STUDY OF X-RAY EMISSION AND ANALYTICAL APPLICATIONS

Sunil Kumar

Department of Physicsj b i iPanjab University

Chandigarh-160014

Overview of presentation

(i) Ion-atom interactions

Introduction

(ii) Photon-atom interactions

Experimental Set-ups

Res lts and Disc ssionResults and Discussion

Summary

INTRODUCTION

NMR Electromagnetic Spectrum

X di ti di d b

10 um - 10 mm

700 to 104 nm

X-ray radiation was discovered by Roentgen in 1895.

Continuous X-rays are generated by400 to 700 nm

10 to 400 nm

Continuous X-rays are generated by bombarding electrons on an metallic anode due to de-acceleration of electrons.

10-1 to 10 nm

10-4 to 10 -1 nm

e ec o s.

Characteristic X-rays are produced due the inner-shell vacancy in an atom.

The inner-shell vacancy in an atom can be produced through direct ionisation using h t h d ti l l t t di ti d i t l iphotons, charged particles, electron-capture radioactive decay, internal-conversion

decay and during the atomic de-excitation. Whenever the inner shell vacancy is produced, the outer shell electrons can fill the i h ll th h th di ti di ti d d th binner shell vacancy through the radiative or non radiative decay process and thereby shifting the inner shell vacancy to the outer shell.

Radiative process: Results in emission of characteristic X-rays.All the transitions from one shell to another do not result in the emission of characteristic x-ray, since there are certain selection rules that must be satisfied for radiative decay mode. The selection rules are

Δl = ±1Δj =0, ±1

Non Radiative process : Emission of Electrons.pIn this process the difference in the energy of two shells is used to eject another outer shell electron. e.g. Auger process, Coster Kronig and Super Coster Kronigprocess.p

L3 L3

n l j2 1 3/2

Schematic of various inner shell processes :

Projectile ion

L1

L2

L3

x-ray photon

L1

L2

L3 2 1 3/22 1 1/21 0 1/2

Δ n ≥ 1,

Δ l =

Δ j= 0, ± 1

± 1

K K(a) (b)

X-ray emissionVacancy production

M1

M2

M3

M4

M5

M1

M2

M3

M4

M5

M1

M2

M3

M4

M5

L1

L3

1

L2

L1

L3

L2

L1

L3

L2

Vacancy

1

K

1

K

1

K

(c) (d) (e)

Vacancy

Auger process Coster Kronig Super Coster KronigVacancy

Energy level diagram showing the K- and L- shell X-ray transitionsEnergy level diagram showing the K and L shell X ray transitions

Ion-atom Interaction

•Ionization Excitation and Electron Capture

• Elastic encounter or Inelastic encounter (Direct Coulomb Interaction (DCI)and Electron Capture (EC) by the projectile ion)•Ionization, Excitation and Electron Capture

Major problem in ion-atom interaction is multiple ionization ofthe target atom which reduces the screening of the nucleus whichin turn increases the binding energy of inner shell electrons Asin turn increases the binding energy of inner shell electrons. Asthe number of electron for the excitation from higher shellsdecreases so this will increase the fluorescence yield anddecrease Coster Kronig transitions which involves two electronsdecrease Coster-Kronig transitions which involves two electrons.

It has been concluded that multiple ionization affects the innershell ionization process. Therefore, there is a need for morestudies with different projectile- target combination in this areafor better understanding of atomic collision processes.

The understanding of these ion-atom interaction processes leads to thedevelopment of Particle-induced X-ray emission (PIXE) techniques which isused in the determination of the elemental make-up of a material or sample. InPIXE, a material is exposed to an ion beam, X-rays specific to an element may be

d d f ll i h i i fproduced following the excitation of target atoms.

Theoretical models

Binary Encounter Approximation (BEA)

Semi Classical approximation (SCA) theory

Plane Wave Born Approximation (PWBA)

Energy loss, Coulomb deflection, Perturbed Stationary state,

Relativistic effects (ECPSSR).

Photon-atom interaction processes in x-ray energy region:Photon-atom interaction processes in x-ray energy region:

Photons interact with atoms, viz.,

(i) Photo Electric effect

(ii) Compton scattering

The photon-atom interaction processes are well established for the incident

photon energy away from the ionization threshold but the agreement between

theory and experiment deviates significantly, close to ionization threshold.

RRS and XAFS processes contribute significantly just below and above the

ionization threshold, respectively.

These processes are also sensitive to chemical environments.

The understanding of these photon-atom interaction processes leads to thedevelopment of X-Ray Fluorescence (XRF) technique, where x-rays from asealed tube are used to produce x-rays by secondary fluorescence in samples ofinterest. XRF has been a bulk analytical tool in trace element analysis.

Application of XRF and PIXE to various analytical problems:Application of XRF and PIXE to various analytical problems:

Quantitative and Qualitative analysis of major, minor and trace element and amongQ Q y j g

them commercially interesting metals in a inhomogeneous structure of environmental,

Chemical, biological, Forensic and geological samples.

Usefulness of XRF & PIXE is that the elemental analysis helps:

T d t i th f th bj t i th f t i l d• To determine the provenance of the objects i.e. the source of raw material, and

• To derive information regarding the technology used in the manufacturing,

thus providing a wider idea about political social details of that particular erathus providing a wider idea about political, social details of that particular era.

EXPERIMENTAL SET UPEXPERIMENTAL SET-UP

Experimental arrangements for the study of ;

1. X-ray production cross-section measurements of high-Z elemental

targets at Inter University Accelerator Centre (IUAC), New Delhi.

2. Trace element analysis of soil samples using PIXE technique at Institute of

Physics (IOP), Bhubaneswar.

3. Contribution of near edge processes to the attenuation coefficients and

analytical applications of EDXRF Facility at Physics Department, PU,

Chandigarh.

Experimental Arrangement at Inter University Accelerator Centre(IUAC), New DelhiCentre(IUAC), New Delhi

15 UD Pelletron

Present experiment was performed Atomic Physics Beam Line at BH-II

Atomic Physics Scattering Chamber layout at IUAC, New Delhi.

Ion BeamIon Beam

450550

HPGe detectorSi(Li) detector

X-ray photon

7.507.50

Target

SSB detectorSSB detector SSB detector

Faraday Cage

Block Diagram of Electronics used in IUAC Experiment:

All the targets were prepared by using High vacuum and Ulta high vacuumAll the targets were prepared by using High vacuum and Ulta high vacuumevaporation set up in the target laboratory of IUAC.

Fluorine ion beam with +6, +7, +8 charge states was obtained from the 15-MVFluorine ion beam with 6, 7, 8 charge states was obtained from the 15 MVpelletron at IUAC, New Delhi, where the data were acquired using PC-basedCANDLE software.

The low energy Si(Li) detector (CANBERRA) was used as X-ray detector withresolution 180 eV at 5.9 keV.

The current at the target was kept below 5 nA in order to avoid the targetdamage due to charge build up.

Th t t i di t d f b t 15 60 i t d h ll t dThe targets were irradiated for about 15-60 minutes and charge was collectedwith charge integrator connected to Faraday cup.

2. Experimental Arrangement at Institute of Physics (IOP), Bhubaneswar

The PIXE experiments were carried out using 3 UD pelletron at IOP, which is h i l d l i lhorizontal tandem electrostatic accelerator.

A multipurpose scattering chamber with 20 inch diameter used for PIXE studies.

Beam size at the target position was 2 mm diameter.

The target was positioned at 450 with respect to the beam directionThe target was positioned at 45 with respect to the beam direction.

Characteristic X-rays detection by Si(Li) detector at 900 to the beam line.

aabc

ef

d

c

gg

a) HpGe X-ray detectorb) Stepper Motor for the rotation of target holder wheel) pp gc) PIXE Chamber, d) Faraday Cup, e) CCD Cameraf) Controls for Turbo Molecular Pumpg) Position for Gamma ray detector or Si(Li) horizontal detector (70 mm endg) Position for Gamma ray detector or Si(Li) horizontal detector (70 mm end

cap)

EDXRF Set Up at Physics Department:

EDXRF facility available at Physics Department, Panjab University, Chandigarh.

Geometrical diagram of Energy-dispersive detection set-upsDirect mode Secondary excitation mode

Low energy measurements were performed in vacuum

Data AnalysisData Analysis

•CANDLE software was used for the evaluation of the Peak areas of different L IUAC N D lhiLx-ray components at IUAC, New Delhi.

•GUPIX code was chosen for qualitative as well as quantitative analysis in PIXE experimentsPIXE experiments.

•GUPIX determines the intensities of characteristic X-ray peak in PIXE spectrum by fitting a model spectrum Mj to the measured spectrum Yjspectrum by fitting a model spectrum Mj to the measured spectrum Yjusing the non-linear least squares technique.

•Genie2K was used in EDXRF laboratory for the peak area evaluation.y p

Results and discussion

L shell X-ray production cross-sections measured for thin (48-120 mg/cm2) solid targetsof 78Pt, 79Au, 82Pb, 83Bi, 90Th and 92U for 76-114 MeV F+6,7,8 ions.

Th d ti ti t t d f diff t L X t iThe x-ray production cross section was extracted for different L X-ray components usingdetector efficiency, number of incident particles and target thickness.

The measured XRP cross sections are compared with the calculated ones obtained usingp gFBA and ECUSAR theories.

Multiple ionization was included from the method suggested by Lapicki et al. [1986] tocorrect the fluorescence yield and Coster Kronig yieldscorrect the fluorescence yield and Coster-Kronig yields.

The multiple ionization reduced the CK yields, and enhanced the L3 subshellfluorescence yields are more than the L1 and L2 subshell fluorescence yields.

The measured values are found to obey the ECUSAR theory within 10-15%, and are 1.2to 4 times lower than the predictions of the first-order Born approximation.

Effect of RRS in EDXRF spectrometryEffect of RRS in EDXRF spectrometry

The RRS process can lead to significant alteration in the attenuationcoefficients from the theoretical values which are generally used for thecoefficients from the theoretical values, which are generally used for thesample matrix corrections.

The Radiative RRS peaks can also interfere with normal x-ray spectrum dueto limited resolution of the energy-dispersive detection set up.

The RRS cross sections depend upon the energy difference between the sthshell/subshell ionization threshold and the incident x ray, Ds (= Ein-Bs), andenergy widths (Γ ) of the shell/subshellenergy widths (Γs) of the shell/subshell.

Dotted lines correspond to measurement error ±2.5%. The symbols enclosed indotted circle and square represent the measured values at near-edge X-rayenergies. The measured values at these energies in general exhibit largedeviations from the XCOM values, which are attributed to the expected XAFSand RRS contributions.

Applications of XRF and PIXE Techniques:

The trace elements present in air, soil and water pose a great risk to the life of the workers and the population which is exposed to various toxic l i h i h h di ielements present in the environment; hence there studies are important

from health point of view

We have studied different fungi species for the heavy metal removal using EDXRF facility.

We have studied soil samples collected from the Ludhiana city and nearby regions for the heavy metal contamination due to industrial waste.

The cross section results are significantly lower for the FBA theory

Summary

after including the Multiple ionization effect. Further improvements inthe existing direct ionization theories are needed.

A detailed knowledge of the cross sections would enhance the accuracyA detailed knowledge of the cross sections would enhance the accuracyof the ultra-trace-analysis based on the x-ray spectroscopic techniques.

The results for radiative RRS can be extended to the metallic andcompound forms of attenuator elements by measuring the attenuationcoefficients.

Work to explore the near edge processes in various elemental targetsWork to explore the near edge processes in various elemental targetsand their compound forms will be interesting .