical insulation and thin (≈ 1 mm) carbon fiber outside
shielding (stopping electrons with E< 500 keV or protons E<
13 MeV)
300
Counts (a.u.)
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250
200
150
100
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No active shielding; but outer layers can be used if needed
for a (“topological”) trigger
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Upper threshold Esum< 300+ keV (total sum) (TBC)
0
60
90
120
150
180
Azimuth Angle 
Fit function: N=A·cos(2(h-f)+½)+B
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G. LAMANNA
30
Maximum effective area for monochromatic photons
·A = Aeff ≈ 350 cm2
Polar angular dependence varies within 15% only
Maximum Modulation Factor is 30% - 40%
Constant values kept up to more then = 30 for offaxis GRB
Polarimetry days in Rome
16-17 October 2008
Monte Carlo Results
Background:
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Cosmic rays removed by upper energy threshold
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Diffuse background E,bg>10 keV Fdif=2.46 /cm2/sr/s – 430 coinc./s
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Non-GRB  sources – e.g. Crab FCrab=0.7 /cm2/sr/s
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S/C induced ’s – ISGRI estimated Find=0.02 /cm2/sr/s
GRB signals:
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MDP =
n / 100 S  (S  B) / T
E=10-5 erg/cm2 → MDP3 ≈ 10%
(example LTC GRB060418 by RHESSI)
Statistics based on BATSE catalogue
MDP
Number of GRB in 1 year
5%
2.9
10%
12.7
15%
24.4
20%
36.8
25%
48.6
30%
59.4
One year observation of POLAR
180
TS2/DM2 FOV = 2π
160
140
(GEANT4 full simulation)
Number of GRBs (N < MDP)
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120
100
80
60
40
20
0
G. LAMANNA
0
10
20
30
40
50
60
70
80
Minimum Detectable Polarization with 3σ (%)
Polarimetry days in Rome
90
100
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Mechanics
G. LAMANNA
Polarimetry days in Rome
16-17 October 2008
The POLAR detector: Space know-how
AMS-ECAL @ LAPP:
Optical coupling
G. LAMANNA
Polarimetry days in Rome
16-17 October 2008
MAPRA ASIC PROJECT
MAPRA: Multi Anode Polar Readout Asic, based on the MAROC -LAL-IN2P3
(64 channels ASIC for ATLAS (CERN) luminometer)
100% Modulation factor vs. Uniformity
Characteristics
– 64 PMT channels input
– Variable gain
– 64 GTL outputs
– Multiplexed direct signal output
– 3 thresholds loaded
Modulation factor (%))
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50
40
30
20
Non-Uniform
Ideal
Energy Smearing
10
0
0
100
200
300
Energy (keV)
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Technology : AMS SiGe 0.35µm
– Area 12 mm2
– Dissipation O(100 mW)
G. LAMANNA
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Lack of uniformity affects the
modulation factor: Non-Uniformity of
PM has a strong influence
Polarimetry days in Rome
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Laboratory Tests
G. LAMANNA
Polarimetry days in Rome
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Laboratory Tests
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Tagged -ray polarized source
– 90 scattering of photons on large
scintillator
– Strong 137Cs source (37 MBq)
– -rays (E90deg≈ 290 keV, Pav≈ 40%, Pmax≈ 60%)
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Measured asymmetry up to 12%
(depending on distance between plastics)
Polarized 290 keV -rays
2
bars 6x6 mm
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Corresponding to (0.12 / 0.3 =) 40 %
polarization
Measured asymmetry (~modulation factor)
confirming
the validity of working principle
Asymmetry (1)
0.16
0.12
0.08
0.04
0.00
10
20
30
40
50
Distance (mm)
G. LAMANNA
Polarimetry days in Rome
16-17 October 2008
SVOM as a trigger for POLAR
The POLAR project is in the middle of an hybrid “Phase A/B”:
- Demonstrator is under test on polarized beam, meanwhile an Engineer Model (EM) is under
development
- The EM project (e.g. Phase B) has recently produced a Preliminary Technical Design Report for all
sub-projects: Mec, Ele.etc..
-The schedule of the EM is mainly driven by the FNS – Swiss funding (April 2007-April 2010)
- French funding for the MAPRA ASIC (2008-2009) +
A scientific collaboration between POLAR and SVOM…
The SVOM community strongly supports POLAR
as a low cost “natural” complementary device for an
extremely important complementary scientific measurement.
Chinese space-flight opportunity.
IHEP-proposing POLAR a Chinese experiment
and a flight onboard the Chinese SpaceLab in 2013.
G. LAMANNA
Polarimetry days in Rome
16-17 October 2008
SPACELAB
- Chinese space-flight opportunity.
SpaceLab in 2013.
Space radiation environment MC
studies:
-Photons scattered by the SpaceLab
-Polarized photons back-splashed from
Earth.
G. LAMANNA
Polarimetry days in Rome
16-17 October 2008
SUMMARY
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POLAR – Compton hard X-ray GRB polarimeter using low Z
scintillators
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40x40 homogeneous array of 6x6x200 mm3 plastic bars
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FoV ≈ ⅓ of the sky and low  energy threshold Emin < 50 keV
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Aeff ≈ 400 cm2 and 100 ≈ 40% at 200 keV
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MDP3 ≈ 10% for GRB total energy of 10-5 erg/cm2; tens of
detections/year
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First asymmetry results obtained demonstrating polarimetric
capability
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Engineering Qualification Model under development
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Demonstrator will be tested in Beam-tests
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Collaboration with IHEP to include POLAR in forthcoming Chinese
satellite experiments
G. LAMANNA
Polarimetry days in Rome
16-17 October 2008
BACKGROUND SOURCES
• Cosmic rays removed by upper
energy threshold
• Diffuse background E,bg>10 keV
Fdif=2.46 /cm2/sr/s – 430 coinc./s
0
10
• S/C induced ’s – ISGRI estimated
Find=0.02 /cm2/sr/s
• Weaker GRBs at lower energies
require careful background
subtraction
G. LAMANNA
-1
10
Photon Flux (a.u.)
• Non-GRB  sources – e.g. Crab
FCrab=0.7 /cm2/s
Diffuse
Crab
-6
2
GRB (10 erg/cm /s)
-2
10
-3
10
-4
10
0
100
200
300
400
Energy (keV)
Polarimetry days in Rome
16-17 October 2008
Monte Carlo Results
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Analysis uses two largest energy
deposits with Ethr= 5 keV
Direction and Spectrum known
Implies less systematic, less MC simulated
cases for cross-checks…
(corresponding to electron recoil energy
from 50 keV photon scattered at 90o)
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400
GRB position is known (e.g. GCN)
Modulation Curve
350
Fit function:
N=A·cos(2(h-f)+½)+B
f – polarization direction
300
Counts (a.u.)
•
250
200
150
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MC predicts clear modulation signal
with period 
100
0
30
60
90
120
150
180
Azimuth Angle 
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Unpolarized photons create pattern
with period /2
G. LAMANNA
Polarimetry days in Rome
16-17 October 2008