University of Geneva - Positron Annihilation Laboratory
WELCOME to our home page !
You find here informations about the positron annihilation laboratory of the University of Geneva.
(Our pages are still under development).
Maintained by Alfred A. Manuel (Alfred.Manuel@physics.unige.ch)
and Ludger Hoffmann (Ludger.Hoffmann@physics.unige.ch).
Revised: December 20, 1995
Angular Correlation of Annihilation Radiation (ACAR) contains information on the momentum density of electrons in solids.
The experimental arrangement consists of a small SAMPLE, irradiated by a POSITRON SOURCE.
In the sample, the positrons annihilate, sending out radiation to the gamma-ray DETECTORS.
The most interesting case is the one of two photon emission, to be detected by two detectors in coincidence.
The two photons carry both the energy and the momentum of the positron and the electron.
In metals, the incident positron loses it's momentum before annihilation, in insulators the situation is less clear;
each gamma-ray carries about one electronic mass of energy and hence a momentum which is large compared to the
momentum of the annihilated pair:
therefore the angle in the annihilation radiation is of the order of milliradians.
Detection of the angle for a large number of events leads to a histogram which corresponds to a two-dimensional
projection of the Two Photon Momentum Distribution (TPMD).
Out of several such projections, and using reconstruction algorithms known from tomography, one can construct the TPMD.
In metals, it is possible to reconstruct Fermi surfaces from the TPMD.
In addition to the angular distribution, one can also measure the lifetime
of the positron in a solid, that is, the time which elapses between the entry of the
positron and the emergence of the annihilation radiation. This time lies between
picoseconds and nanoseconds, and it is easy to see that it will become longer if
the spatial electronic density diminishes in the place where the positrons are
located. Since defects often result in spots with diminished density,
lifetime measurements have become a popular method for the study of the physical and
metallurgical state of matter.
Band structure calculations can be compared to the experiments.
Good agreement has been achieved in metals, but difficulties subsist in the case of oxides, including superconducting oxides.
We believe these difficulties are not just instrumental:
comparison experiment-theory becomes difficult in substances where fundamental questions on electronic structure subsist.
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- International list of positron annihilation labs (by D. Britton, University of Cape Town)
- Links to a few labs:
A few other links:
- University of Geneva
- The home page of the Town of Geneva (in french)
- Informations about Switzerland
- Search engines:
- Other starting points:
Yanoff's Internet Services List,
General Overwiew of the Web
- Telnet to the CARL references database (Colorado Association of Research Libraries,
its UnCover database is accessible at no charge). Type first PAC, then follow the instructions.
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access to home.html since 11.03.96.
Graph of feb 96
