Molecules with approximately 100 electrons exhibit faster annihilation rates with positrons due to their electrons being quantum mechanically distributed over a larger area. The positron lifetime is notably longer in polymers and insulators, while it is shortest in metals, as detailed in the referenced slide 13 from the Positron Lifetime document. Polymers, primarily composed of CmHn or CmHnOp, and metal oxides, which are rich in oxygen, possess lower electron densities compared to metals from period 4 and beyond. This results in higher linear energy transfer (LET) and increased stopping power, facilitating more efficient annihilation events.
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Polymers are mostly CmHn or CmHnOp and insulators are metal oxides, usually richer in oxygen. C, H and O have relatively low electron density, as opposed to metals, from period 4 and greater, have much greater electron density - which means greater LET (linear energy transfer) or greater stopping power, and more electrons with which to annihilateBob S said:Positron lifetime is highest in polymers and insulators, shortest in metals. See slide 13 in
http://www.positronannihilation.net/Index_files/positron lifetime.pdf
Bob S