X-ray and gamma-ray polarimetry

X-ray and gamma-ray polarimetry

Experimental techniques for particle astrophysics

Magnus Axelsson

Outline

● Why measure polarization?

● Previous results

● Photoelectric effect

●

Physical background

●

MPGD

● Compton scattering

●

Basic concept

●

PoGO

●

GRAPE, RHESSI, ...

● Pair production

●

Basic idea

●

Example design

● Summary

Why measure polarization?

Emission mechanism in pulsars

●

Lightcurves look similar – difficult to distinguish between models

●

Polarization properties allow determination

“Polarization measurements provide a powerful probe into the gamma-ray emission mechanism and the distribution of magnetic and radiation fields, as well as the

distribution of matter, around [a wide variety of astrophysical sources].”

PoGOLite proposal

80% 80%

-35^o

30^o 80^o

20^o

70% 70%

10%

0^o

40^o

10%

0^o

0% 10%

-50^o

2^o

18%

Why measure polarization?

X-ray binary systems

●

Determine emission mechanism and geometry/inclination

Jets in AGN and Microquasars

●

Test synchrotron radiation as source for gamma rays

OSO-8

Polarization measured using Bragg reflection,

at 2.6 keV and 5.2 keV

What is the problem?

Previous measurements have been very few and not very successful.

No experiments have attempted to measure polarization in the

gamma-ray band.

Long et al. (1980) 67%

99%

OSO-8

Techniques

● Want to determine degree and direction of polarization (+ energy)

● Three potential processes:

− Photoelectric effect

●

MPGD (planned for XEUS?)

− Compton scattering

●

PoGO, GRAPE, RHESSI

− Pair production

●

In development...

Compton Scattering (~0.1-10 MeV) Photon Crossection Minimum

Scattered photons with long range

Pair Creation (> 10 MeV) Photons completely

converted to e

e

Interaction Cross Section

Energy

Photoeffect(< 50 keV) Photons effectively blocked and stopped

Experimental Regimes

for the Detection of Gamma Radiation

courtesy G. Kanbach

Photoelectric absorption

Analytical expression for differential cross section (K-shell, non-rel. limit):

Projected on the detector plane:

∂

∂ =r

Z

137

 ^{mc h }



^{4 } 1− cos ^{2 sin}

^cos

^

∂

∂ ∝cos



(R. Bellazzini)

Photelectric absorption

Information about the photoelectron direction is in the initial part of the track. Elastic scattering in the detector will

randomize the direction.

R. Bellazzini

Micro Pattern Gas Detector (MPGD)

A finely subdivided gas detector is used to image the photoelectron track.

The photon is absorbed in the drift gap, and the

photoelectron track is drifted down to the gas electron multiplier (GEM) – a thin, perforated foil layer where the charge is multiplied. The charge is read out in the pixel anode, projecting the track onto the detector plane.

The gas used to fill the chamber is chosen to achieve a sufficiently long track, while

trying to keep the track as straight as possible. The energy of the Auger electron (which

is emitted isotropically) should be low enough to not blur the identification.

Compton scattering

∂

∂ = 1

2 r

k

k

[ ^{k k}

^{ k k}

^−2sin

^{ cos}

^ ]

Cross section for Compton scattering (Klein-Nishina):

Observed azimuthal scattering angles are dependent on polarization of photons.

Most of the energy deposited at

photoabsorption site, rather than during

scattering – allows Compton site to be

identified.

High-Z vs Low-Z

● Different material in the scattering block for different energies

●

“Standard” instruments typically use high-Z elements to maximize photon absorption – not optimal for

polarimetry at lower energies

●

At energies of ~30-300 keV, best results are typically achieved with scattering in a low-Z material, and a high-Z material for photoabsorption.

●

At higher energies, the cross section for scattering in

low-Z materials is too small, and high-Z materials are

used for both scattering and absorption.

Polarized Gamma-ray Observatory (PoGO)

Balloon-borne polarimeter, planned to be launched in 2009.

PoGO

Events which enter cleanly through the slow

scintillator tube and scatter in the fast scintillator are kept.

Events entering at an angle will be registered in the slow scintillator and can be rejected.

The well-type design allows for efficient background rejection, but narrows the FoV (2.4' x 2.4').

One PMT reads out all scintillators, so pulse shape

is used to select events.

Events where a small fraction of the total energy is deposited during the first hit are selected.

Results from prototype testing in polarized beam:

Clear separation between fast and BGO/slow scintillator signals.

Fast branch chosen for analysis.

Telescope will rotate - removes systematic bias.

Direction of polarization can be determined from the phase, and the strength of modulation gives the degree of polarization.

Energy of incoming photon is calculated by combining the

scattering and absorption signals.

GRAPE

●

Photon scatters in plastic

scintillators and is absorbed in the central CsI block

●

Scattering site read out by position sensitive PMT

●

CsI scintillator coupled to own PMT for energy measurement and signal timing

●

Significant modulation out to incident angles of ~60º – large FoV

●

Energy range: 50-300 keV

RHESSI

●

Designed as a hard X-ray solar imager

●

Be scattering block placed in the spectrometer array

●

Photons may scatter in the block and be absorbed in the Ge detectors

●

For high energy photons, Ge-Ge events possible

●

Rotation of the satellite gives rise to modulation

curves which can be used to detemine polarization

McConnell et al., 2003

LXeGRIT

Liquid Xenon calorimeter: high-Z element,

provides good scattering qualities from 250 keV – 10 MeV.

Scintillation from first scattering read out in PMTs and marks start time.

Drift time of electrons to sense electrodes used to obtain third dimension (drift velocity known).

Multiple scatterings produce a multi-step signal, which can be identified, and these events do not need to be rejected.

In each event, the strength of the signal is determined by the energy deposited.

Combining all events, one may reconstruct the direction and energy of the incident photon.

Has flown on balloon flights in 1999 and 2000, observed the Crab nebula/pulsar for ~10 hrs.

Data still being analyzed.

Pair production

Pair production cross section dependent on difference between polarization angle ( ψ

) and azimuth angle of e

- e

pair plane ( ψ ):

Technique suggested already in 1950 (Berlin & Madansky), but not tried in practice.

Difficult to determine the e

- e

plane.

Potential design

Three dimensional track imaging detector – reconstruct the tracks to determine pair plane.

Place a layer of micro-well

detectors surrounded by Xe-gas.

Micro-well detectors read the projected two- dimensional track

Third dimension is obtained by timing the drift of the ionization electrons.

Potential problem is the diffusion of the drift electrons in the gas.

Images from P. Bloser

Advanced Pair Telescope

Array of track imaging detectors arranged in Xe- gas filled cylider, 3 m in diameter and 5.1 m high

Modules rotated 90 in relation to each other for better tracking.

However, simulations show that diffusion of drift electrons is a big problem, almost eliminating the modulation:

Measure scatter of electron-positron pair to determine energy – no

calorimeter needed.

Use outer gas layer as

anticoincidence?

Summary

● Polarimetry is a tool which has so far not been used to any greater extent

● Compton scattering most common process exploited in instruments, but plans also for instruments using photoabsorption and pair production

● Potential for effective polarimetry in energy

range from a few keV to MeV