Position sensitive x-ray detectors applied to Compton ... · as a Compton Polarimeter 7 mm 64 mm 64...

Günter Weber EMMI Workshop, GSI, 7. 6. 2010

H. Bräuning1, S. Hess1,3, U. Spillmann1, A. Surzhykov2, S. Fritzsche1, R. Märtin1,2,S. Trotsenko1,4, D. Thorn5, D. Winters1 and Th. Stöhlker1,2,4

1 Gesellschaft für Schwerionenforschung, 64291 Darmstadt, Germany2 Universität Heidelberg, 69120 Heidelberg, Germany3 Universität Frankfurt, 60235 Frankfurt, Germany4 Helmholtz-Institut Jena, 07743 Jena, Germany5 Extreme Matter Institute, 64291 Darmstadt, Germany

Günter Weber1,2

Physikalisches Institut

Position sensitive x-ray detectors applied toCompton polarimetry of hard x-ray radiation


Outline of the Talk

• Introduction to Compton Polarimetry

• Recent measurements at the ESR storage ring→ Radiative Electron Capture (REC)

→ Lyman-α1 radiation

→ Elektron-Nucleus Bremsstrahlung

• Bremsstrahlung measurements at thepolarized electron source SPIN at TU Darmstadt(very preliminary)

• Summary & Outlook


Where do we find polarized x-rays?

AstronomySynchrotron radiation

ESRFCrabnebular

Aligned atomic systems

X-ray scattering(Compton, Rayleigh)RR/REC process Bremsstrahlung

EKIN

ωh

K

L

K-REC

∞

20 40 60 80 100102

103

104

105

photon energy [ keV ]

100 keV e- → Cabon


Why should we care?

General interest

Revealing subtle details of atomic dynamics and structure(Lyman radiation in H-like systems)

Of special interest, if angular distribution is not accessible(EBIT, Astronomy)

Possible applications ?(Ion beam spin diagnosis, Imaging, Material science)

But: Precise polarimetry studies in the hard x-ray regime are quite challenging.


10-1 100 101 102 103 104 105 106 107

Bragg Reflection

Photoionization

Thomson Scattering Pair Production

Compton Scattering

Crystal Optics

Liquid Crystals

photon energy [ keV ]

Polarimetry Techniques

MicropatternGas Counters

x-ray Optics

PrismLCD

Segmented SolidState Detectors


0

30

60

90

120

150

180

210

240

270

300

330

ϕ

E

Polarization Measurement via Compton Scattering

Klein-Nishina Equation for Compton Scattering:

)cos θsin2'

'()'(21 2

c222

0 ϕωω

ωω

ωωσ

−+=Ω h

h

h

h

h

hrdd


E

Standard Solid State Detectors

ScatteredPhotonDetector

Scatterer

0

30

60

90

120

150

180

210

240

270

300

330

E

Compton Polarimeter Setups


Scatteringand Photon Detection

Segmented X-ray Detector

E

Compton Polarimeter Setups


32x32 strips → 1024 pixels

64x64 mm → 4096 mm2 active area

Energy (2.5 keV FWHM) + Timing (100 ns) + 2D Position (2 mm) + Multihit Capability

Position sensitive Si(Li) detectoras a Compton Polarimeter

7 mm

64 mm

64 mm

B implantedp+ contact (32 strips)

Li driftedcontact (32 strips)

Dedicated for efficient and precise polarization studies from 70 keV to a few 100 keV


Compton scattered photon distribution

0 30 60 90 120 150 180 210 240 270 300 330 3600

200

400

600

800

exp fit

Klein-Nishina formular

scattering angle ϕ

ϕ

E

From ϕ Distribution to Polarization

Projection on the ϕ angle

Online analysis already yields qualitative results!

K-REC Xe54+ → H2

S. Hess, Phd thesis (2010)


SIS

ESR

11.4 MeV/uU73+

5 - 400 MeV/uU92+ up to

1000 MeV/u U92+

UNILAC

The GSI Accelerator Facility

Ions: H+ to U92+

Intensity: 108

Frequency: 106


Ion Beam

Gas Target ( H2, N2, He, Ne, Ar, Xe )

The ESR internal gas target

Standard X-ray Detectors&

Novel Compton Polarimeters


X-ray detector

Particle detector(MWPC)

Gastarget

Dipol magnet

Measurements at the ESR storage ring

25 50 75 100 125 150 1750

2000

4000

6000

8000

10000

K-RECU92+→ H

2 @ 96,6 MeV/u

∞-RECM-REC

Ly-α2

Cou

nts

Energy [ keV ]

coincient anti-coincident

L-REC Ly-α1

Bremsstrahlung

Typical HCI x-ray spectrum at the ESR

Inverse kinematics

→ Coincidence measurements between x-rays and ions!


Polarization of the K-REC radiation


EKIN

ωhK

L

K-REC

∞

Radiative Recombination (RR)Radiative Electron Capture (REC)

In the non-relativistic approximation the REC radiation has dipole characteristics:

• sin2 θ angular distribution

• 100% linear polarization

E

B

e-

dipole

E

B

EB

e-

e-Time reversal process of photoionization


ion beam

targ

etga

s

K-REC

hω

E

50 100 150 200 250 300 350 400 450 500 5500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

degr

ee o

f the

line

ar p

olar

izat

ion

projectile energy (MeV/u)

Energy dependence

Observation angle θ=60°

First REC polarization measurements

U92+ → N2

S. Tashenov, Phd thesis (2005)

β = 0.4

β = 0.7

16 Pixel Ge(i) detector


Line

ar P

olar

izat

ion

Observation angle ( deg )

preliminary

Xe54+ → H2 @ 150 MeV/u U92+ → N2, H2

100 MeV/u

400 MeV/u

800 MeV/u

Recent K-REC polarization measurements

Si(Li) Polarimeter S. Hess, Phd thesis (2010)

16 pixel detector S. Tashenov et al., PRL 97 (2006)

Photon emission angle ( deg )

mid-Z high-Z

preliminary

A gain by a factor 3 to 5 in precision!


Polarization of the Lyman-α1 radiation in U91+


Study of Lyman-α1 polarization inH-like, high-Z systems

L-R

EC

1s1/2

2p3/2

Ly-α1

µ=+3/2µ=+1/2µ=-1/2µ=-3/2

99%M2 1% (in U91+)

Alignment of the excited 2p3/2 state:

Ly-α1 linear polarization:

Taking into account E1-M2-interference:E1

Revealing details of the population mechanism


Lyman-α1 Polarization of U91+

Exakt relativistic calculations by A. Surzhykov:

E1-M2 interference in Ly-α1 leads to

Increased Anisotropy

Decreased Polarization

preliminary

U92+ → H2@ 96.6 MeV/u

Hyperf. Interact. 146-147 (2003)

G. Weber, unpublished


EquationObservable Experiment Theoryonly E1 E1 + M2

Decoupling of the collision dynamics (alignment parameter A2) and the atomic structure (amplitude ratio α = M2/E1) leads to a model independent estimation of both values.

-> Precise estimation of the E1-M2 transition ratio!

A model independent estimationof the E1-M2 transition ratio

Amplitude ratioTheoryTheory


Polarization of Electron Bremsstrahlung


REC/RR into Rydbergseries limit

Bremsstrahlunghigh energyend‐point

=

Xe54+ → H2 @ 150 MeV/u

G. Weber, unpublished

First Bremsstrahlung polarization study with inverse kinematics

preliminary

Next step:Bremsstrahlung polarimetry + electron spectroscopy -> correlation study


• 100 keV electrons

• longitudinal or transversal electron polarization (about 80%)

• DC mode

C. Heßler, Conf. Proc. EPAC08 (2008)

Measurements at the polarized electron source SPIN at TU, Darmstadt

Experimental Set-up


Linear Bremsstrahlung polarization measurements(unpolarized electrons)

preliminary

e-→ Au, 90µg/cm2 @ 130°

e-→ C, 210µg/cm2 @ 60°

KαKβ

ReconstructedCompton events for

98.5±3, 92.5±3, 86.5±3, 80.5±3,

74.5±3 keV

Bremsstrahlung

R. Märtin, unpublished

Linear Polarization

No corrections for systematic effects (electron scattering, polarimeter efficiency, etc.) done yet.


100 keV electron beam

E

S

S. Tashenov, unpublished

Longitudinal pol. e-

1

22tan PP=χ

χ

P2 is proportionalto the degree ofspin polarization

The effect of spin polarized collision systems on Bremsstrahlung polarization

Au

Transversely pol. e-

Photon energy: 98.5±3 keVR. Märtin, unpublished

preliminaryχ = 9°


Summary & Outlook

Effifcient and precise Compton polarimeters are available(only small systematic corrections needed)

Experiments have already revisited a variety of radiative processeswith respect to linear polarization (e.g. REC, Lyman, Bremsstrahlung)

Thank you for your attention!

Photon energies from about 70 keV to several 100 keV

Future plans:Studies of scattered x-rays at DESYPhoton-photon and photon-electron correlationsMeasurements at EBIS/T facilities

Date post:	16-Jun-2019
Category:	Documents
Upload:	trinhkhanh
View:	224 times
Download:	0 times

Position sensitive x-ray detectors applied to Compton ... · as a Compton Polarimeter 7 mm 64 mm 64...

Documents