Positron Annihilation Studies of Materials

Abstract

Positron annihilation provides sensitive and versatile probe techniques in materials science that can characterize electronic structure and vacancy‐type, open volume, defects. The techniques utilize the information carried by the photons that result from the quantum relativistic process of the annihilation of a positron with its antiparticle the electron. For the implanted positron, the time to the annihilation event with a host electron depends on the electron density of the local environment probed. Annihilation normally results in the conversion to two anticolinear γ photons, this emitted radiation carries information on the momentum of the electron ‐positron pair at the instant of annihilation. The present article focuses on the standard e + annihilation methods applied to the study of defects in materials. Current developments in e+ annihilation are briefly described.

Light-cured dimethacrylate dental restorative composites under a prism of annihilating positrons

“…4,[6][7][8] This process is characterized by mean lifetime for e + -trapping τ av tr , defect-free bulk lifetime τ b , trapping rate in defects κ d and fraction η of trapped e + , defined in respect to the following equations:…”

“…4 In fact, such a 2-state trapping model describes e + annihilating from 2 distinct states, these being latticedelocalized and defect-localized states, ignoring back escape of trapped positrons. 4,[6][7][8] The Ps decaying forms another channel of the PAL spectrum, which is caused by e + annihilating from the Ps state as free particles or interacting with e − from the environment. [4][5][6] In the ground state, the Ps exists as para-Ps (p-Ps, antiparallel e − -e + spins) decaying intrinsically with 2 -quanta and character lifetime in a vacuum of 0.125 ns, and ortho-Ps (o-Ps, parallel e − -e + spins) decaying with 3 -quanta and lifetime of 142 ns, these states being occupied with a relative formation rate of 1:3.…”

“…However, strict parameterization of this channel in terms of a 2-state trapping model seems rather meaningless under essential input from o-Ps decays. 4,[6][7][8] The more realistic values of defectfree bulk lifetime τ b related to positron annihilation from the Bloch states can be extracted from a simple model assuming additive e + -trapping inputs from trapped e + and decayed o-Ps states (with lifetimes τ 2 and τ 3 given in Table 1). 22 The calculated τ b values (Table 3) occur to be nearly 10% overestimated compared to those presented in Table 2.…”

“…Positron annihilation lifetime (PAL) spectroscopy is known to be an example of such voidsensitive structural methods. [4][5][6][7][8][9] This experimental technique, which probes space-time continuum determination for the interaction between electron e − and its antiparticle (positron e + ), is especially sensitive to tiny atomic-deficient entities (like free-volume defects, vacancies, vacancy-like clusters and their complexes, interfacial voids and pores, intergranular boundaries, etc. ), which are highly deterministic in the functionality of modern biopolymers, using the recent advantages of nanotechnology.…”

“…9,10 But hitherto, the PAL method has been applied mainly to "purely technical" materials, compounds and polymers, semiconductor and metallic alloys, etc. 4,7,8 The attempts to use this technique for composite biopolymers have not been very successful so far because of significant complications in the unambiguous interpretation of the obtained experimental data.…”