POSITRON AND POSITRONIUM INTERACTIONS WITH …userpages.irap.omp.eu/~pvonballmoos/astropositron/presentations... · POSITRON AND POSITRONIUM INTERACTIONS WITH CONDENSED MATTER Paul

POSITRON AND POSITRONIUM INTERACTIONS WITH CONDENSED MATTER

Paul Coleman University of Bath

POSITRON-SURFACE INTERACTIONS

positron backscattering

E = 30 keV

ATOMIC NUMBER0 20 40 60 80

BA

CK

SCA

TTER

ED F

RA

CTI

ON

0

5

10

15

20

25

30

35

40

GOLD

INCIDENT POSITRON ENERGY (keV)0 10 20 30 40 50

BA

CK

SCA

TTER

ED F

RA

CTI

ON

05

10152025303540

THE FATE OF POSITRONS IN CONDENSED MATTER


secondary electron emission

ELECTRON ENERGY (eV)100 200 300 400 500 600

dN/d

E (a

rbitr

ary

scal

e)

2 keV ELECTRONS IN

2 keV POSITRONS IN

COPPER



low-energy positron diffraction

θ2 θ1


Auger electron emission

POSITRON-SURFACE INTERACTIONS THE FATE OF POSITRONS IN CONDENSED MATTER

positron surface trapping, re-emission, positronium formation


thermal or epithermal positrons

ENERGY (eV)0 5 10

CO

UN

TS (a

rb. u

nits

)

0.0

0.5

1.0100eV200eV500eV3000eV


positron implantation

POSITRON-SOLID INTERACTIONS

mono-energetic e+

β+

P(z) = α exp(-αz), with α (cm) ≈ 16ρEm-1.4 (ρ = density in gcm-3, Em = maximum beta positron energy in keV)

α exp(-αz)

2(z/z02)exp(-z2/z0

2)

P(z) = 2(z/z02)exp(-z2/z0

2) where zo = (40/ρ)E1.6 with density ρ in gcm-3 and E in keV

depth distributions at thermalisation (after ~ps):


in-flight annihilation


2.65MeV e+s in Au


positron diffusion and annihilation


free annihilation

defect trapping


positronium formation – insulators, nanoporous materials

2γ

Ps (3γ)


** see later talks in this session **


POSITRON SOURCES

The perennial problem – not enough positrons…

•  Relatively long half-life (2.6 years)

•  Relatively short biological half-life (3 days)

•  1.274 MeV γ can be used as a ‘start’ signal

RADIOACTIVE ISOTOPES – eg 22Na

•  Relatively long half-life (2.6 years)

•  Relatively short biological half-life (3 days)

•  1.274 MeV γ can be used as a ‘start’ signal

RADIOACTIVE ISOTOPES – eg 22Na POSITRON SOURCES

LINEAR ACCELERATORS – eg KEK, Japan

~ 102 MeV electrons

bremstrahhlung

pair production

RESEARCH REACTORS – eg NEPOMUC, Munich

n capture

gamma emission

pair production

= probability that a pair of annihilation gamma rays has total (ie ~ e-) momentum p

2-D ANGULAR CORRELATION

DOPPLER BROADENING SPECTROSCOPY

LIFETIME SPECTROSCOPY

POSITRON SPECTROSCOPIES

ρ(px,py) = ∫ρ(p) dpz

ρ(pz) = ∫∫ρ(p) dpx dpy

λ = ∫∫∫ρ(p) dpx dpy dpz

ρ(p)

ANGULAR CORRELATION OF ANNIHILATION RADIATION

δθ ≈ pt /mc (~ 5-20 mrad)

where p t is the component of momentum of the annihilating pair in a direction transverse to the gamma emission

C of M FRAME LAB FRAME

θ0 θ0

θ1 θ2

γ1

γ2

γ1

γ2

Bristol 2D-ACAR lab

e+ 2D-ACAR – quartz ↑

← 2D Fermi surface

electron momentum densities

POSITRON LIFETIME SPECTROSCOPY

electron densities

∑ )λexp(=)(i

ii tItI

positronium (Ps): e+-e- bound state

-formed in insulators

- para-Ps lives 125 ps in vacuum

- ortho-Ps lives 142 ns in vacuum

- typical Ps lifetime in solids ~ ns

e+

γ

LIFETIME (ns)

0.1 1 10 100

Metals & alloys

Semi- conductors

Polymers

Mesoporous materials

POSITRONS POSITRONIUM

POSITRON LIFETIME MEASUREMENT


e+ Doppler shift in the photon energy = ± cpz /2

If pz is from a 5eV electron cpz/2 = 1130 eV (~ the resolution of a high-purity Ge detector) Doppler broadening around 511keV is measured Good for rapid, low-resolution measurements of e- momenta vs time, temperature etc.

Ge PRE-AMP AMPLIFIER BIASED AMP DIGITAL STABILISER ADC LN2 CRYO- STAT MCA MEMORY

GAMMA ENERGY (keV)

504 506 508 510 512 514 516 518

CO

UN

T R

ATE

(AR

B U

NIT

S)

C

W1 W2


PARAMETERS: Sharpness parameter S = C/A Wing parameter W = (W1+W2)/A

where A = total area under photopeak

TRANSITION LIMITED TRAPPING:

High C, deep but narrow traps: K = νC

DIFFUSION LIMITED TRAPPING:

Low C, large traps: K = 4π r DC

NEUTRAL VACANCY: no detrapping, τ > τbulk , less Doppler broadening: no temperature dependence

NEGATIVE VACANCY: no detrapping, τ > τbulk , less Doppler broadening: stronger trap at lower temperatures

SHALLOW TRAP (eg NEGATIVE ION): detrapping below 300K, τ ~ τbulk , little Doppler broadening

OPEN-VOLUME POINT DEFECTS WHAT CAN BE STUDIED?

sensitive to ~ 1 vacancy

per 107 atoms

VACANCIES, VACANCY CLUSTERS, DISLOCATIONS, GRAIN BOUNDARIES, INNER/OUTER SURFACES

DEFECT SIZE

DEPTH (Angstrom)100 101 102 103 104 105 106 107 108

RE

SOLV

ED

DE

FEC

T S

IZE

(Ang

stro

m)

100

101

102

103

104

105

106

107

108

STM/AFM

X-RAYSCATTERINGTEM

OM

nS

POSITRON SPECTROSCOPY

DEFECT CONCENTRATION

DEPTH (Angstrom)100 101 102 103 104 105 106 107 108

DE

FEC

T C

ON

CEN

TRAT

ION

(app

m)

10-1

100

101

102

103

104

105

STM/AFM

POSITRON SPECTROSCOPY

OM TEM

X-RAYSCATTERING

nS

SENSITIVITY TO OPEN-VOLUME POINT DEFECTS

OPEN-VOLUME POINT DEFECTS: EXTRACTING CONCENTRATIONS

∑=i

iiSfS

for two states - bulk or defect:

BBD

BD C

CSSSS

fλ+ν

ν==

- -

GAMMA ENERGY (keV)

504 506 508 510 512 514 516 518

CO

UN

T R

ATE S

)λ∑ exp(λ=)( tItN ii

ii -

for two states - bulk or defect:

)λλ(=µ=κ dbb

ddd I

ICν -

τ2

τ3 τ1

ν is the specific trapping rate

CONCENTRATION

ν is the specific trapping rate – ν depends on charge state of the defect

VACANCY CONCENTRATION (cm-3)

1014 1015 1016 1017 1018 1019 1020

S/S

Si

1.00

1.01

1.02

1.03

1.04

V2-

VV+

OPEN-VOLUME POINT DEFECTS SIZE

τ and S are related to open volume of defect

Vacancy cluster size0 5 10 15 20

S P

aram

eter

for V

n1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

Si

CHEMICAL SPECIFICITY - ANNIHILATION LINESHAPE ANALYSIS

GAMMA ENERGY (keV)

506 508 510 512 514 516

CO

UN

TS

annihilation lines for Si Ge

Si0.7Ge0.3 Si0.9Ge0.1

Si0.98Ge0.02

GAMMA ENERGY (keV)

512 514 516 518 520 522 524

SP

EC

TRU

M R

ATI

O

0.8

1.0

1.2

1.4

1.6

1.8

spectrum ratios: Ge/Si Si0.7Ge0.3/Si = 30% of Ge/Si Si0.9Ge0.1/Si = 10% of Ge/Si Si0.98Ge0.02/Si = 2% of Ge/Si

information on chemical environment and affinity

POSITRON MODERATION

NEGATIVE WORK FUNCTION BETA SPECTRUM (0 – 540 keV, PEAK 180 keV) RE-EMITTED POSITRONS (0 – 3eV, PEAK 1.5eV)

POSITRON ENERGY (keV) (LOG SCALE)0.0001 0.001 0.01 0.1 1 10 100 1000

NU

MB

ER

OF

PO

SIT

RO

NS

PE

R e

V BETA POSITRONS FROM 22Na

3eV POSITRONSFROM MODERATOR

EMISSION FROM TUNGSTEN

alternatives include moderation by solid rare gases

LAB-BASED POSITRON BEAMS

Magnetic-transport positron beam system

Depth (z) / nm 0 20 40 60 80 100 120 140

Impl

anta

tion

Pro

files

P(E

,z)

0.00 0.03 0.06 0.09 0.12 0.15

E=0.5 keV

E=1.0 keV E=1.5 keV

E=2.0 keV

Si

LAB-BASED POSITRON BEAMS

Magnetic-transport positron beam system

also: TRAP-BASED BEAMS

PULSED BEAMS

INCIDENT POSITRON ENERGY (keV)0 5 10 15 20 25 30

NO

RM

ALIZ

ED

S P

ARAM

ETER

0.96

0.97

0.98

0.99

1.00

1.01

1.02

1.03

1.04

1.05

1112131415

virgin

FZ Si implanted with 2MeV Si+ ions at 10n cm-2 (n = 0, 11, 12, 13, 14, 15)

depth sensitivity

nSi SAMPLES (MANCHESTER)JUNE-SEPTEMBER 2008

INCIDENT POSITRON ENERGY (keV)

5 10 15 20 25 30

S P

AR

AM

ETE

R

0.92

0.94

0.96

0.98

1.00

1.02

1s5s10s100s1has-imp

POSITRON RESPONSE

INTERFACE TRAPPING

Si-rich SiO2 ANNEALED AT 1100ºC

EXAMPLES OF POSITRON BEAM STUDIES OF SOLIDS 1. SILICON NANOCRYSTALS in SiO2 MATRIX


5 10 15 20 25 30

S P

AR

AM

ET

ER

0.92

0.94

0.96

0.98

1.00

1.02

WITH Er

NO Er

→ Er in interfaces

ERBIUM and nSi

TEM Si Er

3-5nm clusters – Er and Si

EELS responses coincident

ILLUMINATION BY BLUE LEDs

TOTAL LED POWER (W)

0 5 10 15 20 25 30 35 40 45 50

MIN

IMU

M S

PAR

AME

TER

(AT

3 ke

V)

0.955

0.956

0.957

0.958

0.959

0.960

0.961

EXAMPLES OF POSITRON BEAM STUDIES OF SOLIDS 1. SILICON NANOCRYSTALS in SiO2 MATRIX

Ps (3γ)

2γ


0 2 4 6 8 10

Ps

FRA

CTI

ON

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

grown at 120k 10-4torr 20mingrown at 120k 10-7torr

DEPENDENCE ON GROWTH RATE


0 2 4 6 8 10

Ps

FRA

CTI

ON

0.0

0.2

0.4

0.6

0.8at 120Kat 150Kat 170K

DURING SUBLIMATION

T (K)

40 60 80 100 120 140 160

Ps

FRAC

TIO

N (A

RB

UN

ITS

)

0.0

0.1

0.2

0.3

0.4

0.5

PORE COLLAPSE

see poster for more information

CLOSED nm PORES:

Ps DECAY TO 3γ and Ps LIFETIME

DEPEND ON PORE SIZE

INTERCONNECTED PORES:

ESCAPE POSSIBLE, 3γ FRACTION INCREASED,

VACUUM LIFETIME

EXAMPLES OF POSITRON BEAM STUDIES OF SOLIDS 2. NANOPORES in ICE

POSITRON BEAMS: APPLICATIONS IN ASTROPHYSICS

DUST in the ISM

N. Guessoum, P. Jean & W. Gillard Applied Surface Science 252 (2006) 3352

back- scattered

free/ trapped e+

o-Ps Ps


DOPPLER BROADENING – CHEMICAL ANALYSIS

IN-FLIGHT ANNIHILATION ON MODEL SYSTEMS

DOPPLER BROADENING – STRUCTURAL ANALYSIS

Ps FORMATION AND DECAY – PORES AND STRUCTURAL ANALYSIS

DUST in the ISM


Thanks to past and present members of the Bath positron group

- and to you for your attention

POSITRON AND POSITRONIUM INTERACTIONS WITH …userpages.irap.omp.eu/~pvonballmoos/astropositron/presentations... · POSITRON AND POSITRONIUM INTERACTIONS WITH CONDENSED MATTER Paul

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