PC4250 – ADVANCED ANALYTICAL TECHNIQUES

Part I – Ion Beam Analysis using High Energy Beams

1. PIXE: A Novel Technique for Elemental AnalysisSven A. E. Johansson and John L. CampbellPublisher: John Wiley & Sons, 1988

2. Materials Analysis using a Nuclear Microprobe M B H Breese, D N Jamieson and P J C KingPublisher: John Wiley & Sons, 1996

3. Handbook of Modern Ion Beam Materials AnalysisEdited by Joseph R. Tesmer and Michael NastasiPublisher: Materials Research Society, Pittsburgh, Pa., 1995

4. Handbook of X-Ray SpectrometryEdited by Rene E. Van Grieken and Andrzej A. Markowicz Publisher: Marcel Dekker, Inc., 2nd Edition, 2002

REFERENCES

Contents1. Introduction to ion-solid interactions2. Ion beam analysis techniques: a summary3. Ion sources4. Experimental setups5. Nuclear microprobes6. Si(Li) detectors (PIXE)7. Surface barrier detectors (RBS, ERDA)

1. INTRODUCTION

TO ION-SOLID

INTERACTIONS

In its passage through matter, an ion may interact with

• THE ATOMIC ELECTRONS

and/or

• THE ATOMIC NUCLEI

The interaction of an ion with an atomic electron is purely Coulomb (i.e. interaction governed by the Coulomb’s law).

• IONIZATION – the electron is ejected from its atomic orbit

Such interaction will result in:

or

• ATOMIC EXCITATION – the electron is raised to an outer orbit

An ionized/excited atom will eventually return to its ground state, accompanied by the emission of one or more x-rays/photons.

• An electron ejected from its atomic orbit is called a secondary electron. It may further ionize or excite another atom, resulting in the emission of more x-rays/photons.

• An secondary electron may also be decelerated by the coulomb field of a nucleus, losing part or all its energy in form of bremsstrahlung (braking radiation).

SECONDARY ELECTRONS & BREMSSTRAHLUNG

The interaction of an ion with an atomic nucleus can be

• COULOMB ELASTIC SCATTERING

• COULOMB INELASTIC COLLISION

• COULOMB EXCITATION

• NUCLEAR INELASTIC SCATTERING

• NUCLEAR TRANSFORMATION

EFFECTS OF ION-NUCLEUS INTERACTIONS:

INTERACTION

FORCEPROCESS EFFECT ON PARTICLE

EFFECT ON

NUCLEUS

Coulomb Elastic scatteringChange of direction, no reduction of energy No effect

Coulomb Inelastic collisionDecelerated, losing part or all of its energy in the form of bremssstrahlung

No effect

CoulombCoulomb excitation

Change of direction and reduction of energy

Excited

Nuclear Inelastic scatteringChange of direction and reduction of energy Excited

Nuclear Nuclear reaction Transmuted or absorbed Transmuted

RADIATIONS EMITTED IN ION-NUCLEUS INTERACTIONS

• An excited nucleus will eventually return to its ground state accompanied by the emission of one or more rays.

• For ions with incident energy of a few MeV, radiations emitted from nuclear reactions are usually p, n, and/or .

Stopping and Ranges of Ions in Matter (SRIM)

• This is a very useful program which is used in many research and technology areas, from ion energies of eV to GeV, in semiconductor manufacturing (why ?), ion beam analysis, nuclear physics, high-energy physics, etc

• It can be used to find the range, electronic and nuclear energy losses of any ion in any material.

• Also used to study recoil events when heavy ions are incident on a light material.

Download from SRIM website http://www.srim.org/

Or from course website here

2. ION BEAM ANALYSIS

TECHNIQUES:

A SUMMARY

TECHNIQUE ACRONYM PARTICLE/RADIATION MEASURED

Particle-Induced X-ray Emission PIXE Characteristic x-rays

Rutherford Backscattering Spectrometry

RBS Elastically scattered ions in backward angles

Elastic Recoil Detection Analysis ERDA Recoiled target nuclei

Nuclear Reaction Analysis NRA Prompt product particles

or gamma-rays (PIGE)

Often two or more of these techniques are carried out simultaneously in order to obtained complementary information.

ION BEAM ANALYSIS TECHNIQUES:

TECHNIQUE ION BEAMENERGY

(MeV)REMARK

PIXE H+ 1 - 4Maximum sensitivity in atomic ranges 10<Z<35 and 75<Z<85

RBS 4He+, H+ 2Non-Rutherford scattering becomes significant for energy >2 MeV

ERDA35Cl+, 20Ne+

3He+, 4He+ 2 - 40

Mass of incident ion must be greater than that of target nucleus. 3He+ and 4He+ are used only for the measurement of H.

NRA H+, D+ 0.4 - 3Reactions used include (p,) (p,p'), (p,), (d,p), (d,p)

TYPICAL ION BEAMS AND INCIDENT ENERGIES USED IN VARIOUSIBA TECHNQUES:

  

High brehmsstrahlung

Comparison between X-ray spectra using EPMA and PIXE

(a) 10 keV electrons

(b) 3 MeV protons

X-ray energy

X-ray energy

Example : RBS spectrum of hard-disk

Layer structure: Protective polymeric material (~200A)Co-Pt-Fe alloy (~200A)Cr (~10A)Co-Pt.Fe alloy (~200A)Cr (~1000A)Ni3(PO4)2 (~100,000A)Al substrate

Composite spectrum

0

1000

2000

3000

4000

5000

100 200 300 400 500 600

Channel

Cou

nt

C O

P

N iCr

Co

Fe

Pt

Elastic Recoil Detection Analysis (ERDA)

• An important application of ERDA is the analysis of hydrogen using 3He or 4He.

• The kinematics of elastic collisions allows the recoil to occur only in the forward hemisphere.

• For ERDA, the mass of the incident particle must be greater than that of the target nucleus.

Nuclear Reaction Analysis (NRA)• Nuclear reaction analysis is based on the detection of the prompt

-rays or prompt particles emitted as a result of the nuclear interaction between the incident particles and the target nuclei.

• The cross sections of nuclear reactions vary rather irregularly. When using light ion beams of only a few MeV, nuclear reaction cross sections are high enough for analysis of only low- and medium-Z elements.

• The most popular application of PIGE is the determination of F in biomedical sample through the reaction 19F(p,p')19F.

• PIGE is often used in conjunction with PIXE for analyzing light elements such as Li, Na, Mg and Al in aerosol and geological samples.

• Deuterons are more commonly used than protons when prompt particles are measured in NRA. Useful reactions for determinations of C and N include 12C(d,p)13C and 14N(d,p)15N.

• The 16O(d,p)17O* reaction has also been used in conjunction with DIXE (Deuteron-Induced X-ray Emission) for stoichiometric analysis of Y-Ba-Cu-O superconductors.

OTHER POSSIBLE EFFECTS & CHANNELING

• Ions incident upon a target may break chemical bonds and produce light, UV radiation or sputter atoms from the target surface.

• For a crystalline target, the incident ions may even channel through the ordered rows of atoms.

Reprinted from “Channeling in Crystals” by W. Brandt, Scientific American

The channelling process

3. ION SOURCES

ION SOURCES

There are a number of methods for ion generation, but the use of radio-frequency power to produce ions from neutral gas in a low-pressure discharge bottle is by far the most popular way.

RF ION SOURCE – THEORY OF OPERATION:

• Neutral gas is bled into the discharge tube from the pressurized gas bottle through the palladium leak. Palladium is porous to low-Z gases and the porosity is a function of temperature. Hence the pressure in the discharge tube can be controlled by adjusting the output of the heating coil power supply.

• Free electrons in the discharge tube are excited into oscillation in the RF electric field and quickly acquire enough kinetic energy to cause ionizations, hence producing +ve ions.

• The ions are pushed by the positively biased electrostatic probe to the tube exit at the opposite end and are drawn into the acceleration tube by the extraction electrode.

Compressedgas

RF oscillator

Palladium leak & heating coil

Heating coil power supply

Extraction electrode (V-)

Discharge tube IONS

Electrostatic probe (V+)

Coupling clips

A RADIO-FREQUENCY ION SOURCE:

4. EXPERIMENTAL

SETUPS

Target

VACUUM CHAMBER

Si (Li)x-ray detector

(for PIXE)

Ge(Li)-ray detector

(for PIGE)

Annular particledetector

(for ERDA)

Annular particleDetector

(for RBS)

Faraday cup

COLLIMATORS

Ion beam

CONVENTIONAL IBA EXPERIMENTAL SETUP:

ELECTRONIC COMPONENTS FOR SIGNAL PROCESSING:

X-RAYDETECTOR

ANALOG-DIGITALCONVERTER

ANALOG-DIGITALCONVERTER

PARTICLEDETECTOR

ANALOG-DIGITALCONVERTER

COMPUTER

ERDA

PIXE

RBS

PRE-AMPLIFIER

DETECTOR BIAS

PRE-AMPLIFIER

DETECTOR BIAS

PRE-AMPLIFIER

CHARGEDIGITIZER

FARADAYCUP

MAINAMPLIFIER

MAINAMPLIFIER

MAINAMPLIFIER

PARTICLEDETECTOR

DETECTOR BIAS

5. NUCLEAR

MICROPROBES

MICROPROBE IBA:

• Using an ion beam with a micron or sub-micron spot size for elemental analysis adds a new dimension to IBA analytical power – i.e. elemental imaging (measuring the elemental distributions of the various elements in specimens).

• Microprobe IBA has applications in a large variety of disciplines, including bio-medicine, earth sciences, metallurgy, solid state physics, electronics, archaeology and aerosol study, etc.

• Although most of the applications of microprobes are analytical, it is now being used for many non-analytical works, such as micro-machining of polymers and semiconductors.

Distance travelled in silicon (m) 1000

Trajectories of 3MeV protons and 3MeV 4He in siliconra

dia

l be

am s

pre

ad (m

)

0

8Very little radial beam spread for MeV ions

• Microprobes using MeV ion beams are difficult to focus because of the high ion mass• However, once the beam is focused, it is this same property which prevents beam “blow-up”, unlike focused keV electrons in a SEM• Microprobes are very good at analysing “thick” layers with high resolution

protons

Helium ions

Unlike keV electrons !

Ions Bi = (MiEi) (singly charged)

Electrons Be = (MeEe)

Proton mass is 1836 greater than electron mass,So a 2 MeV proton requires a magnetic field strengthof 430 times430 times that needed to focus a 20 keV electron !

MeV ions have a high B compared with keV electrons

Very difficult to focus using magnetic solenoid lenses, souse a quadrupole lens focusing system.

Magnetic rigidity BThis is a measure of how difficult charged particles are to bend.

It depends on the particle mass M, energy E and charge Q

B = (ME) Q

Quadrupole Lenses

Lorentz Force: F = q v Bi.e. quadrupole lens gives a focusing force because v and B are perpendicular

COMPONENTS OF A NUCLEAR MICROPROBE FACILITY:

Additional components needed for a nuclear microprobe facility includes:

• A set of object slits to define the geometrical image component of the final spot size.

• A beam focusing device such as magnetic quadrupole lenses.

• A scanning system to raster the beam over the specimen.

• MeV ion beam from Van de Graaff accelerator is focusedonto the sample (target).• Focused beam spot size is 0.05 m to 1 m, depending on the amount of beam current used.• Focused beam is scanned over sample surface and the (x,y) position and relevant detector signal is measured.

Nuclear Microprobe Layout

sampleBeam divergence

Schematic of microprobe chamber

Beam

Viewing microscope

Focusingmicroscope

Trans-missiondetector

sample

X-ray detector (PIXE)

Backscatter detector

(RBS)

Microprobe Chamber

Proton accelerator Switcher magnet

Object aperture

Collimator apertureFocusing system

quadrupole lensesScan coils

Sample chamber

Scan Controller

CIBA Nuclear Microprobe

CIBA. View of 3 beamline facilities: Proton Beam Micromachining (10), Nuclear microprobe (30) and Ion Channeling facility (45)

High-Resolution microprobe beamline

World’s best spatial resolution of 35 nm

Nuclear microprobe PIXE elemental maps from 400 m x 400 m scan over a section of a lung tissue taken from a patient suffered from hard metal lung disease:

SP Ca

Fe WTi

6. Si(Li) DETECTORS

Si(Li) DETECTOR - STRUCTURE

The Si(Li) detector is basically a semiconductor diode fabricated using high-purity p-type silicon cylindrical wafer doped with lithium on one side. The electrode contacts of the diode are formed with thin metal (normally Au) films evaporated on opposing surfaces of the silicon wafer

When the diode is reversed biased, a carrier-free charge depletion region is created and the only current that flows between the electrodes is due to thermally generated carriers.

Au contact

Au contact

Depletion region(Active region)

Li-diffused region

Detector bias (-)

Si dead layerHigh-purityp-type Si

Si(Li) DETECTOR – PRINCIPLE OF X-RAY DETECTION

• In traversing the charge depletion region, an X-ray may interact with a Si atom through photoelectric absorption, spending part of its energy in knocking out an electron from the inner shell of a Si atom and transferring the rest of its energy to the photoelectron (i.e. the electron ejected from the inner shell of a Si atom). It may also scattered by an electron, dissipating only part of its energy in the charge depletion region.

• The dissipation of energy by an X-ray in the charge depletion region of the Si(Li) detector will result in the production of free electron-hole pairs which are swiftly collected by the electrodes as a current pulse. The number of electron-hole pairs produced is proportional to the energy dissipated by the incident X-ray in the charge depletion region, Hence, the amplitudes of a current pulse generated by the photoelectric absorption is proportional to the energy of the incident X-ray.

Si(Li) DETECTOR – SIGNAL PROCESSING

• The current pulse generated by the Si(Li) detector must be processed electronically to such an amplitude and a shape suitable for analog to digital conversion. The pulse processing is done in two stages using two types of amplifiers. The first stage is charge integration which is carried out by using a pre-amplifier. The second is a combination of voltage amplification and pulse shaping which is done with a spectroscopy amplifier.

• It is necessary to operate the Si(Li) at liquid nitrogen temperature (77 K) so the diode is usually mounted on one end of a cryostat finger and is placed inside an aluminum vacuum enclosure. The other end of the cryostat finger is immersed in liquid nitrogen and the vacuum enclosure has a thin Be window for the X-rays to pass through.

• Several components of the preamplifier, a field effect transistor (FET) and the feedback elements, are also mounted on the cryostat finger within the vacuum enclosure to reduce the thermal noise.

Si crystal

Al vacuum enclosureFET & feed back elements

Be window

Cold finger

Vacuum seal

Wire feed through

To detector bias

To pre-amplifier

Si(Li) DETECTOR ASSEMBLY

Pulse shaper

Detector bias

Main amplifier

Pre-amplifier

Vacuum enclosure of cryostat

Si(Li) diode

FET

Si(Li) DETECTOR – SIGNAL PROCESSING ELECTRONICS

• X-rays of the same energy may not produce the same number of electron-hole pairs, and the electron-hole pair production is governed the Poisson statistics and hence its standard deviation is equal to the square root of the average number of electron-holes produced.

• The shape of an X-ray line is near Gaussian and its full width at half maximum (FWHM) is a function of two independent factors: the electronic noise of the detection system and the statistical fluctuation of the electron-hole production:

(FWHM)2 = (Enoise)2 + (Epair )2

and (Epair )2 = (2.35)2 EF

where = average energy for producing a electron-hole pair, E = Energy of the incident X-ray,

F = the so-called Fano factor introduced to correct the departure of the electron-hole production from

Poisson statistics due to other competitive processes, 2.35 = the constant that converts the electron-hole production standard deviation to FWHM.

Si(Li) DETECTOR – ENERGY RESOLUTION

Si(Li) DETECTOR – ENERGY RESOLUTION (cont.)

• Si(Li) detectors are operated at liquid nitrogen temperature (77 K). At this temperature, the average energy for producing a electron-hole pair is 3.76 eV and the Fano factor F is ~0.12. Typical state-of-the-art Si(Li) detectors offers an energy resolution (FWHM) of ~175 eV at 5.9 keV.

• In practice, the shape of the energy peak produced by a Si(Li) departs from Gaussian, and this is due partly to incomplete charge collection by the electrodes and partly to the dissipation of the X-ray energy in the active region of the diode through processes other than the photoelectric effect.

The detector efficiency of the Si(Li) is a function of the energy of

the incident X-ray and also of the following:

• absorption of the Be window

• absorption of the gold contact

• absorption of the Si dead layer

• thickness of the Be window

• thickness of the gold contact

• thickness of the Si dead layer

• photoelectric mass absorption of Si

• thickness of the active region of the Si diode

Si(Li) DETECTOR – DETECTION EFFICIENCY

Si(Li) DETECTOR – EFFECTS OF WINDOW & CRYSTAL THICKNESSES ON ITS EFFICIENCY

The low energy cutoff is determined by absorption of the Be window and the high-energy limit is established by the photoelectric cross section and the thickness of the Si crystal.

Energy (keV)

Det

ectio

n ef

ficie

ncy

(%)

PILE-UP

• The electron-hole pairs or charges produced in a detector require a finite time to be collected by the electrodes and the voltage pulse generated must be shaped by an amplifier to produce a signal that is suitable for analog-to-digital conversion. Charge collection typically takes 0.025 – 0.1 sec and the pulse shaping usually requires 1 – 10 sec.

• When two X-rays strike at the detector at almost the same time, they are recorded as a single event and produce a voltage pulse with an amplitude corresponding to an energy somewhere between that of the X-ray arrived earlier and the sum of the energies of the two X-rays. This is called pile-up and the probability of its occurrence increases with increasing X-ray count rate and depends on the charge collection time as well as the pulse shaping time.

SUM PEAKS

When two X-rays arrive at a detector at the same time or with a time interval equal or less than the charge collection time, the signal generated will have an amplitude corresponding to the sum of the energies of the two X-rays and give rise a sum peak in the X-ray spectrum.

EFFECTS OF PILE-UP & SUM PEAKS

• Pile-up generally affects the accuracy and sensitivity of PIXE.

• Sum peaks affect the determination of the concentrations of those elements whose characteristic X-rays happened to be in the same energy regions of the sum peaks.

• Sum signals cannot be recognized electronically, but their effects can be corrected for in spectrum processing.

• The only way to control sum peak effects is by keeping the count rate low.

PILE-UP REJECTION/SUPPRESSION

• It is obvious that pile-up can be controlled by keeping the X-ray count rate low. This might also means that a longer counting time would be required for desirable count statistics.

• The occurrence of two X-rays arriving at the detector with a time interval about 0.1 sec or longer can be detected electronically by means of a pileup rejector which is a standard built-in component in most spectroscopy amplifiers availably commercially. When this happens, the pileup rejector issues a logic pulse which can be used to inhibit the analog-to-digital converter.

PIXE SPECTRA OF Cu WITH & WITHOUT PILE-UP SUPPRESSIONBY MEANS OF AN ON DEMAND BEAM EXCITATION SYSTEM

0

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100000

Energy

Co

un

ts

k k

pile-up1000

100

Without suppression

0

10

10000

100000

Energy

Co

un

ts

k k

sum peaks1000

100

With suppression

k+ k

k+ k

k+ k

ESCAPE PEAKS

• If the photoelectric interaction between an incident X-ray and a Si atom occurs close to an edge of the Si crystal, the Si K X-ray emitted may escape from the Si crystal without being absorbed. In such a case, the amplitude of the signal produced will correspond to an energy equal to the difference between the energy of the incident X-ray and that of the Si K X-ray (1.74 keV). Hence, a very intense X-ray line usually give rise a small but observable peak (called escape peak) at 1.74 keV below its full-energy peak.

• The probability for the Si K X-ray to escape from the Si detector diode is in the range of ~ 3% to ~ 0.1% for X-rays with energies between 1 to 10 keV.

PIXE SPECTRA OF COBALT

0

40000

80000

120000

Energy

Co

un

ts

k

k

Kk

sum

k

Kescape

Kescape

Kksum

Kk

sum

k

Escape peaks are low intensity

7. SURFACE BARRIER

DETECTORS

The surface-barrier detector is a charged-particle detector fabricated using high-purity n-type silicon wafer. One side of the wafer is chemically etched and a p layer is allowed to form by spontaneous oxidation. Contact to this layer is made by the evaporation of a thin gold layer. When a bias voltage is applied in the reverse direction, a high-resistance depletion (or active) region is formed in the p-n junction. Electron-hole pairs produced by a charged-particle in this region give rise to an output signal with an amplitude proportional the the kinetic energy of the incident charged-particle.

SURFACE-BARRIER DETECTOR – STRUCTURE

Au contact

Metal electrode

n-type silicon

Detector bias (+)

Depletion region

SURFACE-BARRIER DETECTOR – DEPLETION DEPTH & ENERGY RESOLUTION

• To produce an output signal with amplitude corresponding to the full energy of the incident charged particle, the detector must have a depletion region thick enough to stop the particle completely. The depletion depth of a surface-barrier detector increases with the bias voltage, but saturates gradually beyond a certain bias value (~ 50 – 100 Volts). A depletion depth of 30 m is adequate to stop protons of energy up to 1.5 MeV and particles up to energies of 6 MeV.

• The energy resolution or FWHM of the surface-barrier detector is a function of the energy of the incident charged particle and is also affected by the noise from the signal amplification electronics and the detector. The FWHM of the present-day surface-barrier detector is typically in the range of 15 keV for particles of energy around 5 MeV.