Positronium annihilation in LTA-type zeolite

• The annihilation of o-Ps in polycrystalline zeolite LTA pellets, subject to different pressures, was studied.
• The annihilation sites of the long-lived o-Ps in the zeolite LTA were discerned.
• The mean lifetime τ124 qualitatively describes the decrease of IPV as pellet pressure increases.

Dehydrated polycrystalline powder of LTA-type zeolite and its pellets, prepared from 7.86 × 10−2 to 1.5728 GPa, were studied by XRD, SEM, BET and PALS. Crystal size, texture and changes in the inter-particle pellet volume were observed from XRD and SEM results, respectively, as the pellet pressure increased. Positron lifetime spectra of these zeolite samples consisted of four lifetime components (τi, Ii; i = 1, 2, 3, 4), remaining practically constant each lifetime (τi) as the pellet pressure was varied, but their intensities were pressure dependent. The decreasing tendency of the inter-particle pellet volume can be followed by the mean lifetime τ124 + τ1 * I1 + τ2 * I2 + τ4 * I4, where the third lifetime component was practically pellet pressure independent. The third and fourth lifetime components (τ3, I3; τ4, I4) can be assigned to o-Ps annihilating in the dehydrated β and α cages of the zeolite, respectively, as inferred from the correlation between the calculated specific internal surface area of the zeolite and that obtained from PALS data, registered either form the 50 or 800 ns instrumental windows.

(A) The Tao–Eldrup model for the pick-off annihilation of o-Ps, trapped in spherical voids; (B) crystal structure unit cell of the LTA-type zeolite, where the emptied α and β cavities have an approximate spherical shape [19]. In this annihilation process a shell electron picks-up the positron from the o-Ps and annihilates with it. The original electron from the o-Ps is transferred to the electron shell.Figure optionsDownload as PowerPoint slide