https://doi.org/10.1140/epjd/s10053-024-00823-9
Regular Article- Atomic Physics
Ortho-positronium annihilation processes in Y zeolites under different environments by measurements of the energy spectra of the positron annihilation radiation
Department of Applied Chemistry and Biotechnology, Chiba University, 1-33 Yayoi, Inage, 263-8522, Chiba, Japan
Received:
29
September
2023
Accepted:
3
March
2024
Published online:
25
March
2024
Measurements of the energy distribution of the positron annihilation radiation in Y zeolites are carried out to elucidate the various ortho-positronium annihilation processes that take place in these microporous materials and relate them to the ortho-positronium lifetime components observed by positron annihilation lifetime spectroscopy in earlier studies. Y zeolites with various Si/Al ratios in the range from 2.6 to 40 are investigated under a controlled atmosphere of N2 gas and after exposure to the atmosphere. The ortho-positronium annihilation modes are characterized by analyzing the γ-ray spectra through the use of the f3γ index, which represents the fraction of 3γ-annihilation, and the S parameter. The proportion of ortho-positronium self-annihilation is found to be no less than 15% in N2 gas, but drastically decreases by the physisorption of water molecules and possibly positronium spin exchange interaction with oxygen in the atmosphere. The presence of water molecules suppresses ortho-positronium intrinsic annihilation and, at the same time, promotes ortho-positronium pick-off annihilation in the pores. Similarly, the S parameter rises due to an increased contribution from para-positronium annihilation and ortho-positronium pick-off annihilation, which is also consistent with the physisorption of water and positronium spin exchange. In conjunction with the previously measured ortho-positronium lifetime components, the specific ortho-positronium annihilation sites and processes in these porous materials under different environments are determined and a complete picture of the ortho-positronium behavior in Y zeolites is obtained.
© The Author(s) 2024
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