Electric field effect on positronium formation in liquids

Abstract

The comparison of different models (the Ore, spur and blob models) of positronium (Ps) formation is presented. Because in molecular media Ps is formed in the terminal positron blob and not in an ordinary spur, the application of the blob model seems to be the most adequate. We extend this model for consideration of the Ps formation in the presence of external electric field ($<100$ kV/cm). In the simplified limiting case, this approach provides a formula similar to the Onsager one for the geminate recombination probability. The influence of ion-electron recombination and other intrablob processes on Ps formation is taken into account. The role of quasifree positronium in Ps formation process is discussed

Track Effects in Positronium Formation

“…That is why further diffusion expansion of the terminal blob proceeds rather slowly. It obeys the law of ambipolar diffusion, when the diffusion coefficient of this expansion is equal to the double diffusion coefficient of the positive ions (10 −5 −10 −4 cm 2 /s) [11]. This results in the appearance of the local intrablob electric field E which is directed outside the blob.…”

“…3. Dependence of the ortho-Ps formation probability vs. the external electric field D in some nonpolar media at room temperature [10,11]. Dashed lines show fits according to the "white" blob model, Eq.…”

“…Here we are facing a huge contradiction. It is eliminated by using the e + blob model [11], which in the case of small D gives for P Ps a very similar expression, but with a different physical meaning of the parameters involved…”

Positronium as a probe of small free volumes in crystals, polymers and porous media

“…The theory for a more realistic case of Ps formation in the blob containing many ionization products is given by Stepanov and Byakov [89]. An electric field can reduce the Ps intensity, down to about half of that observed without the field.…”

A Comparison of Different Theoretical Models of Positron Lifetime Spectra for Polymers

“…SLP is based on following assumptions: A positron injected into polymer takes part in fast processes inside the positron spur and then leaves the terminal spur (blob) as a free particle or as a loosely bound pair e + -e − [24]. Because of the spin exchange repulsion between the electron of pair e + -e − and the core electrons, the pair is pushed out, into the holes of free volume of larger and larger size.…”