Positrons for scientific research
The positron is an excellent probe particle to study a set of key properties of condensed matter. Upon annihilation with its antiparticle, the electron, it provides us with a direct image of the electronic structure of metals and semiconductors, nanocrystals, near
interfaces and surfaces. Its positive charge and mobility in condensed matter make it a very sensitive, self-seeking probe to monitor
the evolution of vacancy-related defects and embedded nanocrystals. Vacancy concentrations down to the ppm scale can be
detected. Spin-polarized positrons provide a new, atomic scale view on the magnetism and electronic structure of magnetic
materials. Its kinetic energy can easily be tuned, enabling depth-profiling studies of a large variety of sub-micron thin films. The
recent appearance of brilliant, reactor-based positron beams boosts the development of new thin-film positron probes, providing the
research community with new views on nature and with new possibilities in the development of advanced materials for future
applications in everyday life.
Properties of the positron
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positrons are the anti-particle of electrons...
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visualize the properties of electrons in matter
positrons are positively charged...
high sensitivity to detect and identify atomic scale defects
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positrons have a magnetic moment...
direct imaging of the electronic structure of magnetic materials
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300 nm
1000 nm
positrons are non-destructive...
950K
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750K
S-parameter
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in-situ studies on a wide range of materials
1100K
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as impl.
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1200K
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positron kinetic energy...
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can be varied to obtain depth-scans of sub-micron thin films
positron energy (keV)
positronium formation…
sensitive and unique probe for nanoporous materials
Positron Science
Particle Physics
The positron and its anti-particle, the electron, are elementary particles. As far as we know they have no internal
structure. They are characterized by the same mass and spin (magnetic moment), while their charges are exactly
opposite. The magnetic moment of the positron and its anomalous part was extremely accurately determined in
innovative studies on individual positrons confined in ion traps, for which Hans Dehmelt was awarded the Nobel
Prize in Physics in 1989. Alternative routes to test quantum electrodynamics (QED) are provided by the existence
of positronium – the bound state of a positron and an electron.
New intriguing opportunities to unravel the nature of anti-matter have emerged from the production of antihydrogen (the bound state of a positron and anti-proton) at CERN with the first successful synthesis in 1996, and
from the synthesis of di-positronium molecules at the University of California at Riverside in 2007.
Solar Energy
Sun light provides a natural and very abundant clean energy source. Both the possibility of scarce oil supplies and
the need for sustainable energy sources with reduced carbon dioxide emissions form a strong driving force to
develop more efficient and economic solar cell devices.
Positrons are very useful to detect sensitively the presence of defects in solar cell devices, aiding to develop better
materials and improved long-lasting device operation. In particular, positrons can be used to monitor the defects
leading to the Staebler-Wronski effect in silicon-based solar cells and to electrical deactivation of dopant atoms.
Innovative positron methods are developed as a new probe for surfaces of nanoparticles embedded as key
functional elements in innovative ‘third-generation’ solar cells.
Hydrogen Energy Economy
Hydrogen is an ideally clean carrier of energy. A future hydrogen based energy economy will need substantially
better ways of storing hydrogen in a safe, light and affordable manner. Metal hydrides, and ionic compounds of the
lighter elements, appear promising candidates.
Positron methods provide important insights in the nature and evolution of atomic vacancies in metal hydrides. On
the one hand, the presence and tailoring of vacancies in metal hydrides is of crucial importance to achieve fast
transport of hydrogen and metal ions through the metal hydrides during hydrogen cycling. On the other hand,
insight in the trapping of hydrogen in defects and the introduction of defects in the metal hydrides during the
hydrogenation reactions can be gained on an atomic scale with positrons. This knowledge will be of great
importance for rational materials design.
Nuclear Energy
Nuclear fission is a sustainable energy source with no carbon dioxide emission in the primary energy generation
process. In the development of future ‘Generation IV’ reactors, key points are to reach the highest possible
intrinsic safety, very effective economics, reduced waste production and enhanced proliferation resistance.
The materials aspects for structural materials and components are therefore very important. Positrons are
excellently suited to monitor the important phenomenon of radiation embrittlement as a consequence of neutron
irradiation of the reactor pressure vessel (RPV) steels. Further, important insight in the behaviour of nuclear fission
fuel materials such as uranium dioxide under irradiation can be gained from positron studies, and will aid to
achieve optimum operation conditions and to develop optimum waste storage matrices.
Information Technology
ICT is all around us in the world of today, with computers, the world-wide-web, wireless communication systems, iPods and portable GPS car navigation systems. This exponential growth in capabilities of ICT systems mirrors the
miniaturization of their electronic building blocks, following Moore’s law for decades. The ever decreasing sizes of
device elements necessitate the study of alternative materials such as high-k and low-k dielectric thin films, and
drives the need for ultimate material control as the impact of defects rises dramatically with decreasing size.
Here, positrons show up as an indispensable tool to monitor, qualify and quantify the presence of atomic vacancy
related defects with sizes well below the imaging capabilities even of high-resolution electron microscopy.
Further, positron methods are one of only very few methods capable to determine the evolution of pore sizes and
connectivity of nano-pore networks in low-k dielectric thin films.
Magnetoelectronics
Magnetic sensors based on the giant magnetoresistance (GMR) effect can be found in hard disc reading heads,
position sensors for precision tools and ABS systems. GMR sensors exploit the magnetic field dependence of the
electrical resistance in layered magnetic structures. GMR, together with the Exchange Bias (EB) effect that pins
the direction of magnetic moments in a certain direction, allows the construction of spin valves, which are essential
components of magnetoelectronics. On this basis, smart micro-magnetic-media can be envisaged that could
become prototypes for magneto-neural-networks.
Spin-polarized positron methods provide us with a unique way to image the electronic structure of GMR materials.
Low-energy positron beams are well suited to investigate the ultrathin and laterally confined films, in order to
explore the magnetism and, intriguingly, its proposed relationship with the presence of defects.
Nanoscience
Tailoring the sizes of semiconductor nanocrystals typically below 10 nm leads to pronounced changes in their
electronic and optical properties, as can be seen very clearly from the strong changes in colour of their solutions.
Consequently, these nanocrystals have a large potential for applications, which has been exploited during the past
five years to create innovative light emitting diodes, solar cells, and fluorescence monitoring of biomolecules by
quantum dot labeling.
Positrons can detect key properties of these nanocrystals, in particular their surface composition and structure,
and further the effect of confinement on the valence electron states. These aspects are crucial for understanding
and tailoring the nanocrystals’ functioning while they are embedded deep within the layers of thin film devices.
Solid State Physics
Electrons are the glue and the spacer of any solid state material, pulling on the one hand the atoms together, and
on the other hand maintaining the material’s finite dimensions because of the Pauli exclusion principle. The
dynamic character of electrons appears particularly well for metals, providing their large conductivity and a fast
screening on remarkably short length scales. The concept of the Fermi surface, separating occupied momentum
states from unoccupied ones, is a corner stone in theories of the metallic state, and an appreciation of the shape
of the Fermi surface is an important step in understanding the nature of some of the more fascinating electronic
ground states.
Here, positrons are extremely useful as they probe the electron momentum density in a direct manner. 2D-ACAR
studies provide 2-dimensional projections, enabling the reconstruction of 3D Fermi surfaces in a large temperature
range while including the important domain of alloys. Crucial insight in the physics of Fermi surface nesting,
superconductivity, charge- and spin-density waves, ferromagnetism, and shape-memory alloys was thus obtained.
Surface Science and Catalysis
Surfaces play a crucial role in the chemistry of catalysis as applied in many industrial processes. It is therefore
very important to gain a detailed insight into the mechanisms of dissociation and association of molecules at
surfaces, and their local binding properties.
Low energy positrons provide a sensitive probe to this field, since they show a large affinity to trap at surfaces
after implantation in their vicinity. In particular, the surface sensitivity of the well-established technique of Positron
Auger Emission Spectroscopy (PAES) is far larger than conventional Electron Auger Spectroscopy, and further
opens up detection of low-energy semi-core electrons. In the recently accomplished combination with an intense
reactor-based low-energy positron beam, this technique holds large promises for new discoveries in surface
catalysis.
Materials Science
The ultimate dream in the technology of constructive metals is the point where a mechanic can walk up to a
deteriorating airplane and urge: "Wing, heal thyself!" Here, supersaturated aluminium alloys, which age harden
naturally, are promising. Supersaturated aluminium alloys subjected to a mechanical load may perform a healing
phenomenon by aggregating solute atoms in those areas where strain has induced open volume defects. The
aggregated clusters fill the open volume defects, contributing to a restoration of the material’s strength.
Positrons here sensitivity point out the evolution of open volume defects and, owing to the chemical specificity of
coindicence Doppler broadening, the clustering of impurity atoms in their vicinity. This paves the way to optimum
compositions of alloys which one day may lead to wing materials with a seemingly nearly ‘eternal youth’.
Materials Design
One can hardly think of a world today without substances based on polymers. Both the large variation in polymeric
groups and the playground of polymer chain length and shape contribute largely to the widely varying functionality
and physical and chemical properties of polymers. This formed the basis for the synthesis revolution leading to our
modern ‘polymeric world’.
A key aspect in the polymer behaviour and synthesis is the local mobility of polymer chains, which is often related
to the concept of free volume. In many polymers, positronium (the bound state of a positron and an electron) is
formed at these open spaces, and acts as a sensitive probe to monitor the evolution of free volume as a function
of temperature or of the grade of isomerization. Here, the positron lifetime method forms a very suitable in-situ
tool, aiding largely to our understanding and synthesis of polymers.
S.W.H. Eijt, 2008 Department of Radiation, Radionuclides & Reactors, Faculty of Applied Sciences, Delft University of Technology (s.w.h.eijt@tudelft.nl)
Properties of the positron
positrons are the anti-particle of electrons... visualize the properties of electrons in matter
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X
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Positrons annihilate after a small, but finite lifetime after they are implanted in
materials with their anti-particle, the electron. Its lifetime typically between ~100
ps and ~1 ns (in absence of positronium formation) depends sensitively on the
local electron density at the annihilation site.
The emitted gamma fotons carry detailed information on the electron
momentum distribution and electron density at the annihilation site. Positrons
are therefore excellently suited to probe the electronic structure of materials.
positrons are positively charged... high sensitivity to detect and identify atomic scale defects in matter
Neutral or negatively charged vacancies often act as a strong trap for positrons,
as positrons, also in the bulk, naturally try to avoid regions near the positively
charged nuclei of atoms. Their finite lifetime in combination with a high mobility
leads to a very high sensitivity to find the trapping defects, and enables one to
monitor very low defect concentrations and determine the types of defects
present in the material.
positrons have a magnetic moment... easy and direct imaging of the electronic structure of magnetic materials
The positron, being a spin-1/2 particle, carries a magnetic moment equal to that
of the electron. Positrons emitted from a radio-isotope like 22-Na are partially
spin-polarised, a relativistic consequence of non-conservation of parity in the
beta-decay process.
Switching the magnetization direction of a magnetic material allows one
to extract its spin-resolved electronic structure, as the positron annihilates
preferentially with an electron of opposite spin direction.
positrons are non-destructive... enables in-situ studies on a wide range of materials
Positron annihilation methods are to any practical purposes non-destructive for
nearly all materials. With at most one positron in the material present at each time,
which can be sensitively detected with the emergence of gamma fotons upon its
annihilation, it removes merely a single electron. The application of a positron beam
is easily combined with a variety of in-situ sample treatments including variation of
temperature, application of external electric fields and laser excitation.
positron kinetic energy... can be varied to obtain depth-scans of sub-micron thin films
70 130
0,62
300 nm
1000 nm
950K
0,60
The average probe depth of positrons in materials can be varied simply by tuning
their initial kinetic energy in the keV range. This enables positron depth-profiling
studies on a wide variety of thin film systems, typically in the range between
~10 nm and a few micron. In this way, the evolution of individual layers, defects,
interfaces, as well as the nucleation and growth of embedded nanocrystals with
material treatments can be sensitively monitored.
750K
S-parameter
0,58
1100K
0,56
0,54
as impl.
0,52
1200K
0,50
0,48
0,46
0
5
10
15
20
positron energy (keV)
positronium formation… sensitive and unique probe for nanoporous materials
In pores with a size larger than about 0.7 nm, positronium (the bound state of a
positron and an electron) atoms can be formed. The very long lifetime of orthopositronium (up to 142 ns) and the relatively narrow momentum distribution of
para-positronium provide sensitive detection of the dimensions of nanoscale
pores. Moreover, they are indispensable and very sensitive tools to monitor the
connectivity of pore networks and open or closed porosity.