Positron states transitions between

Simultaneous measurement of positron lifetime and the momentum of the annihilating pair can give information on thermalisation and transitions between positron states (and hence on chemical reactions of positrons or Ps). The most recent version uses MeV positron beams [35]. A full description of AMOC can be found elsewhere in this volume. 

AMOC allows time-dependent observations of the occupations and transitions of different positron states tagged by their characteristic Doppler broadening. Chemical reactions of positronium have been studied by beam-based AMOC as well as bound states between positrons (e+) and halide ions (cf. Sect. 2). 

For free particles this point of view has indeed some attractive features. There are, however, situations where the sign of the energy does not distinguish between electronic and positronic behavior. Consequently, transitions from electronic to positronic states cannot be excluded. A famous example is the Klein paradox, where a potential step divides space into two regions with a different interpretation of particles and antiparticles. If the step size is larger than twice... 

The separation of states of different T implies that one of a set of isobaric nuclei is stable and the others unstable against beta decay. The Wigner theory predicts superallowed decay between the states of a given supermultiplet because no change of spatial wave function is needed. This is found (a) for the positron decay of odd mirror nuclei (b) for transitions between the low states of nuclei with mass number 4 + 2 in which both T= and P = 0 states are found in the (1,0,0) supermultiplet. 

In practical situations any intermediate case is possible. In the case of the transition-limited regime, the link between positron states in the specimen and the experimental positron lifetime spectrum is provided by the simple trapping model (STM) [103]. Let m t) denote the probability that positron will be present in the specimen at time t. In the case of an ideal crystal i.e., if no defect is present in the specimen), positrons will be delocalised in the material. The time when positron thermalisa-tion is accomplished is chosen as t = 0, so m t = 0) = 1. The probability m t) decreases exponentially with time ... 

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