Hard X and Gamma-ray Polarization: the ultimate dimension (ESA Cosmic Vision 2015-2025) or the...

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

(ESA Cosmic Vision 2015-2025)(ESA Cosmic Vision 2015-2025)

or the Compton Scattering or the Compton Scattering polarimetery challengespolarimetery challenges

Ezio Caroli, INAF/IASF – Sezione di Bologna

17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna

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 FlaresSolar 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 sourcesOther 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 by-product 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.

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



22

220 cossin2

'

''

2 E

E

E

E

E

Er

d

d

Klein-Nishina cross-section for linearly polarized photons:

(°)

Q f

acto

r

||

||

NN

NNQ

090

090

dd

ddQ

2

2

sin'

'sin

EE

EE

Q

Polarisation direction



P o la r ised B ea m E n erg y ra n g e: 1 0 0 k e V to ~ 1 M e V

C om p ton S ca tter in g

C d Te P ix e l M a tr ix

E

E

1 0 m m

2 m m

2 m m

C d Te P ix e l D im en sio n s

Advantages of a ‘thick’ pixel detectors:

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.



20 - 401 – 5 x109~1002.5 2.53.4 – 5 – 7.5

D.C.(nA)

Resistivity(.cm)

Bias(V/mm)

Pixel(mm2)

Thickness(mm)

+/- 12 VBias

< 3 keVEquivalent noise

< 200 nsRise time

70 mV/MeVSensitivity vs photon energy

2 V/pCSensitivity

CSP Model Eurorad PR-304

The POLCA pixellated CdTe detectorsExperiment at the beam line ID15 of the ESRF in July 2002



1 2 3 4

5 6 7 8

9 10 11 12

13 14 15 16

1

16

1

16

1

16

90°180°

270°

• Corner pixel irradiaton • Rotation of the 4x4 matrix• 90° double events distribution symmetry

Allows extrapolation to a 7x7 matrix

POLCA: Experiment at the beam line ID15 of the ESRF in July 2002



Single events Double events

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:

Nx/y = Mx/y x Nt/Nsxy

M = detected double events

Nt = Total events

Ns = Single events



POLCA experiment

Monte Carlo

POLCA

7.5 mm

300 keV

MC

7.5 mm

300 keV



0,00

0,10

0,20

0,30

0,40

0,50

0,60

100 200 300 400 500 600 700 800 900 1000

Energy (keV)

Q Fa

ctor

Central pixels suppression

First order suppression

Second order suppression

Symmetric module

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

10

20

30

40

50

60

70

80

90

100

1,0E+1 1,0E+2 1,0E+3 1,0E+4 1,0E+5 1,0E+6

Temps d'observation (s)

MD

P (

%)

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).

MDP=5% in 10000 s.

T

BSA

QSA

nMDP F

F

..

...%100

100

Minimum Detectable Polarisation



CIPHER polarimetric sensitivity

Energy (keV)100 1000

photo

ns/(

cm2 s

keV

)

0.001

0.01

0.1

Fmin(Q100) at 3

for T=15000 sCrab TotalCrab Pulsar

)/( 2

100min keVcmsph

EAT

B

Q

nF

The polarimetric sensitivity is (3σ, 15000 s):

~20 mCrab over the 40-1000 keV range

Polarimetric Sensitivity for CIPHER geometry

Crab (10% polarized), 3 hours



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.

Laue Lens Telescope The lens does not affect the linear polarisation status of the incidence beam (Frontera et al., SPIE 1995)



Lens aperture: 400 cm2; Focal length 10

m Double events map for a Crab like incidence (100% polarized) flux in the 60-400 keV rangePolarisation direction

0

10

20

30

40

50

60

70

80

90

100

1,0E+04 1,0E+05 1,0E+06

Observation time (s)

MDP

(%)

3 mm

10 mm

100 1000 10000



ConclusionWide 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

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