Variable Energy Positron Beam System (VEPOS)

VEPOS at MARPOS enables high-precision, depth-resolved investigation of defects and free-volume structures in materials. Unlike conventional bulk PALS measurements, which average information over the entire sample thickness, VEPOS allows positrons to be implanted at controlled depths ranging from a few nanometers to several micrometers. This capability makes it a powerful tool for analyzing thin films, multilayer structures, coatings, interfaces, and irradiated materials.

           

How VEPOS Works

In the VEPOS system, positrons emitted from a radioactive source are moderated via neon crystal to low energies and then re-accelerated to selectable implantation energies (typically 0.1–30 keV). By tuning the positron energy, the implantation depth can be precisely controlled:

Low energies (0.1–1 keV): top surface and near-surface region (1–10 nm)

Medium energies (1–10 keV): thin films, coatings, and interfaces

High energies (10–30 keV): micrometer-scale depth profiling

Once implanted, positrons diffuse through the material and annihilate at defect sites. The annihilation signatures—measured through Doppler Broadening Spectroscopy (DBS) or Positron Annihilation Lifetime Spectroscopy (PALS)—provide detailed information on the type, concentration, and depth distribution of vacancy-type defects.

What VEPOS Can Reveal

The VEPOS system offers unparalleled access to:

Surface and subsurface defects created during polishing, etching, or thin-film deposition

Radiation-induced damage from alpha, beta, gamma, proton, or neutron exposure

Diffusion profiles of dopants, impurities, or vacancies

Depth-dependent phase transformations in multilayer or graded materials

Interface quality in semiconductor heterostructures, oxide coatings, and perovskite films

These measurements are essential for fields such as microelectronics, battery research, thin-film technology, nanomaterials, and nuclear materials engineering.

Applications at MARPOS

At MARPOS, VEPOS is employed in several ongoing research directions, including:

Surface defect profiling of lanthanum-based perovskite thin films

Depth-resolved characterization of irradiated polymers, metals, and ceramic coatings

Investigation of diffusion and structural relaxation in complex oxide films

Quality assessment of nano-layered and composite coatings

Why VEPOS is Unique

VEPOS complements the other positron techniques at MARPOS by offering non-destructive, nanometer-scale depth resolution, making it one of the most sensitive tools for detecting open-volume defects near surfaces and interfaces—regions that are often inaccessible to other characterization methods such as SEM, TEM, or XRD.