positron annihilation spectroscopy

**PAS** (Positron Annihilation Spectroscopy) is a **non-destructive technique that probes open-volume defects (vacancies, voids, pores) by measuring the lifetime or energy of gamma rays from positron-electron annihilation** — positrons are trapped by open-volume sites, and their annihilation characteristics reveal defect type and concentration. **How Does PAS Work?** - **Positron Source**: $^{22}$Na source or slow positron beam (variable energy for depth profiling). - **Lifetime**: Positron lifetime is longer in larger voids (more time before annihilation). Bulk Si: ~220 ps. Vacancy: ~270 ps. - **Doppler Broadening**: Momentum of annihilating electron pair -> chemical environment information. - **Positronium**: In pores, positrons form positronium (Ps) with lifetimes proportional to pore size. **Why It Matters** - **Vacancy Detection**: The most sensitive technique for detecting vacancy-type defects (below SIMS detection limits). - **Low-k Porosity**: PALS (Positron Annihilation Lifetime Spectroscopy) maps pore size distribution in porous dielectrics. - **Non-Destructive**: Positron beam measurements are completely non-destructive. **PAS** is **defect detection with anti-electrons** — using positrons as probes that seek out and reveal open-volume defects invisible to other techniques.