Positron annihilation spectroscopy in materials structure studies - - PowerPoint PPT Presentation

Positron annihilation spectroscopy in materials structure studies

Participants: Kacper and Przemysław Gajos Gontar Supervisor: Paweł Horodek , Ph. D.

Outline

• Basics of Positron Annihilation Spectroscopy (PAS)
• Slow positron beam
• Idea of measurements
• Results
• Summary

• elastic scattering
• nonelastic scattering
• bremsstrahlung

ANNIHILATION e+e-

e+ + e- → 2 (99.8 %, E ≈ 511 keV)

β+ decay isotopes

POSITRON SOURCES

Two gamma quanta emission (511 keV) from the pair e+e- with momentum p annihilation. POSITRON IS AN ANTIPARTICLE OF THE ELECTRON

THE DEVIATION FROM COLINEARITY CHANGING OF GAMMA QUANTA ENERGY as a result of the Doppler shift INTERACTION

• implantation into medium
• thermalization
• diffusion
• annihilation with electron

INSIDE THE MATTER

Basics of PAS

• solid body physics
• material and surface engineering
• metals, semiconductors, thin

layers

Basics of PAS

The examples of structural defects.

• Doppler broadening of

annihilation gamma line (DBGL)

• positron life times (LT)

EXPERIMENTAL TECHNIQUES

• defect concentration
• defect concentration profile
• detection of the kind and size of

defects POSSIBILITIES APPLICATIONS AIM OF THE EXERCISE

Introduction to PAS. Measurement of S parameter profile for sample of Cu after milling

Slow positron beam

SLOW POSITRON BEAM IS THE FLUX OF MONOENERGETIC PARTICLES WITH ENERGYS BETWEEN A FEW eV AND A FEW DOZENS keV TWO TYPES OF MODERATORS FROZEN NEON TUNGSTEN FOIL SCHEME OF THE POSITRON EMISSION SPECTRUM OF A 22Na SOURCE

moderator intensity energy range diameter of the flux vacuum conditions frozen Ne (7 K) 3 [mm] (Low Energy Particle Toroidal Accumulator)

Slow positron beam

Slow positron beam

Slow positron beam

HpGe detector preamplifier amplifier MC analyzer PC computer gamma ≈ 511 keV The energy resolution of DBGL spectrometer at LEPTA project is 1.2 keV interpolated at 511 keV.

Slow positron beam

energy [keV]

507 509 511 513 515

counts

10000 20000 30000 40000 50000 60000

AW

background non-defected defected

A AS

W

A W A =

S

A S A =

S - PARAMETER W- PARAMETER Comparison of annihilation lines for defected and nondefected samples. The rule of calculation of S- and W- parameters.

Idea of measurements

Energy [keV]

10 20 30 40

S parameter

0.37 0.38 0.39 0.40 0.41 0.42 0.43

L+=76.7(2.7)

W parameter

0.005 0.010 0.015 0.020

S parameter

0.37 0.38 0.39 0.40 0.41 0.42 0.43

Results

AVERAGE DIFFUSION LENGTH DEFECTS CONCENTRATION MEAN IMPLANTATION DEPTH

From 1.96 Å for 50 eV up to 1.084 μm for 35 keV VEPFIT program solves it to fit the model function to experimental data

bulk (*)=117 [ps] - mean positron lifetime

(*) 5 1014 [s-1]-the trapping coefficient for vacancies in pure Cu Lbulk (*)=121 [nm] - the positron diffusion length in the bulk

* J.Dryzek et. al.., Tribol. Lett 11, 29 (2001)

Results

• The measurements were performed correctly,

the typical S – parameter profile was obtained

• The domination of one kind of defect, probably

vacancies was observed

• The mean diffusion length equal about 77 nm, as well

as defects concentration on the level 2.3 ×10-5 are similar to the values obtained by other authors for copper after sliding under load of 1.5 N

Summary

Acknowledgement

All workers from LEPTA facility, especially:

• Prof. Igor Meshkov, Ph.D.

Paweł Horodek, Ph.D. Andrey Kobets, Ph.D.

Organizers:

• Prof. Roman Zawodny, Ph.D.

Władysław Chmielowski, Ph.D. Kinga Horodek

Thank you for attention !