Transcript Hard X and Gamma-ray Polarization: the ultimate dimension

Hard X and Gamma-ray Polarization:
the ultimate dimension
(ESA Cosmic Vision 2015-2025)
or the Compton Scattering
polarimetery challenges
Ezio Caroli, INAF/IASF – Sezione di Bologna
Hard X and Gamma-ray Polarization: the ultimate dimension
Conventional analysis (imaging, spectroscopy and timing) of the high energy radiation from
cosmic ray sources often provides two or more different models that successfully explain the
observations; The combined measurement of polarization angle and degree of linear
polarization can provide vital extra information to discrimate among the models.
• Solar Flares Solar flare emission is contaminated by thermal emission at lower energies
and by lines above 1 MeV, the best energy range for polarimetry investigation from solar
flares is from 0.1-1 MeV. Models for solar flares predict polarization levels as high as
20 or 30% in this energy band.
• Gamma-ray Bursts . Since the peak energy of the GRB spectrum is narrowly
distributed at ~ 250 keV, Compton polarimetry is clearly a requirement for this topic. A
high level of polarization (>50%) is expected in GRB if the prompt emission, as
recently suggested, is the result of a relativistic expansion of strongly magnetized
electron-positron plasmas; in the standard (internal-external shock) model,
polarization levels of about 10-20% are expected
• Other possible sources Pulsars: hard X-ray polarimetry to understand the extent to
which gamma-ray photons are related to those at longer wavelengths (e.g. a polarisation
level >20% is expected from the CRAB pulsar). Soft Gamma-ray Repeaters: one byproduct of magnetic photon splitting (50-500 keV) is that the reprocessed photons
would exhibit a polarization level of ~25%. Massive Black Hole: the geometry of the
the accretion disk; For optical thin disks polarization levels as high as 30-60% are
possible while in the optically thick regime lower levels (~10%) are predicted.
17 March 2005
E. Caroli, INAF/IASF-Sezione di Bologna
Hard X and Gamma-ray Polarization: the ultimate dimension
Polarisation
direction


Klein-Nishina cross-section for linearly
polarized photons:
Q
2
 E'   E' E

2
2
     2 sin  cos  
 E   E E'

Q factor
d r02

d 2
d   90  d   0
d   90  d   0
Q
N   N||
N   N||
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sin 2 
Q
E' E
  sin 2 
E E'
 (°)
E. Caroli, INAF/IASF-Sezione di Bologna
Hard X and Gamma-ray Polarization: the ultimate dimension
2 mm
2 mm
Polarised Beam
Energy range:
100 keV to ~1 MeV
10 mm
E
CdTe Pixel Dimensions
E
pton
m
o
C
ing
r
e
t
t
Sca

Advantages of
pixel detectors:
a
‘thick’
Each element of the
dectection plane is both a
scatterer and a detector.
Therefore all the sensitive
area is used as polarimeter
The geometry of the
detector select only events
with a scattering angle close
to 90º. For these scattering
angles we have the best
modulation factor.
CdTe Pixel Matrix
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E. Caroli, INAF/IASF-Sezione di Bologna
Hard X and Gamma-ray Polarization: the ultimate dimension
The POLCA pixellated CdTe detectors
Experiment at the beam line ID15 of the ESRF in July 2002
Thickness
(mm)
Pixel
(mm2)
Bias
(V/mm)
Resistivity
(.cm)
D.C.
(nA)
3.4 – 5 – 7.5
2.5  2.5
~100
1 – 5 x109
20 - 40
CSP Model Eurorad PR-304
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Sensitivity
2 V/pC
Sensitivity vs photon energy
70 mV/MeV
Rise time
< 200 ns
Equivalent noise
< 3 keV
Bias
+/- 12 V
E. Caroli, INAF/IASF-Sezione di Bologna
Hard X and Gamma-ray Polarization: the ultimate dimension
POLCA: Experiment at the beam line ID15 of the ESRF in July 2002
Allows extrapolation
to a 7x7 matrix
16
• Corner pixel irradiaton
• Rotation of the 4x4 matrix
• 90° double events distribution symmetry
16
90°
1
16
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1
270°
1
180°
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
E. Caroli, INAF/IASF-Sezione di Bologna
Hard X and Gamma-ray Polarization: the ultimate dimension
The single events are
used to correct the double
counts maps for pixel
response non uniformity in
order to reduce
sistematics effects on the
evaluation of the Q factor:
Single events
Double events
Nx/y = Mx/y x Nt/Nsxy
M = detected double
events
Nt = Total events
Ns = Single events
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E. Caroli, INAF/IASF-Sezione di Bologna
Hard X and Gamma-ray Polarization: the ultimate dimension
POLCA experiment
POLCA
Matrix number
(thickness)
Polarimetric Q factor
100 keV
300 keV
400 keV
1167/11
(3.4 mm)
0.15  0.051
0.46  0.036
0.36  0.091
1283/26
(5.0 mm)
_
0.40  0.12
0.33  0.13
1186/49
(7.5 mm)
_
0.39  0.060
0.31  0.065
7.5 mm
300 keV
Monte Carlo
Matrix number
(thickness)
MC
Polarimetric Q factor
100 keV
300 keV
400 keV
1167/11
(3.4 mm)
0.49  0.005
0.47  0.003
0.43  0.003
1283/26
(5.0 mm)
_
0.43  0.002
0.39  0.003
1186/49
(7.5 mm)
_
0.40  0.002
0.35  0.002
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7.5 mm
300 keV
E. Caroli, INAF/IASF-Sezione di Bologna
Hard X and Gamma-ray Polarization: the ultimate dimension
Q Factor in function
of energy for
CIPHER telescope
in the following
cases: excluding
only central pixel
double events,
excluding additional
first order pixels
double events, and
additionally second
order pixels double
events
0,60
Central pixels suppression
First order suppression
0,50
Second order suppression
Q Factor
Symmetric module
0,40
0,30
0,20
0,10
0,00
100
200
300
400
500
600
700
800
900
1000
Energy (keV)
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E. Caroli, INAF/IASF-Sezione di Bologna
Hard X and Gamma-ray Polarization: the ultimate dimension
Minimum Detectable Polarisation
The MDP calculated for
the CIPHER (160 cm2)
telescope when
irradiated by a Crab
emission flux in presence
of a background noise
measured by HEXIS
instrument (full active
shielding configuration).
100
90
80
MDP (%)
70
60
50
40
30
20
MDP=5% in 10000 s.
10
0
1,0E+1
1,0E+2
1,0E+3
1,0E+4
1,0E+5
1,0E+6
Temps d'observation (s)
MDP100% 
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n
A. .S F .Q100
E. Caroli, INAF/IASF-Sezione di Bologna
A. .S F  B
T
Hard X and Gamma-ray Polarization: the ultimate dimension
Polarimetric Sensitivity for CIPHER geometry
CIPHER polarimetric sensitivity
Fmin(Q100) at 3
for T=15000 s
Crab Total
Crab Pulsar
photons/(cm2 s keV)
0.1
The polarimetric sensitivity is
(3σ, 15000 s):
~20 mCrab over the 40-1000
keV range
0.01
0.001
100
1000
Energy (keV)
Fmin
n

  Q100
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B
AT E
ph /( s  cm 2  keV )
Crab (10% polarized), 3 hours
E. Caroli, INAF/IASF-Sezione di Bologna
Hard X and Gamma-ray Polarization: the ultimate dimension
Laue Lens Telescope
The lens does not affect the linear
polarisation status of the incidence beam
(Frontera et al., SPIE 1995)
Focal plane detector requirements:
•Sensitive area ≈ 50 cm2
•Efficient at high energy (from 60 to 400
keV) therefore a CdTe thickness from 3 to
10 mm is required,
•Pixel size at mm level because the
expected point spread function is about 10
mm (FWHM) for photon energies of about
200 keV.
A possible implementation: 32×32 CdTe
pixels, where each pixel has a 2×2 mm2
surface area and pixel thickness limits from
a least 3 mm up to 10 mm.
17 March 2005
E. Caroli, INAF/IASF-Sezione di Bologna
Hard X and Gamma-ray Polarization: the ultimate dimension
Lens aperture:
m
400 cm2; Focal length 10
Polarisation direction
Double events map for a Crab like
incidence (100% polarized) flux in the
60-400 keV range
100
90
80
MDP (%)
70
60
50
40
30
20
3 mm
10 mm
10
0
100
1,0E+04
1000
1,0E+05
Observation time (s)
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E. Caroli, INAF/IASF-Sezione di Bologna
10000
1,0E+06
Hard X and Gamma-ray Polarization: the ultimate dimension
Conclusion
Wide field telescope configuration: both experiments (like
POLCA) and simulations suggest that with a thick (5-10 mm) CdTe/CZT pixel
(few mm2) detector modulation factor up to 0.5 can be achieved in the range
100-500 keV. These results allow to predict that an MDP at level of 1-2% can
be obtained with 500 cm2 in about 30 h exposure for a 100 mCrab source flux.
Gamma ray lens configuration: the implementation of the laue
lens technique can provide an improvement in sensitivity of about 2 order of
magnitude with respect to current hard X and soft gamma ray telescope. A
polarimeter in the focal plane of this kind of telescope can attain
unprecedented performance: e.g. with a collecting area of 300 cm2 and 10 m
focal length MDP ≈ 0.2% can be achieved in 30 hr exposure for a 100 mCrab
source flux.
Both mission configuration can be evisaged for the next decades, the choice
depending on the scienfitic requirements and objectives
17 March 2005
E. Caroli, INAF/IASF-Sezione di Bologna