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... 

The disordered polyethylene shows a broader distribution of orthopositronium annihilation irregularities than a more crystalline low molecular weight sample. Above 230°C, which is the Tg claimed for polyethylene,(19,20) positron annihilation effects are uniform for all specimens of varying perfection. 

Adkins, G.S., Salahuddin, A.A. and Schalm, K.E. (1992). Order-a corrections to the decay rate of orthopositronium in the Fried-Yennie gauge. Phys. 

Asai, S., Orito, O. and Shinohara, N. (1995). New measurement of the orthopositronium decay rate. Phys. Lett. B 357 475-480. 

Asai, S., Orito, S., Sanuki, T., Yasuda, M. and Yokoi, T. (1991). Direct search for orthopositronium decay into two photons. Phys. Rev. Lett. 66 1298-1301. 

Asai, S., Shigekuni, K., Sanuki, T. and Orito, S. (1994). Search for short-lived neutral bosons in orthopositronium decay. Phys. Lett. B 323 90-94. 

Gidley, D.W. and Zitzewitz, P.W. (1978). Measurement of the vacuum decay rate of orthopositronium formed in an MgO-lined cavity. Phys. Lett. A 69 97-99. 

McNutt, J.D. and Sharma, S.C. (1978). Dependence of orthopositronium annihilation rates on density fluctuations in methane gas. J. Chem. Phys. 68 130-133. 

Orito, S., Yoshimura, K., Haga, T., Minowa, M. and Tsuchiaki, M. (1989). New limits on exotic two-body decay of orthopositronium. Phys. Rev. Lett. 63 597-600. 

Sharma, S.C., Kafle, S.R. and Hart, J.S. (1984). New features in the behaviour of orthopositronium annihilation rates near the vapour-liquid critical point of ethane. Phys. Rev. Lett. 52 2233-2236. 

Westbrook, C.I., Gidley, D.W., Conti, R.S. and Rich, A. (1987). New precision measurement of the orthopositronium decay rate a discrepancy with theory. Phys. Rev. Lett. 58 1328-1331. 

Four-photon decay of orthopositronium a test of charge-conjugation invariance. Phys. Rev. A 54 1952-1956. 

Fig. 17. Intensity (I2) of the longlived thermalized orthopositronium versus surfactant concentration in various micellar solutions of DAP and AOT, 20 °C. 1J. Am. Chem. Soc. 100, 984 (1978)]...

Applications of the technique to heterogeneous catalysts have been few, but they have demonstrated that the method is useful for catalyst characterization. For example, the lifetime of the orthopositronium species is inversely proportional to the number of Bronsted acid sites present in alumina-silica cracking catalysts. This interpretation was derived from a correlation between the activity for the alkylation of cumene and the lifetime of the orthopositronium species. 

The triplet state of Ps, orthopositronium, decays with rate At into an odd number of photons since an even number is forbidden by charge conjugation. Momentum conservation forbids decay into a single photon thus the minimum allowable number of photons is three. The decay of o-Ps into five photons [21] can be ignored at the level of current experiments. The three photon decay rate, A3, is calculated using perturbation theory. The long-awaited order a2 radiative corrections have been calculated very recently [2] and the decay rate is determined to be2 7.039934 0.00001 /xs"1. 

Fig. 5. Gamma-ray spectrum for vacuum orthopositronium measurement (from ref. [26])...

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. 

The techniques used in the three measurements of the 23S —23Pj, J = 0,1,2 intervals are summarized in Figure 8. In all of these experiments the initial state is the 23S i state formed from positrons striking a metal target with about 100 eV kinetic energy. The first two measurements [15] [16] detected the transition as a 243 nm Lyman-a photon in delayed coincidence with a detected 7 ray from the annihilation of orthopositronium. The most recent and most precise experiment [17], which we detail below, uses only the Lyman-a detection. 

A further improvement can be made to this experiment starting with thermal orthopositronium formed on porous SiC>2 films [31] with pulsed Doppler-free, two-photon excitation to the 23,S i state. More of the initial state for the micro-wave transitions would thus be available and time-of-fiight velocity systematics can also be done. 

Abstract. Order a2 corrections to the decay rate of orthopositronium axe calculated in the framework of nonrelativistic QED. The correction is 45 in units of a./it)2 times the lowest order rate. 

Table 1. Numerical values of contributions to the orthopositronium decay rate...

Table 2. Recent experimental results for the orthopositronium decay rate. The quantity A is the difference between the experimental and theoretical values for the rate in terms of the experimental uncertainty a...

Now we proceed to the one- and two-loop calculations in QED and NRQED. The graphs contributing the orthopositronium decay at one-loop order in QED are shown in Fig. 3. The one-loop graphs contribute [10]... 

Fig. 3. One-loop QED graphs contributing to the orthopositronium decay rate. They are (a) the self-energy graph, (b) the outer vertex graph, (c) the inner vertex graph, (d) the double vertex graph, (e) the ladder graph, and (f) the annihilation graph...

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