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Positronium orthopositronium

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... [Pg.174]

A small fraction of the orthopositronium atoms produced pass through the cw-excitation beam, where they are promoted to the 23Si level and then through a multi-pass doubled-YAG beam at 532 nm, where they are photo-ionized. The photo-ionized positron is electro-statically accelerated and magnetically-guided into a channel-electron multiplier array (CEMA) where it is detected. The time-of-Hight between the incident positron pulse and the photo-ionization pulse determines the range of positronium velocities detected. [Pg.116]

A different type of precisely measured quantities are the decay properties of the positronium atoms, e.g. total decay widths of both ortho (S = 1) and para (S = 0) positronium ground states into three and two photons, respectively. The orthopositronium decay width is a controversial issue because the theoretical prediction differs significantly from the most precise experimental result [8]. The... [Pg.387]

It was found by the American physicist Martin Deutsch that there are two kinds of positronium. The kind in which the spin of the positron is antiparallel to that of the electron is called parapositronium, and that in which the two spins are parallel is called orthopositronium. Parapositronium decomposes with destruction of the positron and the electron and production of two photons, its half-life being 0.9 x I0 s. Orthopositronium decomposes with production of three photons, and half-life 1.0 x 10" s. The existence of positronium was detected by the observation of a delay between its production (by decomposition of sodium 22, which emits positrons) and its annihilation. The time of delay was found to correspond to the sum of two first-order reactions, with the values of the half-life given above. [Pg.696]

Positron Annihilation Spectroscopy (PALS) can investigate the free volume existing between polymer chains. The lifetime of particles (positrons) injected into a sample can thus provide information on the void structure existing in polymers and polymer blends. For immiscible polymers, free volume existing at the interface due to poor adhesion can be detected by PALS. In miscible polymers, densification due to favorable interactions may be capable of determination. This technique involves the injection of positrons into a polymeric system from a radioisotope capable of emitting positrons, such as Na. The positrons (positively charged electrons) combine with electrons to annihilate or to form a bound state called a positronium (Ps). If the spins of the positron and electron are antiparaUel, para-positroniums (pPS) with a lifetime of 0.125 ns are formed. If the spins of the positron and electrons are parallel, an orthopositronium (oPs) is formed with a lifetime of 1-5 ns. The oPs hfetime, Ts, is related to the free volume cavity in which the oPs is formed [388,389]. [Pg.310]


See other pages where Positronium orthopositronium is mentioned: [Pg.269]    [Pg.28]    [Pg.105]    [Pg.377]    [Pg.360]    [Pg.105]    [Pg.377]    [Pg.98]    [Pg.308]    [Pg.653]   
See also in sourсe #XX -- [ Pg.153 , Pg.154 , Pg.159 , Pg.169 ]




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Orthopositronium

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