Positronium quenching

These traps, (Fig. 6) and similar effects in the motion of holes and other charges through polymers, would eventually be correlated also with such structural probes as positron lifetimes in macromolecular solids. Extensive recent studies of positron lifetime are based on positronium decay. In this, the lifetime of o-positronium (bound positron-electron pair with total spin one) is reduced from about 140 nanoseconds to a few nanoseconds by "pick-off annihilation" in which some unpaired electron spins in the medium cause conversion quenching of orthopositronium to para-positronium. The speed of the t2 effect is supposed, among other things, to represent by pick-off annihilation the presence of defects in the crystalline lattice. In any case, what amounts to empty space between molecules can then be occupied by orthopositronium.(14,15,16) It is now found in linear polyethylene, by T. T. Wang and his co-workers of Bell Laboratories(17) that there is marked shift in positron lifetimes over the temperature range of 80°K to 300°K. For... 

The experiments were performed at two values of the magnetic field, 0.375 T and 0.425 T, and at various densities of N2 gas with small admixtures of isobutane to quench the free-positron component (see subsection 6.3.2). Al-Ramadhan and Gidley (1994) derived a quantity A(p) from their measured values of A Ps and Ao-ps, for the mixed and unmixed ortho-positronium states respectively, at a gas density p given by... 

Fraser, P.A. and Kraidy, M. (1966). The pick-off quenching of ortho-positronium in helium. Proc. Phys. Soc. 89 533-539. 

Klobuchar, R.L. and Karol, P.J. (1980). Positronium formation and quenching in argon-oxygen mixtures. J. Chem. Phys. 84 483-488. 

The ubiquitous problem encountered in all positronium decay rate measurements to date is isolating the positronium from the formation medium in order to determine the vacuum decay rate. In both gas and powder experiments the interactions (Ps quenching and Ps polarization) with these media need to be accounted for. This can involve extrapolations of A to zero density in the formation medium, as in the Michigan experiment, and/or spectroscopic corrections for 2q decays as in the Tokyo experiment. At Michigan we decided a decade ago to abandon powder media in future precision experiments because we could... 

When inhibition of positronium formation occurs, it is seen as a decrease in the o-Ps intensity, while quenching of o-Ps, as a result of chemical reactions, will shorten the o-Ps lifetime (x3). While quenching and inhibition of positronuim have been extensively studied in solution (see Chapter 5) it is a factor that has often been overlooked in polymer studies. There are numerous examples of inhibition and quenching effects in polymers [42, 80-82]... 

In the case of quenching the reduction (shortening of the o-Ps lifetime) is explained by the chemical reaction of positronium and the quenching species (which in many cases are additives present in the polymer matrix). The chemical rate constant for the reaction between the Ps and quenching species can be expressed in terms of the concentration of the quenching species [M], and it is found that it can be described by pseudo-first order kinetics as [42, 80] ... 

OXYGEN AND NITRIC OXIDE QUENCHING OF POSITRONIUM IN LIQUIDS. 

The processes that reduce the lifetime of positronium are collectively referred to as quenching of the positronium. 

Positron annihilation lifetime spectroscopy (PALS) studies the lifetime spectrum of ortho-positrons after being injected into the sample [3,4]. This lifetime depends on the probability of the ortho-positronium (o-Ps) particle (a hydrogen-like bound state formed by a positron-electron pair) to be quenched and annihilate. This probabihty is higher in condensed matter than in vacuum. Of all the probe methods PALS is nowadays probably the most versatile one and the most widely used. The o-Ps particle is the smallest probe available and can thus detect the smallest free volume elements furthermore, the method furnishes information on the average free volume size and on the FV size distribution. 

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