Positronium Confined in Nanocavities: The Role of Electron Exchange Correlations

Round 1

Reviewer 1 Report

In the paper submitted by F. Castelli and collaborators and entitled "Positronium confined in nanocavities: the role of electron exchange correlations", a theory of the positronium (Ps) annihilation in nanocavities, being developed during the earlier period by the same authors' team, is briefly reviewed and extended towards obtaining a simple relation connecting the ortho- and para-Ps annihilation rates with the electron density. The relation appears to be totally independent of the geometrical characteristics of the nanoporous medium. Within the theory, the role of the exchange correlations between the Ps electron and the outer electrons is rigourously included, which has not been usually considered so far. Using the experimental data on positron lifetime spectra (PLS), moreover, the relation can be employed to deduce information on the electron density and on the average cavity radius of the confining medium. The theory was examined on a set of PLS data and it was emphasized that a reasonable interpretation of the data could be obtained only if the para- and ortho-Ps intensities follow the ratio 1:3 and the interpretation of the tau_1 lifetime as the para-Ps lifetime is valid.
The manuscript is well-written and the results constitute a further step forward in the field. It is thus worth publishing in the MDPI Nanomaterials Journal in its present form. 

Reviewer 2 Report

Positron annihilation spectrosocpy is essential tool for study of porous materials, soft matter, various polymers, etc. Such success is mainly benefited from the quantitative description of the relationship between ortho-positronium pick-off annihilation lifetime and the size of micro/meso pores. The present work developed a theory based on exchange correlations between the Ps-electron and the outer electrons, which is not seriously considered before by positron community. With the present theory, electron density could be obtained, besides the average cavity radius of the confining medium. The present manuscript is already well written both scientifically and technically. I believe it is qualified to publish in Nanomaterials in its present form.