orbital electron results in

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15 Νοε 2013 (πριν από 4 χρόνια και 1 μήνα)

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X
-
ray Microanalysis

An inelastic collision
between a primary beam
electron and an inner
orbital electron results in
the emission of that
electron from the atom.


The energy released from
an electron replacement
event produces a photon
with an energy exactly
equal to the drop in
energy.

X
-
rays can have an energy nearly equal to that of the
primary beam electron and thus can escape from very
deep within the specimen

Energy Dispersive

Spectroscopy (EDS or EDX)

When an electron from a K
-
shell is replaced by
one from the next closest shell (L), it is
designated as a Kα event

When an electron from a K
-
shell is replaced by
one from the second closest shell (M), it is
designated as a Kβ event

K
a

K
b

L
a
-

When an electron from a L
-
shell is replaced by
one from the next closest shell (M).


The K shell will never donate its electron as this would
require an increase in energy, not a drop.

Certain events such as M
α
, L
β
, and K
γ

are only
possible in atoms of sufficient atomic weight

There are a wide
variety of subsets of

X
-
rays since each
electron shell has
multiple orbitals

An X
-
ray spectrum for a sample is composed of all
the possible signals for that given set of elements.


These will differ in terms of energies (
KeV
) and
probabilities (likelihood) scored as number of such
signals collected over a given period of time.

# Counts

X
-
ray Energy in KeV

Each element has a family of characteristic X
-
rays
associated with it

Positive identification of an element is best done by
evaluating the entire family of peaks for a given
element.

"
Bremsstrahlung
" means "braking radiation" and comes from
the original German to describe the radiation which is emitted
when electrons are decelerated or "braked" when they interact
with the specimen.

Although they contribute to the total X
-
ray signal they contain
no useful information because their energies are nonspecific
and therefore are considered as part of the background .



Bremsstrahlung

X
-
rays are the major part of the
continuum X
-
ray signal that can escape from the
deepest portion of the interaction region.

Chrysotile Asbestos Fibers

Bullet fragments (blue) can be identified on cloth
fibers and distinguished from other metal pieces by
their elemental composition

Gunshot Residue (GSR) Analysis

Gunshot Residue (GSR) Analysis


Particles are very
characteristic, therefore
presence of these particles
forms evidence of firing a
gun.


Particles normally consist
of
Pb

(lead),
Sb

(antimony)
and
Ba

(barium).


New ammunition:
environmentally friendly (no
Sb
).

The proportion of
elements present in
GSR differ slightly
and databases of
GSR from different
manufacturers can
be used to identify
what ammunition
was used in a crime.

GSR is often found
on criminals and also
on victims if shot at
close range.

X
-
ray Mapping

X
-
ray analysis of paint fragments

The combined
(a) backscatter image
and X
-
ray maps of

(b) Au,

(c)
Ba


(d) Ca


Different layers of
paint can be identified

EDS = Energy




Dispersive


Spectroscopy

WDS = Wavelength


Dispersive


Spectroscopy

X
-
ray Detection

EDS

WDS

Pulse Processor

Measures the electronic
signals to determine the
energy of each X
-
ray
detected

X
-
ray Detector

Detects and converts

X
-
rays into electronic
signals

Analyzer

Displays and interprets

the X
-
ray data

Cut
-
away diagram showing

the construction of a typical
EDS detector.

FET

Crystal

Window

Collimator

Lithium doped Silicon (
SiLi
) crystal detector

acts as a semiconductor that carries current in a rate
proportional to the number of ionization events and
acts as an indirect measurement of the energy
contained in the X
-
ray
.


Absorbed X
-
rays create an ionization event
similar to that of a
scintillator


Each ionized atom of silicon absorbs 3.8
eV

of
energy, so an X
-
ray of 3.8
KeV

will ionize
approximately 1000 silicon atoms
.

Collimator

to limit BSE and stray X
-
rays


Window

usually made of beryllium (limited to sodium, atomic
number 11) or thin plastic to detect down to boron (Atomic
number 5) protects cooled crystal from air.

FET

Crystal

Window

Collimator

Detector

: crystal silicon wafer with lithium added in. For each
3.8
eV

from an X
-
ray, produce an electron and hole. This
produces a pulse of current, the voltage of which is
proportional to the X
-
ray energy. Must keep the crystal at LN
temperature to keep noise to a minimum.


FET

:
The field effect transistor is positioned just behind the
detecting crystal. It is the first stage of the

amplification process that measures the charge liberated in
the crystal by an incident X
-
ray and converts it to a voltage
output.

FET

Crystal

Window

Collimator

Multichannel Analyzer (MCA)

The changes in conductivity of the
SiLi

crystal can be
counted for a given time and displayed as a histogram
using a multichannel analyzer.

Multichannel Analyzer (MCA)

MCA consists of an analog to digital converter which “scores”
the analog signal coming from the field effect transistor (FET).
Newer systems employ a digital pulse processor which
converts the signal on the fly

Then

Now

Factors affecting signal collection

Distance between detector and X
-
ray source

Angle at which detector is struck

Volume of signal collected.

For a given angle of electron incidence, the length of
the absorption path is directly proportional to the
cosecant of the take
-
off angle,
φ


Take
-
off Angle

Solid Angle

The solid angle
Ω

of a detector is defined as angle
of the
cone

of signal entering the detector. The
greater the size of the detector surface area the
greater will be the solid angle.

Larger
SiLi

crystals will be able to sample a larger
volume of signal (better
Ω
)
but because of
imperfections in the crystal they have slightly greater
noise and thus slightly lower resolution.

One can also increase the solid angle by placing
the detector closer to the source.

One then tries to maximize both the solid angle
and the take
-
off angle.

One reason that the final lens of an SEM is conical in
shape is so that the EDS detector can be positioned at
a high take
-
off angle and inserted close to the
specimen for a high solid angle.

William Henry Bragg


1862


1942

Nobel Prize in Physics


1915

X
-
ray diffraction in a crystal.

Like an electron beam an X
-
ray
has its own wavelength which is
proportional to its energy

Crystal:

A solid formed by
the solidification of a
chemical and having a
highly regular atomic
structure. May be
composed of a single
element (C = diamond) or
multiple elements.

Cubic

Hexagonal

If a wavelength enters a crystal at the appropriate angle it will
be diffracted rather than being absorbed or scattered by the
crystal

For a given wavelength
λ

there is a specific angle
θ

(Bragg’s angle) at which diffraction will occur.

Bragg’s angle is determined by the d
-
spacing
(
interplanar

spacing) of the crystal and the order of
diffraction (n = 1, 2, 3….).

A WDS detector takes advantage of the fact that an X
-
ray of a
given wavelength can be focused by a crystal if it encounters
the crystal at the proper Bragg’s angle.


To better accomplish this crystals are bent and ground to form
a curved surface which will bring all the diffracted X
-
ray
wavelengths to a single focal point, thus the crystal acts as a
focusing lens.


To change the Bragg’s angle the diffracting crystal and
detector can be moved together relative to the
stationary specimen along a circle known as the
Roland Circle
.

WDS detectors are quite large and must be positioned around
the specimen chamber at an angle to take advantage of
maximum take
-
off angle and maximum solid angle

A microprobe is a specialized SEM that is outfitted
with an EDS detector and array of several WDS
detectors.

Different diffracting crystals can
only diffract certain wavelengths
(even with the changes in Bragg’s
angle) so an array of detectors
must be used if one is to be able
to detect K, L, and M events for
many different elements. Since
WDS detectors do not need to be
cooled they are windowless and
can detect down to
Berylium

LiF = Lithium fluoride; PET =Pentaerythritol; and TAP = Thallium acid
phthalate.

Specimen preparation for WDS

Samples must be conductive since high
KeV

is used
(Carbon coating if not naturally conductive)


Samples must be flat (polished) as geometry of
sample to detector is crucial and also minimizes
artifacts when doing quantitative measurements.

A comparison of two
spectra collected with
EDS and WDS shows
how peak overlap and
energy spread can
serve to obscure the
information in an EDS
spectrum

Quantitative X
-
ray Analysis

If one wants to quantify the relative
amounts of different elements present in
a complex sample one has to account for
a number of factors and carry out a
correction of the data

One must account for other elements
present in the sample and whether their
individual peaks overlap with each other
creating a “shoulder” that can mask the
presence of one element or distort the
midpoint of another.

Several methods to correct the spectra. ZAF
takes into account the Atomic Weight (Z),
effects of Absorbance (A) and effects of
Fluorescence (F) in adjusting the data to give
the correct values.

Applications of X
-
ray Microanalysis

Secondary

Electron image

EDS can be added as a component of a TEM

Requires an angled detector (for take
-
off angle) and
scan coils in the column to function as a Scanning
Transmission Electron Microscope or STEM.

EDS can be used to
identify elements present
vacuoles or inclusions.
Must take into account
elements present in the
embedding medium