Observation of positronium slowing down

In positronium forming materials without optical phonons much longer Ps slowing-down times should be found. This may be tested by AMOC measurements in solid rare gases, crystallizing in the face centered cubic (fee) structure which, being a Bravais lattice, does not have optical phonon branches. 

The observation of delayed Ps slowing-down in all positronium forming liquids and solids investigated so far shows unambiguously that for positronium formation no special sites are required [28,29], 

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