2018 HM PIXE PARTICLE -INDUCED X-RAY EMISSION AN ANALYTICAL TECHNIQUE

PIXE –
AN ANALYTICAL TECHNIQUE
DR. HARSH MOHAN
DEPARTMENT OF PHYSICS
M.L.N. COLLEGE
YAMUNA NAGAR
(HARYANA)

ContentsContents
 INTRODUCTIONINTRODUCTION
 BASIC PRINCIPLEBASIC PRINCIPLE
 BRIEF HISTORYBRIEF HISTORY
 X-RAY PRODUCTION PROCESSX-RAY PRODUCTION PROCESS
 INSTRUMENTATIONINSTRUMENTATION
 ANALYTICAL ANALYSISANALYTICAL ANALYSIS
 APPLICATIONAPPLICATION
 NEW AVENUESNEW AVENUES

IntroductionIntroduction
PIXEPIXE ⇒⇒ PARTICLE-INDUCEDPARTICLE-INDUCED
X-RAY EMISSIONX-RAY EMISSION
 CHARGED PARTICLECHARGED PARTICLE
 USUALLY PROTONS OR OTHERUSUALLY PROTONS OR OTHER
IONSIONS

Basic PrincipleBasic Principle

Brief historyBrief history
 As early asAs early as 1912, Chadwick1912, Chadwick usedused αα particlesparticles
from a radioactive source to induce X-rayfrom a radioactive source to induce X-ray
emission, but with such a low intensity that itemission, but with such a low intensity that it
was not possible to study any details ofwas not possible to study any details of
emission process or to use it for analyticalemission process or to use it for analytical
purposes.purposes.
 The foundation laid inThe foundation laid in 1914 by Moseley1914 by Moseley in hisin his
pioneering study of energy of thepioneering study of energy of the
characteristic X-ray lines of differentcharacteristic X-ray lines of different
elements of periodic table. He used a flatelements of periodic table. He used a flat
crystal spectrometer with photographiccrystal spectrometer with photographic
recording.recording.

Photographic plate of X-ray spectraPhotographic plate of X-ray spectra
recorded by Moseley in 1914recorded by Moseley in 1914

 InIn 1922 Swedish geologist HADDING1922 Swedish geologist HADDING
reported on the analysis of various minerals.reported on the analysis of various minerals.
He detected 10-12 elements in the samplesHe detected 10-12 elements in the samples
and made a comparison with the result ofand made a comparison with the result of
analysis performed with conventionalanalysis performed with conventional
chemical methods.chemical methods.
 A great step forward in X-ray emissionA great step forward in X-ray emission
spectrometry was made whenspectrometry was made when CASTAING atCASTAING at
the University of Paris in 1950the University of Paris in 1950 showed that X-showed that X-
rays emitted by the specimen could berays emitted by the specimen could be
exploited for multi-elemental analysis.exploited for multi-elemental analysis.

 InIn 19501950, when accelerators become, when accelerators become
available in connection with the rapidavailable in connection with the rapid
growth of nuclear physics researchgrowth of nuclear physics research
considerable interest in investigatingconsiderable interest in investigating
the X-ray emission process had beenthe X-ray emission process had been
started with their help.started with their help.

 During theDuring the 1960’s1960’s, a great progress was, a great progress was
made in nuclear detector technologymade in nuclear detector technology Solid-Solid-
State Surface Barrier detectorState Surface Barrier detector werewere
developedfor charged particles. For energydevelopedfor charged particles. For energy
dispersive spectrometry of X-rays thedispersive spectrometry of X-rays the
lithium-drifted silicon detector,lithium-drifted silicon detector, Si (Li)Si (Li)
become available. It has an energy resolutionbecome available. It has an energy resolution
of aboutof about 150 eV150 eV measured formeasured for 5.9 keV5.9 keV X-ray,X-ray,
which make it possible to resolve K X-rayswhich make it possible to resolve K X-rays
from adjacent elements.from adjacent elements.

 InIn 1970, Johansson1970, Johansson et al.et al. at Lundat Lund
Institute of Technology showed that aInstitute of Technology showed that a
combination of excitation withcombination of excitation with 2 MeV2 MeV
proton and X-ray detection withproton and X-ray detection with Si (Li)Si (Li)
detector constituted a powerful methoddetector constituted a powerful method
for multi-elemental, non-destructivefor multi-elemental, non-destructive
analysis of trace elements.analysis of trace elements.

 InIn 1980’s1980’s this new analytical methodthis new analytical method
which soon become known under thewhich soon become known under the
acronymacronym PIXEPIXE was tested and appliedwas tested and applied
in many nuclear physics labs. Therein many nuclear physics labs. There
are several reasons for its rapid growthare several reasons for its rapid growth
and development inand development in 90’s onward90’s onward..

 The growing interest inThe growing interest in environmentalenvironmental
problemsproblems created a need for efficient methodcreated a need for efficient method
of trace elemental analysis. Air pollutionof trace elemental analysis. Air pollution
studies and the determination of toxicstudies and the determination of toxic
elements in environment and humans areelements in environment and humans are
analysed by PIXE.analysed by PIXE.
 Small AcceleratorsSmall Accelerators become available in manybecome available in many
nuclear physics labs.nuclear physics labs.
 A third reason for the interest in PIXE wasA third reason for the interest in PIXE was
the development ofthe development of microbeammicrobeam Technique.Technique.
Such a beam has many interestingSuch a beam has many interesting
applications.applications.

 It is probably fair to say that PIXE hasIt is probably fair to say that PIXE has
now reached a stage of maturity. Anow reached a stage of maturity. A
good picture of the development ofgood picture of the development of
PIXE can be obtained fromPIXE can be obtained from
International PIXEInternational PIXE conferencesconferences startingstarting
from 1977, which are arranged everyfrom 1977, which are arranged every
third year. The proceedings of thesethird year. The proceedings of these
conferences illustrate the increasingconferences illustrate the increasing
use of PIXE in various scientific fields.use of PIXE in various scientific fields.

X-ray production processX-ray production process

X-ray production processX-ray production process
 RADIATIVE TRANSITIONRADIATIVE TRANSITION

Characteristic X-rayCharacteristic X-ray
• K-seriesK-series
• L-seriesL-series
• M-seriesM-series
 NON RADIATIVE TRANSITIONNON RADIATIVE TRANSITION

Aüger effectAüger effect

Coster-Krönig transitionCoster-Krönig transition

Notations for X-ray spectrumNotations for X-ray spectrum

Energy of K and L X-rays as a function ofEnergy of K and L X-rays as a function of
atomic numberatomic number

InstrumentationInstrumentation
 Beam preparationBeam preparation
 CollimatorsCollimators
 On demand deflection plates.On demand deflection plates.
 Beam current measurementsBeam current measurements
 Visual inspection of the beam atVisual inspection of the beam at
various points using quarts viewersvarious points using quarts viewers
whose fluorescent can be seen.whose fluorescent can be seen.
 External beam PIXE.External beam PIXE.

Experimental setupExperimental setup

External beam PIXE setupExternal beam PIXE setup

InstrumentationInstrumentation
 Choice of spectrometerChoice of spectrometer
 Si (Li) & HPGeSi (Li) & HPGe
 HPGe has advantages of maintainingHPGe has advantages of maintaining
effectively approximately 100%effectively approximately 100%
efficiency well beyond 25 keV photonefficiency well beyond 25 keV photon
energy at which this quantity begins toenergy at which this quantity begins to
fall off in Si (Li) detector.fall off in Si (Li) detector.

Si (Li) detector efficiencySi (Li) detector efficiency

Typical PIXE spectrumTypical PIXE spectrum

Analytical Analysis
Qualitative Analysis Quantitative Analysis
Energy Calibration
Comparison
Absolute
Method
Comparison
Method
with standard

PIXE BASICS
• For an incident proton energy of 1 – 4 MeV,For an incident proton energy of 1 – 4 MeV,
elements with atomic numbers up to about 50elements with atomic numbers up to about 50
are generally determined through theirare generally determined through their KK X-raysX-rays
(typically(typically KKαα line). Heavier elements areline). Heavier elements are
measured through theirmeasured through their LL X-rays because theX-rays because the
energies of theirenergies of their KK X-rays are too high to beX-rays are too high to be
detected by using the Si(Li) detector availabledetected by using the Si(Li) detector available
commercially.commercially.
• The concentration of an element is deduced fromThe concentration of an element is deduced from
the intensity of the measured X-ray line togetherthe intensity of the measured X-ray line together
with parameters obtained either theoreticallywith parameters obtained either theoretically
and/or experimentally.and/or experimentally.

Quantitative analysisQuantitative analysis
 Absolute methodAbsolute method

Requirement of different parameters.Requirement of different parameters.

Generalized geometry.Generalized geometry.

Generalized geometry of PIXE analysis ofGeneralized geometry of PIXE analysis of
specimenspecimen

Where
Ω = solid angle subtended by the detector at the target,
Np = number of incident protons that hit the specimen,
nz = number of sample atoms per unit area of the specimen Z
Z= is the atomic number of the element,
συ = production cross-section for υ - line x-rays,
ευ = detection efficiency for υ - line x-rays.
The X-ray yieldThe X-ray yield
The number of counts under the X-ray peak
corresponding to the principal characteristic X-
ray line of an element is called the yield (Yυ ) for
the υ - line. It is a product of 5 quantities:
Yυ = • Np• nz• συ• ευ
Ω
4π

Areal concentration of an analysed
elementThe concentrations of the analysed elements are usuallyThe concentrations of the analysed elements are usually
what one wants to obtain from a PIXE measurement. Forwhat one wants to obtain from a PIXE measurement. For
thin specimens, the areal concentration (i.e. mass/unitthin specimens, the areal concentration (i.e. mass/unit
area) is often the quantity of interestarea) is often the quantity of interest .
where Awhere Azz = atomic mass of the sample element,= atomic mass of the sample element,
NNoo = Avogadro’s number (6.02252 x 10= Avogadro’s number (6.02252 x 102323
).).
mz =
Az • nz
No
This quantity (denoted by mThis quantity (denoted by mzz) is related to n) is related to nzz (the(the
number of element atoms per unit area) by thenumber of element atoms per unit area) by the
equation:equation:

The knowledge on the number of protons thatThe knowledge on the number of protons that
hit the specimen in a PIXE measurement ishit the specimen in a PIXE measurement is
required for quantitative PIXE analysis. Nrequired for quantitative PIXE analysis. Npp cancan
be measured directly or indirectly.be measured directly or indirectly.
As protons are positively charged, NAs protons are positively charged, Npp is oftenis often
measured by charge integration and quoted inmeasured by charge integration and quoted in
units of micro-Coulombs (orunits of micro-Coulombs (or μμC). The chargeC). The charge
carried by a proton is 1.60210 x 10carried by a proton is 1.60210 x 10-19-19
Coulombs.Coulombs.
The charge carried by NThe charge carried by Npp protons is therefore:protons is therefore:
DETERMINATION OF Np
Qp = 1.60210 x 10-13
x Np μC

Direct Np determination methods
These methods involve charge integration carriedThese methods involve charge integration carried
out in various ways.out in various ways.
• Use of a Faraday cup coupled to a chargeUse of a Faraday cup coupled to a charge
integrator.integrator.
Charge
integrator
Faraday cupProton beam
Specimen
This is only suitable when the specimen is not
thick enough to stop the proton beam. For thick
and conducting specimen, one may couple the
charge integrator directly to the specimen holder.

• Use of rotating vane or chopper which periodically
intercepts and samples the proton beam.
Direct Np determination methods

Indirect NIndirect Npp determination methodsdetermination methods

X-ray production cross-section συυ
συ is a product of three factors:
συ = σS • ωS • rυ
where
σS = S-shell ionization cross section (S = K, L, M, …),
ωS = fluorescence yield,
rυ = fractional radiative width of υ-line X-rays.

The ionization cross-sectionThe ionization cross-section σσSS
σS is a probability factor and has the dimensions of area. The
product nz• σs represents the probability for a proton to knock
off a s-shell electron from a target atom.
1 cm
1 cm
Area = σs

σS is a function of incident proton energy and can be
determined theoretically. Three different theoretical
approaches have been used to calculate the cross
sections for inner-shell vacancy creation:
• the binary encounter approximation (BEA),
• the semi-classical approximation (SCA),
• the plane wave Born approximation (PWBA).
Theoretical cross sections for K-shell ionization with
protons calculated with PWBA are found to be in good
agreement experimental data and are adequate for
most PIXE work. For the case of L-shell ionization, the
situation is much less favorable.
Determination of σS

Following the ejection of an electron from an inner
atomic shell through the ionization process, the
vacancy in this particular atomic shell is quickly
filled by an electron from an outer shell. Such an
electron transition will lead to the emission of
either a characteristic X-ray or an Auger electron.
The probability of the X-ray emission alternative is
known as the fluorescence yield.
Although the fluorescence yield can be calculated
theoretically, experimental data are usually used in
PIXE analysis.
The fluorescence yieldThe fluorescence yield

The K-shell fluorescence yield
The K shell of an atom is a single shell and has no
sub-shell. Therefore K-shell fluorescence yield of an
atom is simply:
Ik
Nk
ωk =
where Ik is the total number of K x-rays emitted from a
sample and Nk is the number of K shell vacancies.

The definition of fluorescence yield for the L shell (or any higher
shell) is more complicated because of the Coster Kronig transition
(non-radiative transition between sub-shells). The L shell has 3
sub-shells (LI, LII and LIII), and Coster Kronig transitions can occur
between LI and LII, between LI and LIII, or between LII and LIII. A
Coster Kronig transition will result in shifting a vacancy from a
lower-energy sub-shell to a higher-energy sub-shell with in the
shell
The L-shell fluorescence yield

The fractional radiative width rυ
Electron transitions in the atomic de-excitation are
governed by the following “selection rules”:
∆ n ≥ 1
∆ l = ±1
∆ j = ±1 or 0
Where n, l and j are the principle, the orbital angular
momentum and the total angular momentum quantum
numbers respectively. Although “forbidden transitions” are
observed but their probabilities are usually very small and
not of any significant. The electron that fills up the vacancy
in a particular inner shell may come from one of the many
outer shells allowed by the selection rules, but with
different probabilities which are often referred to as
fractional radiative widths.

K X-ray lines
The K X-ray lines come from electron transitions that finish at the
K shell. The partial energy level diagram below shows the origin
of the main K X-ray lines: By convention, an X-ray line is
denoted by a leading capital letter
indicating the final shell involved in
the transition followed by a small
Greek letter and a numeric subscript
to reflect the intensity of the line
relative to the others (for example
Kα1). α1 is the strongest line, α2 the
next and followed by β1 etc. In x-ray
spectra obtained using Si(Li)
detectors, the α sub-lines and the β
sub-lines usually appear as two
lines because the energies of the
sub-lines are too close to be
resolved. Hence, these two lines
are often referred to simply as Kα
line and Kβ line.
N
M
L
K
VII
|
I
V
|
I
III
II
I
α1 α2 β1
β2β3

The L X-ray lines come from electron transitions that finishThe L X-ray lines come from electron transitions that finish
on the L shell. The partial energy level diagram belowon the L shell. The partial energy level diagram below
shows the origin of the principle L X-ray lines:shows the origin of the principle L X-ray lines:
N
M
L
α1 α2
β1
β2 β3
γ1
β4
ι
L X-ray lines

The detection efficiency of a Si(Li) X-rayThe detection efficiency of a Si(Li) X-ray
detector is dependent on the X-raydetector is dependent on the X-ray
energy. It is usually determinedenergy. It is usually determined
theoretically using the parameters (i.e.theoretically using the parameters (i.e.
thicknesses of Si diode, Be window, goldthicknesses of Si diode, Be window, gold
contact and Si dead layer) provided bycontact and Si dead layer) provided by
the detector manufacturer. However,the detector manufacturer. However,
calibration standards are often used tocalibration standards are often used to
determineddetermined εενν experimentally.experimentally.
Detection efficiency εν

Qualitative analysisQualitative analysis
 Energy and efficiencyEnergy and efficiency
using differentusing different
isotopes.isotopes.
IsotopeIsotope EnergyEnergy
in keVin keV
5555
FeFe 5.8645.864
6.4856.485
5757
CoCo 6.3716.371
7.0247.024
14.36114.361
241241
AmAm 11.86111.861
13.93613.936
17.72717.727
20.76620.766
26.30526.305

RELATION BETWEEN X-RAY YIELD AND AREAL
CONCENTRATION

FORMULA FOR DETERMINING AREAL
CONCENTRATION
Note that yυ can be computed theoretically and is the
yield per µC of charge per unit areal concentration per
steradian for 100% detection efficiency.
mz =
Yυ
Ω • Qp• ευ • yυ
By re-arranging the equation in the previous slide ,
we have the following formula which can be used to
determine the areal concentration of an element in a
thin specimen.

Use of filters
In PIXE measurements of intermediate and thick specimens, the
bremsstrahlung background can be large, especially in the low-
energy region. This will bring about poorer detection limits for
the detection of light elements as well as diminish the sensitivity
for the detection of medium- and high-z elements. To reduce
the low-energy background, a plastic or metal filter of
appropriate thickness are often placed between the specimen
and the detector.
If a filter is used, one must correct for its effect of x-ray
attenuation in calculating the concentration of an element. Thus
the equation for cz becomes:
where fT is the transmission factor of the filter, which
is a function of X-ray energy.

A PIXE spectrum consists of two components:
peaks due to characteristic X-rays, & a background continuum,
X-ray energy (kev)
X-raycounts
as can been seen from the spectrum shown below, which is
obtained from a lung tissue taken from a patient suffered
from hard metal lung disease:

Output from detector
Voltage
Time
• X-ray hits
detector,
produces lots
of charge,
which is
collected by
detector over
a short time.

Conversion to digital (ADC)
• The voltage signal
from the detector
must be digitized
before it can be
stored in the
computer. The curve
is integrated to find
the area.

Conversion to digital (ADC)
• The area under the
curve is measured
and is converted to a
digital number. This
number is a measure
of the energy of the
X-ray seen by the
detector.

Histograms
• The number from the ADC is collected and
stored in a histogram – a collection of bins (we
use 2048) scaled to match the energy spectrum
of the X-rays we want to measure.

Histograms
• As the ADC gives the number for the X-ray
energy, the count in the appropriate bin is
incremented, and over time the shape of the
distribution begins to develop.

Typical Spectrum (INCO 625)
• Here’s a histogram from a run on one of our standards,
INCO 625. There are lots of peaks here.

Equation for concentration of given element inEquation for concentration of given element in
a specimena specimen
( ) ( )zNpCbNzY xziavx σ
π
εω 




 Ω
=
4
( )zYx
avN
ω
b
iε
π4
Ω
Np
zC
( )zxσ
: X-ray yield
: Avogadro number
: Fluorescent yield
: Line intensity
: Efficiency of the detector
: Solid angle
: Number of protons
: Concentration of element
: X-ray production cross-section

Comparison with standardsComparison with standards
( )
( )
( )
( )dardSY
SpecimenY
dardSC
SpecimenC
z
z
z
z
tantan
=
• Detection Limits

Detectable concentration as a function ofDetectable concentration as a function of
atomic number and proton energy in a typicalatomic number and proton energy in a typical
PIXE arrangementPIXE arrangement

ApplicationApplication
 Air PollutionAir Pollution
PIXE spectrum of air pollution collected on a thin foil

 Industrial applicationIndustrial application
K X-ray spectra recorded for (a) 4x10-11
g of Ti, (b) 8x10-10
g of Cu

Botany
PIXE spectrum from analysis of Shaji plant material

 ArcheologyArcheology
PIXE spectrum of potshred (broken piece of pot)

 ArtArt
PIXE external beam setup for analysis of an art document

 Medical scienceMedical science
PIXE analysis of Bromine
in blood plasma

New AvenuesNew Avenues
 Micro PIXEMicro PIXE
 Proton MicroprobeProton Microprobe
 Applications of mapping in differentApplications of mapping in different
areasareas

Micro PIXE setupMicro PIXE setup

Proton microprobe arrangementProton microprobe arrangement

PIXE elemental maps from a bonePIXE elemental maps from a bone

3-dimensional element plot from micro PIXE3-dimensional element plot from micro PIXE
analysis of single cellanalysis of single cell

3 MeV PIXE elemental maps of Ag, Cu and Si seen on3 MeV PIXE elemental maps of Ag, Cu and Si seen on
the surface of fragment of lusterware from 9the surface of fragment of lusterware from 9thth
centurycentury
IraqIraq

STIM (Scanning Transmission ION microscopy)STIM (Scanning Transmission ION microscopy)
maps of aerosol using 2 MeV protonmaps of aerosol using 2 MeV proton

Example of direct PIXE analysis of humanExample of direct PIXE analysis of human
finger nailfinger nail

2018 HM PIXE PARTICLE -INDUCED X-RAY EMISSION AN ANALYTICAL TECHNIQUE

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