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					              CSEM’s

            Materials Monthly
           May 2006                                                 Making materials matter

                  New light on the dark side of matter
                  CAMS and working with positrons
                                     Mention antimatter in public and it’ll          are negative where normal protons
                                     be assumed you’re either a sci fi fan            are positive. Indeed, it’s believed
                                     or a theoretical boffin. That’s because          there’s an antimatter equivalent
                                     most people see antimatter as an                for each of the known subatomic
                                     abstract concept with little place in           particles.
                                     the real world. And yet the truth
                                                                                     “If an antimatter particle comes in
                                     is a bit different – antimatter is a
                                                                                     contact with a normal matter particle
                                     commonly used substance with many
                                                                                     they annihilate and are converted
                                     important applications. PET scans,
                                                                                     into energy. The amount of energy
                                     for example, are a common form of
                                                                                     released is predicted by Einstein’s
                                     diagnostic imaging used to dectect
                                                                                     equation: E = mc2; where m is the
                                     tumours and it works by releasing
                                                                                     mass of the particles and c equals the
To make sure that Australia          antimatter in our bodies. Firing
                                                                                     speed of light. That energy is released
captures the benefits of              antimatter at the surface of materials,
                                                                                     in the form of gamma rays.”
antimatter research an ARC           as another example, is a technique
                                     for characterising those surfaces.              And therein lies the reason why
Centre of Excellence devoted
                                     Indeed, antimatter plays a number of            antimatter is considered so exotic, so
to antimatter-matter studies
                                     roles in the real world and its value           ‘other wordly’. If it comes into contact
has been established. Known          in materials science, medicine and              with normal matter it dissappears in
as CAMS – the Centre for             biology is only set to increase.                a burst of gamma rays so how can it
Antimatter-Matter Studies
                                                                                     serve any useful function in a world
– it’s hosted by the ANU and         So what is antimatter?                          made from normal matter?
includes other universities and
government laboratories, and         “As the name implies, and as most               “There’s no question that antimatter
promises to raise the status         people would have heard, antimatter             is an exotic substance and it’s not
                                     is the same as normal matter but with           easy to work with but there’s a lot
of antimatter from something
                                     an opposite electrical charge,” says            more to it than abstract physics,” says
obscure and abstract to
                                     Professor Stephen Buckman, Research             Professor Buckman. “When antimatter
something real and now.              Director of CAMS. “Antimatter                   meets normal matter it takes only a
                                     electrons are identical to normal               split second before its gone but in that
                                     electrons but carry a positive charge           split second of interaction it’s possible
                                     where normal electrons carry a
                                     negative charge. Antimatter protons                                 continued on next page



         Inside this MM
  2-3     CAMS & positrons (cont)
  4       Slivers away
  6       CAMS & positrons (cont)

        Volume VII, Issue IV
        Materials Monthly
        is produced by the ANU
        Centre for Science and
        Engineering of Materials




                                     Professor Stephen Buckman (right), Director of the Centre for Antimatter-matter Studies
                                     (CAMS) with Dr James Sullivan. The two scientists were responsible for the design and
THE AUSTRALIAN NATIONAL UNIVERSITY   construction of the positron beamline. (Photo by Tim Wetherell)
Antimatter-matter research                  to be mapped, usually down to a
(continued from previous page)              resolution of 2-3mm.                      Thinking up antimatter
                                            “PET scans are a well-developed           Antimatter is a good example of
to learn a lot about the physical           diagnostic tool and yet little
                                                                                      science predicting the existence
environment around the antimatter           is known about how positrons
                                                                                      of something before it’s actually
particle.                                   interact with biomolecules despite
                                                                                      observed (or even suspected).
                                            the sophistication and cost of the
“On top of this, when antimatter                                                      In 1928 the British physicist Paul
                                            technology,” says Professor Buckman.
annihilates with a normal matter                                                      Dirac combined quantum theory
                                            “One of the aims of CAMS is to study
particle, gamma rays are released.          the interaction of positrons with bio-    and special relativity in one
Gamma rays are easily detected              molecules and try and shed light          equation in an effort to better
allowing researchers to pinpoint            on what happens between positron          model the behaviour of electrons.
where the antimatter-matter particle        emission and positron annihilation.       His solution, known as the Dirac
pair was when it disappears. These          In particular, we want to look at ways    equation, worked exceptionally
particles are too small to see with any     in which the efficiency of the process     well describing many attributes
microscope but when they annihilate         might be improved. In so doing we         of electron motion that previous
the gamma rays they give off are a          think we can improve the resolution of    equations could not. However, it
crystal-clear signal that is easy to pick   PET scans.”                               also suggested the possibility of
up. These aspects of the interaction                                                  the existence of electrons with
between antimatter and matter lend          Positrons are also a useful tool in the   positive charges – anti-electrons.
themselves to some very useful              analysis of materials. When positrons     But where would you look to find
applications in the areas of medical        are fired into a material they tend        such a strange beast?
scanning, materials characterisation        to drift towards any open volumes
and theoretical particle physics.”          — very small holes. This is because       The existence of anti-electrons
                                            they like to be away from the positive    created a real problem for
If antimatter can’t coexist with normal     charge of the fixed nuclei in the          the growing field of quantum
matter for more than an instant and         material — like charges repel. When it    physics. Everyone accepted the
our world is made of normal matter,         finds a hole in the material, there are    mathematics of Dirac’s equation
where does it come from and how do          no electrons to annihilate with, so the   but anti-electrons did not
you use it?                                 size of the hole determines how long      correspond to anything known
“There are two basic sources of             the positron lives.                       in the physical world. Werner
antimatter on Earth,” says Professor        “By looking at the lifetime of the        Heisenberg, one of the world’s
Buckman. “It’s produced when                positrons in the material, we can         leading quantum physicists at
subatomic particles collide, as occurs      get information about the size and        the time, initially called this
in particle colliders, and from the         distribution of holes, or defects, that   “the saddest chapter of modern
radioactive decay of some isotopes.         are as small as a nanometre in size,”     physics”. However, as events were
Antimatter used in diagnostic scanning      Says Professor Buckman. “Holes of         soon to demonstrate, it was to end
and materials characterisation uses         this size are related to important        in intellectual triumph.
antimatter produced by radioactive          properties in some materials, such as
isotopes.                                                                             Four years later when studying
                                            porosity and conductivity. They can
                                                                                      the particle tracks left by collisions
“The commonest form of antimatter           also be an early indicator of material
                                                                                      with cosmic rays a particle was
used is the positron – the antimatter       degradation. In CAMS we plan to
                                                                                      identified that had the same mass
equivalent of the electron. Indeed, the     use one of the positron beamlines
                                                                                      as an electron but the opposite
positron is identical to the electron       for the study of materials for various
                                                                                      charge – the anti-electron or
in all aspects except that it carries a     applications, from new generation
                                            plastics to silicon wafers.               positron had been found.
positive charge.”
                                                                                      Since then it’s been determined
Putting positrons to work                   Crossing to the dark side                 that an antiparticle exists for each
One of the most common applications         Professor Buckman’s interest in           of the known sub atomic particles
of positrons is in medical imaging          positrons and antimatter arose from       and that these are naturally
and PET scans. While many people            his earlier research on electrons. He     generated by cosmic rays
have heard of these, few would              says it’s not an enormous jump to         impacting with normal matter.
connect them with antimatter, and           switch from electrons to positrons.       Amidst the cascade of resulting
yet PET stands for Positron Emission                                                  particles antiparticles will often
                                            “Several years ago I spent some time
Tomography. The procedure involves          over at the University of California,     result, existing for nanoseconds
injecting a patient with glucose            San Diego, where they have a              before combining with normal
containing a radioactive isotope,           beamline set up specifically for the       matter and annihilating.
usually fluorine-18, which emits             study of positrons,” he explains.
positrons. The body directs the                                                       In 1933 Dirac (pictured below)
                                            “When I saw what they were achieving      was awarded a Nobel Prize for
glucose to areas of high metabolic          over there I realised that antimatter
activity, often indicating the presence                                               his modelling of the electron and
                                            science was a real opportunity for us
of a tumour or some change causing                                                    prediction of
                                            in Australia because our strengths
increased blood flow or immune               in electron atomic physics meant we
                                                                                      antimatter. He
system activity.                            already possessed the core expertise      was just 31
                                            to undertake these studies. You might     years of age.
The fluorine-18 has a half life of
a few hours. As it breaks down it           say my exposure to this positron
emits a steady stream of positrons.         research convinced me it was worth
These combine with electrons in the         crossing over ‘to the dark side’.
surrounding tissue, they annihilate         “When I returned to Australia I got in
and give off energy in the form of          contact with several research groups
gamma rays. The gamma rays are              around the country with an interest
easily detected and allow the source        in antimatter and we were successful
of the increased metabolic activity         in obtaining funds in 2004 from the
                                                         page 2 - May06
Australian Research Council to build a     These positrons are then confined
low energy positron beamline here at
RSPSE.”
                                           in an electrostatic potential well
                                           where they cool to room temperature
                                                                                          The stuff of sci fi
                                           through collisions with nitrogen               One of the reasons antimatter
The funding, which amounted to
                                           gas molecules. The positrons don’t             has such as an exotic aura is its
around $1million, was part of the ARC
                                           combine with electrons of the nitrogen         frequent use in science fiction as
National Facilities program. The other
                                           atoms because their energies are too           a source of seemingly unlimited
partners in the project were Griffith
                                           low. This technique allows for the             energy or as a weapon. Mix
University, Flinders University, Charles
                                           production of a high resolution beam.          a little antimatter and normal
Darwin University and CSIRO.
                                           When the trapped positrons have                matter and their combined mass
“In the couple of years that it has                                                       is converted to massive amounts
                                           cooled sufficiently the beam is formed
taken us to build the low energy                                                          of pure energy. It sounds pretty
                                           by carefully raising the confining
beamline we’ve discovered there’s
                                           potential and allowing the trapped             appealing, and has been the
a lot more interest in antimatter-
                                           positrons to spill out. This produces          driving force behind the Starship
matter studies around the country
                                           a very high resolution beam (with              Enterprise since the inception of
than we first realised,” explains
                                           an energy width of around 25 meV)              Star Trek. However, it overlooks
Professor Buckman. “To capitalise on
                                           which can then be used for various             the fact that antimatter is very,
this interest and the diverse range of
                                           experiments. The beam is confined in            very expensive to create. To
skills available around the country the
                                           the radial direction by magnetic fields,        produce a single gram of sodium
original concept has been expanded
                                           while electrostatic potentials allow           22, the radioactive isotope used
with an ARC Centre of Excellence
                                           the positrons to be directed along the
grant worth $7 million over five years.                                                    by CAMS to produce positrons,
                                           apparatus.
In addition to this the institutions                                                      you’d need about twice the
involved are chipping in an extra $3
million. This funding will enable us
                                           Positrons & materials                          GDP of the United States (as
                                                                                          measured in 2004).
to build an additional high energy         The materials side of the CAMS
beamline which will greatly enhance        experimental program will be
our capacity for positron studies on       undertaken by ANU, ANSTO and                 with a positive charge, with electrons
materials.”                                the University of Western Australia.         orbiting around it. In the case of
                                           Each institution brings a range of           positronium, the positive nucleus is
And the beamline is the key to             experience in positron handling and          replaced by the positive positron.
studying positrons as it allows you to     materials characterisation.                  Positronium is really an exotic type of
control the energy of the positrons                                                     atom. It is very light and only lives for
and precisely measure its interactions     “The goal of the materials research          120 picoseconds or 142 nanoseconds,
with matter.                               is to provide new tools for Australian       depending on the configuration of
                                           scientists to investigate the properties     the electron and positron. The key
A beam of positrons                        of novel materials, such as polymers,        to these studies is understanding
                                           thin films and semi-conductors,”              the formation and annihilation of
The source of the positrons in the
                                           says Professor Buckman. “It’s also           positronium.”
CAMS beamlines is a tiny speck of
                                           hoped that our research and that of
radioactive sodium 22. It’s a only a                                                    Positronium can be formed in two
                                           our collaborators, will be useful in
few nanograms in size, so tiny that                                                     configurations: para- and ortho-
                                           the investigation of such properties
you can’t see it with the naked eye.                                                    positronium (the singlet and triplet
                                           as surface effects, fatigue in some
It was produced by a nuclear reactor                                                    form of the positronium ground
                                           metals and even for some medical
in South Africa, the only place in the                                                  state, respectively). Para-positronium
                                           applications.”
world capable of creating this isotope,                                                 decays via 2-gamma emission (with
and costs around US$25 thousand.           Three types of experiments in                both gamma rays having an energy
                                           materials analysis are possible within       of 511 keV) with a lifetime of 120 ps.
“That would make it the most
                                           CAMS: Positron Annihilation Lifetime         Ortho-positronium, on the other hand,
expensive material I’ve ever worked
                                           Spectroscopy (PALS), Doppler                 decays via 3-gamma emission with
with,” observes Professor Buckman.
                                           measurements and surface studies.            a lifetime of 142 ns. However, inside
And, if you do the sums, he’s not                                                       a solid material the lifetime of ortho-
                                           PALS relys on the detection of 511
joking. A gram of this material, about                                                  positronium depends on how easily it
                                           keV emitted gamma rays as a function
the size of pill, would cost billions of                                                can annihilate via ‘pickoff’ annihilation,
                                           of time elapsed from the entry of the
dollars.                                                                                where the positron annihilates with
                                           positrons into the sample. The lifetime
Fortunately, for CAMS’ purposes, a few     of the positrons in the sample changes       one of the other electrons in the
nanograms is all they need. Sodium         as a function of various properties                                  continued on page 6
22 has a half life of around 2.6 years     of the material, specifically on the
meaning this solitary speck can supply     presence of voids
their beamline with a serviceable          and defects. On
number of positrons for around three       entering the sample,
to four years.                             positrons thermalise
                                           and either annihilate
The sodium sits in a lead lined            or form positronium
chamber at one end of the beamline.        with one of the
It emits around a billion positrons        target electrons.
every second, firing them off in all
directions and with a wide range of        “Positronium is the
energies. Most of the positrons have       substance formed
energies that are too high to be of        when a positron
practical value so before they enter       and an electron are
the beamline they pass through a thin      bound together,”
film of solid (frozen) neon. Around 1%      explains Professor
of the positrons make it through this      Buckman. “In a
step.                                      normal atom, there     Professor Buckman with the positron beamline. The cylinder on the
                                           is a heavy nucleus     right houses the radioactive sodium 22, the source of the positrons.

                                                        page 3 - May06
  Slivers away
  Thinking outside the square = thinking inside the wafer
Few industries can boast a sustained     that simply cutting the silicon
growth rate of 23% but that’s            wafers thinner would only produce
just what’s been happening in the        incremental improvements and we
photovoltaic (PV) industry over the      were looking at more than merely
last 15 years. And that growth is        improving what we already had. So,
set to continue as an energy hungry      we began to think outside of the
world searches for alternatives          square or, for this problem, inside
sources of electricity (23% was          of the wafer. It occurred to us that
an average growth rate; the PV           maybe we should be working with
industry grew by 65% in 2004!).          the volume of the wafer rather than
However, the PV industry is facing       with its surface.”
a major challenge in the form of
a critical shortage of hyper-pure        What they did was cut narrow
silicon at an affordable price.          grooves into the silicon wafer using
                                         an alkaline etch. The alkaline etch
Solar cells are traditionally made       attacks most of the silicon quickly
from wafers of hyper-pure,               but the (111) crystal plane slowly.
crystalline silicon (see box on how      A resistant mask is placed on the        The output of 72 conventional cells is the
this is produced). These wafers          surface of the silicon wafer. The       same as that from two wafers when the
– typically 15 cm in diameter and                                                wafers are manufactured into Sliver® cells.
                                         mask has long narrow slots cut into
less than half a millimetre thick        it. The silicon below the mask is       was most efficient using wafers that
- are used in 95% of photovoltaic        oriented such that a (111) plane is     were 1-2mm thick.
modules. The shortage of hyper-          perpendicular to the wafer surface.
pure silicon is proving a major          Etching commences at the surface        One important aspect is that after
barrier to reducing the price of PV-     of these narrow slots and continues     creating the grooves the slivers
based energy, and it’s a shortage        down through the entire thickness       are still attached to the wafer. This
that only promises to grow over the      of the wafer. The result is a large     allows the slivers to undergo further
next decade.                             number of thin silicon strips in the    steps in the fabrication process
                                         centre of the wafer, held together      before they are cut from the wafer.
There have been many attempts to
                                         by the unetched surrounds of the        Each sliver is now an individual solar
get around this bottle neck. Some
                                         wafer.                                  cell, and these are then assembled
have involved using non-silicon
                                                                                 into modules.
semiconductors based on materials        In effect the wafer has been cut into
such as gallium or indium, while         a series of slivers, with each sliver   Cells can be laid out with no gaps
others are based on using lower          approximately 50-100mm long and         between them or with any spacing
grades of silicon. However, none         40-60μm thick. Each sliver is as        between slivers up to about 1.5
of these efforts have produced a         wide as the wafer is thick and the      times the width of each cell. With a
solution that can compete with           researchers found that the process      scattering reflector attached to the
cells based on wafers of hyper-pure                                              rear of each module, most of the
silicon in terms of cost per watt.
There’s also been work on using
thinner wafers of silicon but
only incremental savings can be
achieved here – you can only cut a
traditional wafer so thin and there
are significant losses of silicon in
the process of slicing up the original
ingot of silicon into wafers.
But researchers at the ANU Centre
for Sustainable Energy Systems
(CSES) have found a way around
the problem and their solution lies
in a little lateral thinking and a lot
of clever materials engineering.
The lateral thinking involves cutting
thin layers of silicon out of the
wafer itself. The clever materials
engineering is how they achieved it.
“The main issue we needed to
tackle was how to reduce the
amount of high grade silicon being
used in solar cells,” says Dr Klaus
Weber from the CSES. “We knew
                                                   page 4 - May06
                                                                                     Six strengths of
  Silicon for solar cells                                                            the Sliver
  As materials go, surely there is no element as versatile and useful as
  silicon. After oxygen it’s the most abundant element in the Earth’s                1. More bang for your buck:
  crust - it makes up a quarter of the planet’s crust by weight. How                 Sliver® cells use approximately
  is it then that hyper-pure silicon is in critically short supply for the           one tenth the amount of
  manufacture of solar cells?                                                        expensive silicon compared with
  The silicon in solar cells is hyper-pure silicon, and making it is neither         conventional cells.
  easy or cheap. It involves loading a crucible with high-purity silicon
  along with small amounts of either boron, phosphorus, arsenic                      2. Designer transparency: any
  or antimony (these impurities or dopants give the final product                     degree of module transparency
  different electrical properties). The silicon is then melted at a process          can be easily achieved by
  temperature of 1400°C in an atmosphere of pure argon. Once the                     adjusting the Sliver® cell spacing.
  proper ‘melt’ is achieved a ‘seed’ of single crystal silicon is lowered
  into it.                                                                           3. Flexible fitting: The thinness
  The temperature is then adjusted as the seed is rotated and slowly                 of the slivers makes them
  pulled out of the molten silicon. The surface tension between the seed             flexible meaning modules can
  and the molten silicon causes a small amount to rise with the seed.                be designed to flex and bend
  As the growing seed is pulled it cools to form a perfect single-crystal
  ingot with the same crystal orientation as the original seed. This ingot
                                                                                     (opening up countless new
  is then sliced up into individual wafers which are used as the basis of            architectural possibilities).
  solar cells or integrated circuits.
                                                                                     4. High voltage: Many Sliver®
                                                                                     cells in series still take up very
                                                                                     little area, so high voltages can be
light incident in the space between       as that from as little as two wafers       obtained in very small modules.
the cells can still be captured.          when the wafers are manufactured           This makes Sliver® cells ideal for
Sliver® cells, as they are known,         into Sliver® cells.                        powering small consumer items.
are truly bifacial as they respond
equally well to light on either           As part of a research program              5. Bifacial response: The perfect
surface. They can be connected in         substantially funded by Origin
                                          Energy, the Sliver process concept         bifacial response of a Sliver®
any series and parallel configuration
                                          was invented by Dr Weber with              cell means that Sliver modules
to deliver the output voltage
required for any application.             Professor Andrew Blakers, Director         respond equally well to light
                                          of CSES, in 2000. Since then ANU’s         falling on either surface. This
So, how much hyper-pure silicon is        commercial collaborator Origin
saved using the Sliver approach?          Energy has constructed a factory in
                                                                                     allows for novel applications of
Consider this: A typical silicon wafer    Adelaide to produce Sliver® cells          Sliver® modules. For example,
configured as a conventional solar         commercially. First sales occurred         highway round barriers can utilise
cell will require about 10kg of silicon   in July 2005 and full commercial           Sliver® modules that are mounted
for each kilowatt of peak output          production is expected to get              vertically facing east-west.
power. However, a wafer, when             underway later this year.
processed to produce Sliver® cells,                                                  6. Energy payback: The energy
can achieve the same output with          In the meantime, research at
                                          CSES is further improving Sliver           payback time of a Sliver® module
1kg or less of silicon, which is 10
times better than for conventional        technology. Recent results indicate        is short because the quantity of
technology. Moreover, far fewer           that with careful engineering              energy-intensive silicon is sharply
                                          using well-known and established
wafers need to be processed to
                                          techniques, Sliver technology could
                                                                                     reduced. The energy payback
give the same output compared to                                                     time is 1.5 years, two thirds of
conventional cells. The output of         reduce the costs of PV technology
72 conventional cells is the same         to the point where it will be              which is due to standard module
                                          competitive with wind energy and           components (glass, aluminium
                                                ‘zero emission’ coal. Given the      frame etc.) compared with 4 years
                                                vast worldwide solar energy
                                                resource, this has major             for a conventional module.
                                                implications for climate change
                                                policy.
                                               More info: http://solar.anu.edu.au/
                                               or email solar@anu.edu.au

                                                                       MM




                                                      page 5 - May06
Antimatter-matter research                    Two beamlines
(continued from page 4)                                                                      Partners in
                                              “The high energy beamline being
                                              built at CAMS will be dedicated to             antimatter-matter research
target (not the one it is bound to).          materials studies, ”says Professor
                                                                                             CAMS is hosted by ANU and has a
The lifetime against direct annihilation      Buckman. “This beamline will allow us
                                                                                             total of 15 collaborating partners.
is typically 100’s of picoseconds.            to control the energy of the positrons
                                              being injected into the sample to be
                                                                                             Australian members are Flinders,
If annihilation gamma-rays are                tuned up to 20 keV. This will allow the        Murdoch and Griffith universities,
detected as a function of time after          samples to be probed as a function of          the University of WA, and ANSTO.
entering the sample (usually the two          depth, and we can go into the sample           Overseas partners are The Open
511 keV are detected in coincidence           to approximately 1 micron.                     University, The Universities of
to improve timing resolution), then                                                          California in San Diego and Davis,
a series of lifetimes can be seen in          “Our existing low energy beamline              The Lawrence Berkeley National
                                              has a peak energy of 100 eV. This is
the spectrum. These lifetimes give                                                           Laboratory, Drake University, the
information about the structural              suitable for investigating biological
                                                                                             University of Nebraska, Tohoku
makeup of the sample, and the ortho-          systems and fundamental atomic and
                                                                                             University, and the University of
positronium lifetime is particularly          molecular interactions with positrons.
                                                                                             Munster.
sensitive to the size and distribution            “The fundemental investigations
of voids and defects. PALS, for                   involve understanding the interactions
                                                                    of positrons with       understanding, we expect to greatly
                                                                    atoms and molecules     increase our understanding of the
                                                                    in a way that gives     interactions of positrons and matter at
                                                                    us insight into their   a fundamental level.”
                                                                    behaviour at a
                                                                                            The design of the two beamlines
                                                                    quantum mechanical
                                                                                            are based on a positron beamline
                                                                    level. Despite that
                                                                                            operated by the University of
                                                                    fact that quantum
                                                                                            California, San Diego, though many
                                                                    mechanics has been
                                                                                            aspects of these new facilities are
                                                                    around for a long
                                                                                            unique to Australia.
                                                                    time the interactions
                                                                    of positrons with       “Our two beamlines incorporate a lot
                                                                    single atoms is         of second generation improvements
                                                                    notoriously difficult    to the technology,” says Professor
                                                                    to model correctly.     Buckman. “We will have a more
                                                                    Instead, numerous       intense beam with a broader program
                                                                    approximations          of study.
                                                                    have to be made
                                                                    to describe the         “This Australian facility will place us
                                                                    interactions and a      at the global forefront of positron
                                                                    lot of computing        physics. There is no other centre in
                                                                    power is needed to      the world with such an adventurous
                                                                    solve the equations     focus or such a breadth of activities
                                                                    numerically.            involving positrons.”
                                                                    The facilities of       The high energy beamline is scheduled
Dr James Sullivan, an ARC Research Fellow, and PhD student Ms      CAMS will allow          to be operating by the end of this
Violaine Vizcaino by the positron trap. (Photo by Tim Wetherell.)  the experimental         year.
                                                                   investigation of these
example, has been used extensively                interactions at an unprecedented          More info: www.positron.edu.au
by scientists at CSIRO to relate the              level of accuracy. In addition, CAMS      or email Professor Buckman
average pore size in polymers to their            includes some of the best theorists in    <Stephen.Buckman@anu.edu.au
oxygen porosity.                                  the world in this area. By combining
Doppler measurements provide                      experiments with a new theoretical                                    MM
additional information on the site of
annihilation. Positrons that annihilate
with a fast moving electron (ie, a core
electron) will emit Doppler-shifted             Contacting CSEM
gamma rays. Measuring this shift can                                                        Materials Monthly comes out 10
then provide information about the              Director
                                                Dr Zbigniew Stachurski                      times a year (February to November).
chemistry at the annihilation site.                                                         We welcome your feedback and
As positrons tend to be attracted to            Phone: (02) 6125 5681
                                                                                            contributions. Please send them to
defects in the materials, this then             Email: Zbigniew.Stachurski@anu.edu.au       David Salt, Editor, Materials Monthly,
gives us the ability to determine the
chemical makeup at defect sites, such           Editor, Materials Monthly                   care of CSEM.
as voids, which may be sites for solute         David Salt                                  Please let us know if you wish to
atoms forming clusters or precipitates.         Phone: (02) 6125 3525                       be added to our electronic or postal
Surfaces can also be studied                    Email: David.Salt@anu.edu.au                mailing lists.
using antimatter. Variable-energy,
thermalised positrons will be scattered         CSEM Office                                  Electronic copies of Materials
from thin films, surfaces and                    CSEM                                        Monthly, useful links and additional
interfaces for metals, semiconductors           Department of Engineering                   information about CSEM can be found
and insulators, thereby yielding a              Bld #32, ANU ACT 0200                       at our website:
range of information on the nature of                                                       www.anu.edu.au/CSEM
the surface.

				
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