all EM beam machines produce x-rays because of beam-specimen interactions.

They come from the entire monte carlo shell of beam interaction--the x-rays given off are photons and less likely than electrons to lose their energy traveling through the sample. Typically, resolution of adjacent element locations is much poorer compared to spot resolution. For example, spatial resolution might be 4 nm, but the elemental resolution might be 30 um in a biological sample
 
 

Each x-ray produced is called a line and has a characteristic energy and wavelength.

Each x-ray comes from an electron shell vacancy being filled by another electron.

K, L, M, N

the subscripts denote how many shells the filling electron had to jump down
 

e.g., a K shell vacancy filled by a M shell electron ==> Kb X-ray.

X rays will only be produced if the acc. voltage is high enough to knock out an electron

==>critical excitation energy or absorption edge ---this is the minimum amount of energy required to knock an electron from a given shell in a given element.

These values are known and can be looked up in the tables--use 1.5-3X the critical energy for a particular line and to insure a sufficient number of x-rays are produced (to be detectable)

K lines are higher energy x-rays than L lines.  You need to dump more energy to jump into the K shell than the L shell.

 

Table describing examples of the minimum accelerating voltage (1.5x) necessary for K and L x-ray line generation

Element	K line absorption edge/minimum kV	L line absorption edge/minimum kV
Si	1.7/2.55 kV	---
Fe	6.4/9.6 kV	0.705/ 1.1 kV
Pd	21.2/31.8 kV	2.84/4.26 kV

Different elements have different excitation energies: the penetration of the beam into the sample leads to dissipation of the beam. Elements with lower critical excitation energies can be excited from deeper within the monte carlo shell. This leads to differences in spatial resolution and is more important in whole samples compared to thin sections.
 
 

In addition to the characteristic X rays, we see background, continuum or bremsstrahlung X-rays (different words for the same thing).

These arise as a result of the beam interacting with the nucleus and tightly bound electrons. See peaks superimposed over the background
 
 

X-rays have both a wavelength and an energy, and detectors can collect information on either

==>WDS wavelength dispersive spectrometer
 

the detector must be tuned for a given wavelength

works best at higher energy--greater sensitivity

==>EDS energy dispersive spectrometer
 

cheaper detector--simpler design, robust

high efficiency

can measure entire energy spectrum

use smaller beam size, currents

advantage of WDS==> spectral sensitivity-- the size of the window of detection for each detector
 

The WDS can see a window 10eV wide vs. 135 eV wide for EDS

If peaks overlap from various elements, can't resolve them as easily with the EDS compared to WDS

EDS DETECTOR
 

Collimator to limit BSE and stray X-rays

Window usually made of beryllium(limited to sodium, atomic number 11) or thin plastic to facility detection down to boron (Atomic number 5)

Detector crystal silicon wafer with lithium added in. When hit by X-ray, creates a free electron and a hole. Keep at high voltage. For each 3.9eV 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
 

1. prevent redistribution of lithium

2. reduce electronic noise

3. to keep crystal electrically isolated
 

Field effect transistor (FET) amplifies signal from the crystal--

Pulse processor: convert output of detector to voltage pulses,
 

the heights of the pulses are proportional to the X ray energy

when too many pulses come at one time, processor shuts off==>dead time

analog to digital converter (ADC) converts each pulse from processor to several pulses of equal height. Greater energy, more pulses

multi channel analyzer (MCA ) sorts pulses into different channels of different energy
 

 
Diffractor--this is a crystal that reflects x-rays coming into the detector so they are positioned to hit the counter.  Only those x-rays of the correct wavelength are correctly reflected to the counter.  To do more than a few elements, you need several different crystals with different spacing in them that will reflect different wavelenght 
 

Gas proportional counter (AT ROOM TEMP) --tube with wire in it

The  X-rays that strike the counter ionize the contained gas and cause an increased current
 
 
 
 

FET Field effect transistor (FET) amplifies signal from the counter

Pulse processor: convers the output of detector to voltage pulses,
 

The heights of the pulses are proportional to the X ray energy

when too many pulses come at one time, processor shuts off==>dead time

SINGLE channel analyzer
 

OUTPUT

spectrum plot==>analysis of total area by elements

       plot the x axis as energy, the y as counts of x-rays per second

dot maps ==> sums x rays of given window of energy or wavelength and portrays it on CRT.  An element is chosen, and then each dot in the image is colored black if no x-rays from that element are detected for that spot, vs given a color if x-rays from that element are observed.  By overlapping the maps of all the elements selected for observation, it is possible to ascertain the composition and distribution of elements in the surface (SEM) or in the slice(TEM) of the sample.
 

Qualitative analysis vs. quantitative analysis
     qualitative analysis--what elements are present

    quantitative analysis--what elements are present, and what is the concentration of each

quant is much more difficult

SET UP

1. accumulation time=> 30sec-10 minutes. Longer times give better statistics

2. beam current =>greater current, more X-rays. However, can overload detector and get too much dead time (the detector shuts off and catches up in the counting, then turns back on)

3.take off angle want to minimize absorption of x-rays by sample --usually 35 degrees off the horizontal (mount the detector on a tilt, or tilt the sample 35 degrees)

4 solid angle --want larger angle to collect more x-rays

ARTIFACTS

Systems peaks---created by interactions in the system
a) the beam can generate x rays from the aperture, which is thin and allows the x-rays to come out the other side

use molybdenum aperture coated with tungsten and graphite stubs to reduce these effects

b) the X-rays can generate X-rays by fluorescence

very tough to control, use a collimator to block x-rays not coming from the sample

--sum peaks: two x rays arrive at almost the same time==>added together

collect with different dead time and look for changes in curves

--escape peaks: x-rays hit crystal and produce silicon x-rays instead of electron hole pairs
 

the peak will therefore be reduced by that much energy

--secondary fluorescence: X-rays producing other x-rays of lower energy

--specimen topography: optimum is flat surface

otherwise get absorption, variations in take off angle

--mass loss: can lose up to 30% over several minutes, lowering concentration of element or increasing apparent concentration of retained element

ZAF correction