Positronium emission from MgO smoke nanocrystals

L Gurung,B S Cooper,D B Cassidy,T J Babij,A L Shluger,A M Alonso

doi:10.1088/1361-6455/ab0f06

Abstract

We report experiments in which positronium (Ps) atoms were created in a thick layer of MgO smoke powder deposited on a thin silicon nitride substrate. The experimental arrangement was such that a positron beam could be implanted directly into the top of the MgO layer or be transmitted through the substrate, allowing Ps to be produced within ≈100 nm or 30 μm of the powder-vacuum interface. The transverse kinetic energy of Ps atoms emitted into vacuum was measured via the Doppler broadening of 13SPJ transitions, and found to be Ex ≈ 350 meV, regardless of how far Ps atoms had traveled through the powder layer. Our data are not consistent with the model in which energetic Ps atoms emitted into the internal free volume of a porous material are cooled via multiple surface collisions, and instead indicate that in nanocrystals lower energy Ps is generated, with negligible subsequent cooling in the large open volumes of the powder. Our experiments also demonstrate that SiN substrates coated with MgO smoke can provide a simple and inexpensive method for producing Ps transmission targets.

Highlights

In 1968 Paulin and Ambrosino discovered that long-lived Positronium (Ps) [1] atoms are produced in fine granulated powders of SiO2, Al2O3, and MgO following irradiation with energetic positrons [2]
We report experiments in which positronium (Ps) atoms were created in a thick layer of MgO smoke powder deposited on a thin silicon nitride substrate
Our experiments demonstrate that SiN substrates coated with MgO smoke can provide a simple and inexpensive method for producing Ps transmission targets

Summary

Introduction

In 1968 Paulin and Ambrosino discovered that long-lived Positronium (Ps) [1] atoms are produced in fine granulated powders of SiO2, Al2O3, and MgO following irradiation with energetic positrons [2]. Using positron annihilation lifetime spectroscopy (PALS) they measured a long-lived Ps lifetime component that was comparable to the 142 ns vacuum lifetime [3], and whose intensity was dependent on the specific surface area of the sample. These observations were explained by Brandt and Paulin [4], who suggested that slow positrons. Recent studies have indicated that in SiO2 both surface and bulk Ps formation occurs [6], whereas in Al2O3 [7] and MgO [8] Ps formation is most likely generated only via surface processes

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Conclusion