Positrons for Applied Science & Materials Science K.G. Lynn and M.H. Weber and many others!! Washington State University, Pullman, WA JPOS 09 International Workshop on Positrons at Jefferson Lab Thomas Jefferson National Accelerator Facility Newport News, VA March 25-27, 2009
Positrons for Applied Science & Materials Science. K.G. Lynn and M.H. Weber and many others!! Washington State University, Pullman, WA. JPOS 09 International Workshop on Positrons at Jefferson Lab Thomas Jefferson National Accelerator Facility Newport News, VA March 25-27, 2009. - PowerPoint PPT Presentation
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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
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.
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
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
• 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