Gamma- and X-ray polarization

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Gamma- and X-ray polarization

Mózsi Kiss

May 28 2007 – 5A5461: Experimental techniques for particle astrophysics

PoGOLite collab. RHESSI home page / NASA OSO-8 / NASA

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Outline

1. Background

• Polarization – Why bother?

• What can polarization tell us?

• From modulation to polarization

• Photon interactions in matter 2. Photoelectric absorption

• Theory

• How to measure?

• Case study 3. Compton scattering

• Theory

• How to measure?

• Case study 4. Pair production

• Theory

• How to measure?

• Case study?

5. Summary

6. References

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P t

r

E , ˆ , , ˆ

• Photons can be characterized by their energy, direction, time of detection and polarization

• Polarization is usually not measured  measuring polarization gives two new observational parameters (pol. angle and degree)

• Gives new information about the emission mechanism, geometry and magnetic field of the observed source

M. Owen, J. Blondin, North Carolina State University

t r E , , ˆ Polarization – Why bother?

“Most instruments”:

Polarimeters:

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What can polarization tell us?

Pulsars

• Three different emission models: polar cap, caustic, outer gap

• Models predict different origins for the high-energy photon emission

A. Harding, NASA Goddard Space Flight Center (2004)

caustic

Charged particles near surface

emitting synchrotron radiation

Charged particles confined

between last open B-field lines

Acceleration in outer magneto-

sphere  pair prod. cascades

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What can polarization tell us?

• Relative flux almost identical for all three models  cannot be used to identify correct model

• Polarization (angle and degree) different for all models  can be used to identify correct model!

PoGOLite collab.

33 ms

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What can polarization tell us?

Direct soft photons

Cold outer disc

Scattered hard photons

Black hole

Hot inner disc

A. A. Zdziarski, et al. (2002)

Accretion discs

• Primary component of photons directly from the accretion disc – unpolarized

• Secondary component of photons reflected in the accretion disc – polarized

• Polarization  information about the geometry and inclination of the disc

Neutron stars

• QED predicts absorption of photons polarized perpendicular to the magnetic field lines

 reconstruct the geometry of the magnetic fields

Astrophysical jets

• Polarization observed in radio and UV range, polarization of HE emission unknown

 study emission mechanisms and magnetic field of the host galaxy

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Modulation factor:

difference / average

From modulation to polarization

Azimuthal scat. angle (degrees) ^{M. Pearce}

Number of counts

The distribution of azimuthal scattering angles in photon interactions is modulated by polarization  modulation factor

The polarization degree and polarization angle can both be determined from the modulation factor

Polarization degree:

(M

₁₀₀

: modulation factor for a 100% polarized source)

M

100

M

P

_source



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Photon interactions in matter

WATER

Photon energy (MeV) Mass at te n u ation (cm

2

/g)

B. Coursey, NIST (2001)

Total

< 30 keV  Photoelectric absorption dominates

 30 keV – 10 MeV  Compton scattering dominates

> 10 MeV  Pair production dominates

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Photoelectric absorption – Theory

1. Photon absorbed by the material 2. Energy is transferred to an electron 3. Electron is emitted

M. K. Yip (2007)

z

x

y Angular distribution in the xy-plane of

K-shell photoelectrons (ellipsoids)





Emitted photoelectron Angular distribution in space of

K-shell photoelectrons (lobes) Incident photon

Photon polarization (in yz-plane)

4 2 2 2

2 7 4

5 2

0

( 1 cos )

cos sin

2 4









 



 

 

 

 h

c Z m

d r

d

_e

Differential cross-section for an electron emitted from the s-orbital of an atom in the non-relativistic limit (R. Bellazzini, et al., 2003):



₂



 cos

 d

d  the emission angles are modulated by the polarization

: azimuthal angle of the emitted electron

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Photoelectric absorption – How to measure?

Monte Carlo simulation of 5.0 keV photoelectrons in neon. Incident

beam is 100% linearly polarized

Photon polarization

R. Bellazzini, Pisa (2003)

Need information about the initial part of the track, before the directions

are randomized by elastic scatterings  high resolution is needed! How?

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Photoelectric absorption – Case study

Optical Imaging Capillary Gas Proportional Counter

T. Masuda, et al., IEEE (2000)

Gas: Ar + CH

₄

+ N(CH

₃

)

₃

at 1 atm (gives high gain)

Electron cloud divided into several capillaries

Gas multiplication + light emitted by de-excitation

of gas molecules

Charge signal E from gas multiplication

Light signal from CCD camera

DAQ

Reconstruct initial point and first scattering point of the photoelectron

 directional information  polarization information!

But! Quoted spatial resolution 100 m  probably not enough...

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Compton scattering – Theory

1. Photon scatters off an electron 2. Electron gains kinetic energy 3. Photon loses energy

G. F. Knoll (1999)

Recoil electron Incident photon,

momentum k₀

Scattered photon, momentum k

The Klein-Nishina differential scattering cross-section formula (T. Mizuno, et al., 2004):

 

 





   



₀ ₂ ₂

0 2 0

2 2

0

2 sin cos

2 1

k k k

k k

r k d

d

cos

²

 term the scattering angles are modulated by the polarization

Azimuthal scattering angle Polarization

vector (E)

Polar scattering angle





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Compton scattering – How to measure?

Polarization

The problems:

• Choice of material, low-Z or high Z?

• Multiple site events?

• Sufficient energy resolution?

• Nasty background, e.g. from neutrons The idea:

Use a segmented detector  Compton scattering in one unit and

photoabsorption in another unit  reconstruct the path of the photons

from the relative energy deposition in the detector cells  distribution

of the azimuthal scattering angles  polarization info!

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Compton scattering – Case study

PHENEX – Polarimeter for High ENErgy X-rays

S. Gunji, et al., 28th ICRC conf.

MAPMT

Hamamatsu Photonics, H8500

5 cm

• Small 8x8 detector array, 36 plastic scintillators (low-Z) and 28 CsI(Tl) scintillators (high-Z)

• Multi-anode photomultiplier tube (MAPMT) used for read-out: multiple PMTs in a single

housing  spatial resolution

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Compton scattering – Case study

PHENEX – Polarimeter for High ENErgy X-rays

Balloon-borne experiment Energy range: 40–300 keV Flight altitude: 38 km

Beam tests at KEK-PF

First flight was in June 2006 Four 8x8 counter units,

one monitor counter (in the middle)

Telescope assembly

• No published results  problems?

• Detectors cross-talking  event misinterpretation

• Multiple scattering events

• No veto system  background

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Pair production - Theory

1. Photon enters the detector material

2. Interaction with a nucleus to conserve momentum 3. Positron-electron pair is produced

R. Nave, HyperPhysics

Incident photon

Photon polarization (in the xz-plane)

G. O. Depaloa, Astro. Ph. (1999)

: Azimuthal angle of e⁺–e^–pair plane

Last year: nice formula for cross-section (M. Axelsson)

 

 



₀



0

1 cos 2

) 2

(  



 

   PR 

This year: nasty formula for differential cross-section (T. H. Berlin & L. Madansky)

Depends on the polarization vector   modulation!

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Pair production – How to measure?

Pixelized Micro-Well Detector (PMWD)

• Gas proportional counter with micro-wells

• Drift region and avalance region  spatial resolution (two-dimensional)

• Timing information  third dimension

• Spatial resolution 200 m (in theory), timing resolution  10 ns, energy resolution 20%

• Problem: diffusion of drift electrons  track information is lost

Drift region (drift voltage)

Avalanche region (higher voltage)

Insulation Insulation

P. F. Bloser, LHEA (2004) P. F. Bloser, LHEA (2004)

Xe + C0

₂

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Pair production – Case study?

• No such instrument for polarimetry yet – too difficult to accurately determine the e

⁺

–e

^–

plane!

• Low angular resolution is sufficient to identify if incident radiation is polarized or not, but a very high resolution is needed to determine the polarization angle

• For comparison: minimum detectable polarization with pair production:

80% from 1 Crab source in 10

⁶

s, about 12 days (P. F. Bloser).

Minimum detectable polarization with Compton scattering technique:

10% from 200 mCrab in 6 hours (PoGOLite). Different energy range, but

still gives a feeling for how difficult this kind of measurement is...

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Summary

Which processes can be used for polarimetry?

In theory? All!

The cross-sections for all three interactions depend on the polarization of the incident photons

In practice?

Compton scattering: feasible! (PoGOLite, PHENEX, RHESSI, POLAR, GRAPE…) Photoelectric absorption: trickier! (Optical imaging CGPC)

Pair production: difficult! (No experiments yet)

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Summary

• Detection methods cannot be combined, since different photon interaction processes dominate in different energy regions

• Most experiments are based on Compton scattering, but experiments based on photoelectric absorption and pair

production may become important in the future, if the detector technology is improved

• X-ray and gamma-ray polarimetry has an enormous scientific

potential! The question is not IF polarization will be measured

in this energy range, but WHEN...

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