Positron-Annihilation Lifetime Spectroscopy for …...MePS – PALS timing resolution Page 16 Lifetimes of Silicon (218 ps) and Yttria-stabilized Zirkonia can be reproduced, timing

SIP-IMASM Symposium, Tsukuba, JP 27.-30.9.2016 | Andreas Wagner I Institute of Radiation Physics I www.hzdr.de

Member of the Helmholtz Association SIP-IMASM Symposium, Tsukuba, JP 27.-30.9.2016 | Andreas Wagner I Institute of Radiation Physics I www.hzdr.de

Andreas Wagner Nuclear Physics Division Institute of Radiation Physics

Positron-Annihilation Lifetime Spectroscopy for Materials Science


Member of the Helmholtz Association

• Motivation Positron Annihilation & Materials Science

• Positron sources From reactors to radio-isotopes to LINACs

• Pulsed beams: annihilation lifetime spectroscopy for thin films, bulk materials, fluids, and gases

MePS – the Monoenergetic Positron Source GiPS – the Gamma-induced Positron Source

• Some apps porous glasses w/ D. Enke (Univ. Leipzig, D) low-k dielectrics w/ A. Uedono (Univ. Tsukuba, JP) MnSi skyrmion lattices w/ C. Hugenschmidt (TU Munich & FRM) 3D tomography w/ some medical physicsts

Outline

Courtesy: R. Krause-Rehberg / M. Butterling

Page 2



Fate of positrons in matter

Positrons slow down to thermal energies in 3-10 ps. After diffusing inside the matter positrons are trapped in vacancies or defects.

Kinetics results in trapping rates about 105 times larger than annihilation rates: defect concentrations down to 10-7 are accessible.

e+

free

trapped

self

capture

self

spin-flip

para-Ps

ortho-Ps

2 g

2 g

2 g

2 g

3 g

2 g

0.2 ns

0.5 ns

0.125 ns

1.0 ns

142 ns

1.0 ns

insulators voids

metals (screening)

Page 3



Typical types of defects in condensed matter

grain boundaries

dislocations (cluster) vacancies

impurities decorated vacancies

Page 4

- Local electron density changes at defects - Lack of positively charged nucleus causes trapping



Positron trapping model

Page 5

- Measurement of decay lifetime is characteristic for defect type - Intensity is sensitive to defect concentration

defect-free bulk

annihilation radiation

defect λbulk

λdefect

κdefect

201

02

2

1

2

2

1

1

1

1

)(

21

III

II

eI

eI

N

nndt

dn

ndt

dn

defectdefectbulk

defect

defect

defectbulk

tt

annih

bulkdefectdefectdefect

defect

bulkdefectbulkbulk

defect lifetime

reduced bulk lifetime



Sensitivity limits

Positron Annihilation Lifetime Spectroscopy

x-ray scattering

optical micro-scopy

n- scatt.

STM/AFM

TEM

optical micro- scopy

TEM

x-ray scattering

n-scatt

Positron Annihilation Lifetime Spectroscopy

STM/AFM

Page 6



Depth distributions for mono-energetic e+

Page 7

10 keV 5 keV

3 keV

1 keV

Positron diffusion can be described as random-walk problem with dominant scattering from longitudinal acoustic phonons (in semiconductors and with T < 300 K). Positron trapping or positron annihilation stops further diffusion. Internal electric fields may significantly change diffusion (Fokker-Planck).

surface bulk film substrate

Positron depth in Si



Sources? Isotopes, reactors, accelerators …! Production of positrons in weak interactions (mediated by W’s)

proton

neutron 27Al(p,n)27Si(β+νe, 4.2 s) 27Al (u d d)

(u d u)

νe e+

W+

Sumitomi Heavy Industries Cyclotron 18 MeV protons, 50 µA beam current

Page 8

22Na(β+νe, 2.6 a) 22Ne



Sources? Isotopes, reactors, accelerators…! Production of positrons through electromagnetic interactions (photons)

e- e+

γ

e-

e-

Use intense source of photons for pair production Capture-neutron gamma-rays from reactor 113Cd(n,γ)114Cd

FRMII Munich KUR Kyoto TU Delft Mc Master North Carolina

AIST, Tsukuba, Japan ELBE, Dresden

Bremsstrahlung from electron accelerators (AIST/Tsukuba, HZDR Dresden) or upcoming high power lasers (Changsha/Wuhan, Osaka/Hyogo, ELI-NP Bucuresti)

Page 9



SC-LINAC in CW mode EPOS facility

38 ns / 26 MHz

~ 2.8 GHz / 1 µs / 100 Hz / 10 µA

Positrons from accelerators NC-LINAC in bunched mode

e+

converter

moderator linear storage

chopper buncher A buncher B

sam

ple

1 µs 10 eV 3 ms 5 ns 2 ns 250 ps

e- converter

moderator magnetic transport

chopper buncher

sam

ple

10 ps 2 keV 2 ns 250 ps

e-

e+

Page 10



1.6 mA, 40 MeV (64 kW) CW electron accelerator

coherent IR-radiation 3 – 230 µm

coh. THz radiation 100 µm – 3 mm

electrons 34MeV radiation biology

detector tests

Bremsstrahlung 16 MeV

Gamma-induced Positrons

pulsed, mono-energetic positrons 0.2 – 20 keV

Positrons from accelerators

SPONSOR: mono-energetic positrons 0.2 – 30 keV

from 22Na

Page 11



mode

High-frequency standing-wave mode with 1.3 GHz corresponding to unit cell length of 11.5 cm.

74,0 W250

mA 1keV 250

P

I

E

1kW 20

mA 1MeV 20

P

I

E

1.8 K LHe tank with LN2-cold shield and super-insulation

Superconducting Accelerator Structures

Page 12

Superconducting niobium resonators developed by TESLA Technology Collaboration. Used in FLASH, XFEL,

H. Büttig, et al., Nucl. Instr. Meth. B 704 (2013) 7



The Mono-energetic Positron Source MePS

Page 13

SC-LINAC beam • 30 MeV, 0.1 mA • 1.625 MHz repetition rate

615 ns spacing

2 keV magnetic transport system

4 ns chopper 78 MHz buncher

post-accelerator -1.5 … +20 kV

sample

20 cm Pb + 3.2 m concrete shielding

max 2.5 kV/cm max 500 V/cm

30% HPGe detector for DBS BaF2 detector for PALS

Flux: 1.2·106 e+/s on sample



Accelerators can produce intense and pulsed slow positron beams. LINear ACcelerators have some advantage due to their high beam power and time structure.

Converter issues

A) normal conducting LINAC E ~ 50 MeV Ipeak ~ 100 mA beam power tbunch ~ 1 µs 500 W frep ~ 100 Hz

B) superconducting LINAC (HZDR) E ~ 35 MeV Iaverage ~ 100 µA beam power frep ~ 1 - 10 MHz 3500 W

sophisticated converter designs and heavy shielding needed

stack of 50 x 100 µm thick W foils

Water-cooled electron to bremsstrahlung converter

Page 14

nELBE photoneutron source: LPb loop (450°C) can take 50 kW power in ~ 1cm3

e+

converter

e-

moderator

e+

A.R. Junghans, et al., Nuclear Data Sheets 119 (2014) 349



The Mono-energetic Positron Source MePS

Page 15

PALS (BaF2, plastic scint.)

DBS (HPGe)



MePS – PALS timing resolution

Page 16

Lifetimes of Silicon (218 ps) and Yttria-stabilized Zirkonia can be reproduced, timing resolution is 230 ps with an additional (and stable) lifetime component of 720 ps. Signal to noise ratio: 5·105

Si YSZ



Experiments at MePS – porous glasses D. Enke, G. Dornberg (U Leipzig): Porous glasses feature a tunable pore width and adjustable surface properties for membranes, chemo-sensors, drug delivery, optical coatings, etc.

Stimulated phase separation in sodium borosilicate glass into silica and an alkali borate phase generates sponge-like porous structures with unknown porosity. H. Uhlig, G. Adouane, C. Bluhm, S. Zieger, R. Krause-Rehberg, D. Enke, J. Porous Materials 23 (2016) 139

Page 17

20 µm

glass beads

thin film thin film



Experiments at MePS – porous glasses Annihilation lifetime experiments reveal depth dependent pore size distributions.

Pore sizes according to modified Tao-Eldrup model by K. Wada and T. Hyodo, JPhysG 443 (2013) 012003.

Page 18

2.6 nm

1.8 nm

1.3 nm

1.0 nm

5.0 nm

245



Experiments at MePS – porous glasses Effect of tempering -> unexpected reduction of porosity.

Page 19

2.6 nm

1.8 nm

1.3 nm

1.0 nm

5.0 nm



MePS – low-k SiO2 Nano-porous organo-silicate glasses

Depending on the specific treatment processes nano-porous structures can be stabilized or not. A. Uedono et al., Appl. Surf. Science 368 (2016) 272

Page 20



Future work in films – In-situ AIDA

Page 21

Gas loading reactor 20 bar 77 – 900 K

XPS analysis unit

Climate chamber

Motivation • Hydrogen loading and

deloading process, e.g. Hao et al, J. Phys. Chem. Lett. 1 (2010) 2968

• Hydrogen storage capacity: decoration of defects, e.g. J. Cizek et al., Phys. Rev. B 79 (2009) 054108

• Magnetic properties and open volume defects in Fe60Al40 M.O.Liedke, et al., J. Appl. Phys. 117 (2015) 163908

Duoplasmotron N, O, H < 30 keV 2 mA

Raman spectrometer

e-beam evap. Fe, Co, Ni, Au, Ag, Cr, Pt, Mo



Future work

Page 22

High-voltage cage on top of MePS

UHV system at the 6 MV ion beam for H-depth profiling

XPS analysis and PALS chamber

@KEK



What about bulk materials, fluids, gases, or biological stuff …?

e- converter

moderator magnetic transport

chopper buncher

sam

ple

10 ps 2 ns 100 ps

e- radiator

γ

10 ps

sam

ple

e-

e+

10 ps

GiPS The Gamma-induced Positron annihilation Spectroscopy

e+

Page 24



Positron beams for material research

Nb foil: 10-3 X0

ps 10σMHz 26fµA 900I

MeV 16E

t

e

e

photon beam 2 cm diameter 108 cm-2 s-1

Annihilation Lifetime Spectroscopy (Coincidence) Doppler Broadening Age-momentum Correlation

Positron production using electron-bremsstrahlung

F.A. Selim, et al. Nucl. Instr. Meth. A 495 (2002) 154 M. Butterling, et al., Nucl. Instr. Meth. B 269 (2011) 2623 Y. Taira, et al., Rad. Phys. Chem. 95 (2014) 30

studies performed so far: - animal tissue - semicondudtors, alloys (ZnO, MnSi) - (neutron-activated) reactor materials - water, glycerol from 10°C to 100°C

Page 25



Positrons: backgnd for nuclear physics exp’ts

Seite 26

Hard bremsstrahlung produces a huge amount of positrons via pair production inside the target material. High-energy photons act as a volume source of positrons throughout the entire volume.

Kapton at 16 MeV electron energy



science cases

Page 27 Page 27

• porous glasses D. Enke (U Leipzig) • Si nano-channels R. Brusa (U & INFN Trento, I) • low-k dielectrics A. Uedono (U & AIST Tsukuba, JP) • low-k dielectrics E. Zschech (TU & IKTS Dresden) • low-k dielectrics N. Köhler (TU & ENAS Chemnitz) • reactor pressure vessel steel F. Bergner (FWI) • ZnO luminescence and scintillation F. A. Selim (U Bowling Green, OH) • MnSi skyrmion lattices C. Hugenschmidt (TU & FRM Munich)

S. Mühlbauer, C. Pfleiderer, et al., Science 323(2009)915

MnSi shows spontaneously formed and topologically stable magnetic vortices. Defect identifications and concentrations in Mn1+xSi published in: M. Reiner, et al., Scientific Reports 6(2016)29109 19 nm



• porous glasses D. Enke, U Leipzig • Si nano-channels R. Brusa, U & INFN Trento, I • low-k dielectrics A. Uedono, U & AIST Tsukuba, JP • low-k dielectrics E. Zschech, TU & Fraunhofer IKTS Dresden • low-k dielectrics N. Köhler, TU & Fraunhofer ENAS Dresden • reactor pressure vessel steel F. Bergner, HZDR • ZnO luminescence and scintillation F. A. Selim, U Bowling Green, OH • MnSi skyrmion lattices C. Hugenschmidt, TU & FRM Munich

science cases

Page 28 Page 28

Green luminescence in ZnO results from defects (both Zn and O vacancies) passivated by hydrogen. Very fast scintillation lifetimes obtained! Published in: J. Ji, et al., Scientific Reports 6(2016)31238



High resolution lifetime spectrum with signal to noise ratios of better than 105:1 using gamma-gamma coincidence techniques for background reduction. Lifetime spectra are free from artefacts. → Long lifetimes reveal atomic defects caused by neutron-induced damage. → Can (and how) defects be removed by thermal annealing?

conventional LINAC mode pulsed RF, highest energy typically pile-up problems F.A. Selim, D.P. Wells, J.F. Harmon, et al. Nucl. Instr. Meth. A 495 (2002) 154 SC-LINAC in CW mode highest average power – high yield and low pile-up

38 ns / 26 MHz

~ 1 µs / 100 Hz

Intrinsic radioactive materials: RPV steels

Page 30



Physics with GiPS: RPV steel Reactor vessel steel becomes brittle due to neutron-induced defects like open-volume defects seeding cracks.

→ Preferential formation of double vacancies → Thermal annealing (290°C) not sufficient to remove defects!

Collaboration with Reactor Safety Division at HZDR. To be published …

Page 31



Physics with GiPS: Kapton®

Annihilation lifetime in Kapton has been under debate for quite some time - > get a measurement without source correction.

→ consistent single positron lifetime of (381 ± 1) ps two components show larger χ2

applied cuts on Germanium and BaF2 detector energy signal reduce background from interactions outside the sample

Kapton

Page 32



Physics with GiPS: Fluids Conventional lifetime measurements: → dissolve 22Na and dispose it afterwards Positrons from bremsstrahlung → homogeneously distributed, sharp time stamp Target is temperature-stabilized, continuously circulated, degassed, dry-nitrogen flushed.

Positron Physics Ortho-Positronium (o-Ps) in a fluid forms a bubble given by its zero-point energy and the surface tension. We know estimate the change of the o-Ps pick-off annihilation lifetime with temperature in a bubble created by the o-Ps itself…. R.A. Ferell, Phys. Rev., 108,167, 1957 S.J. Tao, J. Chem. Phys., 56,5499, 1972 M. Eldrup et al., Chem. Phys., 63,51, 1981

fluid

e+

e-

142 ns

125 ps

o-Ps

p-Ps

Page 33

Kapton tube target


Member of the Helmholtz Association Page 34

Physics with GiPS: Fluids

e+

e-

fluid

20

22

20

2

0

0

0

2

48

sin

2

rmrm

hE

kr

krkrj

krjRrR

rR

ErUm

ePs

l

Ps

stationary Schrödinger eqn.

Ansatz: spherical Bessel fct.

zero-point energy

1st non-trivial solution

A 06.14

A 3.416

082

0

4

22

20

0

4

2

0

030

22

00

20

cm

c

ema

mr

rrm

EEr

rE

e

e

surf

surf

°

°



Physics with GiPS: Positron(-ium) Chemistry Experiments with water are in variance with a simple bubble-type model. Extension: chemical reactions between radiolysis products of the slowing-down of the positron → Ps chemistry.

→ Radicals are positron scavengers which reduce annihilation lifetimes. → Extended bubble model including chemistry [S.V. Stepanov et al., Mat. Sci. Forum 607] and energetics of hydrated e+

aq and e-aq (what are the

relaxation times of un-/polar media?) → Chemistry of radiolysis directly accessible since the probe creates the ionization itself

Courtesy: Maik Butterling, S V.Stepanov

Page 35



Towards imaging of defects / porosity Material failures impose a significant threat to the integrity and the safety of technical systems. A thorough understanding of the microscopic origin and the development of defects requires advanced methods.

… early days of material analysis … and the quests of today

Page 36



Motivation Establish a non-destructive and non-intrusive method which allows for spatially resolved positron-lifetime spectroscopy. Reconstruct PET-like images plus positron annihilation lifetime. Possible Applications (list not complete): • Porosimetry • Medicine in-beam positron lifetime spectroscopy during hard x-ray

tumor therapy • Engineering pre-failure diagnostics of micro fractures

fuel rod inspection

APS Physics & Society Newsletter 2011. R. Hargraves, R. Moir

Page 37



Prerequisites • Intense source of positrons with deep penetration (cm) ≈15 MeV X-rays • Accurate time-stamping of positron creation (<10 ps) CW LINAC • Position-sensitive positron detectors (mm) Siemens LSO PET • Time-resolution for lifetime spectroscopy (~100 ps) in-house (physics) • Efficient data acquisition in-house (physics) • 3-D image reconstruction in-house (medicine)

e-

e-

sample

Gamma-induced Positron Spectroscopy

Page 38



Towards 2/3-D positron lifetime tomography • Two position-sensitive photon detectors with 169 elements each

LSO-based commercial 13x13 PET pixel detector

4 mm x 4 mm x 20 mm LSO crystals

• Each crystal array read out using 4 PMT • Summed PMT signal -> gamma energy • Correlation of individual PMT signals ->

position • Positron annihilation time given by sum

over all 8 PMT involved

Lutetium oxyorthosilicate Lu2SiO5:Ce

courtesy: university hospital Dresden ©Siemens

Page 39



Bremsstrahlung beam

γ

0° 180°

3D reconstruction

Fe

Cu

Al

PTFE

γ

3D tomography applied for the first time using bulk volume positron production. Target is rotated in 2 deg. steps and the image is reconstructed using a cubical (30 mm)3 voxel space and back-projection algorithm.

Page 44



Copper

Aluminium

Steel

Maximum Likelihood Expectation Maximization Iterative method for image reconstruction based on a algorithm developed in PET [L.A. Shepp, Y. Vardi, IEEE-MI 2 (1982) 113]. Solves the inversion problem numerically where one

has a system matrix M, an a-priori unknown source

distribution s and a measured distribution r.

rsM ˆ The system matrix has a size of 132 x 132 x 180 x 303 = 138 x 109.

all prompt long step 1

Page 45



2nd Iteration

all prompt long

Copper

Aluminium

Steel long gate

short gate

step 2

MLEM

Page 46



5th Iteration

all prompt long

Copper

Aluminium

Steel

step 5

long gate

short gate

MLEM

Page 47



10th Iteration

all prompt long

Copper

Aluminium

Steel

step 10

long gate

short gate

MLEM

Page 48



20th Iteration

all prompt long

Copper

Aluminium

Steel

step 20

long gate

short gate

MLEM

Page 49



all prompt prompt / all

Copper

Aluminium

Steel long gate

short gate

MLEM

Page 50



all prompt prompt / all

Copper

Aluminium

Steel

MLEM Gating on positron lifetimes with 225 ps timing resolution. Now the Al is clearly discriminated against the surrounding Teflon.

Page 51



Courtesy: Jochen Wosnitza

Lifetime-sensitive analysis B-field coil

Cut through the record coil which reached 91.4 T peak field. Coil is fed by the world’s largest capacitor bank w/ 50 MJ stored energy.

Page 52



Copper

Zylon® Teflon

Tomography: B-field coil

48 h measurement time, 316 GB, 1.6 G events 324 M filtered coincidences

y

x z

Page 53



Lifetime-sensitive analysis: B-field coil

Page 54



Lifetime-sensitive analysis: B-field coil

Now, we select specific voxels and determine the annihilation lifetimes for spatially separated regions. Since the voxel is identified as an ensemble over all possible lines-of-response between two detector crystals, the lifetime distribution is a convolution as well. Some real physics questions needed …

τ = 1.48 ns

Page 55



Credits Helmholtz-Zentrum Dresden-Rossendorf W. Anwand, M. Butterling#, T. E. Cowan*, J. Ehrler*, M.O. Liedke, K. Potzger, T.T. Trinh*, A.W. *also at Technische Universität Dresden #now at TU Delft and many collaborators S.V. Stepanov, D.S.Zvezhinskiy (ITEP, MEPhI) e+ chemistry C. Hugenschmidt, M. Reiner (TU Munich) skyrmion-lattice MnSi F. Selim (U Bowling Green) ZnO luminescence M. Kraatz, E. Zschech (Fraunhofer IKTS) low-k dielectrics A. Uedono (Univ. Tsukuba) low-k dielectrics R. Brusa, L. Ravelli (Univ. Trento) nano-Si for anti-matter D. Enke, G. Dornberg (Univ. Leipzig) porous glasses for catalysts Funding provided by BMBF (05K2013) and the Helmholtz Energy Materials Characterization Platform

Martin-Luther-Univ. Halle-Wittenberg

R. Krause-Rehberg, A. G. Attalah, M. Elsayed, E. Hirschmann, M. John, M. Jungmann, A. Müller



Positron Studies of Defects 2017 domo

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Documents

Positron-Annihilation Lifetime Spectroscopy for …...MePS – PALS timing resolution Page 16 Lifetimes of Silicon (218 ps) and Yttria-stabilized Zirkonia can be reproduced, timing