Ortho-positronium particles

Positron annihilation lifetime spectroscopy (PALS) provides a method for studying changes in free volume and defect concentration in polymers and other materials [1,2]. A positron can either annihilate as a free positron with an electron in the material or capture an electron from the material and form a bound state, called a positronium atom. Pnra-positroniums (p-Ps), in which the spins of the positron and the electron are anti-parallel, have a mean lifetime of 0.125 ns. Ortho-positroniums (o-Ps), in which the spins of the two particles are parallel, have a mean lifteime of 142 ns in vacuum. In polymers find other condensed matter, the lifetime of o-Ps is shortened to 1-5 ns because of pick-off of the positron by electrons of antiparallel spin in the surrounding medium. 

Despite all simplifications the model of particle in the rectangular potential well, extended to include the population of excited le els. describes quite well the dependence of ortho-positronium lifetime on the pore radius. In this model the o-Ps lifetime is ruled entirely by geometrical factors, however, maybe the chemical composition of the medium should be taken into account. The lifetime vs. average radius dependence is particularly steep below 5 nm. and in this range the positron annihilation method can be useful for determination of average pore radii. The specific surface determines the distribution of o-Ps between small voids in the bulk and pores. 

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. 

Another important interaction of positronium is the ortho-para conversion. It occurs when the substance contains paramagnetic particles with unpaired electrons. When colliding with such a particle, the orientation of one of the parallel spins of ortho-positronium may be reversed, simultaneously with the reversion of the spin of the unpaired electron of the colliding molecule. This interaction takes place via electron exchange between the molecule and o-Ps. The pflra-positronium formed by this process annihilates very rapidly, according to its short mean lifetime. Consequently, this effect also leads to the decrease of the lifetime of positronium. Ortho-para conversion can be demonstrated by the following reaction scheme (vertical arrows show the directions of the spins) ... 

Thus, the lowest gluonic intermediate state which has the right quantum numbers to couple to a 1 particle has three gluons, just as ortho-positronium decays into at least three real photons. The J/ decay is now visualized as proceeding via the quark-gluon diagreuns of Fig. 11.6, where quarks are denoted by continuous lines and gluons by helices. 

The mean lifetime of the free para-Ps in vacuo is Ts = 1.25 x 10 s, and that of ortho-Ps is 1.4 x 10" s. These lifetimes appear to be very short, but if they are compared with the shortest time intervals experimentally measurable, or, for example, with the period of intramolecular vibration of atoms (10 s), it can be seen that they are sufficiently long for the positronium atom to take part in chemical reactions or to enter into other interactions with the particles of the medium, and to allow the following of these processes in time. This is particularly the case as regards ort/jo-positronium. 

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