Positron Annihilation Spectroscopy
e+ Lifetime Measurements
Positrons enter a material, slow down, and annihilate with an electron in the ‘electron sea’. The lifetime is typically very short ~100 ps. Some materials have ‘voids’ in the sea of electrons, and positrons that stop in the ‘voids’ have longer lifetimes (~ 200+ ps). Decay curves may therefore have multiple components.
Fig *. Information contained in a decay-time measurement.
The lifetime of the slow component is related to the nature of the ‘void’ and the strength of the slow component is related to the relative number of ‘voids’.
2. 511-keV Annihilation Line Doppler Broadening
Suppose electrons in a material are classified as either conduction electrons or core/valence electrons. Since positrons are positively charged they will avoid the nuclei and thereby avoid the regions where the core electrons live. Most e+ annihilations therefore occur with the conduction electrons.
Conduction electrons have momentum and this additional momentum adds & subtracts from the equal & opposite 511-511’s from the annihilation process itself. The broadening of the 511 line is determined by the range of momenta in the conduction band. The Fermi momentum (and Fermi energy) can be extracted from the distribution.
Positron interactions with the core/valence electrons are ~ 10 3 less apparent than those discussed above. The core electrons are fast moving and therefore the energy spread of the 511-511s is much greater.
Fig *. Shape of the 511-keV annihilation line.
Nuclear Structure and Spectroscopy
Studies of Exotic Vibrations in the Atomic Nucleus
Studying the properties of low-lying nuclear states reveals much information about the behavior of nuclear matter. In the heavier nuclei where many nucleons are involved, it is not possible to keep track of the orbital motion of "hundreds" of individual particles and one is forced to use almost exclusively the collective model description. The collective model treats the nucleus as a fluid undergoing vibrations and rotations.
As in any system, the nuclear fluid will oscillate in it's normal modes. (In what shapes do bubbles blown from a soap-bubble wand oscillate?)
But in fact, the nucleus is composed of neutron and protons -- distinguishable particles, so the nucleus should be considered as a mixture of two separate fluids. These two fluids do not necessarily have to oscillate in phase. These exotic oscillations are commonly referred to as mixed-symmetry states or isovector states.
As is the case in many systems undergoing oscillations, it is becoming apparent that the actual motion of the nucleus is not in pure normal modes. The extent of the fragmentation of isovector vibrations has not been measured in spherical nuclei.
Present research concentrates on the Tellurium nuclei -- there are 7 stable nuclei. The mid-shell nuclei 120Te and 122Te are clearly dominated by vibrational characteristics. The heavier nuclei such as 130Te are clearly dominated by the particle orbital structure. By studying the whole range of tellurium nuclei, one should gain some insight into the evolution of these structures and perhaps those aspects of the nuclear force which control the balance.
Most of the measurements are done at the University of Kentucky Nuclear Structure Laboratory. Isotopically enriched samples are hung in a neutron beam which was produced by use of the Van de Graaff accelerator. Most of our experiments focus on things that can be learned by measuring the gamma ray reaction products.
Occasionally the required reactions cannot be performed with the Univ Ky accelerator, and we have traveled to the Swiss nuclear physics lab: the Paul Scherrer Institut. There we utilized beams made from one of their cyclotrons.
There are a few measurements which remain to be done on the tellurium isotopes, and we have also traveled to the Nuclear Structure Lab at the University of Cologne, Germany. --- no pictures yet..