Transcript The Australian Positron Beamline Facility

Experiments at the APBF
From Atoms to Materials – Fundamental to Applied Science
Positron binding
Interaction fundamentals
3.0
N2
Positron binding has been predicted for a number of atomic
systems. Experimental verification of this process is still lacking.
The high resolution positron beam allows the design of experiments
to measure positron binding to atomic systems and study of this
type of “exotic ion”.
2.0
Cross section (a02)
a 
a' 1
e- a 1
1
e- a' 
2
Cross section (a0 )
2.5
16
1
1.5
1.0
14
12
10
8
6
4
0
8
10
12
14
16
18
20
22
Energy (eV)
9.0
2
Cross section (a0 )
8.5
H2 total
8.0
7.5
7.0
6.5
6.0
5.5
10.0
10.5
11.0
11.5
12.0
Energy (eV)
14
2
Cross section (a0 )
12
Bio-materials design
Total elastic cross section
10
12.5
13.0
0 10 20 30 40 50 60 70 80 90
Angle (degrees)
The atomic and molecular physics
program will initially concentrate on
the measurement of low energy
positron scattering cross sections.
We will be able to measure positron
cross sections o unprecedented
accuracy and precision, shedding
new light on the fundamental
interactions between positrons and
matter.
We also plan to develop new
experimental
and
analysis
techniques to extend the range of
cross sections measured.
Lima (2001)
8
6
Studies of biosystems
4
2
0
12.0
New materials are being developed to revolutionise drug delivery.
The function of these materials depend critically on the open
volume present and the interplay between this and the desired
drug molecule. The positron beam can aid in the design of these
materials by analysing the pore size, distribution and connectivity,
allowing the tailoring of drug delivery to optimise drug
effectiveness and minimise side effects.
1.0 eV
2
0.5
0.0
Argon
12.2
Positron interactions with biomolecules are the
fundamental drivers of PET (positron emission
12.4
12.6
12.8
tomography). However, there have been no
Energy (eV)
studies of these interactions and the way in
which they can affect the ultimate resolution of
this imaging technique. A program of study of
positron interactions with relevant molecules,
such as water and typical positron delivery
drugs, will shed light on the possibility of
improvements
to
PET
through
the
understanding of the underlying, fundamnetal
interactions.
Materials analysis
Positrons can be used for the analysis of materials and
material structure. Two main techniques are planned for
experiments at the APBF, PALS and 2D-Doppler.
Positron annihilation lifetime Spectroscopy, or PALS,
measures the time it takes for positrons to annihilate inside a
material. The lifetime of the positrons is related to the
vacancy structure. Positrons are particularly sensitive to
nanometre scale defects, which have an important role in
determining material properties, such as permeability.
Oxygen Permeability
(Barrer)
Polyphenylene oxide
10
oPs (ns)
2.47
2.5
Polybutyl methacrylate 2.32
Polyether urethane
Polyarylate
1
Polyvinyl acetate
0.1
0.01
2.03
2.0
1.94
Polyethylene terephthalate 1.65
Polyethylene naphthalate
Polyvinyl alcohol
0.001
2.25
3.3Å
Permeability for Oxygen
(Barrer)
The variable energy of the positron beam will allow us
to do materials experiments as a function of depth.
As the energy of the incident positrons is increased to
10 keV, different depths of the materials are sampled.
This capacity will be the first of it’s kind in Australia
and allow studies of thin films and defects as a
function of depth in materials.
100
10
PPO
PBMA
PEU
1
PAr
PC
PVAc
PMMA
0.1
0.01
PCT
PBT
PET
PVOH
PEN
0.001
1.58
1.5
1.40
2.3Å
0.3
0.4
0.5
0.6
1/oPs (ns)-1
Figure 15 Schematic comparing oxygen permeability and PALS free volume parameter
oPs. The graph shows the variation in permeability for oxygen of a range of polymers as
a function of inverse free volume (as represented by oPs-1) in the form of the CohenTurnbull198 relationship. Adapted from ref. 197.
0.7
0.8
The 2-D Doppler broadening technique
gives information about the chemical
environment of the annihilation site.
Annihilation of a positron with an inner
shell electron produces a distinctive
doppler shift to the annihilation gamma
ray energies, which can be related to
the atom at which the annihilation took
place.
This technique can give
important information about the make
up of impurity clusters in metals.