Positrons for Applied Science& Materials Science
K.G. Lynn and M.H. Weberand many others!!
Washington State University, Pullman, WA
JPOS 09International Workshop on Positrons at Jefferson Lab
Thomas Jefferson National Accelerator FacilityNewport News, VAMarch 25-27, 2009
My Concerns in a low energy positron facility
• Intense positron sources have not fulfilled its promise to DOE/NSF• The intense sources have been small groups trying to move into a larger
facilities based on the researchers interests and lacked the support of the facilitiy and funding agency.
• The beams that have operated have not provided the needed user support and users have gone elsewhere.
• Neither the brightness nor the intensity has been routinely been achieved
• If the Jefferson Lab is planning this facility a real commitment is needed from OS/DOE and local management
Number 1:
Positrons madethe Newsweekhitlist
The positron’s death
= g(p)
red shifted
blue shifted
Eventually, in our world of matter, the positron will annihilate with an electron.Two (or rarely three) photons (gamma rays) emerge.
The number of electrons (density) determines how fast this occurs
Basic laws of nature (physics) force certain conditions:2 gammas in opposite direction with small changes in energy (Doppler shifts) and direction.Doppler shifts
Angular deviation from opposite
JPOS 09 Newport News (March 2009)
Positron characteristics• Unique quantum numbers
– No exchange at the present time
• Annihilation with electrons radiation can be detected– Little interaction with specimen after annihilation
• Electron momentum encoded in -rays– Doppler broadening– Angular correlation
• Lifetime is electron density dependent– Positron lifetimes
Hits in “Defects”• Vacancy formation enthalpies in metals (90%) (1975-
present)• Voids in neutron irradiation and deformation of metals• Observation of vacancy migration at stage III (1980)
– Major controversy resolved• Vacancies observations in compound semiconductors (1990)• Vacancy character of EL2 in GaAs (1993)• Role of defects in hi-Tc superconductors (1988-92)• Open volume measurements in polymers (ongoing)
– Gas diffusion, Mechanical properties, Aging • Defects at semiconductor interfaces (ongoing)
Annihilation at high relative momentum• 2D spectrum:
• x: p-parallel <==> Doppler shift• y: Sum energy <==> rest mass + kinetic energy
1022 keV
1092 keV
0 keV340 keV=> 91.3 a.u.-340 keV
JPOS 09 Newport News (March 2009)
Channeling
Angle
Nor
mal
ized
Yie
ld
Positron Holography (Never fulfilled)
CdSe
- Electron-electron interaction- Multi-layer contribution
- One positron at a time- Topmost layer only
Now: with electrons Future: with positrons“If positrons were routinely available, all diffraction would be done with them” S.Y. Tong
Fermi SurfacesYtterbium
Experiment
Theory
Now: 16 dataset Future: Super ACAR 1 shot and depth profiles
Resolution limited by acquisition time
Quantum dots
0 1 2 3
0.8
1.0
1.2
Rat
io to
bul
k C
dSe
Doppler momentum (a.u.)
1.8 nm
6.0 nm
4.5 nm
“baseline”
JPOS 09 Newport News (March 2009)
Fe Fe
Cu
Zero point motionenergy
Potential wellin Fe
e+
Cu in Fe
Precipitates-Critical in ReactorSteels
Atomic scale defects• Missing atoms in crystals are called vacancies• They play a key role in the properties of many metals,
semiconductors and insulators• How to tell the difference between impurities and
dopants– One makes the PC work the other turns it to a pile of junk
• Understanding them drives progress– Electronics, solar cells, sensors, optics, detectors (airports),
lasers– Silicon, silicon carbide, ZnO, GaN, GaAs, YAG,…– Lasers to cut steel, transparent conductors for monitors, sunlight
to electricity, longer lasting cell phones, more gigabytes on DVDs beyond Blue-ray, shorter queues at airport baggage scanners…
JPOS 09 Newport News (March 2009)
Trapping in negative or missing atoms
<100>-direction (a.u.)
05
10
05
10
-1
0
1
2
3
<010>-direction (a.u.)
05
1015
05
1015
-1
0
1
2
3
<100>-direction (a.u.) <010>-direction (a.u.)
Delocalized Bloch stateLocalized trapped state
A positron and many electrons
Doppler broadeningConduction electrons: delocalized; low momentum
Bound electrons: localized; highmomentum
Potential of atomic cores
JPOS 09 Newport News (March 2009)
A positron “likes” vacancy
Doppler broadeningConduction electrons: delocalized; low momentum
Bound electrons: localized; highmomentum
JPOS 09 Newport News (March 2009)
2400
200016001200800400
Temperature (K)
Ope
n vo
lum
e pa
ram
eter
Tc
Vacancy formation energy
Mo
Now: 1D depth profile Future: 3D map with lifetime
Depth profiles
0 5 10 15 20 25Positron energy (keV)
0.99
1.00
1.01
1.02
1.03
1.04
1.05
1.06
Nor
mal
ized
S p
aram
eter
0 10 20 30 40 50 60 70 80Depth (nm)
Def
ect c
once
ntra
tion
(cm
)
Mean implantation depth (nm)
d = 150 nmMB E
100 300 500 1000 1500 2000 3000
-3
1020
1021
1019
Now: layer averaged Future: 3D map with nm3 resolution
SiO2-Si interfacePs trapped in microvoidsat the interface
With broad component
Without broad component
0 1000 2000 3000 4000 5000
1.00
1.01
1.02
1.03
1.04
1.05
1.06
Ope
n vo
lum
e/da
mag
e
Mean depth (nm)
Sample surface treatment: as cut; polish 1x; polish 2x etch polish after etch Vendor M etched reference
Bulk material level
Colloidal silica (50 nm)
JPOS 09 Newport News (March 2009)
Defects in matter
The mesh represents electrons “flowing” around atoms in silicon. The atoms are indicated by the red spheres. One atoms is missing and a different atom (green) is replacing a neighboring silicon.
This is hard to “see” but can be detected with positrons.
JPOS 09 Newport News (March 2009)
Looking for defects
Doppler shift momentum
Total energy
Highly porous material
JPOS 09 Newport News (March 2009)
Chemical environment
Coincidentpositron annihilationsensitive tocore electrons
Now: 12 hours for 1 sample @ 1 selected depth Future: within hours a full depth profile
0 1 2 3 4 5 6 7 8
1.0
1.5
2.0
2.5
3.0 Si Cu Nb W Pb
Rat
io to
Al
Doppler momentum (a.u.)
x 1/2
Micro probes
News item in Nature vol. 412, p.764 (2001)W. Triftshauser et al, Phys. Rev. Lett. 87, 067402 (2001)
Combined positron (1-5) and electron (7-6) Microscope (9-10) to probe cracks in metals (11,13). An electrical prism (6) switched between electrons and positrons to combine electron microscope and defect images.Greif et al, Appl. Phys. Lett. vol 71, p. 2115 (1997)
Positron probe thatMeasures the electron density of patterns on silicon with 2 micrometer resolution
JPOS 09 Newport News (March 2009)
CracksLifetime scale120 170 350 (ps)
Dislocations
Void
Matrix
The future of Defects 2D lifetime maps
Simulation of the future with e+
Vacancies
Dislocations
Matrix
Precipitate
Small void
TEM
Lifetime scale120 170 350 (ps)
Stress-Are you feeling some??
stress relieved
under stress
Direct observation of dislocations in metals during elastic deformation
Lifetime
Intensity
Now: stop frame Future: movie
Lifetime apparatus
Stop: detector
discriminator
Data collectingcomputer
positron beam
Start: e- detector
discriminator
JPOS 09 Newport News (March 2009)
Positron lifetime
0 40 80 120 160 20010-6
10-5
10-4
10-3
10-2
Are
a no
rm c
ount
s
Time (ns)
Samples: Al(100) 4.1 keV low-k non porous; 2.0 keV low-k 10% porosity; 2.0 keV
Background subtractedNo pores
big space between moleculeslarge pores
JPOS 09 Newport News (March 2009)
Positron Lifetime
125 150 200175 225
Unit-cell volume (a.u.)
220
260
300
340
Pos
itron
life
time
(ps)
Now: bulk averaged Future: 3D map
Positronium in Voids & Open Porosity
poro
sity
inte
rcon
nect
ivity
surface
+
JPOS 09 Newport News (March 2009)
o-Positronium Lifetime
0.1 1 10 1
100
10
Pore radius
o-P
s lif
etim
e
SILICAGEL
ALUMINAGEL
POROUS VYCOR GLASS
SILICAGEL
SODALITE
MS-4A
MS-5A
a-CYCRODEXISTRIN
MS-3A
13X
3A4A
4A13X
13X4A
5A
Now: bulk averaged Future: 3D map
Separating closed and open porosities (at 2 keV)
0 10 20 30 40 500
5
10
15
20
25
30
35
0.0
0.2
0.4
0.6
0.8
1.0
Por
osity
(%)
(wt%)
closed open total
Ope
n fra
ctio
n
Open/closed porosity differ qualitatively :
25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
3/2 r
atio
: Diff
eren
ce to
bul
k S
i
Temperature (K)
3.4 % Porosity - Closed 10.4 % Porosity - Closed
17.3% Porosity - Open Contribution 17.3% Porosity - Closed Contribution
Closed vs open porosity
JPOS 09 Newport News (March 2009)
Percolation Threshold; Open Porosity
0 10 20 30 40 50
10
20
30
40
PALS, 140 ns lifetime
Inte
nsity
, I4
(%)
porogen load (%)
0 5 10 15 20 25 300
20
40
60
80
100
120
140
porogen load (%)
L Ps
(nm
)
0 5 10 15 20 25 300
10
20
30
40
3 o
-Ps
(%)
porogen load (%)JPOS 09 Newport News (March 2009)
Two pore diameters
0 200 400 600 80010
20
30
40
50
1.67
2.02
2.43
1.37
1.02
Closed Pore: diameter
Open Pore: Channel Diameter
Pore S
ize (nm)
(ns
)
depth (nm) x density (g/cm3)
Pore 1 Pore 2
JPOS 09 Newport News (March 2009)
Pores in materials• The size of pores determines
– what size molecules pass– how long a pill can deliver drugs– the function of fuel cells– the mechanical properties of plastics– how fast a computer can calculate– the purity of filtered water
• Filters, membranes, drug-delivery, microelectronics
• How to measure the size? – These are nanometers.
JPOS 09 Newport News (March 2009)
Ce:YAG Boule
JPOS 09 Newport News (March 2009)
0 100 200 300 400 500 600 700 8000
1000
2000
3000
4000
5000
Cou
nts
Energy [keV]
After air anneal After Al sputtring and 1st Ar anneal After 3rd Ar anneal
JPOS 09 Newport News (March 2009)
JPOS 09 Newport News (March 2009)
#2
Zn
A
B C D E
F G
H I J
As rec.: clear
Zn Ti(H) Ti(D)Ti(H dep)
Ti(H dep)Zn
Ti(H dep)O2
Ref [24]#1 #3
JPOS 09 Newport News (March 2009)
Oxidation of a layer on Si
0 100 200 300 400 500 600 700 800
0.96
0.98
1.00
1.02
1.04 Exposure: 0 min 10 min 120 min
S
depth (nm)
layer Si
Zero Temperature Limit of 3/2 ratio Extrapolate to 0 K
• Initial Amount of Ps with in T c.f results of Goworek.• Increase in R due to increase in pore lifetimesÞ Less initial Ps but less pick-offÞ “Purification”: Greater relative intensity of self-annihilation
Ps does not die out
3.5% 10.4% 17.3% 21%
0.0
0.2
0.4
0.6
0.8
1.0
3/2 r
atio
: Diff
eren
ce to
Si
Lim
it at
0 K
Porosity
JPOS 09 Newport News (March 2009)
Top Related