High-energy Photonics: Positronium photons characterize gummy-bear nanopores

Gummy bears and other forms of gelatin are used in the pharmaceutical industry to encapsulate biologically active substances such as medication and vitamins, protecting against oxidation and overly quick release. Nanopores in gelatin have a significant influence on the material’s properties, yet are difficult to study. Now, using gummy bears, researchers at Technische Universität München (TUM; Germany) have determined relevant gelatin properties using positron-annihilation-lifetime spectroscopy (PALS).¹

In PALS, a beam of positrons is directed at the gummy bear. If a positron encounters an electron, the two particles briefly form an exotic atom-like object called positronium, then annihilate, producing two photons, each with a 0.511 MeV energy (in positronium decay, three photons of lower energy are very occasionally produced instead).

Using sodium (²²Na) as a positron source, the scientists bombarded red gummy bears at various stages of wetness with positrons to model gelatin capsules dissolving in the stomach. Results of photon measurements showed that positronium lasts 1.2 and 1.9 ns (avg.) in dry and saturated bears, respectively. From these lifetimes, the scientists then deduced the number and size of nanopores in the bear material. The nanopore size was inferred by applying a semi-empirical quantum-mechanical model in which the the radial Schrödinger equation was solved for a spherical cavity containing a single unit of positronium. Saturation with water, which increases a gummy bear’s volume by 2.5 times, also leads to formation of voids with a size of 84.3 Å (avg.). As-purchased bears had 51.6 Å pore sizes (avg.); and dry (dehydrated under heat and partial vacuum for 3 days) and superdry (packed in silica-gel desiccant and left for 2 years) gummy bears had pore sizes of 47.1 and 33.5 Å (avg.), respectively. The experiments were redone many times to ensure repeatability.

“The larger the free volume [of the nanopores], the easier it is for oxygen to penetrate it and harm the medication, but also the less brittle the gelatin,” says Christoph Hugenschmidt, a physicist at TUM. The positronium nanopore-determining technique can also be applied to other substances such as polymers and inorganic glasses.

REFERENCE
1. C. Hugenschmidt and H. Ceeh, J. Physical Chemistry B (2014); doi: 10.1021/jp504504p.