Positron is the antipartie le of electron and its existence was predicted “by Dirac’s relativistic theory of electron. Positrons were experimentally discovered “by Anderson in cloud chamber photographs of cosmic rays. In a material medium, positrons do not live long because they can either annihilate with electrons in the medium, or can form a metastahle system of a positronium atom which is short-lived. In the first type of decay, called prompt or free annihilation, a positron, after thermalisation, annihilates with an electron in “the medium either througji a singlet collision (spins antiparallel) or through a triplet collision (spins parallel). Selection rules governing the annihilation show that a singlet collision results in emission of two photons in exactly opposite direction and each one having an energv of about 511 (= m0 c2) KeV. This annihilation has a life time ~10-10 s. On the other hand, a triplet collision gives rise to an annihilation into three photons with a lifetime ~10-7 secs. In this case the total rest energy (= 2 m0 c2) of the electron-positron pair is shared between the three photons which are emitted in one plane. The ratio of singlet (2 photon) to triplet (3-photon) annihilation rates was calculated by Ore and Powell2 to “be

$${\lambda _s}/{\lambda _t} = 1115$$

(1)

Positron Annihilation in Solids

The angular correlation of photons from the two-gamma decay of positrons in some substances (paraffin wax, nylon, teflon, fused quartz, liquid and solid water) has been measured in the presence of an external magnetic field up to 21 kG. The narrow-component intensity of the angular spectrum, due to the rate of positronium (Ps) which self-annihilates from the singlet state, results enhanced by theH field. The effect, produced by the magnetic mixing of two spin states of Ps, is shown to be in agreement with theory. Measurements in powdered air-saturated aluminium oxide reveal a different behaviour of Ps which can be explained by the ortho-Ps quenching property of the O2 molecules absorbed.

Positronium annihilation in magnetic fields up to 21 kG

Positron lifetime spectra have been studied for ThO2. powders, calcined at different temperatures and having different particle sizes. Three lifetime components could be resolved, the longest component being of low intensity. An observed strong dependence on the particle size of the annihilation process and the variation of positronium diffusion constant is explained on the basis of defect density variations in these powders.

Positron lifetime studies on thorium oxide powders

Positron lifetimes in some ionic compounds

Positron annihilation has been studied in some ionic compounds such as hydroxides and sulfates by two-quantum angular measurements. It is concluded that a fraction of the positrons, those not forming positronium, annihilate almost exclusively in the neighbourhood of the negative ion, oxygen, while the remaining fraction forms positronium atoms. This fraction has been derived for each sample and plotted against the percentage of volume which in matter is external to the ionic spheres: a quasi-linear dependence is seen and is deduced by starting from considerations on the « Ore gap ».

B. Fiscella

Papers

Positronium in ionic compounds.