Positron lifetime picosecond timing system

Positron Annihilation Lifetime Spectroscopy (PALS)
The existence of positrons was first proposed by Dirac and confirmed experimentally in the 1930s. The positron is the antiparticle of the electron. The collision of a positron with an electron will cause the two particles to annihilate and emit two characteristic 511-keV gamma rays.

This phenomenon helps test quantum theory's predictions about the differences between electrons and positrons interacting with matter. Additionally, positrons have proven to be useful tools for studying a variety of structures and processes. The lifetime of a positron can be used to measure the local electron density at the annihilation point. Annihilation can be easily detected with the help of emitted gamma rays. The positron lifetime technique is one of the few methods sensitive to single-atom-scale holes.

Since the early 1970s, ORTEC has been providing modular instrumentation to elite-level positron physicists and chemists, as any academic paper on positrons can attest. The availability of positron sources has stimulated interest in positron lifetime systems in teaching or research laboratories.

ORTEC simplifies the testing process by offering complete PLS-System models.

Advantages

• Fully tested and integrated "turnkey" system... all you need is a positron source.
• A complete set of labeled cables and connectors.
• Two Type 905-21 detector assemblies.
• Documentation of testing procedures and results.

PLS-System includes

• 2 model 905-21 detector assemblies.
• 2 Type 583B constant score discriminators.
• 2 x 556 high voltage power supplies.
• 1 type 414A quick fit.
• 1 Model 567 time-to-amplitude converter/single channel analyzer.
• 1 multi-channel analyzer type 928-MCB with MAESTRO software.
• 1 DB463 type delay box.
• 1 4001A/4002D type NIM chassis and power supply.
• 1 type 113 preamplifier.
• 1 type 575A spectral amplifier.
• 1 PC.
• 1 set of labeled cables and connectors.
• 1 procedure and factory test documentation.

The system has a guaranteed time resolution of 200 picoseconds (typically measuring less than 180 picoseconds) using a Co-60 source for measurements in a narrow energy window. (This system does not include sources.)

PALS: To the nanoscale.
One of the most important structural issues in soft condensed matter is the presence of unoccupied or free volumes between molecules due to irregular packing, density fluctuations, and topological constraints. Free volume is considered to be the fraction of volume capable of molecular rearrangement and is important in determining the physical and mechanical properties of a system.

PALS is a mature, versatile technology that can directly measure these sub-nanometer molecular free volumes. The PALS experiment injects a positron into the material being tested and then measures the length of time before it annihilates with one of the material's gamma-ray-producing electrons.

When a positron enters a molecular material, it thermalizes, after which it can diffuse as a free particle in the material or capture the electron to form positron atoms (Ps). Ps is an electron-positron bound state, which has two spin states: paraelectron even prime (p-Ps, a singlet state with zero spin angular momentum) and positron prime (o-Ps, a triplet state with unit spin angular momentum). In PALS experiments, we are mainly interested in the o-Ps state because its lifetime depends on the environment. In molecular materials, o-Ps, once formed, localize in the molecular free volume and remain unchanged throughout their lifetime. Because the o-Ps collides with the "wall" of the hole in which it is located, the o-Ps' positron can annihilate with the electron of the opposite spin. Therefore, the o-Ps lifetime provides information about the average free pore volume size in the material. In addition, PALS can probe the size of the molecular free volume over a wide temperature range, allowing the identification of important phase transitions occurring in molecular materials.

Acknowledgments: Department of Physics, University of Bristol, UK
Dr M Roussenova.